Freesteel Blog » Pencil overthickness nightmares

Pencil overthickness nightmares

Monday, January 4th, 2010 at 12:59 pm Written by:

I thought I was doing well with the pencil overthickness routine, until I got this Christmas present from Cimco.


The strategy works by making (bitangent) pencil milling passes with an enlarged cutter, and projecting the smaller cutter from each location on the pass towards the half-way point between the two contact points.

(The black “stitching” lines show the perpendicular passes of the smaller cutter sweeping between the two contacts points for the enlarged cutter.)

The idea is to produce useful pencil passes when the model has fillets with equal or greater radius than the cutter, as in this diagram:


The difficulties include:

  • avoiding the noise from the positions of the contact points producing a wobbly toolpath
  • missing the corner when there are bitangency points for the smaller cutter because there is no fillet
  • joining the projected passes up at the junctions
  • two bitangency points for the same pass because the overthickened cutter runs up a channel

Now, I either have to argue that this example above is asking too much of the strategy, or try and solve it and possibly cause other problems if it doesn’t get it right.

The expectations that I should produce a reliable algorithms are unfortunately high.

Cimco have done well. Their example displays almost everything that can go wrong with this algorithm. Notice, for example, how the shelf causes the projection to hit the floor instead of anywhere near the corner.


Also, you can see a fold-back at the 3-way junction near the bottom of the picture as the larger tool directs the smaller tool to project back into the channel.

Have I got so much else to do! Just got back from 7 days of not caving (and one day of caving) in the icy wastelands of Yorkshire. (Becka did 7 days of caving and one day of not caving.) Pictures to follow eventually.

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