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How to back chamfer threaded holes?

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mlitzkow
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How to back chamfer threaded holes?

I want to drill and thread 480 holes into the aluminum table of a CNC router using the router itself to do the work. Tapping is out of the question because the minimum speed of the router is 10,000 RPM. The table is made from a hollow extrusion. This means the holes aren't exactly blind holes. They go all the way through the surface of the extrusion and into the hollow part. Still, I can't access the bottoms of the holes directly. The holes I need are 8-32 and 1/4-20.

 

My operations are:

  Spot the holes with a spotting drill

  Drill the holes to the recommended tap drill size with a twist drill

  Thread the holes with a multi-form threadmill

  Deburr the tops of the holes using the spotting drill

 

After some experimentation with scrap aluminum, I have this working pretty well, but I still have a problem. This leaves a burr at the bottom of the hole where my fastener will extend into the hollow part of the extrusion. While I am using a multi-form mill to create the threads, during my experimentation I also aquired a single-form thread mill of the appropriate size. Now I want to use that single-form threadmill to create a back chamfer so as to debur the bottom of the threaded hole. I have tried to re-create the geometry of the single-form threadmill to create a custom "form mill" for this operation, but I am running into some trouble.

 

1. What operation (2d chamfer, circular, 2d contour, etc.) is appropriate for this job?

2. I have sketched my single-form thread mill so I can turn it into a form mill, but I am confused about the "compensation point" and "tip offset". Do I need custom settings of one or both of those so that Fusion realizes it needs to insert the tool below the bottom surface of my part and cut on the bottom to make that chamfer.

 

Thanks in advance for your help.

 

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Message 2 of 2
rengfx
in reply to: mlitzkow

I use 2D contour, with a sketch on your tool profile of the various chamfers I use (.005" edgebreak + .010" / .015" / .020" chamfers) you can determine the negative Radial / Axial stock needed to drive the chamfer, I don't mess with compensation point adjustments, just use the negative stock to drive it

 

chamfers.png

 

Experiment in CAM / Simulation then it will work

 

Obviously you could use a 60 degree threadmill or a 90 degree dedicated backchamfer or double angle shank cutter, just needs to be smaller diameter (accounting for lead in / lead out) than your threaded hole

 

Save as a template, reuse when needed

 

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