You don’t really need a particularly wide piece of tool steel to cut the profile in the wheels for tube benders (or any other part that requires a radius for that matter.)

After several attempts at free hand profiling the groove in wheels for my bender, I finally remembered other members talking about using a spreadsheet to calculate the cuts for such jobs.

The penny dropped when I realised that when two circles touch, the centre’s of the two circles are always on a straight line through the tangent point, whether the smaller circle is outside or inside the larger one. The location of the tangent point can then be defined in terms of one parameter, the angle through the two centres.

It is then relatively simple geometry to calculate the plunge depth and x location for each cut to generate the circle. By using a single cell for the required circle radius, and another for the tool radius, and absolute references for those cells, it is easy to print out the table of cuts. I used an angle increment of ten degrees between points, but easy to adjust in the spreadsheet for closer points or for fewer cuts. I have a cheap dro for the x axis on my lathe, but not for y, so the plunge depths were done purely using the handwheel scale, withdrawing after each cut and plunging in for the next to eliminate backlash in the normal way. They are not heavy cuts as one side of the tool is always clear, and only shallow plunge cuts on the other, so I don’t think there would be any problem even on a small lathe. I did appreciate having the dro for the x axis, but the hand wheel graduations will also work as the travel is always the one direction.

I had a small piece of tool steel with an appropriate radius on the end, given to me in a box of useful bits when I was starting out. Clearly hand ground, as it’s not a perfect circle, but it did the job well enough. Really interesting watching the profile develop.

You can see the result in the close up of the wheel. Aluminium is now a bit worse for wear and showing some corrosion after a year or two, it would be better in steel for frequent use. The next project will require some smaller wheels, so I will just get out the spreadsheet and adjust the radius required, using the same tool.

The same method would work on the mill using a ball nose cutter, eg for Chris’ “gear bag” to avoid those interference issues.

Last picture is the bender. No credit to me, it was in Model Engineer a while back.

Apologies for not posting the spreadsheet, it’s in Apple Numbers, and while it is easy to attach pictures even on the iPad, I can’t work out how to attach a file. (If you need it, pm me an email address and I will send it.)

I hope this helps.

MJM460