Some research notes on gear cutting

  With trying to keep this project as authentic as possible, one of my great concerns has been the cutting of the gear teeth.  No documentation that I am aware of exists for how medieval clock makers would have cut the gear teeth.  I have gone back and forth on this but I believe I have a completely feasible production method.  First, the list of the different techniques.

  1)  Filing
  2)  Chisel cut then filed for cleanup.
  3)  Punch then filed for cleanup.
  4)  Sides of teeth cut with hacksaw then hot chisel on bottom of tooth gap to remove material.  Filing for cleanup.

  I think the fourth option is the most likely scenario.

  The first scenario, all filing, works for small gears made out of material like brass.  However, with the large sizes of the wheels and the fact that the material is iron it would involve a lot of filing.  Also, this would wear through many files.  I think this technique would just be too labor intensive and expensive on the files.

  Chisel cutting was a method I have thought about frequently but it has problems.  For those not as experienced in blacksmithing, the metal is heated and a chisel is hammered into the material while it is still hot.  This is usually in the 1400-2000 degree range.  This is a very common blacksmithing technique.  It has some problems as far as the teeth of the gear are concerned.  The biggest problem is that metal is always displaced as part of the chiseling process.  In other words, as I cut one tooth, the adjacent tooth would be pushed some.  I could clean this up with some careful forging.  Another problem is that much hot cutting makes it very easy to accidentally soften the chisel due to the heat.  This would probably require several touch ups and rehardening of the chisels during the work.

  I have briefly thought about punching the teeth.  This would involve creating a punch that matches the exact shape of the gap between the teeth.  While the gear is hot, the punch could be used to knock the gap out.  Problem is that punches also displace some material and they are very easy to not place exactly where you want the punch, especially when talking about a difference of 1/32" of an inch being important.

  So I am now looking at hacksaws.  When I first started this project, I believed hacksawing the sides of the teeth were the way to go.  And when I received a copy of a research article on medieval clocks from the British Museum it also indicated a belief in using hacksaws to cut teeth.  However, when I contacted the gentleman who wrote the article, he indicated that he was no longer sure about using hacksaws.

  Well, it is possible to find hacksaws in period.  Indeed, a search I did over the weekend uncovered several more examples that I was not aware that they existed.  There is one issue to be aware of with this.  Hacksaws can be used for many things.  Examples include cutting for decorative pieces such as horn or bone or cutting softer metals for jewelry.  Hacksaws really are not good for wood because of their tooth size and spacing.

  My research a few years back led me to the Viking tool chest, the Mastermyr Chest.  I made a reproduction of this about 2 years ago with a working lock and key.  In this chest was a hacksaw.  This hacksaw, from what I can see, almost exactly matches a modern equivalent.  The tooth count, tooth pitch and blade thickness all compare to a hacksaw blade that you can buy at a local hardware store.  Also, there are other metal items in the chest that have cuts that match the thickness of the blade.

  Getting the shape of the hacksaw was easy.  The difficult part may have been getting it hard enough.  When cutting iron or mild steel, you need a fairly tough blade.  I think that this was well within the capabilities of the smiths of the later middle ages.  A thin saw blade could easily be case hardened.  If the item was surrounded with carbon material such as bone or fat then wrapped in clay, it could be baked in a fire and case hardened.  I believe that this would give a hacksaw enough carbon to harden it sufficiently to cut the relatively soft wrought iron.  Also, files date back to the Romans and would have needed just as much toughness.

  I also believe that locksmiths probably used hacksaws.  Why so few examples of hacksaws then?  Very simple, they are thin quality steel and blacksmiths are the ultimate recyclers.  Once a hacksaw went dull there was a very good chance that the smith just reused the thin blade as a small spring.  And if the blacksmith did not reuse the blade and just discarded it, the thin material would have easily corroded away.  Most examples of hacksaws from the middle ages come from bogs where the material was preserved.

  So, within the next few weeks I hope to try actually cutting the gears.  I am going to cut the sides of the teeth using a hacksaw.  When I do this, I am going to use a template to make sure all the teeth are at the same angle.  Then the excess material between the teeth will be popped out by heating the gear and using a flat sided chisel to cut the bottom of the gap.  The only metal displaced should be the waste material.