Making a gear hob

[cessna]

Hi, I am trying to cut three small gears, I have in the past made a gear hob which worked very well, however they were brass gears, I am trying to cut a small module 0.7 12 tooth gear in steel. After making the hob, I heated until it lost its magnetism, cooled in spindle oil, put in small oven for 30 minutes at 400 deg. to temper. The material I am using for the hob is drill steel. It does cut but by the time I get to the twelve teeth the center cutters on the hob are worn out resulting in a useless gear. Any suggestions would be welcome on how to get a harder hob.
Terry

 

[gbritnell]

Terry,
For one-off type work like this and where the cutter doesn't have a real small cross section to it I would just harden the cutter and leave it untempered. I have made drill rod cutters and hobs in the past and have then cut more drill rod with it and they hold up quite well. That's not to say they have the same quality as a H.S. cutter but they will work.
gbritnell

 

[cessna]

Thank you for your reply, I wondered about tempering the cutter, I will try another one without, would it help to anneal the gear blank first?
Terry

 

[Swifty]

Terry, are you sure that your drill rod is the oil hardening type, I always purchase water hardening so there can be no mix up. I feel that it is easier to get small parts hardened in water. I always check that it has hardened by rubbing it with an old file, as gbritnell has mentioned, just use it without tempering. Unless you are machining a lot of parts with it, you will get away with no tempering.

Paul.

 

[gus]

Have done my fair share of heat treatment of alloy tool steels which must be quenched in oil during my time with Ingersoll-Rand,Singapore to make hole punches and forming rolls for air receivers.Swedish Tool Steels preferred. There are three grades of quenching oil-------slow-------medium--------fast. I preferred Medium though I have all three on hand being a '' Born Loser''
For the hobby craftsman w/o electric furnace and built in pyrometer, I found it best to heat treat at night as eye-ball gaging to dark red, chilly red is more accurate. Butane Gas Torches can be used but Oxy-acetylene torches is ''no no''. For every inch thickness,you must hold and soak hardening temperature one hour. Half Inch means 1/2 hour. New mineral engine oil is best quenching oil for us hobby crafts man. For now I use mostly Silver Steel. I use Japanese Silver Steel to make the engine cams and quench in oil. The file test indicated the heat treatment was OK.
Tubal Cain's book on ''Hardening,Tempering & Heat Treatment'' will be very useful. I bought mine from '' Australian Model Engineering Magazine''. All my heat treatment know-how came from here.

 

[cessna]

Thank you for your replies, I do have that book on hardening etc. I am making another hob this AM and will skip the tempering part. It is sure nice to be able to ask questions of people in the know and get suggestions that help.
Terry

 

[abby]

The quenching of small tools is best done using cold water , temper by carefully heating the tool until a previously polished area takes on a straw colour then quench immediately.
As already pointed out the tempering is not absolutely necessary but why risk chipping a tool that has taken time and effort to make !