I decided to make a form cutter for the front knobs. The cutter would eliminate countless hours of measuring and remeasuring trying to make 20 knobs consistent. If done properly, it should be able to be plunged into a spinning blank and make a knob quickly. I did not do it for the quick factor, I am more worried about consistency. I have never made a form cutter prior to this, it is something I have been meaning to try for a while.
I started by making a full size drawing of the front knob on paper. Using a copy machine I scaled it down to 25% and measured to make sure it was the right size. I cut the small paper knob down the center, then cut the profile. I glued the outside of the shape to a piece of A2 tool steel .5" wide and 3/32" thick.
Using a jewelers saw, I sawed out the rough profile. I then used files to refine the profile down to the paper template. Once the profile was filed to the template, a relief was filed on the cutting edge to clear the work piece. Filing the relief becomes tedious because if the cutting edge is worked down it changes the profile of the cutter.
With the cutter portion all set, I needed to put a shank on so it could be held in the lathe tool post. I made the shank from a piece of 3/8" stock. I first used a abrasive wheel to make a slit in the end, than heated it cherry hot and bent it open a bit. I bent it so I could get a file in there to square it up. I would have done this slitting operation with a slitting saw on the mill, but I did not have a cutter the proper width on hand.
I then heated the shank up and hammered it closed sandwiching the blade in. With the two pieces tight, I brazed themtogether. Since I used a air hardening tool steel, it became super hard after brazing. I don't have a hardness tester but a file would skip off telling me its quite hard. I relieved the hardness by tempering the cutter in the oven at 400 degrees for a hour. Once the cutter was cool, I used a slip stone to touch up the cutting edge.
To make a knob, I first cut a square blank a hair bigger than my desired final diameter. I turned the square blank round before loading up the form cutter. Before tackling the form, I drilled and countersunk the through hole. I mounted the cutter in the tool post, spending a bit of time shimming it on center. With the cutter centered, I carefully plunged it in. The first attempt was a failure because the sharp corner at the bottom of the knob was not sharpened enough. Once it hit that point, it snagged and tore off a chunk (scared me good). I took the cutter and touched up the edge under the microscope to make sure it was sharp. On the second attempt instead of plunging straight in, I moved it side to side slightly as I went in. Only the last few thousandths of inch I went straight and took a full width shaving. It worked.
I have only made a handful of the knobs so far, I will do another post when I get through the whole set to report on how the cutter held up. I am also debating fuming the boxwood with nitric acid to darken them.
These form cutters can be dangerous. It is a lot of cutting edge contacting the wood at the same time, which can result in bad things if one of the many variables is not right. This post is for documenting purposes only, do not try to make one unless you understand the dangers involved. I am not responsible for form cutters gone bad.