What is causing this?

[Mark Farris]

Hello all,
I am in process of cutting a 3D project that I have cut previously a few weeks ago. In fact, I have cut numerous 3D objects in the last few years with excellent results. Yesterday I started cutting again with a new, different brand, 1/8" tapered bullnose and the background has striations of different depths. I let the model finish, put one of my older proven 1/8" bullnose bits in and started to re-cut the model taking off and additional 1/64". It has the same lines. It only appears in the flat areas where there is no Z movement and simply cutting in a straight line. When it is cutting over the model with the Z movement the finish is very smooth. This makes me think I have a wear or adjustment problem somewhere in the Z axis or spindle bearings. Has anyone experienced a similar problem or have suggestions? This is a 5 1/2 hour cut and it is driving me crazy. I have attached a couple pics. I have a 2010 PRS Alpha with the 2.2 hp spindle. I am cutting MDF with a 1/8" Centurion bullnose bit, 3 ips @ 18,ooo rpm.

[Brian Harnett]

Check your rollers, make sure no crud is built up on them. I have had that happen cutting foam.

[Mark Farris]

I had actually just polished the rails and rollers for an HDPE job, so I'm pretty confident it's not that, but I'll check again. The striations are pretty consistant. Thanks for the suggestion.

[Brady Watson]

Looks like some backlash in the Z. Shake the entire YZ assembly, wiggle in all directions with the control box on.

Pay particular attention to the bottom V-rollers as Brian points out. Grab the spindle and force it up and down. Any play?

Move the spindle off the table, say -1 on the X. Now move the Z down to the height you were cutting. Wiggle it up/down and all around. Where is the play?

First impression is some backlash between the Z rack and pinion.

Just listen & feel...(that's what she said) - and you'll track down the issue.

-B

[Mark Farris]

I found it. After the run I found that the set screws for the Z motor pinion had come lose. That is the first time that has happened in my Shopbot career. I guess I need to add that to my list of things to check weekly. Thank you to those that replied.

Mark

[jerry_stanek]

does yours have 2 set screws one on top of the other like mine

[Brady Watson]

Blue Loctite will ensure it doesn't happen again...

-B