Fixing a major mistake

After sawing the halves of my pod mold apart, the first thing I did was to fit the two halves together to see just how much clearance there was (should be right about 0.010″).  Unfortunately, this is what I got:

The two halves should fit fully together with no gap, so I started trying to figure out what went wrong.  As it turned out, this was simply a result of picking precisely the wrong surfaces to be machined, and I wound up with the exact inverse of what I wanted.  I was guessing that I’d have to simply recut the molds (I say ‘simply’, but it would be kind of a pain), until I realized that I should be able to simply make molds from the molds, thus flipping the surfaces back to how they’re supposed to be by turning the male half into a female half and vise versa.  Copying molds in this way is not uncommon for composite work – a male master or ‘plug’ might have a number of molds pulled from it, with each mold being used to create dozens or perhaps hundreds of parts before it starts getting warped or damaged.  If another mold is needed, you simply cast a new one from the master.

The first step was to polish the surfaces (which I would have had to do without a screw-up anyhow), for which I wet sanded each half with 400 grit sandpaper to eliminate the ridges left from machining (I could have done a little better, as there are some ridges left, but I think it will certainly be good enough).  After that, I applied a coat of Partall #2 and buffed it off. After applying and buffing 4 more coats (to ensure that there are no missed spots – if the epoxy adheres to the mold surface itself, you have a ruined mold), I was ready to apply a coat of PVA. I diluted the PVA about halfway with distilled water and then brushed it on the surfaces with an acid brush, then set the parts aside to dry (you can see the green tint of the PVA pooling in low spots on the Corian mold halves).

Next I needed to dam up the sides of each half, so I used some sheet foam and hot glue, taking care to leave no gaps between the Corian and foam (lest epoxy leak out the side).

Then it was time to mix up some tooling resin, which is basically just epoxy mixed with graphite powder to provide a nice dark surface (which makes it easier to see when you have fiberglass properly wet out against the mold surface).  I used a little bit of West 404 filler as well to thicken the mix a little.

I carefully brushed the tooling coat over the entire mold surface and up the sides of the walls.  I then poured the remaining resin into each half, and took a blurry photo.

I tossed the halves into the oven for an hour on a very low temperature to help the epoxy ‘kick’ and start polymerizing (which is what actually causes it to harden).  With the tooling coat thickened, I was ready to fill in the remainder.  Since epoxy is expensive (and epoxy fillers aren’t cheap, though certainly less costly than the epoxy itself), I figured I’d try using aquarium gravel as an aggregate filler since it comes in relatively small bags for just a few bucks.  I mixed in a bit of 404 filler as well, though my mixes were a bit unbalanced – one half was gravel poor, while the other had an abundance.

The aquarium gravel turned out to be not as strong as I’d like (it chips and breaks easily), so perhaps I’ll just use sand as a filler in the future.  After letting the halves cure for a day, I used a utility knife to slice away the foam dams and a pick to dig out the hot glue.  In retrospect, I should have really used plasticine clay, as the hot glue was difficult to remove.  Separating the Corian master from the epoxy mold was simple, if brutal – just whack the block on a hard surface a few times until the epoxy half pops away (this was how I discovered that the aquarium gravel isn’t the greatest in a structural sense).

When I had a look at the molded halves, I was amazed at the level of detail captured by the resin – not only the minute milling ridges in the Corian and smoother areas where I had wet-sanded were visible, but even the edge of the puddle of dried PVA could be clearly seen.  I did a little more wet sanding on these halves, then applied 5 coats of wax, and then a coat of PVA (this time standing the mold halves up on end to keep the PVA from puddling).

Meanwhile, I realized I had neglected another part of the sailplane that will probably take a bit of wear from landings – the tail skid.  While there is a strip of plywood embedded, I’m sure large gouges in the foam will result the first time I miss a grassy landing strip and plow through a gravel driveway.  Rather than haul the fuse back over to Frankie’s studio to digitize the tail, I thought I’d try making a flexible mold with some OOMOO 25 silicone rubber that was past its shelf life – best to put it to use than throw it out.

I brushed it on the tail skid and threw in a few strips of fiberglass to give it a little more strength (a technique I recall seeing in a book long ago where strips of burlap were used to strengthen and stiffen a latex mold).  The silicone started setting up quickly, so I blobbed the remainder on and covered it with another piece of fiberglass.

Once cured, the silicone popped right off the tail skid.

I then made a plug in the silicone mold, using more aquarium gravel for bulk.

Just as with the foam, the silicone separated very easily from the cured epoxy.  Note that the texture of the original foam is perfectly captured.  The plug was thoroughly waxed, brushed with PVA, and set aside to dry.

Tonight I finally attempted using both the wing pod and tail skid molds to make actual parts.  I used a layer of 3oz and a layer of 1.4oz glass for the wing pod and mashed the mold halves together (I should clamp them together, but they seemed to be sticking together quite well on their own).  The 3oz glass wasn’t draping well over the tail skid plug, so I abandoned that weight and went with a layer of 1.4oz. and a layer of 0.75oz.  Trying to fit the silicone mold over the fiberglass covered plug was a bit tricky, as the shape doesn’t ‘key’ together as well as it could, but I finally called it good enough and set it aside to cure.  In 2 or 3 days I’ll see if all this work has actually yielded anything useful when I crack open the molds.

