Yesterday (Sunday) my friend had a free afternoon where I could use his CNC machine to work on my orrery parts.  He was in the process of carving some guitar necks and when he was done, we put my stuff on the machine.

It was fascinating to watch it carve those two guitar necks.  Once you work out all the kinks in your process, the CNC session can go pretty quickly.  Each neck seemed to take about 20 minutes but I wasn’t timing it.  The machine will tell you exactly how long each session is going to take but I never remember that sort of detail.  Plus, there’s the time to mount the blank stock and all that stuff.  Even so, I’ll bet each neck only took about half an hour to complete.  He uses a “ball mill” (i.e., round nosed router bit) to do this work and if the router only moves over a very small distance with each pass, you get a really smooth surface left behind that requires very little sanding.  Much less sanding that you’d need if you were cutting everything on a bandsaw.

Well, on to the orrery.

Machining The Base

Machining the Base

I had my glued-up panel of pine and we clamped that to the bed of the machine with bolts and quickly cut that out.  I already have a top made from MDF but I think pine looks much better.  At least it looks like wood and not cardboard.  This is a pretty simple shape and the machine made quick work of it.

We then stuck down a piece of 1/8” plywood for some small parts; spacers and other small pieces.  This went pretty well.  We left tabs to keep the parts from flying off and as you can see by the photo, everything went as intended.  In the photo, I’m breaking the last tab and removing the parts from the stock.  I forgot to bring my camera to his house so all these pictures were taken after I got back to my house.  We actually did have one or two parts get loose.  This illustrates a problem that is not uncommon and that is your wood not being perfectly flat or not perfectly stuck down to the bed of the tool.  Thin plywood is often curved a bit.

200 years of tool history in one day!

Cut with a CNC mill, clean up with a spokeshave.

It’s pretty easy to clamp it to the bed of a CNC router with something around the edges but difficult to clamp it in the middle unless you plan ahead really carefully so that you can have a bolt in the middle somewhere.  As a result, the middle can bow upwards slightly; you might not even notice it.  But the tabs you leave in your parts are really thin.  These have to be thin so that you don’t have to use a saw to pop them loose after the cutting is done but if the stock is slightly lifted off the surface, then the router may still cut through the tabs as well as the stock which will cut your part loose.  As a result, one of my parts ended up on the floor under the machine and a few others ended up in the dust collector bin. It’s just something that happens when you work with CNC and with proper clamping and preparation you can avoid. The trick is in knowing that you have to think about it at all; this is not the sort of thing that ever happens when you’re using traditional woodworking methods.

Punching Out Some Parts

We then put on a piece of half-inch plywood to cut the gears.  We were now cutting the largest and most complex gears.  Again, we clamped the wood along the edges.  Half inch material is pretty stiff and we apparently didn’t get any lifting.  At one point though we did have an issue.  I’m not sure if we forgot or if he just thought that we could get away without the tabs but the first gear we cut had no tabs to hold the scrap when we cut out for the spokes in the interior of the gear.  I was off doing something else at the time.  The scrap got loose and the bit grabbed it and the scrap got wedged between the remaining stock and the bit.  Right before the bit pitched it away, the entire piece of stock got scooted over a tiny bit.  As a result, my center shaft hole ended up off center.  Again, this isn’t a disaster, it’s just something that happens occasionally and the lesson is that we have to think of that next time.  The worst thing that can happen is that during brief moment of wedgification, the bit can flex and it could break off.  That really would be an expensive mistake.  Generally it’s easy to cut another part but not so easy to have to buy another router bit.  We got lucky I think.

So we made some changes before cutting the next one.  And by “we” of course I mean “him”.  The first time you ever make a part on a machine like this requires some tinkering.  It’s after you’re done tinkering and have the process down that the next time goes so smoothly.  I guess this is something that engineers in the mass production industry have known for years that I’m must learning now.  I’m not so much interested in mass production as I am in having parts that extremely precise and are in almost finished form when they come off the machine.  Just look at those edges in the photos.  Even the scrap pieces have perfect edges.  It’s almost a shame to use them as kindling in his wood stove.

The Cleanest Piece of Scrap You'll Ever See


And if you happen to have what is called a compression bit, then both sides of the part look like this and no sanding will be required.  Huzzah!

Well, we finished up with two gears for the earth and one large gear for Jupiter.  By this time I was getting pretty tired after having been on my feet all afternoon and he decided to break for dinner.  Looking at the clock, I realized that we had been at this (guitar necks and gears) for a little over six hours.  I was unwilling to impose on his free time any more than that so I went away without my last two gears:  the drive gear for Mercury and the largest and most complex gear – Saturn.  I ordered a thin bandsaw blade last week since I realized that it will come in handy for any fine tuning of these parts and also they’re cheap.  Perhaps I can do these two gears by hand.  We’ll see.

If it weren’t for the fact that the Mercury gear set goes on the bottom, I would almost be able to put the thing together and see if it fits.  I may do that anyway and leave Mercury out for the moment.

The Completed Parts

While I was waiting for one of the CNC sessions to run its course, I cut out the pattern for the dial.  This is something that the plan designer included and is a round piece of paper that you’re supposed to glue onto the top.  It has months and signs of the zodiac on it and just generally looks cool.  The designer says in his notes not to use a traditional copy machine to make copies of it because copy machines are notorious for distorting things just a little; probably because of slight irregularities in how the paper is pulled through the machine as it prints.  He recommends using an inkjet printer or, better yet, a plotter.  Just for test-fitting purposes, I used a plain old copier and the resulting drawings are in fact distorted.  Because the dial is larger than one sheet of paper, it comes in two parts; you cut these out and join them afterwards.  So a distorted drawing will not come together at the seams as I found out.  I expected it though; I was just running a quick test so I’ll print another one on the inkjet.

At any rate, I made much progress and spent the rest of the evening until bedtime sanding away all the fuzzies.  Now, on to the grand test fitting of all the gears on the shafts!