Cutting Miters on the CNC

      Advice on bits and methods for cutting miters and miter-folds on CNC routers. October 14, 2010

I am new to CNC and would like to cut mitres on my parts. Will a large V-bit or 45 degree bit work well or am I better to still use my slider? I only have 3 axis. I am sure it would be easier with a 5. I nest, so should I cut out the part first with a compression bit and then profile it with the mitre? Any help is appreciated.

Forum Responses
(CNC Forum)
From contributor M:
I cut mitres with a large 45 degree bit frequently. I would recommend cutting the mitre first, then finishing with the compression bit. This helps hold the part in place, as the 45 cuts a large path.

From contributor K:
I use a miter fold bit to cut 90 degree corners. Amana makes several 90 degree miter fold bits with replaceable cutters. They work very well.

From contributor I:
I would recommend buying the insert type tool for folding. We use them for straight and curved mitres on a regular basis. As mentioned, the tool removes quite a bit of stock, but you can take multiple passes to get to your desired depth. Talk to your tooling supplier. You can buy bodies that cut 90, or 90+/- depending on your application.

From contributor O:
Make your final pass a trim of about 1-2% of final depth for best accuracy. A CNC is never going to be as fast as a saw for doing mitres but it has other advantages of course.

From contributor L:
I cut miters on the CNC all the time. I don't cut them in the nest, I cut them as a second operation along the edge of the table. Just set up a second operation that you keep loaded in your machine. I have four for miters (2 in X and 2 in Y) 1 each 22.5 and 45 deg. in X and Y.

I remove the bearing and grind off the bearing mount and call it a 1/2" diameter tool and set it to run the outer edge of the bed at flush with the spoilboard. Also using a modified bearing bit you don't have a pointed tip that gets dull before the rest of the cutting edge, it all gets dull at the same rate.

A CNC router can be fast at cutting miters and it does a much cleaner job of it also. Once your program is written and stored in your machine, there is no fence to set, no board to push no test cut to make. Its very safe, quick and accurate.

From contributor R:
I have worked with a couple of my clients to get them up and running using CNC's for miterfolding. Ideally, use a 91 degree bit, not a 90. The gap allows for glue without opening the tip of the miter. Taping the joint after milling is fine, and can allow for far better yields in many cases.

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