Reverse engineering for molds

On the Diamond 2500 powered sailplane, there are small ‘pods’ on the underside of the wings in front of the servos for the flaps and ailerons.  I’m guessing that these pods are intended to serve as some form of protection for the servo arm and linkage on landing, but the problem is that the pods will then be torn to smithereens (being foam, just like the rest of the wing).  While my quest to protect these 4 measly foam bumps seems to be ever-increasing overkill, it’s turning out to be a fun project and I’m learning a number of new skills from it.

While there were several ways to approach this, I decided to try making conformal covers for these pods out of fiberglass.  I could have just applied fiberglass directly over the pods, but I wanted to try something a little more precise (and replaceable, though I don’t know why I’m clinging to that notion when the plane is likely to be damaged in far more horrific ways).  I’ve been watching Tom Siler’s work on building his own fully molded F3K competition planes, and his videos are fascinating. He uses Corian for his molds, as it machines really nicely and is easily sanded and polished to provide an excellent finish on the composite parts pulled from the molds. I don’t yet have a vacuum system to bag parts, but I figured if I made a 2-piece mold, I could perfectly form pod covers without needing any sort of vacuum.

First things first – I needed to model the pod in SolidWorks.  Normally I just grab my calipers, radius gauges and other measuring tools, but the pod had me stymied – it’s a more complex feature than I initially thought and isn’t as simple as a truncated swept profile.  What’s worse is that it’s located on an airfoil, so I don’t even have a flat plane to reference.  I started to consider making a rubber mold of the pod, then casting an epoxy plug from the mold, then digitizing the plug with the touchprobe I have for the Taig (but have yet to finish wiring up), but that was turning into quite a production.  I remembered that one of Frankie’s toys is a NextEngine scanner, which would be perfect for this application, so I took the wing along during one of our Zcorp hacking sessions.

First step was to position the wing in front of the scanner itself.  The base will automatically rotate in increments if needed, but I just needed a 1-pass scan.

Once in place, let the scanner rip – a few of the laser beams are visible sweeping over the scan area, and the monitor screen shows a rough pass of the scanned pod.

I brought the generated STL into MeshLab and did some minor cleanup before bringing it into SolidWorks.  SolidWorks actually has some impressive mesh-to-surface capabilities, but since I was working with a mesh with a few holes in it, it would have taken a bit of work to get usable output (and I didn’t see a way to define a symmetry plane, but maybe I didn’t look hard enough).

Instead, I did my own surfacing, which took me quite a while.  I’m not good at it, and I know some of my techniques are wrong, but the final output should serve its intended function.

After finishing the male side of the mold, I thickened the surface by 0.010″  (I figure that should be plenty of fiberglass) to create a solid and extracted the far surface as the female side of the mold.  I set the two halves side-by-side in an assembly and exported it to GibbsCAM.

Once in Gibbs, I created my toolpaths (this shows the paths for the second operation, which uses a 0.250″ ball end mill).  After posting the file, I was finally ready to start cutting material.

Not having yet found any 1″ thick scrap Corian (everything I’ve gotten is 1/2″), I glued two pieces together.  The cold temperatures meant that the epoxy hadn’t fully cured after 24 hours, so I stuck it in front of a space heater for a day, and that firmed everything right up.

I drilled and counterbored mounting holes and then bolted the block to the tooling plate on the Taig.  This shows the results of the first pass, which was roughed with a 0.250″ flat end mill.  Note the curvature in the parting plane to match the airfoil surface.

This is the third and final pass, which used a 0.125″ ball end mill (and a generous amount of WD-40 as cutting fluid).

Once washed off, this is the result.  The pattern of the Corian makes it impossible to see any fine detail in the photo, but the surface finish is phenomenal – I used a 0.010″ stepover for the final pass (overkill, but it’s my CNC, so I’m not paying any extra for machine time) and it looks superb when you hold the machined surfaces up to the light.  All that remains now is to chop the two halves apart, then sand and polish the mold surfaces.

Just one word… Are you listening?… Composites.

‘Composites’ (which for a very long time I mistakenly equated with ‘carbon fiber’) were always one of those mysterious materials to me – rare, expensive, difficult to work with, but with incredible strength. Something of a real-world parallel to Mithril or Adamantium, if you will. It turns out that there’s nothing really magic about it (though production of carbon fibers is still difficult and expensive) when you consider that the hard fiberglass chairs in a school cafeteria are composite (the fiberglass being the base reinforcing material and the epoxy being the binding matrix).  I always wanted to learn more about composite fabrication methods, and lately I’ve been learning more than I perhaps wanted to.

