Sunday, August 15, 2021

Weekend Shop Update on the 1 by 1 - part 5

Last weekend, I had essentially just finished the outsides of the two side plates. 

As the week began, I tapped the five holes that will get screws holding the sides together, #10-24 - a size screw I don't have a single one of - and then it was time to take my aluminum tooling plate off the mill and put the milling vise back on.  Now it was time to start working on some of the internal complexity of these parts.  As refresher, here's a rendering of the side plate 1 model I made from the pdf drawings. 

There are three large, cylinders of metal removed to make three different diameter and different depth areas in the metal.  I've numbered them arbitrarily: area 1 is 2.375 in diameter by 0.557" deep; area 2 extends the center of area 1 deeper and is 1.125 diameter; the bottom face, which has the four screw holes in it, is 0.375" below the floor of area 1; finally 3 is a separate cavity 0.903 in diameter and 0.875" deep.

So how do you remove that much metal?  There are a few main ways to do that.  

On a milling machine, you can mount the plate rigidly in some sort of fixture, remove as much metal as you can with a drill bit or end mills, and then switch to a boring head, which allows you to move a cutter outward from the center line and cut larger diameters.  A company named Criterion is the originator of the most copied designs, and there are now dozens of clones of their design, like this one made of Chinesium and sold by Grizzly.  I've done this before with a boring head like that.

Alternatively, you could put a rotary table on the milling machine, positioned so that a cutter with a flat end (any end mill) cuts while the rotary table rotates the work into the cutter.   

A completely different way that can work is to hold the work on a four jaw chuck on a lathe and use a boring bar on the tool holder to size the hole.  I did this for some of the parts on my CNC conversion of my big milling machine, like this.  All three of these methods are going to involve tons of making a cut, readjusting the machine and repeating. 

The basic thing this is doing is cutting a circle, and the ability to cut smooth nice circles is built into the programming language we use in CNC tools, G-code.  Plus, while the commercial CAM program I have can cut circles, it ends up being a fairly coarse approximation - I can see and feel the steps the CAM generates.  I've hand programmed circle cutting, too while making the parts for the big mill CNC conversion and while working on my Webster engine.  Since I consider having CNC a strong feature of my shop, I thought I'd try to do them that way.  

The first thing I did was use the biggest end mill I have, a 3/4" diameter cutter, in the centers of the cylinders and not cutting to final depth.  Just removing bulk metal and creating a starting point.  That works out to be two holes.  Next, and I'm not gonna lie, I had forgotten how to do the code for cutting circles and dug out the code for that motor mount in that video as a refresher.  

The basic approach is to use that 3/4" hole in the middle as an entry into the cut.  That hole is where I lower the cutter (so I'm not drilling with an end mill) and then move back out to the rim of the circle to cut around the perimeter. I have to keep straight in my head that the tool path marks the center of the cutter, not the diameter of the hole it's cutting.  When using the CAM program, it will work from the diameter it needs to cut keep track of the tool radius offset.

After testing a couple of things, I figured the way to cut the big cylinder (1) is by cutting two passes around the circle, one with cutter center at the edge of the 3/4" hole, which doubles its size to 1.5", followed by a second cut with the center of the cutter 0.375 in from the final size.  Since the radius is 1.188, the cutter goes at 0.813 from the center.  This is after the first operation on cavity 1 and a video is here

The changes in the way the bottom of the cylinder reflect the light can't be felt, and you can see marks from both passes around the circles.

Today, I wrote toolpaths to enlarge and deepen the center of cavity 2 and then enlarge and deepen cavity 3, bringing both to final size.  Except I misread the drawing and made cavity 3 too small.  It's always easier to correct making something too small as opposed to making it too big, and with the file that enlarges and deepens that cylinder taking about 15 seconds, it took me more time to fix the G-code than fix the part by running it (video of the process - 28 seconds long at actual speed).

Both cylinders 2 and 3 are to be sliding fits for some ball bearing sets, and both fit well.

If you compare this view to the rendering above, that shows a large semicircular cutout along the top edge.  I'll cut that like these holes but later, when I have both halves screwed together.  As of now, there need to be two screw holes on the top, left and bottom sides and a couple of other holes to manage.  I call this 80% done. 

EDIT Aug 15 10:00PM and 10:20 PM EDT - I didn't include a link to a video I meant to include - I hadn't uploaded them yet.  


  1. Amazing and amusing, I hope this little engine that could can keep your webpage powered as things get crazier.

  2. Hey Sig, is a pocket sub program available for your software?

    1. Not that I know of, but I haven't looked in years.

      Good idea. I should go looking.