Machine 'Slop'

[nat_wheatley]

I have a PRS Alpha and there's a fair amount of 'play' in the machine. I'm wondering what other have done to help minimize this. I've adjusted the bearings on the Z-axis, as outlined in the SB literature. On the X and axis, I've snugged the motors against their 'tracks' as much as I was able, short of clamping them in place. Still I do notice I'm able to move the gantry a bit with the machine powered on.

On perimeter cuts I'm able to work around this doing first a climb, then conventional pass, on all cuts. On dadoes, and tenons, though, I'm running into issues as a result of this play.

It's the one glitch I'm finding to an otherwise very good machine, and I'm curious as to how others have addressed it. Thanks.

[thewoodcrafter]

If you have the motor pinons tight up against the rack and still have play you may need new pinon gears.
Did you keep these lubed to minimize wear?

[nat_wheatley]

X pinions have both been replaced within the past 6 mos. Machine is well lubed.

[coach]

check the set screw. did you use lock tite?

[nat_wheatley]

I've pulled everything apart several times, and all connections seem to be tight. I should add that I've had this issue from day one.

[coach]

have you contacted shopbot tech support?

[nat_wheatley]

David,

I haven't. I don't think there's necessarily anything wrong with the machine. I would guess that it's a combination of things that need to be adjusted/tightened. Just wondering what others have do to 'tighten up' their machine. Thanks.

[wberminio]

Nat

I've clamped the the motors to tracks and tightened them down.This has eliminated any movement in my machine.

Erminio

[carlcnc]

My 2cents on pinions

I had errors with my rack-n-pinion setup
it wouldn't happen on every direction change ,but
over time it happened more frequently and seemed to get larger,
even with locktite on the set screw the pinion would start "rocking" a couple of degrees on the key.
My solution was to plug and rebore the pinion
about .0015 under size for a tight shrink fit., when I remounted I heated the pinion to dull red ,
and slipped it onto the shaft, problem fixed.
Carl.

[nat_wheatley]

Thanks for the posts. I took X and Y motors off and checked them, all looked OK. I did notice that there is some play in the motors themselves. I put them all back on, pressed snugly against the racks. Though I didn't do any cutting, I did power the machine on, and there is less movement now, when I try to move things manually, then there was before. We'll see how this carries over to cut quality.

Thanks again for the posts. Any additional input is definitely still welcome.

[knight_toolworks]

there is some play in the machine.and you learn to compensate for it. all machines have slop you need to deal with. I tweaked my machine and finally just realized that there is some slop and how to deal with it. the depth annoys me. when using a downcut bit I tend to undercut about .02. till I compensate for it and it cuts too deep (G)

[joe]

All this is so unfortunate.

It's my opinion that bolts and screws should never be expected to stay in place. Call up your local welder and have the thing welded up.

[khalid]

I think You should Tack weld the nuts to the structure... This will keep them in place for ever.. This will avoid to weld the whole structure...

Tack Welding of nut will also help you in assembling and disassembling...

Before tack welding i will advise you to drill some hole between mating parts in machine assembled conditions and insert some dowels inside...

Also you can tack weld the nuts when bolts are fully tightened ( In this scenario you will have to avoid the weld spatters on the extended bolt threads beyond the nuts.

[rhfurniture]

Hi,
I think that one advantage of the old PRT was that the stepper motors and pinions were spring loaded against the rack in all but the Z axis - please say if I am wrong but a quick trawl through the pictures on the shopbot website show this is not now done on the PRS. I am now planning on spring loading my Z motor, as some others (Paco?) have done and not regretted. This would mean that even if the geometry changes slightly along the axis, the tightness between rack and pinion would remain.
From what I have been told, welding tends to distort things like crazy, therefore I have avoided it and taken to adding steel instead.
Sorry if this doesn't solve anything, but it might help to describe it instead.
R.

[nat_wheatley]

Ralph,

On the my PRS alpha, the pinions are not spring loaded against the rack. If you do end up spring loading any of yours, I be interested in seeing pictures of the set up. Thanks.

[Gary Campbell]

Joe, Khalid, et al...

The problems that Nat is experiencing seems common to those of us that use our machines hard and often. The PRSa does not experience the extreme flex when under load as the PRTa did. The gantry, albeit top heavy, is a much more rigid piece. The bolts, in my experience, do not come loose. Nor can you weld the connections as virtually all of them are aluminum bolted to steel.

Between my rails and the bit there are 29 rollers. Some fixed, some adjustable. If each of these has a few thousandths play the total can be .020 to .030 at the tool. When this is added to the inherent flex of the table assembly, the X rails and side flex of the gantry, there is a measureable amount of "slop". In mast cases toolpath strategy and/or lighter cutting loads can solve the innacuracies. In some cases it can be very difficult to find. It very seldom shows up unless cutting speeds approach 400ipm.

The biggest problem may be that the gantry is retained on the machine by the pinions. If there is any play in that area the gantry may rock or allow its wheels to climb the rails slightly. This condition is very hard to diagnose and solve without dial indicators and equipment to place loads equal to cutting in all directions at the collet.

When all is well we have the ability to cut 3/4 ply onionskin at near 600ipm. This generates a great deal of force. Force beyond what many machines can handle. But it does take some work. Ive been thru it. Nat will find his gremlin, but there is little info from the thousands of PRT's out there than can apply to the PRS.
Gary

[rhfurniture]

This is how the steppers used to be held on the prt's:

8630

[rhfurniture]

Gary,
"Nat will find his gremlin, but there is little info from the thousands of PRT's out there than can apply to the PRS."
I agree - the PRS is a different design and uses the motor and pinion as the hold-down. A spring loaded motor would not work as that. I have tried it by removing the prt hold-down (which was difficult to adjust right at its best) with comical results as I could lift the whole carriage with aggressive cutting. I now have much improved hold-down devices as well as the spring loaded motors. For me it makes good sense to keep the two functions separate - but I still have an old prt (benchtop).
R.

[Gary Campbell]

Ralph...
I agree completely. If you add the torque and speed of the current PRSa 7.2 motors into the equation the gantry would never be able to be contained by the spring tension. Perhaps the PRT with alpha motors is the reason for the redesign into the PRS configuration.
Gary

[joe]

I'm with Gary.

If anyone can make one of these work, he's the man.

But, it's such ashame to worry with such problems. We have confidence in you so stay at it.

[carlcnc]

Gotta chime in here

[Gary Campbell]

Joe...
Thanks for the vote of confidence, but like you, some days I wonder if it's worth it. Sure do like the way it works on the good days tho. We all need a few more of those.
Gary

[carlcnc]

whhops,
fat fingered!
I was about to say, I tried the rigid mounted pinion method and had problems
my current motor and pinions are spring loaded with valve springs from a small block chevy[actually 1"die springs] much better.
there is a formula for pressure needed at a given torque, it is very difficult to get a rack +- .010 for 10ft.
Carl

[coach]

Nat, I was looking at my machine today. I had an issue with one side always had some movement.
everytime I had it tight after a few runs it was sloppy again. I am not sure why but I had no flat washers on the allen head screws and they dug into the material and seemed to move back to the spot that dug into the metal.
After placing flat washers on them I have never had an issue.