It started with stumbling across a video of some East European F3K competitors having a bit of fun with their DLGs (F3K is a class of aeromodeling competition with hand launched RC gliders). I never had any sort of interest in gliders whatsoever – if it didn’t have a motor, what fun could it be? But after watching the video and seeing the incredible capabilities of a 1.5m airframe that you hurl into the air yourself, I was intrigued. I attended a DLG contest in September to learn more about this particular segment of RC flying, and was smitten with the graceful purity of unpowered flight.  I now understand what some people see in ballet – I simply had to experience it in a form I could comprehend.  After the contest, I decided that I’d like to give it a try myself. Fortunately, Steve Meyer, one of the fellows at the contest (a seasoned competitor who actually qualified for the USA 2011 team) had an old plane that he was willing to sell for a good price and a few weeks later I had my own DLG to try out. Launching it the first few times was equal parts terror and exhilaration, but I started to get the hang of things quickly enough and even managed to catch it a few times (outside square loops on the other hand, well, I’ll give that one a few years). I gained more valuable experience in RC flight during the first half hour with the DLG than I had all summer. I then began to fly it every chance I could get, though the creeping wind and colder weather kept thermals to a minimum. On the last reasonably warm weekend of November, I was flying at the local park, trying to put more power into my launches. When spinning around, my fingers slipped off the launch peg, and I felt more than heard a sickening crack as the plane made a beeline right for the nearest swingset. In retrospect, I had enough time to get my fingers on the sticks and pull up, but was shocked enough that all I could do was watch the impact.

Would you believe that this is what it looked like post-impact?  Incredibly, airframe damage was pretty minor – a few small creases on the wings, ailerons and horizontal stabilizer.  Really a tribute to the sturdy design of Dr. Mark Drela and Aradhana Singh Khalsa’s beautiful execution of it in kit form (and certainly Steve’s expert assembly of the Light Hawk II).  The right aileron servo was completely stripped, but I hoped that it wouldn’t be too horrific to replace and ordered a new Dymond D47 while I surveyed the rest of the damage.

This is the worst of it – a crease on the underside of the right wing (with a slightly smaller matching crease on the top surface), and the reflective tape removed to reveal the stripped servo.  The servo has been glued in place, so I’ll need to Drem-mill (Drem-mill?  Dremel? Get it? Eh?  Er, never mind…) it out of the pocket.  Fixing the crease itself would probably be easy, but fixing it well seems a more daunting task.  Steaming small dents out of the wing (which is a high strength blue foam covered in Kevlar) works quite well, I found – just pour boiling water over the dent, and the foam magically reverts to an undamaged state (or use an iron pressed against a wet paper towel on the wing).  These creases however, don’t steam out – trust me, I tried.  So I’m saving them for later while I get up to speed with the basics of composites.

This is the left wingtip, where the launch peg is located.  You grasp the peg with your index and middle fingers, whirl around, and release the plane (a left handed pilot would have the launch peg on the right wing).  I think the crease in the Kevlar skin just above the peg may be the result of the sickening crack I felt/heard (honestly, I though the peg itself had snapped off).  From what I’ve read, this is caused by swinging your arm downward on release (a natural movement, as it’s how you would throw a baseball).  This makes sense, as the buckling happened only on the underside of the wing.  My plan is to poke a few small holes into the crease, use a syringe to inject epoxy into the damaged area and clamp it to a flat surface while it cures.

For anyone wondering “why is it made out of Kevlar – you expecting to be shot at?  Har har!” – this is the reason.   The nose took the brunt of the impact from what I can tell, and you can see the cracks at the 4 corners of the rear cutout of the pod where the epoxy matrix entirely failed.  If this was just carbon fiber or fiberglass, I’d have a lot of pieces and fragments in the photo and would be looking for a whole new plane.  Carbon and glass are strong but brittle, while Kevlar is extremely tough and the fibers will remain intact to hold the airframe together even with such damage.  The nose is one of the most interesting parts of a DLG, if for nothing else than the sheer density of components.  The battery is crammed all the way forward with two small servos behind it (I don’t even want to think of the difficulty in getting the control rods connected), followed by a voltage regulator and low voltage beeper, with the radio receiver bringing up the rear.  There’s even a ballast tube further back sized for tungsten weights, but I haven’t even gotten to that point yet.  The red cord coming off the side of the nose is the charging plug, which you remove to turn on the electronics (this eliminates a standard power switch, saving precious grams).  Custom wing airfoils, specific weave patterns of fabrics, CNC made molds for airframe components – these are standard fare for top competitors, making DLGs truly the F1 cars of the air.

So as a new driver with a broken F1 car, it seemed prudent to start small on the repairs.  The vertical stabilizer and rudder (light blue) were unscathed, but the tailplane (in yellow-brown Kevlar) sustained a very slight crease just to its left of the centerline.  It was in good company – Steve had pointed out the damage on the right side and had explained to me how he fixed it with a little scrap of lightweight fiberglass cloth and CA (CA is short for short for cyanoacrylate, better known as ‘superglue’ for those who don’t frequent hobby shops or RCG).  I steamed out the crease as much as I could, and then applied a small rectangular patch of fiberglass cloth wetted with foam safe CA (regular CA will partially dissolve polystyrene foam, so you have to be a bit careful with adhesive choices).

I used a hex wrench set as a weight to keep the stabilizer flat as the CA cured (a piece of kitchen plastic wrap keeps it from bonding to the cutting mat).

The fix turned out quite well, and I had to position this shot just so in order to glint the light off of the repaired area.  The ever-so-slight delamination of the Kevlar from the foam core is no more, and the tailplane is ready for action.  Still, the rest of the airframe remains.  I decided that getting up to speed with epoxy and fiberglass would be worthwhile before diving into more involved repairs.

