Circle is perfectly round but 1/32 of an inch undersized

I used v carve and tried to cut a 4.875 inch circle in a profile tool path operation, following inside the line (to cut all the way through the material i was using to create a circular template for another project). I exported the file and ran it with UGS. The file ran perfectly, cutting all the way through and creating tabs as well. However while the circle is perfectly round (no oblong oval or football shapes) it is .09375 or 1/32 of an inch undersized. I measured the bit and it is exactly 1/8 of an inch… so I’m a little lost as to what the issue is.

I tried a second time, running a pocket operation on a 2" circle cut to only 1/16 of an inch deep (just to check) and it is about the same… just over 1/64 actually undersized.

Anyone have any ideas what my problem could be… I’m kind of at a loss. I suppose a calibration issue… but if one of the x or y axis were off, wouldn’t I get an oval, and not a circle?

Thanks in advance for all the help.

When I cut using VCarve pro, I have the option of cutting on the inside of the line, on the line or on the outside of the line. Have you chosen the correct option?

Have you created the code for a slightly different sized tool? Many tools are not quite the the size that they’re advertised as. Similarly, I’ve found 90 degree tools are often 'nearly 90 degree. If you’re able to, measure your tool bit accurately at the cutting tip using a good vernier or micrometer and then adjust your gcode accordingly.

There are probably more complex errors possible with the type of post processor you’re using - try the above first and I’m sure other more knowledgable folk can assist with the PP’s

I should have stated… i meant i used v carve pro * as software* the bit I was using was a 2 flute straight bit.

also, i chose to cut inside the line.

Cutting ‘inside’ the line may be the issue when cutting pockets or profiles. When I need to cut accurately sized items (wooden clock gears), I need to set it to cut ‘outside’ the line. This means the outside-size of the object will be accurate

Note however, when cutting accurate holes, pockets etc, you’ll typically need to cut ‘inside’ the line. This ensures the inside-size of the hole is accurate.

When size doesn’t matter and accuracy is important, cut “on” the line. Adjusting between “inside”, “on”, and “outside” can make significant differences in both the finished size of an object and what happens in corners and fine detail.

You also need to make sure the tool size is accurately entered. “Select” a tool from the list that is closest to one you’re using. Then “edit” the settings for that tool (from within the profile or pocket cutting menu, not in the database itself) to accurately reflect the true size of the tool.

Settings made in the “edit” portion of the program will only apply for the job you’re doing, they don’t change the settings in the tool database. This mean this is where you can correct for toolsize errors, tweak the feeds & speeds to suit the job etc.

I’ve checked all the settings and chose the inside tool path in this particular situation in order to arrive at a circle template with a cutout exactly the size i needed. I keep getting parts undersized. I appreciate your efforts to try and help me, but I still can’t seem to fix the issue.

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Have you tried to calibrate the steps/mm? If you’re using the default settings, they’re probably close but not quite exact. There’s a good video showing how to do the calibration at Steps/mm Calibration

Sorry, I assumed you were trying to cut out a circular piece of wood. Not cut a pocket or circular hole.

Are you using the correct post processor?

I believe so, it’s the x code post processor I download form inventables. Tomorrow I will try and do a few calibration tests and see if I can’t figure out what the issue is. I suppose it could be a steps issue ( $100 and $101). Do you know how to adjust those. Or check what my current steps are?

This forum (Z axis not accurate) has a link to the video, as well as a description on how to adjust the settings in Easel’s machine inspector. I don’t know if the settings transfer to other programs, or how to adjust it in other programs, but this’ll at least get you calibrated in Easel. I ended up buying a Shinwa 600mm steel scale off Amazon for $30, it’s marked in 0.5mm increments, and used that to measure out 500 mm for calibration. It took a few rounds of calibration to get it dialed in, but it’s pretty much gnats-■■■ now.

In UGS type $$ and it’ll return a list of all the settings that are current.

To change the 100 setting from its current setting to 40.099 for example, you’d type “$100=40.099” (excluding the quotation marks).

Typing $$ again will list the current settings which should show the new value.

I just checked my X and Y on my 1000x1000 machine and only needed to change the value of Y from 40.000 to 39.95. x remains ok on 40.000.

Keep in mind, this test was done visually, using a 1000mm engineers steel ruler, over approx 800mm. Not exacting by any any means, but the results were repeatable, from both directions over multiple passes. Certainly accurate enough for woodwork.

When I get a chance, I’ll set up a dial indicator for the Z axis and check that.

Did you imput the actual measurement of your bit?

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Thanks for passing along that link. It will be really helpful.

I did, to the best of my abilities with the current fractional caliper I have (which I use for woodworking) 1/64 inch markings. So I don’t think it is the bit. I’m guessing the steps must be the issue. Because as I cut larger shapes the issue is more pronounced. They must be off by exactly the same amount though righ? Because if only one of the axis was off I would get an oval… Correct?

Thank you very much for all the help so far.

The amount you’re off on each axis would be linear. For example, if your X axis is short by .010" per inch (cuts .990"), and your Y axis is long by .013" per inch, then a 3 inch circle would be .030" short on the X axis and .039" long on the Y axis.

Your settings may be off by the same amount, but that would likely be coincidence rather than design. In theory, the steps/mm can vary depending on belt tension, among other things, so in reality the settings will be unique, but similar.

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I second what @RobertA_Rieke says. Both axis being off by the same is a coincidence if it is steps/mm and so that is what made me think of the bit measurement.

If you haven’t already, you will want to go through and calibrate your steps/mm. Use this video as a guide. For the Z-axis just use something that is a known thickness.

Thank you to all, I have calibrated my x carve and it seems to have made a difference. Things are very close to dead on now. My caliper shows as 1/128th of an inch smaller than its supposed to be but that is plenty close for me I think.

I have really been impressed with the quality, character and dedication of the members of this community to helping people out. I have had really nothing but good experiences with my x carve. Assembly went perfect, it worked pretty much spot on the first time i used it and I have made so many fun projects!

I still have lots to learn, and by far it seems that the machines abilities far eclipse my cad drafting abilities. So I have nothing but high hopes for the future.

Thank you again @sketch42 @RobertA_Rieke @DavidWestley you are true gentlemen.


Glad we could help @BrianRizzo

Happy carving!