ShopBot Problems Continue

[Steve Wonser]

I had been having problems with my ShopBot Desktop slowing down and then stopping in the middle of a cut. I disabled USB power management and that seemed to go away. However, now the it seems like the ShopBot is loosing the X position.

I was engraving an acrylic piece using a spring loaded 90 degree diamond drag bit with the spindle off and no dust collection attached. The job ran for 1:35:05 without stopping. The attached pictures show the following problems:
- The hatched area is not consistent. The acrylic is not flat (0.002 difference from the thinner left side to the right), but the spring loaded bit should take care of that. I set z-zero to the top of the acrylic. The engrave depth/pressure setting is 0.01 and the fill stepover is 0.004. Feed speed is 1.4 inches/second.
- There are vertical lines in hatched areas that should not be there.
- After the ShopBot finishes hatching an area it outlines the perimeter. It is clear in the pictures that the outline on the right has move about 1/8" to the right.

Any ideas as to what is happening and what I might do to fix it?

Thanks,
Steve

[Tim Lucas]

looks like an off set on the outline?

[steve_g]

Steve…
Just to be sure you know… You can choose whether or not to have that last profile pass. See photo.
What I’m seeing Appears to be a loss of steps. That would account for the perimeter being offset one direction, it could also account for some of the inconsistency. Also, I’m wondering if the hatching started from one direction, went to the opposite end of the area and started coming back the opposite direction. That scenario would be the perfect storm of all accuracy issues working against you!
SG

[scottp55]

Steve, Is it rastering along the Y axis starting on the left and going to right?
Safe Z and Z2 settings are?
Are "Bubbles" cut last or first? Are "Bubbles" clean?
Am I not seeing it in pic or is the Tail and beginning of S pretty good?
Is Drag tool mounted IN spindle collet, or in an offset mount?
scott

[Brady Watson]

It appears to me that the depth of the drag in certain areas could have been too deep for the tool to be able to drag it - and it lost steps. This would be indicative of the offset on the profile you see to the right of the main part. Acrylic is 'catchy' and it would not be hard for it to really create some resistance against movement - especially during short, quick moves.

Diamond drag tools are typically not used to create the effect you show here. Nothing wrong with it - just that it isn't a very efficient strategy. You could create the same effect with less 'whiting' using a 1/32 or 1/16" end mill or tip-off engraving tool with the spindle running.

'Engraving' as it relates to using a diamond drag or engraving tool usually involves what they call centerline engraving using single stroke fonts and line art, much like a Profile-On toolpath with a tool at a fixed depth; or a quick engrave toolpath with cross-hatching to create a fill. It usually isn't done with such a fine tool to clear out a big area.

I don't mention any of this to be critical - just as information.

-B

[scottp55]

Brady, ZERO acrylic experience, But lots of guys on Vectric site doing edge lits similar to this.
I was just wondering, IF Steve is Y rastering and the stepover was SLIGHTLY too close, AND there was ANY play in insert...Would acrylic try to pull it back into the previous groove? Then it might be twice? as deep, and as Z isn't lifting could it be causing missed steps.
My age is showing, but even with the lightest stylus pressure on Vinyl records, once a "skip" started, it got harder and harder to jump "Out of the Groove".
I was wondering if acrylic and diamond drag Y rastering would cause the lost steps, trying to force its way out of a 2-3 time cut groove.
Doug or Jeff, has it ever happened to you?
Should be following "Bradys Rule" "If you don't know...Stop talking and Listen!"
Jus' Cain't seem to help Ma se'f