One aircraft I’m preparing for the upcoming flying season is a Diamond 2500, which is a giant (at least as far as I’m used to) 2.5m powered sailplane. I chose it because my 2m Radian has gotten a bit boring, and because I wanted a good platform for FPV flying. The Diamond 2500 has a generous amount of room in the fuselage for extra gear, and I’m thinking of making a replacement canopy/cover that has the camera and associated equipment attached (so I can easily swap back and forth between camera ship and sport flyer). So the first thing I tried was to just make a rough copy of the existing canopy.

I started with a piece of plastic wrap taped over the canopy (it easily peels away from cured epoxy) and then draped a piece of 1.4oz (the weight for a square yard of material) fiberglass cloth over the canopy and carefully wetted it out with laminating epoxy by using an acid brush.  Once that layer was done, I thought I’d be adventurous and put down a strip of Kevlar tape (which is just the name for a ribbon of woven material – there’s no adhesive involved), which I also wetted out.  Finally, I added one more piece of 1.4oz fiberglass and wetted that out, then covered the whole thing with another piece of plastic wrap (hoping that it would help squish down the layers).  Well, I now see the value in vacuum bagging, as the Kevlar strip has air inclusions all along it (you can see that it’s only partially wetted out in the center).  I was trying not to overdo the resin (very dry layups are the norm when building DLG gliders – every gram counts), but without bagging, there’s no good way to eliminate the trapped air without simply adding more epoxy.

Once I peeled off all the plastic wrap and trimmed the edges, it didn’t look that awful for a first attempt.  Rather floppy, though – a little less rigid than if it were made from blister pack plastic.  Still, I had absolutely zero concept previously of how a completed part would feel, so now I know that more layers are needed to get the rigidity I need for the part (and that skipping the Kevlar tape is a good idea).

Now that I have some bits of fiberglass, carbon fiber and Kevlar, I realize how useful these can be in simple household repairs.  Superglue and epoxy by themselves rarely hold up as well as hoped, but adding a bit of reinforcement can do wonders.  On a replacement rearview mirror I purchased for my truck, the plastic tip was broken off in the package.  I figured a superglue-only repair would likely fail in short order, so I used a few small pieces of Kevlar and carbon fiber tow sandwiched between patches of fiberglass to act as reinforcement.  Dousing them all with CA and letting it cure provided a reasonably stout repair that has held up just fine for several months (and will probably outlast the truck itself).

Ugly, but far more expedient than getting a replacement (and being on the backside of the part, nobody will see it anyhow).

The Zcorp lives!

A few weeks ago, a sharp-eyed coworker mentioned that he saw a rapid prototyper on Craigslist that I might be interested in. It turned out to be a Zcorp Z402C powder bed machine (a technology developed at MIT, which lays down complete powder layers and fuses them selectively rather than depositing material in a specific path like my Stratasys). The machine came with a depowdering station and a wax infusion machine, which are nice bonuses – not all used Zcorp machines come with them. The machine was residing at a high school (which was blessed with a very nice technology program) and was currently inoperable (though had been mechanically sound when put into storage some 3 years ago).

The Z402C has a powder supply on the left, which it wipes in a layer to the right (covered by the gantry) and then fuses the powder with an inkjet printer head inside the gantry. After each pass of the inkjet head, the supply is raised a little, the model bay is lowered a little, and a new layer of powder is spread.
Once the part has been printed and is carefully removed from the loose powder (which is basically plaster in this case), excess powder is dusted off in the depowdering station, which is essentially a fancy sandblast cabinet. Within the cabinet are a very nice air compressor and vacuum - no hardware store grade stuff here.
After depowdering the part, it is still quite fragile and needs to be strengthened. This can be done by dousing it with superglue, though this can warp the parts. An alternative is to use this nifty wax dipper, which dunks the part into a heated bath of paraffin for a selected time.

The seller said that they had diagnosed the problem to be a faulty motherboard (thankfully meaning a standard PC mobo, and not a custom proprietary PCB), and later, upon reviewing the photos I had taken when inspecting it, the issue jumped out at me:

Ho, ho!  All that ails the motherboard are some blown capacitors.  Getting this thing running seemed like a distinct possibility.  Just one problem – I really didn’t need (or have space for) another rapid prototyper, much less a powder bed unit.  But for a measly $500 for the whole system, I couldn’t just let it go.  Fortunately, I managed to convince Frankie (not that it took much arm-twisting) that it would be a perfect addition to the technology lab that he’s setting up for the art school.  Plus, much like with the Cole drill, I like knowing where I can put my hands on a particular tool, even if I have no use for it at the moment.  So I told the seller to consider it purchased, and I’d be by in a week to take delivery.

