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.

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