Circles, what have I missed?

CNC Machines Shapeoko
1

Shapeoko 3XL/Dewalt

Another circles vs ovals situation.

When I mill a pocket for say, a .375 bolt, the pocket ends up .370 in the x and .365 in the y direction.

When I mill a 1" pocket the X width is approx .997 and the Y is .992.

It is consistently wider in the X and narrower in the Y.

My spindle runout is negligible, about 5/10000, if that.

When I use a drill toolpath, i.e., just plunge the cutter straight down, with a .247 endmill, I get a circle with a .247 diameter.

I’ve run the circle,diamond,square test and the circle should be 2.25 and it comes out as 2.245 but it is round. Round because it’s an outside profile and not a pocket?
On that test as well, the diagonals of the square come out exactly the same.
The smaller square’s od was 2.50 and I measured 2.50.
So my outside profiles, especially the squares are good.

I’ve read dozens of posts/articles/tips I could find, including the wiki.

I squared the extrusion ends prior to assembly.
The machine is square corner to corner.
The Y rails are parallel to each other and square to the X rail.
The v-pulleys are tight, the belts are tight. I use an arbitrary tension of approximately 4 pounds for each of the belts.
There is no wiggle/movement at all in the Z structure.

I know how to calibrate my steps per mm and I have adjusted the X/Y steps per mm such that the machine moves exactly as far as it should. If I jog it (x) mm it jogs exactly that distance, both X and Y directions. My ratio is set at X=40.14 and Y= 40.070.

I the tried resetting the ratios back to x=40.000 and y=40.000 but the result did not change, the pocket was oval by approx 5 thou.

I’ve also tried milling the pockets using a climb cut as well as a conventional cut, no change.

I get that it’s a hobby machine and I’m probably OCD but if any of you technical gurus have something else I could track down I would like to hear it.
Or, if this is as good as it gets, then so be it.

Thanks.

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2

If the holes are large enough, try leaving a roughing clearance and taking a finishing pass?

3

Thanks Will,
Yes, the finishing pass option was used in VCarve.
I’m basically just curious if anyone wants to pocket a .375 hole - what is the resulting diameter?
And is it round?

John

4

As the error is 0.005 for several feature sizes, I might guess that there is some (backlash) somewhere.

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5

Good idea.

I calibrated the steps/mm so when I send the cutter a linear x amount it is exactly correct.

But when directed in a circle what happens? The stepper gear makes more changes in direction so it accumulates some small error?
Sounds plausible since I get the same error with a climb or conventional pocket.

On the other hand, why wouldn’t the error cancel itself out as it went around?

How would you ever nail this down and correct for it?

My belts are pretty tight with no discernible slippage.

6

I chased that rabbit for months and found out my spindle was just slightly unsquare to the spoilboard and my router had some runout. I was also getting a small amount of flex in the z axis v-wheels on the x axis. All that together made me crazy. Stay with it, you will get that little bit out eventually. Mine is now down to .001 in all directions.

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7

Backlash comes from a number of places. Given the design of the machine and other peoples experiences, I wouldn’t look for flex at first.

For an example of backlash, pick up a bolt and nut, maybe 12mm dia (1/2") or so. Run the nut up the bolt half way. Now push and pull on the nut. There is a little play. It can shift a little. If the nut were attached to the x axis of your machine and the bolt were pushing it back and forth (this is a leadscrew configuration), when the direction revered, there would be a little rotation of the bolt before the threads started pulling, as the contact changed from one face (the push) to the other (the pull). The same effect happens with gears, for similar reason. There needs to be a little clearance or the surfaces will bind.

In a Shapoko, there isn’t a leadscrew, but there are places where there can be backlash, and there are places where there can be flex.

For example:

A drive pulley can rotate the slightest bit on a shaft as motion changes.

A joint can shift slightly

A mated face can be the slightest bit out of flat and allow a little rocking

A bearing or wheel can shift slightly

A spindle bushing could have a little play in one axis

A belt attachment could have a little play

The guide wheels might not be properly preloaded

Or a number of other options. If the axes are calibrated for the proper motion, then something else is the issue. I would start by lightly pulling back and forth on the spindle (when the spindle is NOT POWERED) and feel for any play. 0.005" (0.12mm) can often be felt quite easily. If you feel play, then it is time to track it down. Instead of pulling the spindle directly, pull the power head body (router, whatever). Keep working out the chain until there is no play. Now you can narrow down the bounds until you find the actual source or sources and address the issue.

If I were to make a wild guess, the first two things I would look at are the preload on the guide wheels and the fit of the x axis pulley., followed by checking the mounting screws for the x axis positioning motor. Then work your way from the spindle out. Pretty much any part or attachment con allow for some slop/lash and you may need to get creative to track it down.

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8

Thanks,
I’ve done all that several times. Nothing loose, all pulleys, screws tight, etc.
I’m now resurfacing the spoilboard and retramming the spindle.
To be continued…

9

One of the other things I did that helped alot is replace all the motor set screws with cap screws. The cap screws give you more torque on the motor shafts and help hold it in place better. May not be the issue but it will solve problems later potentially.

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10

Thanks,
These are the really small set screws for the motor shafts?
Do you happen to remember the size of the new cap screws you used?

11

I dont remember for sure but think they are M3. I would take one of you set screws with you. You can get the cap screws at Home Depot, Lowes and Ace Hardware I know for sure. Other places probably sell them.

Here is a picture for reference

image

12

Correct. M3m the set screws are ~4.75mm in length, so 5mm probably too short, 10mm seems to have clearance.

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