After replacing the trailer hitch on the truck as well as the long-missing passenger side rear view mirror, I had a vehicle that met U-Haul’s rigorous requirements for trailer rental (in retrospect, I could have pulled up on a moped with a ball hitch and they still would have gladly taken my credit card).  Loading the equipment onto the trailer was easy – being in a high school’s shop, there were plenty of kids to heave the stuff onto the trailer and strap it down.  Once at Frankie’s studio that evening, we had notably less manpower at our disposal, but Frankie and I managed to manhandle all 3 pieces into the lab with no casualties. Charles dropped by later, and we all surveyed the bounty of the haul. We found the original machine invoices, and I forget what it all cost, but it was well over $50,000 when new. There were a few bottles of binder, a box of powder, documentation, little odds and ends… But we still had a dead machine. I took the motherboard and hard drive home to source replacement caps and image the hard drive (I recall reading somewhere that it’s a good idea to have a backup image of your Zcorp’s drive). Meanwhile, Frankie found the exact motherboard on Ebay as a refurbished unit, and bought it right away for about $175. Expensive, but when dealing with decade old PC equipment, it can be hard to find specific items. At any rate, we can always replace the caps on the original in the future if the refurb unit blows up someday.

The other week, we all met again at the studio to install the replacement motherboard and flip the power switch for the first time.   I had wisely snapped a few photos of the cable connections before taking out the busted motherboard, so installing the new one and putting the drive back in place went quickly.  Powering on the machine didn’t work, though – after about 20 minutes I found that I had plugged a ribbon cable in 2 pins off – after fixing this, the power button actually functioned (interestingly, the power button connects right to the pins on the PC motherboard, which remains in full control – I would have expected some other power/monitoring board to control the system at such a base level).

After that, we had a machine that was powered on, but not actually doing anything.  We poked at it for a good long while, trying to coax it into doing the startup dance that the manual indicated it should be running (the video output that we hooked up really wasn’t of much help for anyone who wasn’t a trained Zcorp technician, but it did seem to indicate that the machine was at least reading some encoder feedback).  After much web searching, Frankie tried jumpering some photosensors to bypass them, and suddenly the machine began to move the gantry of its own accord.  Victory!

We still didn’t have a null modem serial cable to communicate to the Zcorp with (needed to actually download files to it for printing), so we called it a night, pleased that we had resurrected the beast.  This past weekend, Frankie managed to scrounge up the needed cable and set the machine running a few layers.  It still needs calibration, the water cleaning the binder lines needs to be flushed through, and the screws and shafts could use a bit of cleaning and lubrication, but it’s a very healthy start to what I hope will be a great machine for the lab.

Hot stuff

I started building a Goodman-Holmes heat treat oven some years back, but never got terribly far with it, as I was to the point of puzzling over how best to cast the refractory cement (once the slabs are in place, the whole unit gets welded together, so replacement of the refractory is pretty much impossible).  Plus, the refractory cement suggested in the book (sourced from McMaster-Carr) turned out not to be rated for direct contact with flame. I’ve since started looking more at just getting a kiln for the purposes of heat treating, as temperature control should be much more precise with an electric rather than gas system (plus never having to worry about running out of propane).

As such, I’ve been checking craigslist from time to time for kilns, and found one for $150 that looked like just the sort I’d find useful.  Roomy interior, front opening door, 220VAC power.  Little old lady drove it only to church on Sundays (actually, the seller’s aunt used it from time to time to fire ceramics – close enough).  Looked like something out of the 1950s and even smelled like the piece of vintage lab equipment that it is.  I bought it the next day and somehow managed to single-handedly manhandle it into the garage on its accompanying stand.

The only thing I’m not sure of is whether an electric kiln will be more cost effective than a gas oven.  Probably, but the mere fact that the unit has a much larger interior than the gas unit already puts it ahead.  It’s large enough to heat treat a big knife blade, which is a project I keep hoping to attempt one of these days (I’ve had the O-1 bar stock patiently waiting for me for a very long time).

After purchasing a length of appropriately sized (the meter goes up to a whopping 50 amps, after all) cable to form a suitable extension cord, I plugged the unit in and let it heat up.

It drew a constant 25A and I let the temperature climb all the way up.  Note that this unit has two temperature settings: on and off, depending on whether you have the cord plugged in or not (no power switch).

I thought “what the heck” and let the kiln climb as high as it liked.  At just over 2350°F the current draw fell drastically, which I figured was just some sort of transition point in the elements that caused a sudden increase in resistance.  Well, in a sense I was correct – driving the elements to failure will indeed cause them to break, and you can’t beat the resistance of an air gap.

I heated up the broken ends with a propane torch and twisted them together as per the notes at, but it looks like I have a lot of additional breaks to track down and repair.

Stupid broaching tricks

One of the little trinkets I sell in my online paintball store is nothing more than a modified stock part that adjusts the gun’s muzzle velocity.  The modification is straightforward – I simply drill a big hole through the middle to increase the airflow (just like porting the cylinder head of an engine, this improves efficiency) and then broach a hex through that hole so that the velocity can be adjusted with a hex wrench.  From the beginning, broaching the hex has been the fly in the ointment when modifying these parts.  The concept is simple enough – a hardened steel plug with a hexagonal cross section (and cutting edges shaped appropriately) is pressed with great force into a hole drilled the same diameter (or a teeny bit bigger) as the distance between flats on the hexagon.  The points of the hexagon carve into the circle, peeling six chips downwards along the walls of the hole.  In this case, I’m using an ‘F’ size drill bit (0.257″) and following up with a 1/4″ hex broach.

