Optimizing Machining Techniques

I spent way too much of yesterday evening making a caster set for the Meade LX200 tripod--and end up with something that wasn't good enough to ship. It would have worked, sure, but the bore was eccentric and off-center. That's fine for friends (I mean, it's fine to have friends that are eccentric and off-center--most of mine are), but not okay for products!

So this evening, I scrapped about $15 worth of Delrin and three hours of labor, and started over. This time, it took less than hour, and I ended up with a much better result.

The core problem here was that the version that I make that is an insert that slides into the legs of some tripods has to be pretty precisely machined on the outside diameter--2.35" +- .005", for example, for the Losmandy G-11. For the sleeve version, like I build for the Meade LX200, it is the inside diameter of the sleeve that matters--not the outside.

But because I started out making the insert version, I have gotten into the unnecessary habit of turning the outside diameter of the sleeve to a particular size. Partly, it was just a habit, and partly, I rather like the very machined look it produces.

Worse, because I was trying to turn the entire length of the cylinder in one operation, I had to drill and tap a hole in one end, put it on a special holder, rather than just use the 3-jaw chuck to hold the cylinder in place. I have never produced a perfectly centered hole, so if I start with a 2.265" outside diameter piece of plastic, I typically have to turn it down to 2.20" to get all the outside at the same radius around the center of rotation. This is slow (hence the long operation yesterday evening)--and even then, when it comes time to bore in the center of the sleeve, it is surprisingly easy to end up not being centered.

Anyway, I figured out that I should just put the cylinder in the 3-jaw chuck, and sand off the external surface (which often has brand markings on it) with #80, #180, #800, and #1500 sandpaper. It is fast, and produces an acceptable looking finish that is very smooth. This speeds up the process a lot!

I am also thinking that for the more precisely machined cylinder that are inserts, instead of going through the hassle of trying to get an exactly centered tapped hole in one end (something that often does not happen), it makes more sense to add 3/8" of an inch to cylinder so that I can turn it down for the entire length of the cylinder with the cylinder in the 3-jaw chuck. When it comes time to cut it at the 30 degree angle, I just move the up 3/8" an inch, and have a bit more scrap--but a lot less time spent turning the cylinder to size. Most of the Delrin rods that I start with cost a bit more than $2 an inch, so this turns out to be a materials cost of about $2.50 per set--but a saving of easily six minutes a set, and a more consistent result.

I still have to put a tapped hole in one end, because that's how I attach the cylinder to a fixture when running it through the chop saw to get the 30 degree cut. (This way, I get to keep my fingers attached.) But it no longer has to be a precisely centered, perfectly square hole. It can be even a bit sloppy, because it is just to have an easy way to lock down the cylinder when I put it on the chop saw.