Several years ago we began making floats. I followed some of the instructions above and found hand filing the teeth in mild steel very tedious. After a whole day of filing I had a side float with some oddly spaced teeth.
I figured there had to be a better way and contacted local machine shops. No one wanted to get involved but one machinist took the time to really explain what was involved. His advise was to use tool steel and heat treat it. He explained the even if it wasn't heat treated it was much harder and would hold a better edge. He even gave me the O-1 steel to make some.
It took me two full days of filing to make a side float and an edge float from that steel. I hardened and tempered it by color. Not very successfully, I might add.
Our next attempts used a guide on a metal cutting band saw to set the kerf of the previous cut intoto space the teeth and depth was just eye-balled. From the simple kerf the teeth were formed with a file and we could make the floats twice as fast. We were able to heat treat these in a furnace and that proved much more reliable.
We had moved our shop next door to a private mint with their own machine shop and gained access to their shop. We discovered the milling machine was the way to make these. We bought 60 degree carbide dovetail bits, set the head at 20 degrees and milled the teeth. The only carbide dovetail milling bits we could find were C-2 carbide. These were expensive and just didn't hold up. When they get dull, they introduce all kinds of stress into the steel which warps it and also causes distortion in heat treating.
We didn't want to become a problem in their machine shop and bought our own equipment and tooling so we could do our metal work without working around or messing up their schedule. We also found indexable tooling that uses replaceable cutting inserts. These are a little cheaper and better quality. Our cutter uses three inserts and they cost about $8 each but have three different cut faces.
Feeds and speeds in a metal work is a trade-off between cutter life, time and quality of cut. Through experience we found that we could cut a tooth in a 1" wide piece of steel in about 1 minute and 20 seconds while getting a relatively good finish to the cut and keeping stress in the steel to a minimum. The inserts are good for one float before they need to be changed. The floats we made have 10 teeth to the inch and have slightly over 6" of teeth. So we could make a float blank in 1 hour and 20 minutes. Then it had to be cut to the proper shape and the life of metal cutting band saw blades became an added factor. These have to be fine tooth blades and the weld leaves some soft teeth once one of those is damaged you start loosing the teeth behind it in a hurry. Sometimes we couldn't even shape one float with a $10 blade.
Edge floats are best made in a fixture you can machine to hold them for cutting the teeth. This means they can be shaped before cutting the teeth and you'll get more floats per bar of steel.
Oh, don't forget to use a coolant mist for milling or your cutter life will fall of dramatically.
After you machine the sawn surfaces, make your handles and bore for the attachment of the handles you can heat treat. You'll need a good sized furnace to properly heat treat if you make them with enough length to be properly functional. Ours were 9" long and you'll need a furnace a few inches deeper than that or the ends won't likely be at the proper temperature.
After quenching, you'll have a short period of time to straighten the warps caused by heat treating. Your floats need to be straight but you won't have time to get fancy. Do it by eye. Once they've cooled they'll only break if you try to straighten them. My machinist friend uses a big electro-magnet to straighten freshly hardened steel. This isn't one that you'd want to stand near with credit cards in your wallet and it looked like it'd suck the transformer off a power pole when it's turned on. We don't have the money to pay the electric bill on one of these so we don't have anything like it.
We temper floats to about RC 50 and this means holding them at 800 degrees for an hour. This is getting into the danger zone for hydrogen buildup in the steel but we're working with thin material and quenching again will limit the risks. A floatthat develops problems here would break during the initial sharpening or the first use.
The steel needs to be cleaned and then sharpened after tempering. We sharpen with the same 6" double extra slim taper file you probably use to sharpen your back saw. We also remove all the milling marks during the initial sharpening. This takes about the same amount of time as milling the floats.
After mounting and finishing your handles you're ready to make some planes. This is pretty much the basics of what we've learned. There's more and my personal set has 25 or 30 different floats. I'd like to have a bunch more.
If you think reading this was long and boring wait 'til you make some floats 8-) Ron Hock, several metallurgists and some machinists have been a great help and resource for us in our float making efforts.