The problem with the broaching operation has been twofold – generating sufficient force to press the broach through the hole to the needed depth while at the same time keeping the broach as centered as possible with the hole it is cutting.  My initial method was to use a 5C collet fixture on the bed of the mill to hold the workpiece while gripping the broach in a drill chuck (with a short piece of metal rod as backing) in the spindle.  I’d crank down hard on the spindle feed lever (spindle not rotating – this is entirely linear) and plow the broach through the part with a fair bit of effort – while this worked just fine, I always felt like I was abusing the machine (though my buddy tells me he knows of a shop that punched metal parts in exactly this way on their Tree mill – they are certainly rugged).

As such, I’ve looked for alternative ways to broach the holes.  The most interesting method is rotary broaching, which is also more descriptively known as wobble broaching.  Some years ago in either The Home Shop Machinist or Machinist’s Workshop was a project for a nice rotary broach holder that could be fabricated out of mostly scrap with a bit of work.  Unfortunately, not only am I cheap, but I am also lazy.  The concept did give me an idea, though – the Taig headstock now comes standard with an ER16 spindle.  If I could simply bolt a headstock to the toolpost on my lathe, I’d be set.  I ran the idea past Nick Carter, who agreed that the idea seemed sound in theory, so I purchased a Taig ER16 headstock from him.

Taig ER16 headstock and mounting plate

Ideally I’d make my own dovetail block to bolt the headstock to (the dovetail then mates to the quick change toolpost), but laziness got the best of me here (and then bit me as will be seen shortly) and I bolted the headstock to an unused boring bar holder.

Boring bar holder with mounting plate attached for the ER16 headstock
ER16 headstock mounted awkwardly on toolpost

Well, I overestimated the torsional rigidity of the toolpost mounting.  Really, I should have seen it coming – that’s a heck of a long moment arm between the centerline of the broach and the centerline of the shaft that bolts the toolpost to the compound.  When I ran the assembly into a part to be broached, the whole works just pivoted around the toolpost shaft, making a small (though roughly hex-shaped) indentation in the part.  The idea still has merit, I think, but even if I used a shorter block, I still worry that it wouldn’t be rigid enough.  I should probably think about mounting the headstock to the side of the toolpost so that there’s no moment arm to push it out of alignment.

I considered using the small arbor press I have to do the broaching, but it needs a more secure mounting to the benchtop, and I’d have no way to make sure that the broach is correctly centered with the part.  Unless, of course, I built some sort of system to always keep the part aligned with the broach…  Quick, to the scrap pile!

Alignment sleeve on top, with the broach and part holder underneath. Below that is the broach backup, the broach itself, and the part to be broached.

I dug up a piece of stout stainless tubing and some hardened shaft material that slid nicely through the tubing once I turned off a thou or so from one side (I love the big Keiyo Seiki – while the finish isn’t great on the cut areas of the hardened shaft, I wouldn’t have had a prayer of accomplishing such work on the 9×20 lathe).  The ‘from one side’ part is because I still haven’t addressed the 1.5 thou runout on the 5C collet chuck…  I made a short backup stop from a bolt to sit behind the broach – this let me adjust the depth of the broach without having to drill the hole in the broach holder to a precise depth.  Everything looked pretty good, so I stuck the broach in place with a bit of RTV silicone (I didn’t want anything too permanent).  I slipped a part into the part holder, then dropped the two holders into either end of the alignment sleeve.  Since the assembly looked a bit large for the arbor press, I threw the mess into the bench vise.

Broaching assembly pressed in bench vise

The part to be broached acts as a stop, so I merely needed to crank the vise down until I felt abrupt resistance.  After pulling the holders out of the sleeve, things looked like a success.

Broached to full depth

A little wiggling and the broach pulled out of the part easily.

The broaching on the part was superb – the chips were more symmetric than previous broaching done with the quill on the mill, and all that remained was to drill the chips out.  The batch of parts came back from plating on Friday, and they nearly sold out over the weekend.  Guess I better start making more.

Anno 2011

And now for another installment of “I should really get around to posting this one of these days”.  This summer’s anodizing class was cancelled (along with the pewter casting class) due to low enrollment.  Fortunately, the open sessions went ahead, so I managed to get my anodizing fix after all.  I wanted to try a few new masking techniques this time around, so I stuck to using the 3 same colors for simplicity.

For the left two samples, I masked the blue with UHU Por (sold in the US as UHU Creativ), which is similar to rubber cement but comes in a tube.  It masks just like rubber cement, but since you squeeze it from an applicator tip rather than brushing it on, you can make finer lines (of course, you could do the same with rubber cement in a blunt syringe).  On the top left sample, I masked the green by spraying some 3M 77 aerosol adhesive through a perforated aluminum plate for the polka dot effect.

The green in the center two samples was masked by sponging on the absolute cheapest white spray paint carried at Wal-Mart.  However, the ‘sponge’ used was actually the filter from a wet/dry vac – the very large open celled structure of the filter makes for a pattern that masks with harder edges due to how heavy a paint load the filter will pick up.

