spindle movement, unround holes

[bbortnick]

Hi,
Wondering about some things (related?) with our shopbot prsalpha 48x48.
1) Small holes made with profile CUT are slightly oblong and therefore one dimension is a little smaller than the requested dimension.
2) When changing bits in the middle of a file, it's very easy to move the spindle, loosing your x/y position. I can reset with proximity locations, but if x/y was elsewhere, it's very hard to restore to that spot.

Everything seems tight on the gantry, but is the most likely cause of oblong holes worn pinion gears?
Is there a method to changing the bits in the middle of a file that does not risk moving the spindle and loosing your x/y?
Thanks for the help!

[coryatjohn]

I have a 2013 PRS Alpha and when the system is powered up, the gantry is solid as a rock. No way to move it at all. I've changed bits hundreds of times and never lost position.

I suspect your machine needs a little TLC. When was the last time the pinions were changed and/or adjusted? They are a wear item and have to be changed every so often. If they're loose or worn, you'll have a lot more slop in the system.

Try shaking the gantry with the power on and the reset button pressed. At that point, the motors are energized and should lock the gantry solid. If you detect any movement or even a clicking sound, then there's too much slop in the system and something has to be adjusted or replaced.

[bbortnick]

Thanks for the tip! - can you point me to a pinion adjustment/check routine?

[bbortnick]

I should mention I hit the red stop switch and the key is removed from the control box (which powers the VFD/spindle I believe) during a mid file bit change. Note that I have found the shopbot docs on checking play and replacing pinions.

[coryatjohn]

I suggest refraining from hitting the red stop button during bit changes. It's not only unnecessary, it also cuts power to the motors, rendering them free to move. All you need to do is remove the key to access the wrench.

The purpose of the red stop switch is to immediately cut power to the system in the event of an emergency, such as the being caught in the machine or having the machine do something obviously wrong (like crashing into the work). I've used that button maybe twice in a decade and both times it was due to a screwup on my part.

[bbortnick]

Hi,
So I removed all the spindles to inspect the pinions. All 3 of the x/y pinions were a little loose on on their shafts: I don't know if the key, or the slot in the pinion is warn out, but it was not holding the pinion stationary. I tightened all the set screws and that removed the slop. I am guessing that will not last, however. Should the pinions be rock solid on the shafts with they key, and the set screws not yet tightened? check out the video:
https://photos.app.goo.gl/emoixKN1oYmB7U8z9

[dmidkiff]

You might want to check if there are 2 set screws on the pinion. There were some models that came with 2 set screws one on top of the other.

[coryatjohn]

I suggest two things for the pinions: 1. Get a allen socket to crank in some torque on those set screws. The angle wrenches just don't have enough oomph to do it. 2. Use Loctite.

[dlcw]

Hi,
So I removed all the spindles to inspect the pinions. All 3 of the x/y pinions were a little loose on on their shafts: I don't know if the key, or the slot in the pinion is warn out, but it was not holding the pinion stationary. I tightened all the set screws and that removed the slop. I am guessing that will not last, however. Should the pinions be rock solid on the shafts with they key, and the set screws not yet tightened? check out the video:
https://photos.app.goo.gl/emoixKN1oYmB7U8z9

I've been using, and abusing, my PRSAlpha since 2009. The last 2 years with an ATC. I cut at least 1000 sheets of plywood every year. Plus throw in furniture parts, 3D carvings and other stuff. I replace my pinions every 2 years.

At the start of each cutting project, I loosen all my drive motors, check tightness of the pinion screws and reseat the motors against the racks. I also clean and regrease the racks. I've had zero rack wear. I don't take chances with the pinions as they are the life blood of the machine working properly.

It's a good idea to give your machine a REALLY good inspection every 6 months (I do mine every 3 months since I cut so much). I check to make sure every nut and bolt holding the machine together are tight. Check all wires for wear, etc. If the machine is a money maker for you, you need to treat it very kindly.

In your case, I would give a a very detailed inspection and check EVERYTHING to make sure it is not worn. Make sure every nut and bolt is tight. Clean an regrease all your racks and pinions, clean everything really well.

[coryatjohn]

I think the amount of pinion wear that's acceptable is up to the user. For me, I value accuracy and repeatability over unit cost so I change my pinions whenever they start to show slop. It took 7 years for the first set. My use is considerably less than commercial but what I cut I want cut right with a minimum of backlash.

[bbortnick]

The pinions teeth don't look too bad, a little corrosion, maybe. It hasn't been greased in a long time. I am hearing that it's ok for the pinion to be a little loose with the key installed, but set screws not installed. Will use some lock-tite on the set screws.