Inconsistent widths and backlash (long)

[jlawren6]

After following these forums for many years, I finally took the plunge and bought a lightly used (approx 30 hours) 2002 PRT96. At this point, its all setup, the vacuum table is in place, and I have maybe 3 to 4 hours of cutting time on it myself from surfacing the table, cutting the vacuum grid, and just playing around. My first real job for it involves cutting a lot of dados and rabbets in 3/4" Birch ply. Before cutting the actual pieces, I decided to cut some test dados to dial in my feed speeds, RPM, and cut direction in the material I will be using (the sheet I have acutally measures 0.7"). This is where the problem arose. All of the 0.7" wide by 4" long test dados that I cut were too narrow when the 0.7" dimension was along Y, but not when along X. Depending the the cut direction, they ranged from around 0.65 to 0.68. The more undersized cuts occurred when using the conventional cutting direction. Some basics on how I was making these cuts:

Bit - 3/8" two flute downcut carbide spiral
RPM - 10,000 setting on the PC router
Feed Speeds - Ranged from 1.5 in/sec to 2.5 in/sec, but the majority of them were run at 2 in/sec after my first trials.
Depth of cut - Normally two passes at 0.175" depth.
Method - Have used one of three methods with the same results; CR command in SB3, Part Wizard area clear (inside out) generated SBP file, and manually programed M3 moves. I've use the CR command the most though.
Holddown - Vacuum table (8 Zone, one active, powered by Fein) with one clamp on the edge for good measure.

As far as troublshooting, I'm about 98% sure at this point that it is a mechanical issue. Per the display, the tool is going where it is told and I have not found that I'm losing any steps. The dust collector is also out of the picture for these cuts so there should be no electrical interference there. Everything is also grounded well (Router to Z slide, Z slide to Y carriage, Y carriage to X carriage, X carriage to frame, frame to control box). All the bolts and pinions are tight. I've readjusted the X and Y motors and I believe they are set correctly. I've also gone through the Z axis and adjusted it to minimize any backlash and eliminate lateral movment. The bearings are tight and my slide has an additional set of bearings near the bottom of the slide similar to some early Alpha photos I saw and, I believe, what Gerald has setup on his machine.

When adjusting the X and Y motors, I noticed that the Y motor has some noticable backlash. Turning the pinion by hand a small amount, I can hear a light click when it stops in both directions (motor off and free from rack). The shaft is moving with the pinion. I get the same thing with the X1 motor, but to a lesser degree and the X2 motor is solid. Given this, I did a very crude experiment to quantify the Y axis free play. I plunged the bit about 0.3" into the material, retracted it, and verified that the hole was 0.375". I then plunged again and pushed/pulled on the Y axis with light to moderate force (I'm guessing around 10 - 20 lbs). While doing this, I can see the axis move a small amount including it's bearings and the pinion/shaft. I can also hear the bit cut more material. After retracting the bit and measuring again, my 0.375" had expanded to 0.4225" in the Y direction. Doing the same procedure in X resulted in a 0.3825" hole. The amount of freeplay in Y corresponds to what I saw with the undersize dados and seems excessive to me. As far as I can tell, the only thing that gives the Y carriage rigidity in that direction is the Y motor and the pinion to rack engagement.

Do these number see high to anyone else with more experience and am I missing any other adjustments that might influence what I'm seeing?

I plan to run some more controlled cuts tonight and document things a little better. Anys suggestions on what I might try would be appreciated. I also want to determine if I can come up with a workaround for this one job so that I can get it out the door and spend some more time tuning. I'm hoping that a lighter second pass with bring me to the right size.

Thanks,

Jon

[gerald_d]

"All of the 0.7" wide by 4" long test dados that I cut were too narrow when the 0.7" dimension was along Y, but not when along X"

and then later:

"I noticed that the Y motor has some noticable backlash.......the X2 motor is solid"

Why not swop Y-motor and X2-motor with each other?

Is your router/spindle mounted as near to your y-motor as possible? (Less "twist" effect on the y-car)

[jlawren6]

Gerald,

I'd considered swapping motors after my test cuts tonight. The only reason I had held off till now is 1) I needed to make sure the motors were identical (other than possibly flipping two wires) 2) If the Y motor has too much backlash then I would only be moving the problem to the other axis. Since I didn't know if this amount of play is unusual, I thought I would wait for some input here before swapping.