The purple in the right two samples was masked with Krylon webbing spray paint (silver in this case, but the color doesn’t matter since it’s used for masking and then removed).  This worked quite well, and I’m wondering if the nozzle from the can can be used to spray a webbing pattern with other (cheaper) spray paints.  However, I recall reading somewhere that the webbing effect as practiced by custom car painters requires a thicker paint blend, so perhaps the same holds for aerosol cans.

These samples show more of the same, along with swipes of rubber cement on a brush (which is still a favorite technique for throwing down ragged bursts of color).

After playing with samples, I dove into actually anodizing a paintball gun that I had polished and prepped.  Frankie took some excellent photos of the process and posted them on his blog.  I was very pleased with the result, though I messed it up a little during the sealing on some of the parts.  I had the above samples in the same boiling water bath, and the green on the samples leached out and colored the blue on the gun parts into a slightly aqua color.  Additionally, there’s a spot on the trigger frame where the anodizing can be scratched off with a fingernail, so something isn’t quite right there (of course, this is the first time I’ve tried anodizing parts that are actually intended to be used).  I wound up purchasing some proper anodizing sealant solution, an immersion heater and an insulated cooler to hopefully improve the sealing stage.  Now I just need some free time to go visit Frankie and try it out with another gun.

I made a Thingi

I’ve made several upgrades to my trusty Taig CNC mill over the years, but one of the best was replacing the original headstock with an ER16 headstock. This upgrade has thankfully become standard on the Taig machines, as the original proprietary collets were pretty lousy (and only allowed tool shanks of 5/16″ diameter, versus the far more versatile 3/8″ offered by ER16 collets). I quickly became enamored with the ER16 collets and now have a pair of 3/4″ shank collet holders for the Tree mill, one of which permanently holds an edge finder (this serves as a much more affordable alternative to an actual 3/4″ shank edge finder).

As my collection of ER16 collets grows (and I haven’t even started acquiring any metric sizes), I found that my storage method (consisting of keeping them on whatever relatively horizontal surface is available – oddly enough, also my storage method for everything else) was rather lacking.  Dropping a precision ground object on a concrete floor is seldom beneficial, so I looked for a better system.  While storage caddies for R8 and 5C collets are readily available (and I have a 5C collet organizer that is immensely helpful – when I remember to return the collets to it, that is), I’ve found no comparable options for the diminutive ER16 collet.  [edit – Naturally, after completing this project and post, I managed to find just such a thing.]

Of course, the obvious solution is to make something myself. A simple tray with appropriately sized holes would be functional enough, but I wanted something with just a little more elegance. While some of my most treasured tools have wooden cases, I have no problems with a good plastic case (and have on at least one occasion purchased a really crummy tool for no other purpose than for the halfway decent plastic case that it comes in).  So I whipped up a box in SolidWorks that could contain 15 ER16 collets with a matching lid.  I added some bumps around the outside edge between the halves to key them together so the lid would stay in place, and then sent it off to the printer.  The result was the box mentioned in this post from a few months ago. It wasn’t a great quality print as mentioned in the post, and I had incorrectly estimated the sizing of the cutouts for the collets which left them sticking up too far to let the lid close fully.

I gave it another try with a fixed model in Insight with the black Bolson ABS material, and things fared much better.  The interface between the support and model material wasn’t great, but I quickly discovered part of the problem:

The lid (upper right) has a darker triangular patch on the bottom left corner.  This is because no material was deposited there on the first model layer – there was so much ooze from the model material that a good deal of it flowed out during the lengthy build of the support layers.  As the machine was trying to print the perimeters of the first layer and the start of the infill on the lid, no material was coming out since the liquifier wasn’t full. Thankfully, this can actually be accounted for in Insight, as you can instruct the printer to purge material for longer than normal in order to top off the liquifier – I’ll need to remember to do this on builds with lots of base layer surface area.

No matter – collets fit this version just fine, and I’m not terribly concerned about aesthetics on something that’s going to get knocked around in the garage.

My original plan was to build hinges into the model – I wanted to have lugs coming off the back of the base and lid with circular recesses into which small disc magnets would be glued.  The magnets would attract each other and act as a hinge axis while hopefully providing enough friction to keep the lid open even when at an angle.  I’d still like to explore that concept in the future, but to finish this project in a hurry I simply used a pair of small hinges from Lowe’s.  I now have all my collets in one place next to the Taig in easy reach, and was pleased enough with how it turned out that I uploaded the design to Thingiverse. Much to my delight, it was chosen as a featured item!

Quick CNC work

I’m always impressed by Frankie’s ability to machine one-off parts on his Taig with minimal time spent on generating the toolpaths, which is something that I want to become better at. I’ll frequently over-think and puzzle over the CAM side so much that I wind up just bashing out parts manually on the Tree. However, for a recent project I needed to do a lot of cutting in odd shapes that would be crazy to do manually for a single part, so I gritted my teeth and dove in.

As part of my growing RC aircraft addiction, I had purchased an Ikarus SU27-XXL kit as a fun ‘zoomy’ plane to advance beyond my trusty Slow Stick. My original plan was to use the brushed motor included with the kit – why discard a perfectly good motor, even though it may not be as powerful and efficient as a brushless one?  Well, a good reason is that brushed ESCs (electronic speed controller) are much more difficult to find these days than their brushless counterparts.  So I went brushless anyhow and purchased a motor and ESC.

