Pocket bottoms having uneven steps

[blackhawk]

I was cutting a sign plaque this weekend using Aspire with a pocket/v-carve toolpath. I have a PRT Alpha with upgrade, PC router, ver 3.6.1 control software, using a 1/4" downcut spiral from Centurion, 2 ips XY feed, and 0.8 ips Z feed, 70% stepover. I first did a practice run in MDF. I noticed that the bottom of the pocketed area had quite a few uneven places left by the cutter. It was noticeable in the mdf, but not bad (sanded out easily). I then ran the same toolpaths using red oak. The uneven places in the pocket were quite worse, up to .010" of variation. The steps were nearly identical to what I saw in the mdf just deeper. I then jogged my 1/4" cutter to a Z position of -.001 and then re-zeroed there. I re-ran the same file and the uneven steps were about 75% eliminated. How did lowering my cutter by only .001" clean up a step .010" deep?

My pocket depth was .16" deep and I cut that in one pass. I used a smooth ramp set to 0.5" in Aspire. My pocketing style was spiral (inside out). I checked my Z pinion and it was tight. I never really noticed this problem until I switched to the 3.6.1 software. Anyone else seen similar problems?

[bill]

Brad,
Very possible the "down" spiral bit is riding on the material lifting the head up. Try the same thing with an "up" cut.
Bill

[knight_toolworks]

yes this is a problem with downcut bits. one solution is to make the pocket .01 less deep then planned and then do another pocket toolpath that will cut full depth. also have a smaller stepover on that final pass for a nice clean cut.

[ed_lang]

Find a dial indicator and place it under your Z. Move the Z up and down and verify that the Z returns to the same point. Move the Z in very small steps and watch the indicator. Is the Z moving to where you tell it to?

Are you running the stock direct drive motors or the 7.2:1 (PRS) gear motors on the PRTalpha when you say "with upgrade"?

Next try to move the Z up and down by hand while looking at the dial indicator.

Then place the indicator under the gantry and move the Z up and down by hand.

These tests will tell you where to go and look next.

[blackhawk]

Thanks gentlemen. I will play around with the upshear.

Ed- I have the 7.2 motors. I have checked the Z with an indicator in the past and it has been spot on. I haven't checked since going to 3.6.1, so I will double check again.

One thing that I have noticed with 3.6.1 is that many times I will put in MZ=0.500, for example,but the readout will end up on 0.501. I mean, one thousandth is nothing working in wood, but it bugs me because this never happened in any of the 3.5 control software. Why does the 3.6 software do this? Does the new software not correct an overshoot on the position as accurately as before?

[blackhawk]

OK, I checked the Z with my indicator. On my commanded steps from the control panel everything is spot on. I command .015 and it goes there on the money. I return to zero and it goes right back. That part is good.

With the machine on, I jogged the Z to a normal height for my type of work. I had .009" of slop just in the pinion/rack mesh. The motor pinion did not move at all, just the rack, so the motor is holding tight. The .009" seems like a fair amount of play and lines right up with the .010" steps that I was seeing in my pockets. At the far extremes of Z travel, I had about .004" of play, but I had to push harder to see it there.

Everyone agree that I should replace my Z pinion and rack? I thought about just flipping the rack 180º to get me to a new spot, but I figured if I goto that much trouble I might as well put in a new set. The Z rack is short so I hope it is not too expensive for the set.

[ljdm]

The pinions will wear before the rack does. Replace the pinion, not the rack, assuming the set screws are tight in the pinion.

[rhfurniture]

Brad,
Replace the pinion and re-set the Z.
Resetting the Z is quite an art, but you can fine-tune the tightness of engagement between rack and pinion by turning the eccentric bushes that mount the V rollers on the Z, being careful to keep it vertical. It is a good idea to replace all the pinions on the machine (best bought from shop-bot - even in UK) as these do wear.

[knight_toolworks]

I have the problem of my z motor loosening so the pinion is not tight against the rack. also the factory goofed and left out a washer behind the rack and it mashed down.

[blackhawk]

Lou - I agree that the pinion wears faster, but since I can move the pinion to a different area on the rack and eliminate 60% of the play, I feel like the rack has some excessive wear in that one spot.

Ralph - Yea, I was wondering about how to adjust the mesh of the pinion to the rack. Adjusting those V-rollers is an art. I adjusted mine before in order to get my Z movement perfectly vertical in the Y plane. I had to take the V-rollers to the max end of adjustment to get things right. If I have to move anything to get the gear mesh right, I probably will need to add more shim under the rack. Right now my rack has almost a 1/16" thick shim from the factory.

[erik_f]

I bought my PRS buddy about a year ago, but have not had a ton of time to use it...so I know my pinions are good...the point? I was doing some very shallow cutting for some dado joints (milling the edge of the part that was to go into the dado). Using a down spiral bit threw everything off...went to Lowes out of frustration, bought a 2 flute straight bit and the same file came out perfectly. Bit selection can play a larger role than we would like to believe.

[rhfurniture]

Brad,
You can adjust the vertical with shims above the brackets that hold the Z, giving you more to play with on the v-rollers. You can also take the Z right out to adjust the position of the stepper motor, but make sure you write down which way you want to move it and by how much first (learnt by experience).
r.

[blackhawk]

Ralph - I'm not sure what brackets that you are talking about. My Z doesn't have any brackets that hold it to the Y carriage. The Z is bolted through the Y carriage extrusion with bolts directly into the Z steel frame.

To take out your Z, did you just slide out the T-rail where the router is mounted?

[blackhawk]

Last night I pulled out the Z T-rail as Ralph suggested. (Pretty easy to do.) I then put an indicator on the the pinion to be able to measure the motor position. I then loosened the motor mounting bolts and slid the motor closer to the rack by about .008 inches. I could probaly stand to have moved it more but I ran out of adjustment in the bolt holes. The .008" move took out half of the slop that I had in the Z. The .004" or so of slop that I could measure was firm slop, not the real loose slop that I saw before. Things probably aren't perfect, but they never are and I'm happy with it.

One interesting thing that I did find is that I had one spot on my pinion where I could feel a catch in the movement. I cleaned out the gear teeth and found a small piece of aluminum embedded down in there. I cleaned that out and things were smooth as silk.