CNC: Should I use forstner bits or a 1/4" milling bit?

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CNC: Should I use forstner bits or a 1/4" milling bit?

Post by ohmbrew »

Don't know if this is an appropriate forum for this question...

I had previously used forstner bits to manually drill out large holes in my pinball playfield. They created very nice holes using a hand held drill.

But now, I'm creating a FreeCAD model to create gcode to get the whole thing drilled out nicely using a CNC. Question: should I just be using a standard milling bit (i.e. 1/4") to mill out the 2.5" hole, instead of setting the Path Workbench tool for a 2.5" drill hole bit? It will decrease my tool bit changes and I started researching online and sites were saying not to use forstner bits in routers/CNC because they spin too fast.
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Re: CNC: Should I use forstner bits or a 1/4" milling bit?

Post by sliptonic »

That's not really a Freecad or Path question. You'll get better information from cnczone or the community for your machine. Also a woodworking site will be helpful. A lot of those guys are using cncs now.

The important thing is to get the right tool and use it correctly (ie correct feeds and speeds). There are a million different tools with different geometry like upcut, downcut, compression, different flute counts and all that will affect your surface quality. A tool that works well on solid maple may perform poorly on Baltic birch ply.
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Re: CNC: Should I use forstner bits or a 1/4" milling bit?

Post by Steveco »

The answer depends on your CNC. Personally I haven't seen a Forstner bit rated for over 750 RPM, and the slowest my CNC spindle will go is 6,000 RPM, so I can't use Forstner bits with my CNC. I use a 1/4" endmill instead. If I had a CNC that could do 500 RPM I might try a Forstner bit in that. (I once overheated a Forstner bit while drilling with a manual drill press, and that ruined the cutting edge, so I am cautious with them.)
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Re: CNC: Should I use forstner bits or a 1/4" milling bit?

Post by CandL »

I have a Onefinity Journeyman machine, using Makita router (a nice spindle will be the next upgrade)....kinda typical for a "home" machine.

I am a bit of a "apprentice" when it comes to machining but here are my thoughts. ( those with more wisdom feel free to chime in.)

You do not mention the material or the material thickness. Let's assume 3/8" inch thick wood ( ply or solid )

With that said, I use a 0.25 inch compression bit ( both up and down cutting action) for a lot of my operations, but not if it is this thick. The compression bits transition spiral directions at ~0.25" right in the middle of the thickness. I just have concerns about cut quality in the transition.

For your holes, I would try a 0.25 downcut bit with an adaptive toolpath. Adaptive, because it uses the length of the bit, not just the end. by the way my copy of VCarve pro does not have adaptive cutting :o

With the 0.25 bit loaded, you can just cut your profiles .... fewer bit changes are good.

This is the approach I used when creating some mft squaring triangles.

Hope this helps

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