Since the new 400 brushless was mounted at the base rather than the face, I couldn’t use the included light plywood motor mount.  I needed to build my own custom motor mount, and I happened to have some 3″x3″ squares of 1/16″ G-10 fiberglass sheet left over from a project that would make for very sturdy construction.  I drew up the needed parts in Cadkey and then tinkered with GibbsCAM at work to hopefully output usable toolpaths.  Fortunately, I’ve gotten much better in this regard, and the G-code worked out just fine.

I used a hunk of scrap polycarbonate bolted to the tooling plate on my Taig as a sacrificial base.  Frankie recommended using carpet tape to hold sheet stock flat for machining, and it worked like a charm.  I’m using a 1/32″ carbide cutter to do the milling – I think it cut through the sheet in 3 or 4 passes.  When done, the parts were easily pulled off of the base.

After removing the tape and adhesive with a ‘Goo Gone’ type of solvent, here’s the parts I had.  As it turned out, I could have skeletonized them far more than I did, and using 1/32″ G-10 may have been an even better material.  I did have to do a bit of filing by hand to make things fit – the original plywood parts were laser cut and so had perfect square cornered slots, which obviously can’t be done with a round endmill.

Glued together, it looks pretty good!

Fits beautifully on the plane and certainly looks like the beefiest part of the entire airframe.

Unfortunately, she would never look this good again…  On the final flight (just after I had moved up to a larger prop that finally provided the performance I wanted), I pulled out of a fast low level loop right into a tree, and the brittle Depron foam snapped all over.  The motor and mount tore free from the plane and was unscathed, however, so I’ll be dropping the unit into a scratchbuilt MiG-29 made out of pink sheet foam from the home improvement store.  I have a feeling that the motor mount will easily outlast that airframe as well…

Retro drilling

My interest in hand tools is generally rather limited.  While I admire (and am frequently awed by) the skill of artisans the likes of Roy Underhill (who is arguably the patron saint of human powered woodworking), I rarely find metalworking hand tools to be anything more than quaint when powered alternatives exist. There are exceptions, of course – I still don’t have a bandsaw in the garage, as my hacksaw is far more space efficient and far less expensive (plus, I can bring my bar stock to work and use one of our large cutoff saws). Hand files are almost always more useful than a powered filer, etc. Drills, however, are another matter – you would have to be daft to want to use a hand drill rather than an electric drill. Or so I had thought.

Many years ago I purchased The Machinist’s Bedside Reader series from Guy Lautard. The third volume had a fascinating description of a very old (though at the time, still in production), very simple hand cranked drill. What made the device so interesting was that it was able to drill holes through almost anything thrown at it – steel armor plate, bearing raceways, high speed steel, even plate glass. By hand. The secret to the Cole Drill was a threaded collar below the crank arm that applied massive downward pressure.  “Low speed, high feed” in machining parlance.  Not only that, but the drill was designed in a very modular fashion – the column is nothing more than a piece of pipe or solid rod, as the drill is generally intended to be bolted to whatever it is that you’re trying to make holes in.  Rather than taking your work to a drill press, you take the drill press to your work.  Granted, you can do the same with a portable electric drill, but the Cole has the advantages of rigidity, extreme feed pressure, no electricity needed, and won’t tear your arm off if the bit catches.

As mentioned, the Cole Drill was still being manufactured up until maybe 2005 or so by Cole Tool Mfg.  Despite being ‘old’ tech, they still commanded a rather hefty retail price (presumably limited demand led to the product being discontinued).  While the drills have been routinely available used on Ebay since that time, it seems that prices have been going up – I seem to recall the going price to be around $60 or so a few years back, but now it seems that getting one for under $100 is a bargain.  Admittedly, I have zero use for a Cole Drill.  However, given the ‘field expediency’ of such a tool (drilling holes in a truck frame miles away from a power source being a good example), it’s a tool that I’d really like to be able to put my hands on in a hurry should the need ever arise.

I finally found one on Ebay that wasn’t horribly expensive, owing to a fairly rusty look to it.  However, the seller said that the drill had been purchased new, had been barely used, and had been sitting in an Arizona workshop for the past 30 years or so.  There was a pretty good chance of it cleaning up very nicely, so I bought it.  When it arrived, I eagerly opened up the box to have a look.  ‘Stout’ would be the adjective at the top of the list when attempting to describe the unit.  It was a little larger than expected, and most certainly heavier.

I hosed it down with Gibbs spray (another product touted by Guy Lautard) and set it aside to soak in and and help remove some of the rust. After wiping it off, things looked a bit cleaner, and I took it along to metalworking class so that Frankie could make patterns from it and hopefully bang out a few castings.

Once I had the drill back in my hands, I printed out a few pieces of information from the web and gave them and the drill to my dad as a long-planned present.  Dad is one of the few people I know who has the mechanical ingenuity to use such a tool to its full potential, and will probably have far more opportunities to put it to good use than I ever will.  But at least I now know where I can borrow one in a hurry if I ever need it!