This is another post from my bike business. I’m reposting here for posterity, and shows a local maximum I was at circa 2014. There are better ways to notch tubes for welding, but this was a nice, flexible, and relatively inexpensive way to do it.
When I built my first three frames, I used the best hole saws that I could find, the 6 tooth per inch lennox ones available at my local hardware. They were too coarse for thin walled 4130 and I ran into a lot of problems. Trying to improve on their performance, I switched to end mills, but I couldn’t get enough flutes to get around the edge of the tube catching and kinking in. I knew that abrasive notchers existed, but was far too much of a cheapskate to buy one, so I was off to look for an inexpensive lathe. After some looking, I settled on this one.
Early attempts to notch tubes in it were done with a poorly designed setup that hung off the tool post. It was easy to adjust, but there were tons of parts and the tubes were hard to change out. I did some measuring to make sure that my tube blocks would be on center and came up with the predecessor to the tool posts I use now. I sell a version of them here (this was a link to my old site. It doesn’t work anymore.)
To use it, clamp the tube in a tube block that measures 2” square. I make my own, but paragon sells them here, and they’re very reasonably priced. To keep the notches on either end in plane, always keep one of these blocks on your tube until it’s notched on both ends.
Then wrap the right size mandrel in sandpaper. Tape it on at the bottom of the sandpaper.
Then wrap the sandpaper on the mandrel. It’s important to wrap it in the same direction as the lathe turns. If it’s wrapped the opposite way, the tube unwraps the sandpaper.
Keeping the wrap tight, tape it to itself with small strips of tape at either end. There’s no need to tape it to the mandrel, all these wraps do is keep it wound up on the drum.
After the sandpaper is in place, spin up the lathe and gradually work the tube into it until a full notch is formed. Then deburr the end and check the fit. If the tube is too long, put it back in the lathe and take some more off.
I’ve been using this process for about a year and a half. It will slowly destroy your lathe, as there are loads of tiny abrasive particles that make it into your machine ways. This can cause chatter and slightly off center notching. With a little planning and lots of cleaning, it’s possible to make shields for the ways to prevent this damage.