Sometimes a Hundredth is Good Enough

Smartflix sent me a very interesting product this week. It’s a DVD by a clockmaker named W.R. Smith, and the title is Tooling the Workshop for Clockmakers & Modelmakers. You can find this gentleman’s website here.

I don’t know too much about W.R. Smith, but he’s pretty clearly a Southerner. I know he has been doing what he does for a very long time, because in the video, he presents a tool he made at the age of 14. He takes small machine tools and modifies and adapts them to do all sorts of things.

So far, I’ve only seen him use two machine tools: a Sherline lathe and a small Myford lathe. He’s extremely creative. He uses them in ways you would not expect. For example, he does milling operations on both lathes, using the chucks as indexing tables. If that sounds hard to understand, maybe I can give an example that makes it clearer. He placed a board across the ways of his Myford. He mounted a motor on the board, and he connected it to a spindle mounted on the ways, close to the Myford’s chuck. He mounted a brass disk behind the Myford’s chuck, and the disk had regularly spaced holes in it. He put a part in the chuck and rotated it by hand, using a catch and the disk holes for indexing. Each time he moved the part, he used a tool mounted in the spindle to cut a gap in the part. In this way, he was able to make a clock gear.

I guess you’d have to see it to get a clear picture. I’m pretty sure I’m describing it correctly.

It was a neat thing to watch, because it reminded me that not all machining is about anal retention and 0.00001″ tolerances. In the kind of work he was doing, a hundredth of an inch one way or the other probably made little difference. It’s like the difference between technical drawing and oil painting. Any artist knows that a soft brush is better for your creativity than a sharp pencil.

I have to wonder if a person who has a nice 12 x 36 lathe can get by without something small, like a Taig or Sherline. I would assume so; people make a lot of small parts with big lathes.

I’ve been re-reading one of my favorite books, Machining Fundamentals by John R. Walker. If you have any tools to speak of, this book is a great investment. He begins with things like hammers and files and works his way up to CNC milling and so on. Believe it or not, simple tools like hammers and files are not as simple as you think. You don’t just pick them up and start whacking, if you want the best results. You would be surprised.

The lathe chapters are packed with information.

I have been trying to understand why people were telling me I don’t need an extremely precise lathe to do good work. I just found a forum comment that seems to explain it.

All rotating bodies have an axial center which is theoreticaly true and if you remove material from the rotating circumference of that body with a STATIONARY tool the surface that you generate will be theoreticaly circular and centered with the axis of rotation.

The money in high precision lathes is spent in ensuring that the bearings are capable of absorbing the axial forces of cutting without allowing excess deflection, and in ensuring that the tool travels along the bed and across the saddle in a geometricaly precise relationship to the axis of rotation when under load.

Forums are funny. You can go to a forum where there are supposed to be a lot of experts, and you can get 40 answers to what should be an easy question, none of which make sense. Then some guy who actually knows what he’s talking about drops in, and he ends the confusion in a hurry. Maybe that’s what happened, above.

I thought runout was a big deal because it meant the work as whole, while rotating, would also travel in a circle around an imaginary ideal axis, and the diameter of the circle would be two times the runout. My head hurts when I try to picture it, but I believe that would mean sloppy results. If the axis of rotation is actually nearly fixed, and the cutting tool is mounted firmly, I would assume good work is possible with less-than-spectacular machine specs.

I’m learning a whole lot, even though my closest scrape with machining to date has been cutting a piece of aluminum on a table saw.