A Big Project For A Small Lathe

My lathe is rated at a seven inch swing, meaning that it can accommodate a piece of metal 7″ in diameter. You could maybe mount something that big somehow, and it would be able to rotate. But you couldn’t get a cutting tool to reach the outside diameter of the piece. In truth, the lathe really can’t work over about 5″.

A friend of mine is a cigar afficionado, and smokes outside on his patio. He was lamenting that he needed a better ashtray. He wanted an ashtray with wide enough grooves to accommodate a fat cigar. It must stay on the table even in high winds. And it had to look “industrial”.

The Design

He was much taken with the idea of a machined steel ashtray, so I worked up a 3D model in Onshape.

We worked out a few further details, and I ordered a 5″ diameter chunk of steel. Once it arrived I set to work.

A Big Project

Just for fun I weighed the blank: it was over 9 pounds. And weight wasn’t the only issue. Unfortunately, the biggest chuck I have for this lathe is a 4 inch. And the blank was 5 inches. But you can reverse the jaws, and hold bigger pieces. So that was fine. But the jaws are stepped, so only about 3/8″ of the jaws would have hold of the blank. The nine pound blank. And the blank was rusty, so the grip was going to be a little suspect anyway. I therefore chose the 4-jaw chuck rather than the 3-jaw, to give myself a fighting chance.

And a Slow One

As soon as I started the motor the second problem became apparent: the lathe was underpowered for the job. Or rather, the lathe lacked torque at low speed. And I had to do the work at low speed.

As the material gets bigger around, it has to be turned slower in order to get a good cut on a lathe. The issue is the surface speed. You can think of it as the speed that a given point on the surface of the material is moving while it turns. The bigger the material is, the faster the surface moves for a given number of RPM’s. But the cutting tool can only stand so much surface speed. As the speed increases, the cut generates more and more heat. Eventually the cutting edge gets overheated and fails. So, as the material gets bigger, it has to be turned more slowly in order for the cutting tool to survive.

Machinists worked out the details of “how fast is too fast” ages ago. Nowadays you can find online calculators like this one to do it for you. According to the calculator, I should be turning the blank at 115 RPM.

There was just one little problem. My lathe has a variable-speed motor. But when it’s turning that slowly, the motor has almost no torque. As soon as the cutting tool touched the metal the motor stalled.

A New Lathe Was Not An Option

Fortunately, there was another way. I do most of my work with tools made of a special steel alloy called High Speed Steel, or HSS for short. But industrial machinists long ago started using tungsten carbide tools instead. With carbide tooling I could run the work at 600 RPM, and at that speed the lathe has plenty of torque. And so I pulled out some carbide tools and set to work.

I faced one end and started turning the outside. My plan was to turn part of the length, then flip the part around in the jaws. That would give me a better surface for the chuck to hold. Then I would face off the other end and turn the remaining length of the outside.

And for once, the plan worked! I ended up with a nice, shiny cylinder of steel. Now for the hard part.

A Really Big Hole

You can’t make a four inch hole with a drill bit. At least, not using any machine that would fit in my house. However, I had a plan. I had some tools called boring bars, that will cut the insides of a hole. It was going to take a while, but I was pretty confident that it would work.

And it did, sort of. After I fine-tuned settings, I found I could cut about 0.01″ per pass. It took about two minutes per pass. I was removing about 2″ worth, so that’s 200 passes. Or all of my spare time for a week.

The process of cutting the inside of a hole with a boring bar is called, unsurprisingly, boring. And it really was darn repetitive. However, as you can see from the video, it really wasn’t boring at all. In fact, I’d say it was pretty darn lively.

Those bits that are flying out of the hole are hot steel. They’re sharp enough to cut you, too. Between the noise, the smoke, and the hot flying metal I had no trouble staying awake.

Eventually the hole was “about big enough”. I wasn’t aiming for a specific size in this case, I just wanted good proportions. It needed to look substantial.

A quick trip to the milling machine cut the notches to hold the cigars.

The Finishing Touch

My friend wanted a finish that would look suitably industrial, and that would acquire some surface rust after a few seasons outside. We agreed that a black oxide finish would work well. I preheated the ashtray in a 450 degree oven for about half an hour, then hit it with a couple MAPP gas torches. I heated the ashtray until it glowed a dull cherry red, then dropped it into my oil quench tank. This is the result.

My curiosity got the better of me, and I weighed the finished piece. It was right at four pounds. I had machined off more than five pounds of steel!