Final part dimensions smaller than CAD design

Hi all,

I use the X-carve with the robotics team that I coach, and we’re finding that the dimensions of the final part are smaller than the CAD. They look great, but we have to constantly re-work parts we create for our 'bots.

We have had some issues with belt slipping, in the past, but that usually results in a failed attempt to carve. These, however, run “perfect”, with the exception of dimension errors; for example, WxL is off by about 1/8"
on either side. Holes are about 50 thou too small.

Has anyone else encountered this issue? Is this a problem with the CAM Software, perhaps?
We use SolidWorks with HSMWorks XpressCAM and the Easel output configuration file.

Did you calibrate your steps/mm in the controller? Have you measured your router bits to see their actual dimensions? They’re often a bit small.


Hmmm, didn’t calibrate the steps/mm. Might be the issue. Same with the Router bits. Didn’t think about that.


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run some calibration checks on your machine

grab a scrap piece of wood and make some holes, lines, pockets, etc and let use know your findings

remember to the x-carve is only going to be accurate down to .010"-.005" or so on a finely tuned machine

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another thing to double check is the tool paths… make sure to be cutting on the intended side of the line (usually outside the part)

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If you lose steps the shape of your design will be off and as the machine moves from the last known position it will not return above zero after the job.

Normally we use stepper motors with 200 steps per revolution, the electronics behind it can produce half, quarter, eights of a step electronically. every revolution the pulley drags the belt a certain amount depending on the number of teeth. It depends on the type of belt you use what distance it travels in one revolution.
200steps/ number of microsteps = number of steps to travel number of teeth * dispance per teeth.
So now you can calculate the number of steps per mm you need to move your machine in the right direction. More microsteps sounds like more detail, that is partially true as more microsteps reduce the motors power and increase the risk of losing steps.

On the Z-axis you have an extra component if the two pulleys are not the same.

When your machine moves the correct distances you still have to deal with the quality of your spindle. The runoff means that depending of the quality of your spindle it will run off center increasing the cutting diameter of your tool.

On my machine, 500mm with reinforced X axis and Z axis I can cut very precise parts, The “Oval shaped” part on the top is 20mm and 40mm high out of 1.5mm plywood, the rim should have been 2mm wide but is 1.8 on all sides, as this is oon all sides I made the conclusion that my X and Y axis run as good as possible and thet the spindle is wobbling around for .1mm cutting outsides too small and holes too big.

If your part is 1/8 too small but with the right shape I expect your number of steps to be wrong.

If you’re off by 1/8" and using a 1/8" bit, the first thing I’d check is that you’re cutting on the correct side of the line (such as cutting outside the line instead of on the line). That should provide the largest potential source of error that isn’t due to lost steps, and is usually an error that you can figure out because it is a multiple of the bit diameter.

Smaller sources of error are incorrect bit diameters and uncalibrated stepper settings (steps/mm):

  • You should measure your bits with a micrometer or calipers and use the measured size as your bit diameter. It’s very rare to find a 1/8" bit that is actually .125", and so on. Another way to measure the effective bit diameter is to cut a single line with the bit and measure the size of the line itself.
  • I have made a video that shows one method of calibrating steppers that you may find useful, it’s in the Maintenance and Troubleshooting thread: X-Carve Maintenance/Troubleshooting Videos - Add Your Own!. There’s other ways to do it, but this was the way that worked best for me.

These should help, assuming you’re not losing steps. If you’re losing steps, it will typically (but not always) show up in only one axis. Typical sources for lost steps are loose pulley screws, tight V-wheels, tight/loose bolts, and incorrect pot settings. If it looks like your project has been scaled in only one direction, or if the solutions above don’t work for you, a good thread to check out is the Guide to Lost Steps: [Guide] Offset cutting, Lost steps, Positioning errors, etc.

Good luck, and let us know what you figure out!

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