Make Your Mill More Precise with Circles, Squares, and Diamonds

Posted by Owen Smithyman on Sep 16, 2015 9:40:00 PM
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It’s hard to manufacture stuff repeatably. Unlike the software universe, where you can make exact copies, the physical universe isn’t uniform and nothing is created exactly the same way twice. Everything manufactured from physical materials falls on a spectrum, and it’s up to you to decide what part of the spectrum you’re willing to accept.

If you want parts to be made very precisely, that means you have to toss everything that doesn’t pass your specifications or buy a more precise (and thus more expensive) machine. Both options increase the cost of the parts, and even with the $100,000 Haas mill we have in our machine shop, parts still fall on a spectrum. It’s just a smaller spectrum, hence the cost.

This spectrum is a daily reality for manufacturers, but consumers only see the parts that pass, so the scrap goes unnoticed. When we designed the Othermill, we recognized that many of its parts needed to be precise to within a few ten-thousandths of an inch. It took us a lot of effort to find manufacturing partners with the right combination of price and quality. Even with these precise parts, we run every Othermill we make through multiple tests and rework any that don’t pass.

So now you’re using a CNC mill, and you want to make precise parts yourself. What can you do to maximize the machine’s precision? One way to dial in the precision of your mill, and to simultaneously experience the phenomenon described above, is to mill a few circle-square-diamond tests. The video at the top of this page shows the process on an Othermill. Below is an interactive model of the circle-square-diamond test we used.

Circle-square-diamond (CSD) tests have been used by machinists for decades to gauge the precision of their machines. The process is to mill a few of them, measure the dimensions of each feature (the square, the circle, and the diamond) on each one with a micrometer or other precise instrument, compensate in your CAM software, and then mill a few more.

Step 1: Run your first set of circle-square-diamond tests
The CSD tests you initially make will most likely not match your CAD model, and they also likely won’t match each other. We went over why they won’t match each other (physical universe not uniform, stuff falls on spectrum, etc.), but the reason they may not match your CAD is because the cutting tool’s diameter can vary up to 0.002". This is normal - it has a tolerance that, for many quality tools, such as the ones we sell in our store, is +0.000"/-0.002" (+0.00mm/-0.05mm). When I milled my first three CSDs out of aluminum with a 1/8" flat end mill, I found that the dimensions of all my features were 0.001"–0.003” (0.02mm-0.08mm) larger than the CAD, with an average of about 0.002" (0.05mm).

Step 2: Offset the tool diameter in your software
After taking measurements, the next step is to offset the tool diameter in your software to compensate. I offset my tool diameter by -0.002" in Fusion 360 because my CSDs were, on average, 0.002" too large.

Step 3: Mill a few more tests and measure them
When I milled my second round of CSDs, I found that they matched my CAD much more closely, which is what I wanted. In fact, the average error for these tests was less than +/-0.001"! As expected, they still fell within a spectrum relative to each other, as did the measurements of different parts of the same feature. By testing my tool in a runout detector, I found that it was 0.0013" smaller than nominal, which means that it was the main reason the precision was affected. Very satisfying!

Step 4: Mill with precision
Now you’ve got your machine and software dialed in for that particular tool. Great job! As long as you don’t push the tool too hard (which would make it deflect into or away from the material, making your dimensions smaller or larger), this configuration should work with other materials as well.

If you’d like to try this yourself, here is the Fusion 360 file or a STEP file if you don’t have Fusion 360. To open the .f3d file in Fusion 360, go to the File menu and choose New Design From File. Then navigate to the file on your computer and click Open.

Enjoy diving into the nitty-gritty of manufacturing, and let us know what your results are!

Topics: How-To, Precision, CAM, Tutorials, Videos, Othermill, CAD, Metal