PRT Alpha tune-up proceedure?

[ron_churches]

Hello everyone,

I was cutting a "hand carved" texture toolpath today and at 85% completion the .25 ballnose I was using plunged to about twice the depth it should have been at, totally out of nowhere. The PRT cut a nice 16" gouge right in the middle of my work piece by the time I was able to get across the shop and hit the spacebar. The sign I was making was cut from a White Oak glue-up, and for the 5 or 6 hours I worked on it yesterday and for 45 minutes today, it was cutting beautifully. Has anyone ever had that happen before?

After talking with Shopbot support today, it seems likely that the pinion slipped on the Z stepper and lost it's Z zero position. The red Shopbot Control window showed the Z was at 1.00, but when I physically measured the bit was only at around .5625. I tried to resume the file and use the Nudge command to correct the Z height but ended up screwing it up even more - now it's scrap. I've decided that before risking another $50 worth of material that it might make sense to go over the machine and work out any problems I find. I'm wondering if any of the forum veterans have produced a document detailing the proper way to calibrate and adjust the PRT to ensure squareness, accuracy and reliability. I did a search but didn't find I know this can be done by looking at the manual and following the assembly instructions, but I'm hoping to find something more straightforward.

Thanks,
Ron

[John David]

My brother was having a problem similar when planning down some Corian material on the PRT ALPHA. The Large bit kept plunging into the material and it looked like it was ramping down, we figured it was loosing steps and then as a last resort we retightened the bit in the collet and all of a sudden the ramping was over. The collets do wear and over time you find yourself having to tighten more and more then you did when they were new.

This is a long way to say "Maybe your collet is loose"?

Good Luck
JD

[myxpykalix]

Did you lose your Z height in the chaos? What was your clearance height set at? How long was the bit sticking out of the collet?

I have had this happen to me with a longer bit where i had a higher clearance height set with a longer bit then where it got to a point inthe 3d carving where it needed to go higher (cut less) it topped out of my Z height and then the Z fell onto the material and dragged across ruining it, sound familiar?

[Brady Watson]

Replace and grease all 4 pinions. Correct motor turnbuckle tension is 3.5 turns. This is one of the only consumables on the entire machine & replacing is cheap. Worn pinions on a PRT Alpha with direct drive motors is more critical than any other model ShopBot. They should be 20 tooth.

Replace your collets. Worn collets will cause the tool to get sucked down into the material - which is especially common when using a spiral bit. If your bit slips even one time - it is time to throw that collet out.

Load the latest & greatest control software and firmware. Check machine rails for wear. File and/or sand burrs as needed. Check v-rollers to make sure they are ALL contacting the rails correctly. Adjust as needed.

Take care when replacing the Z axis pinion - you will need to carefully adjust the v-rollers on the Z axis to A) correctly set the lash on the rack and pinion & B) correctly adjust the t-rail to v-roller tension.

Flatten your spoilboard & observe the cut pattern, indicating if the Z axis needs to be tweaked a little one way or another.

-B

[ron_churches]

John - Yep, it appears to be a collet problem. The bit is definitely sticking further out of the collet than when I put it in. I didn't even consider this possibility, thanks for the tip. The bummer to the whole situation is that I have to go buy another 1/4 inch collet when I have a new HSD 4hp 3-phase spindle with brand new collets I can't use because I'm having problems finding an affordable electrician to re-wire my garage.

Brady - Thank you, that was exactly the kind of info I was looking for.