My dad collects antique scales, and I’ve done some refurbishing work on a few of them. Recently (OK, 3 or 4 months ago – I’ve been busy) he asked if I could make a reproduction pin for a couple of other collectors who owned scales where this part was missing. Since I could charge enough to at least cover my materials and class expenses, I decided to take it on. A good time to learn how to use the lathe.
Step 1 was creating a drawing I could work with in Autodesk Fusion, and taking the machine shop safety/mill and lathe class at The Village Workshop so I could use the machine. the model was easy enough – a friend of dad’s measured the part (using one from a scale in my dad’s collection) and sent me the dimensions. From that information it was pretty easy to create a model and drawing in Fusion. This should weigh 3.488 oz according to Fusion – we’ll see how close I get at the end of the process. CAD is so cool…
Signing up for the machine shop class was easy; taking it showed me how much I needed to learn to make this part.
So life happens, and now it’s end of March and it’s time start making this thing.
My first learning experience was how to make a tool for cutting the round cutouts on the ends. This was kind of intimidating, but it turned out to be surprisingly easy – get some tool stock, go to grinder, grind off everything that isn’t on the drawing. Done!
It takes time, and to do something that’s for a critical dimension would be a LOT harder, but for a non-critical shape like this it really wasn’t too bad.
Next up – learning to dimension the part correctly so it’s easy to make on the lathe. “Correctly” means requiring the operator do as little math as possible, therefore minimizing the chances to make a mistake doing the math. Especially a new operator like me, who’s pretty much focused on not breaking the machine and is likely to add when he should have subtracted. By starting all the dimensions at one end and dimensioning to the center of features, you make it a lot easier to make. On the lathe you set the machine zero once at the end of the part (minimizing the chances of setting the zero wrong) and make your cuts from there. You will have to do a little math when you do the final cutoff – you add the cutoff tool width to the overall length of the part, otherwise your part is short by the width of the tool. That calculation should be done at the machine anyway, since you should measure the actual width of the tool you’re using, not just assume that a 1/16″ tool is in fact 1/16″ wide.
My high school drafting teacher Mr. Stroh would be proud. I hope.
For me, the big challenge was changing how I was thinking of the cutouts and understanding what the readout on the lathe was telling me. I dimensioned cutouts on the part as “go this deep from the surface”. The lathe readout is a diametrical dimension, but it’s from the cutting surface in, not from the center out like you think of when looking at the size of a circle. So my 0.140″ deep from the surface cut makes no sense on a lathe unless you do some math. And as a new operator I’m going to mess that up. What you need to do is express the dimension as how much you’re taking off the diameter. In this case my OD is 0.625″, my ID is 0.344″. So the change in diameter is 0.625 – 0.344 = 0.281. Zero the tool on the surface, cut in until the readout shows -0.281″ (moving out is +, in is -). Easy enough once I understood what I was looking at.
My first experiment was on the brass stock. I figured, what could go wrong. I’ll have a good part at the end of this and only one more to go! It was looking good at the start, but my bad dimensioning caught up with me – somewhere along the way I made a mistake and added two values I should have subtracted, or subtracted when I should have added. In any case, the piece ended up too short with the first cutout too far from the shoulder and the and the second one mostly cutting air.
After my first learning experience, I decided to practice on steel – much cheaper than the brass stock! After a few hours playing with the lathe on Saturday and Sunday, taking my time and making lots of measurements, things started looking better.
Progression of parts from left to right –
- The first failure in brass
- Closer in steel, but some tool chatter prevented cutting the big round cutouts. Even though the tool wasn’t too hard to make I didn’t want to make it twice!
- More practice and a better model yields much better results
- The real part. This is from the 1800’s. Pretty impressive given they didn’t have digital readouts, YouTube lathe videos, or calculators on their cellphones!
The final steel prototype with the real part – looking pretty good! Next week I’ll be cutting the real parts in brass.