KevCAM Part 2 – Controlling Toolpath Orientation and Stepover

So if you’ve read through my first post on KevCAM you get the general idea on how the method works.  On this post I’m going to show you how to control the stepover of the toolpath, and the toolpath orientation to your mold.  Sticking with our very basic “bent half pipe” mold from earlier, the toolpath was oriented along the “length” of the mold, like this:

But let’s say that you wanted the toolpath to travel in the other direction, across the “width” of the mold.  How would you do that?  Well it’s quite easy.  Let’s start by making new hatch, exploding it, using Joiner to link the ends, joining it, and moving it up 0.25″ from our reference surface.  I re-created the reference surface so that it would be the exact size of the hatch linework just using a Corner to corner surface.  Takes all of 30 seconds to do, and looks like this:

All we’ve done is make a set of base toolpaths that rotated 90 degrees relative to our base surface.  Now we just FlowAlongSrf to apply the toolpath to the mold:

Ta da!  Our toolpath is now traveling along the surface in the other direction.  Now let’s say you’re feeling wild and crazy, and you decide you want to orient your toolpath at a 45 degree angle to your isocurves.  Well, that’s just as easy:

And then…..

The above idea I just thought of while doing this post, and now I’m very eager to try it out on the mill to see what kind of a finish it gets.  So the next thing to consider is stepover.  By the nature of what we’re doing, the stepover will not be constant, so what we really mean when we talk about controlling stepover is how to limit the maximum stepover.  Typically with a 0.5″ ball end mill I find that a stepover of 0.02″ will give me a fine enough finish so that I don’t have to do any hand finishing.  So I would like to keep my max stepover below 0.02″.  All it takes is a little simple math.   Let’s go with the example where the toolpath is running down the “length” of the mold.  The mold looks fairly constant with regards to width.  If it didn’t, let’s say one end was wide and the other was narrow, then we would want to do our calculation from the widest part of the mold.  In this case, let’s just find the width of the mold surface at any given point – say one of the ends.  Just use the Length command, and click on one of the edges along the end of the mold that run across the width:

So the length of that edge is 5.95 units, let’s call that 6.  Divide 6 by .02 and you get 300.  So we need a hatch with 300 lines running along the length of our base surface.  Let’s do this another way just for fun – draw a line 10 units long running in the x direction, then run the Array command.  Make 300 lines in the y direction, with a y spacing of .02.  Then go about this like before:

FlowAlongSrf, and let’s see what we get:

Certainly no surprise there, same as we had before just more dense.  Now, I will freely admit that when you start to do large pieces, with lots of dense linework, you run the chance of maxing out your RAM and crashing Rhino.   Never fear, there is a very simple work around – just explode your base linework before you FlowAlongSrf, so that it’s just line segments and not one huge joined polyline.  After you FlowAlongSrf, just join the lines.  Okay, so let’s check our stepover.  It looks to me like the stepover is smaller in the bottom of the mold, and larger at the top edges.  Like I said, this method will by definition not produce a completely even stepover, what you’re looking for is to limit your MAX stepover.  So, using the Distance command to measure between the two points shown here, we get that the stepover at the top of the mold is 0.025″:

I happen to know from experience that’s good enough for what I need.  If you wanted to bring it down even more, you could just go back and redo your array with even more points – bump it up by say 20%.  At the bottom of the mold, the stepover is quite small – 0.009″:

So there would be a bit of wasted machine time here, but again the density can be modified to balance that if you want.  We would take the same approach with this if we wanted to mill in the other direction.  Instead of measuring the “width” of the mold, we would find the longest part of the “length.”  That would of course be the outside edge of the mold:

So that edge is 20.5 units long, so let’s base our calcs on a 21 unit length.  21 divided by 0.02 is 1,50.  So make an array with 1050 line segments in the x direction, yadda yadda yadda, I think you know the rest from here:

Incidentally, this was too dense for my little machine the first time and caused it to crash, so I just exploded the linework and then FlowAlongSrf’ed, then joined together.  Max stepover was exactly what I wanted it to be – 0.02″:

In reality, I would not use this toolpath, because if we orient it this way, the tool spends nearly all of its’ time doing stepovers that are smaller than we need, and this means even more wasted time than the previous scheme.

On the next post I’ll show you some little script shortcuts to this, and a really cool trick if your mold forms a “ring” that allows you to spiral mill in one continuous path.  Till then….

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