Ratchet Gear Replacement

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I'm in the middle of restoring a Sheldon #3 arbor press. The ratchet is in poor condition, so I'm planning to machine a replacement. I've got a 70 degree dovetail cutter on the way which will match the profile of the existing gear.

The only question at this point is what material to use. Research so far, points to 8620 as a good choice. I've no idea what the existing ratchet is made of, but it's resistant to a file. BTW, this will be my first attempt at making a gear.

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Regards,
Brian
 
It is not a gear in any way, it is a ratchet. Some kind of higher carbon steel will need to be used, and it will need to be hardened and tempered after machining. It mostly needs to be strong and tough, and and helpful if it also has low wear qualities. 8620 should work but something like 4130 or 4140 should also work. When machining those metals, keep real chips coming, if you dwell in the cut you will get work hardening from rubbing, which can be difficult to deal with. You want real chips coming off whenever the cutter is in contact with the work. A heat treating oven would be a good idea for getting the most amount of hardness and strength without it being brittle. There are good charts available online for that information.

Edit: You will need some sort of rotary indexing equipment to cut the ratchet teeth evenly spaced and equally deep, at the correct angle.
 
Hi Bob,

Point stipulated, it is a ratchet, not a gear, with very different requirements in terms of tolerance, wear and shock. Thank you for your insights on material selection, machining and the need for heat treating. When researching this, I noticed that sometimes gears are made with low carbon steel that is case hardened. I'm assuming that choice is not appropriate in this case with the need for strength over shock and wear resistance.

We have a glass fusing kiln with a programmable digital control that is capable of 1700F, so I think we are good there. This little project will also give me a chance to put my new spindexer to work.

The ratchet is still functional, even the most badly damaged tooth still has half of its engagement. Therefore, its last use will be to drive the broach to make its replacement.
 
It sounds like you are entirely on the right track. Hard with excellent strength and toughness beats extreme hardness alone in this application. Temper carefully.
 
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