A question on couplers.

[Drools]

Lets say you have Anti-backlash couplers but you are exceeding their torque value. The couplers are not breaking during usage because they have rubber centers. What would the routing look like? lets say on PCB material.
I'm trying to troubleshoot what is causing my odd patterens. I had them before upgrading my screws to 2 start ones but now it is really noticable.
http://www.carlsoncomputers.com/cnc/...nc_coupler.jpg
http://www.carlsoncomputers.com/cnc/...s/backlash.jpg

[microcarve]

Just curious....are you cutting at full stepping?

What drivers are you using?

John

[Drools]

Driver is a G540. I had the same issue with my old drivers, AVRSTMD.
Default for the G540 is 1/10 microstepping IIRC.

[microcarve]

That's an oddly consistent pattern. Maybe somethings
mechanically a bit loose. Could be the board being cut
itself.

It always does that?

[Drools]

With the single start screws it was there but you had to look very close with a magnifying glass. Ya it has been there since day one and the only consistant piece is the coupler, I have changed everything else. My intent is to build a smaller CNC 12x12 using the parts I have upgraded on the bigger one.

[microcarve]

Yeah, building a smaller one for that sort of fine work
is a good idea....

Those look like $$ quality couplers, so I wouldn't suspect them
without seeing one. But I'd still personally use solid couplings
with a smaller machine that's better suited to a job like that.

Not to say there aren't big machines that can do that...

John

[ger21]

Is there any flex in the machine?

Can you post the g-code for that? Those two problem areas are exactly the same, so I'd look at the code first.

[microcarve]

What kind of linear motion are you using....

skate bearings, $$ linear rails, etc....?

Is there a picture of your machine posted somewhere?

[Drools]

using CNCrouterparts carriages and CRS. I upgraded from Angle and skate bearings.

[microcarve]

Without checking the code yet...
It looks to me like there's some small point in travel
that's the tiniest bit loose.

There's a way to tell for sure and that's to make a simple
file that's quick to run that'll cause the same effect.
Maybe a couple circles and zig-zags.

Try to reproduce it....make it do the same thing at least
very similarly twice in the same spot on the table.

Then move the 'job' somewhere else on the table and see
if it can be reproduced closely again. I'd bet the error would
look a lot different.

Looks like part of your file runs very well....until it gets to
a certain spot on the table.

Maybe some very small gantry skewing on direction change.
That's enough with fine traces.

Holes look well placed.

John

[ger21]

The code looks like it's for a different board.

But the code doesn't appear to be the problem. My guess is that something on your machine is loose, or flexing.

[microcarve]

Thickness wise....CRS is generally Very good.

Width wise...the same.

Straightness...no guarantees with that. I've bought 3 different
times to make a small machine. The last time it was worse than
I'd ever seen it with at least .070 variation in straightness.

So if you have a little wave that's just, say .020...about the thickness of
a fingernail...the bearing won't make contact and there can be a practically
unnoticeable bit of gantry skewing that won't even matter on a wood project.
But can show badly on fine work....

It looks like your Z depth is good, so I'd guess the upper and lower bearings
are good. Thickness.

Straightness looks to be out a tiny bit. May be the side edge
riding bearings aren't making full contact the length of travel.

Without the screw in place, just a matter of rolling the gantry and using
a strip of thin cardboard to check bearing "tightness" against the rails edges.
But, it's a PITA to remove a screw for that...so the test file may help. Though
that's also a PITA....

Lots of people have better luck than I with straightness of CRS. 3 tries at it
and I give up....

Maybe I'm wrong, but that's what I'd be looking for.


John