Making Bolt Hole Chamfers with an End Mill on your CNC.

I spent the last 10 years using CNCs without a tool changer. Whenever the job required a bit change normally I thought about how I might be able to do the job using just one bit and some creative use of the CNC software I was using. This is one example.

Projects that needed perimeter or internal chamfers were done most efficiently with a bit change to a V bit.  I have a project that is put together with flat head bolts. The need to chamfer the bolt holes could have been done using a drill press after the CNC was done. Figuring out how to chamfer a hole with the same end mill I used for all the other cuts turned a 2 process job into one CNC  job with no bit changes. 

Vectric.com's Aspire and VCarve programs have a fluting toolpath.  Fluting takes a vector and ramps the bit down or down then up with the bit being centered on the vector. A circular array of 9 (or more) short vectors centered over the bolt hole, using a bit no larger than the diameter of the hole, can effectively chamfer the perimeter of the hole from the bolt head diameter down to the hole edge.  Using a small bit (1/8" or 1/16") may require a greater number of vectors to result in a relatively smooth chamfered edge. The chamfers in this image were made with a 1/4" diameter end mill. 

2 x 2 Hole Array

The slope of the chamfer and the diameter can be easily changed.  Aspire/VCarve also have a circular array layout tool that will use one vector that spans the hole and make any number you specify of rotated copies around the same center. Using an odd number of vectors results in a circle of lines with alternating start points.  Alternating the start points reduces the travel time needed move to each successive vector.  

Vectors Used

The center of the bit used is what follows the line. The depth the bit will slope down is set when creating the fluting toolpath.  In the image above the shaded area is a shadow of the area the 1/4" straight bit travelled over.  The short vectors used are shown in red. The center circle was the hole for the 1/4" bolt shaft. Depth of the fluting chamfer cuts was 1/8".

I made the vectors for one bolt hole, then simply copied them to all the other bolt locations. 

You can find downloadable  .CRV3d and .CRV file of the sample above on Vectric's user forum: Vectric.com

Questions or comments are encouraged!

4D