Good point on the router mounting. No. It is actually at the furthest point from the Y motor. Seems dumb in hindsight. I think I orginally put it in that position to allow room for the dust collector hose. Since I'm planning on going a different route for dust collection now, I can move it forward. I did notice when pushing on the carriage, however, that I seemed to get nearly as much movement when pushing near the motor as at the far end. I'll give it a try though. Thanks.

[bill.young]

Hey Jon,

I would also suggest double-checking the pinion gear setscrews and make sure they are good and tight on the shaft.

[gerald_d]

I recently purchased some slightly used stepper motors from ShopBot and was amazed to see that the previous owner of those motors had not put his setscrews on the flat spots of the shafts!

[jlawren6]

I'll do a tripple check on the set screws. I had checked them twice before, but I've been known to make a mistake or two (just ask my wife....).

How much were the used stepper motors by the way?

[mikea@mts.net]

Hello Jon
I had the same problem and never did figure it out. Shopbot said it must be the collet which I replaced with no change. If you can come up with a solution would you post it here as maybe it would help my students out as well. thanks
Mike

do a search for " Bit deflection-how much is to be expected " and you will see my post and replies on this topic. ( I don't know how to just give you a direct link.

[srwtlc]

Jon,

Try the same cuts with a upcut bit once. I have noticed that upcut and downcut bits deflect differently (upcuts pull into the cut and downcuts push away). Also, running a 3/8" bit at 10,000 at 2 in/sec with a PC router is pretty slow and could greatly increase deflection and machine flexing.

[jlawren6]

Back from the shop with an update... The set screws are TIGHT and on the flats of the shaft.

I just finished running a series of test cuts all still with the same bit, feed speed, and RPM (did not see Scott's suggestion till now). By the way, I chose those values based on the chip load calculator.

I cut 4 Dados (0.7" Y x 4" X) each with one pass at 0.32" depth and then another at 0.35" depth to see if the light cut would clean things up. The SBP files were generated in Part Wizard with an area clear toolpath - 1 Conventional inside out, 1 Conventional outside in, 1 Climb inside out, and 1 Climb outside in. In general, the Conventional inside and Climb outside were oversize 0.04" (both passes) and the Conventional outside and Climb inside were undersize 0.08" on the first pass and 0.03" on the second. I also outside profile cut two 4" squares, 1 conv and 1 climb. The conventional was undersize 0.03" and the climb was oversize 0.04". Finally, I used the CR command to pocket cut (inside out, CCW) a 2" square at 0.35" depth. I ran it back to back 5 times to see what dimensions changed. The X dimension started 0.03" undersize and went to within 0.005" undersize on the remaining runs. The Y dimension started at 0.08" undersize, then progressed to 0.04" and finally 0.025" undersize. On the last run, I lightly pushed on the Y gantry as it cut and the Y dimension ended up 0.005" undersize.

All that basically confirms to me is that I have too much play in the Y direction and I don't have a workaround for it. It does not take much force to move the Y gantry (motor on) and I immediately see the pinion and motor shaft move when I push on it. It seems like there is a lot of freeplay in the motor either because the gearbox has too much backlash or the motor does not have enough holding torque to counter the push. I guess my next option is to swap motors with one on the X gantry, but that won't really solve the problem.

Can anyone confirm that the numbers I'm seeing are unrealistically large?

I'm open to suggestions.

Mike - Thanks for the link. I had read it once before, but I will revisit it. I'll be sure and post whatever solution I find.

[jlawren6]

Another update...

I experimneted with different bits last night. Tried the same downcut spiral, a 3/8" 2-flute upcut spiral, and a 3/8" straight bit. I bumped up the router speed to 13000 for all the cuts and also readjusted (again) all the motors before I began.

Basically, I found that the downcut spiral is the worst case for cutting undersize dados and seems to deflect the most. I still can't get the correct width dado in Y no matter how light of cuts I take. The upcut spiral was the next best choice with the downside being the expected fuzz on the top of the cut. With it I could eventually get the 0.7" dados I was looking for. The best in terms of width was the straight bit (2 flute). It cut the correct width on the first pass and the edge quality was pretty good. I was surprised though how much noisier it was (makes sense if you think about it though). In all cases, conventional cutting with and inside out toolpath gave the best size results. I still think that I have too much play in the Y axis. I wish my Y motor and 2nd X motor had as little play as the first one. In the meantime, I'll get what I have to get done with the straight bit for the job I have waiting and then play with some more tuning. I think a second Y-motor setup might be in my future as much as I hate to spend the extra money on it.