Revenge of the Vibratory Deburrer

After a brief vibratory stint on some Roundhead bodies to test out the efficacy of my thread insert plugs during the ceramic deburring cycle, I tried turning on the motor once more only to be greeted with a loud angry hum from the motor but no movement (and an ominous dimming of the lights).  A bit of investigation showed that a wire nut had fallen off of a pair of wire ends, and they had probably contacted the steel tubing that served as the frame of the unit.  I replaced the wire nut after checking for any other signs of damage, and tried plugging in the motor once more.  Again, the lights dimmed and an angry hum flowed forth.

blownmotor

So I started disassembling the deburrer frame so I could pull the motor off of its mounting plate.  Once I had the motor entirely free, I tried plugging it in again, and it spun up like a champ.  I figured I must have knocked some bit of debris loose or something, and re-assembled the frame.  Wouldn’t you know it, I tried plugging it in once more, and again got the ‘growling of extreme displeasure’ that unhappy motors are so very good at making.  At this point I surmised that the eccentric weight that I had bolted to the motor shaft (which is what provides the vibration) was the culprit, and that the motor would spin up only when entirely unloaded.

I asked one of our EEs at work about the issue, and he indicated that single phase motors are generally an annoyance.  I mentioned that the motor had a bulged cover on one side that probably housed a capacitor, and he said that this would be the start capacitor.  Furthermore, toasting this cap as a result of the dangling wires was certainly a possibility, and that a slighty loaded shaft could be just enough to keep it from starting without the cap, giving the angry hum.  “Pop off that cover, and you may find the gooey innards of the cap coating the inside”, he suggested.  However, removing the cover revealed a fully intact, apparently functional capacitor (crude tests with a multimter indicated that yes, it was indeed holding a charge).  I brought the motor in to work and disassembled it on a bench (with the help of the EE, who had once worked for Leeson Electric, and hence knows a thing or three about motors).  Everything inside looked just fine – we couldn’t see any signs of trouble.  I re-assembled the unit and tried powering it up.  As before, it would spin up happily when unloaded, but when I held my foot against the shaft and tried turning it on, I got the angry hum.  The EE surmised that the motor may very well have a bad winding – using a clamp-on ammeter seemed to indicate that the motor was drawing quite a bit of current at idle.  To fix the winding (if even possible for one of these cheap overseas made motors) would almost certainly be more than simply buying a new motor.  Off to Ebay I went, and found a beauty of a 1/2HP Marathon for much less than I paid for the 1/3HP Worldwide Electric.

marathonmotor

Note the internal plate indicated by the red arrow – I’ll mention this in a bit.  The Marathon motor (manufactured for Graymills for use in a pump) was pleasant to hook up, as the connections are all spade terminals inside a cover plate on the tail end.  Despite being more powerful, the motor is actually smaller in diameter than the 1/3HP I had used.  I assembled the vibratory deburrer with the Marathon, and set the whole thing going on a Saturday.  A few hours later, the noise in the basement ceased, and I wondered what the heck had happened.  The motor case was really really hot, so I let it cool off for the night.  The next day, I tried spinning the counterweight by hand to see if perhaps it had seized or something, and I got a nasty shrieking squeal in return.  Great, “another blown motor”, I thought.

I disassembled the deburrer and had a look at the motor.  Thankfully the squeaking wasn’t a failed bearing as I had feared, but was the internal fan scraping against a sheetmetal plate of some sort (the thing pointed out by the red arrow in the above pic).  I tried using a screwdriver to push the plate back down towards the windings (away from the fan), but this was near impossible to do correctly, as there were vents on only one side of the housing, and I managed to tip the plate even further.  I disassembled the motor at work and carefully pressed the plate down into place (our EE said I could probably remove the plate altogether, as it was probably just for splash protection, but it’s captive due to a pressed bearing that I didn’t care to try removing).  I’m wondering if perhaps the excessive temperature walked the plate out of place and up into the fan.  Sticking a desk fan next to the motor seemed like a good idea, though I really needed to keep the motor from overheating in the first place.  Excessive slip was probably the cause of the heat according to the EE, which meant I was simply trying to use too heavy a weight.  Why I didn’t have this issue on the 1/3HP motor is a mystery to me, but at any rate I removed the hunk of steel from the weight.  I immediately noticed that the unit was a good bit quieter, and the media hardly moved anymore.  Well, at least the motor wouldn’t be turning itself into a puddle of slag while I come up with a better system.

I also noticed something slightly disturbing – some nasty chain wear.  I guess this is the cause of the sprinkling of metal dust that appears on the tub cover, and indicates that a top suspended deburring system simply isn’t going to work very well.

chainwear
This chain simply won't survive long-term

I then have two big changes that I intend to make to the deburrer – switching to a DC motor and going to a standard bottom supported tub rather than a suspended tub.  Using a DC motor will allow me to vary the speed with a motor controller, and I also won’t have to worry about excessive slip causing increased current consumption and massive heat buildup.  Even better, I can vary the motor speed in order to best match the load’s characteristics and find a sweet spot of maximum amplitude.  In order to support the unit from beneath, I bought some beefy spring stock and will have to come up with a base of some sort.  My plan is to also detach the motor from the tub plates, and have the motor attach to the new base instead.  Then I’ll use a flexible coupling (a piece of hose, actually) between the motor shaft and the shaft of a weight system.  Not only will this be kinder on the motor’s bearings, but the motor will no longer be useless dead weight in the vibrational system.

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