Distorted circles

[wheatwang]

              Cannot seem to get a nice circle.  Checked the X and Y axis travel and both are correct.  Machine is only on its 9th job.
                                                  The strange thing is that the thin sides are directly across from each other.  please help

[gerry]

What bit, feed & speed?

[chuck26287]

Which circle gets cut first? Where does it start the cuts? You have a very pronounced wobble in the inner cut at just left of 12:00. How deep is the cut per pass? What is the diameter of the bit? Do you ramp-in the cut, or is it a straight plunge? Is the work piece absolutely secured to the table, or could it move a bit? The tabs are what I would call too small. Could you be getting any flex of the tabs? Could you be getting severe bit deflection from cutting too deep, or fast (ipm), or slow (rpm)? Try running the same cuts, but only a single pass, and only about 1/32" deep. Just enough to see the bit path. Is it still non-concentric? If that test looks good, you're probably driving the cut too aggressively.

[garylyb]

That looks like 1/2 inch birch plywood. If you're cutting it in 1 pass, thats too much, use 3 passes at 150 ipm.

I used to get something like that. I used to cut some engineered aluminum substrate 4 layers thick.

That seems like that put too much pressure on the x axis, and the nut at the end of the x axis loosened. You have to remove the cover at the end of the x axis and tighten the nut.

Once I switched to cutting 2 layers instead of 4, I haven't had it happen again.

[wheatwang]

.25 endmill W/c, 20in/min, 1200rpm
2 passes
0.125 birch plywood
tabs are .075in
no wobble in the material, secured well.
where is the nut for x-axis? is it the cover at the end of the gantry?

[stevem]

The plywood looks thicker than 1/8" as you say. Looks more like 1/4 or 1/2 thick.
The speed you posted is 1,200 rpm! I think you meant to say 12,000 rpm.
Should be using a .25 spiral down cut for the plywood.
Are you sure you are at 20 ipm? If so, that is to slow. Should be 80 ipm or possibly faster.

[wheatwang]

Just ran the same job and a test 1/16'' deep, both in basswood- same problem- so i doubt its deflection. everything seems stable on the machine.....i don't know what to do.

[wheatwang]

So i went back and check the travel again. Without changing the pulse, Im getting different Y measurements every time. tested controller at 100mm results vary: 99-101mm in the Y direction. X axis is perfect every time. ??

[Deleted]

First off, wood is not rigid, you'll need larger tabs, and they should be inline with each other so it's a more rigid setup. You may also need more tabs then you've provided. Second of all, if you need a 0.25" width of cut and you want it to be good on both sides, you should use a cutter narrower than the slot and allow for finish cuts on both edges. You don't want to be cutting on 180 degrees of the bit, preferably smaller is better but try to avoid more than 90 degrees. You should also be cutting the inner slot first.

[garylyb]

I think it's time to call Chad. Ask him to look at this thread.

[wheatwang]

Called Chad, problem solved. loose Y ball screw. easy fix. case closed.