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

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ohmbrew
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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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sliptonic
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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.
Steveco
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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.)
CandL
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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. https://wiki.freecadweb.org/Path_Adaptive.... 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

Regards
Carl
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