My dad asked me to build something for him that was going to require a series to rollers. I could have built custom roller for this application, but it was most cost effective to buy off the shelf conveyor rollers. The downside of these conveyor rollers is that they have a hexagonal shaft. While I could have probably gotten away with drilling an over-sized round hole and just let the hex-shaft be loosely constrained that didn’t sit right with me. I also could have gone a very labor intensive route and drilled round holes and then filed them to shape, but I didn’t particularly want to do that either. What options were I left with? The proper way to cut a non-circular hole in material is to use a broach; hexagonal and square broaches are quite common. I could buy a hexagonal broach for a few hundred dollars, but this seemed silly for a tool I would only use for one project. So, instead, I decided to make one.
Having decided to make a custom broach I did research on the design of cutting tools. I found a few helpful internet articles (google: making a broach) but ultimately the most helpful resource was the section on broaches in Machinery’s Handbook (a great resource for anyone who does mechanical design). I then went through the process of designing a broach based on the equations listed in Machinery’s Handbook; I will outline the design process in a future post. Ultimately, however, after watching this excellent video by Chris at Clickspring, I decided to use the method he used. His method was simpler than the one I was going to use for fabrication and as I would only need to cut a few holes, the ideal design of my approach was perhaps overkill.
So, armed with a section of 7/16″ hex stock (the size of the broach) I headed to the shop (not my shop, many thanks to Ben for lending me the use of his lathe). The first step of the fabrication process was to part off a section of the stock that was 3-4 inches in length. The overall length of the part is not critical, there is a minimum length to achieve the proper taper, and a longer piece of stock is easier to work with.
After parting the off the stock I changed tools to face the end. After facing, I turned the one end of the down to just under 7/16″. This end is slightly undersized to fit into the 7/16″ holes I will drill into the part this cutter will be used on. This end was also slightly chamfered to aid with insertion of the broach.
The next step, which varies for the method used by Chris, was turning the taper from the 7/16″ round to the 7/16″ hex of the raw stock. To do this, I set the tool post at about 1 degree and started cutting. I had cut my stock a bit too short and ended up having to flip the part (and my 1 degree angle) and cut the taper from both ends.
After cutting the taper, I cut individual teeth into the broach. The spacing of the teeth, combined with the taper of the broach, define how much material each tooth removes. The teeth are spaced at approximately 0.25″ intervals, allowing each tooth to remove about 0.007″ of material. I cut 2-3 teeth at a time and gradually moved the stock out of the 3-jaw chuck to minimize the cantilever load on the tools as I was cutting.
After cutting the teeth, I used a file to smooth out the back of the cutting teeth to give them some relief. This, combined with the method I used for cutting the taper and the teeth is perhaps not ideal. In the future I would follow the method used by Chris more closely and would ultimately end up with a better tool.
I will next attempt to heat treat the cutter. I have a torch and oil for quenching and I will hopefully try to film the process and include it in a future post. I may end up making a second broach before then using some things I learned in the process of making this one.