Saturday, July 23, 2016

"I Studied the Trigonometry"

Great line as delivered by Mr. Bean.  I was wondering why my slope cutting program was not cutting at the right angle.  It has been a long time since I took trig; Nixon was President.  But after some analysis, I discovered C's trig functions expect the angle to be in radians, not degrees.  Problem solved!

Some video of the mill cutting slopes in scrap acetal:

At the bottom of the slope, I move back up the slope to clean debris and smooth the surface.

I was still struggling with the program at this point, so it was cutting about a 30 degree slope when I asked for 45, but eventually I straightened that out.  You will notice that the mill is moving on a diagonal, not cutting a staircase.  Of course the diameter of this mill requires several passes a few fractions of an inch apart.  

Someone had told me you have to use a ballnose end mill to get a smooth diagonal.  That's not true.  If you use a roughing mill (as I did at one point) you get a very rough diagonal.  This is a finish mill doing the cutting.  I have a variety of sizes of end mill from 1/16" diameter finish up to a 3/4" finish mill and a 1/2" roughing mill. The larger the diameter the longer they are, allowing deeper plunges into the material because the extra diameter makes them stronger and less likely to break.

Obviously, you use a chop saw for cutting large pieces to 0-45 degrees, but my chop saw won't go beyond 45 degrees, and there is no easy way to hold a narrow workpiece at the inverse angle (cut a 60 degree angle by flipping it 90 degrees and cutting a 30 degree angle).  The mill is also useful if you need an angle cut in a piece with a very precise length, or a part not easily held in a chop saw.


  1. Radians-vs-degrees is a pitfall that many modern students fall into, when using programmable calculators.

  2. Would this process work better if you had a milling bit with a 90 degree taper tip, giving you a 45 degree angled edge for surfacing? Could you accurately step the mill to match each horizontal pass as you face the entire slope?