square broaching

[kd0afk]

What is the smallest that you can square broach? I've seen 1/8" but can you go smaller? I would like to make a set of tuning machines for my guitar and I want to cut the threads for the worm and have it driven by a square shaft mounted in the knob. 1/8 might be fine but Im not sure.

 

[sssfox]

If you are using plastic knobs, you can drill out the hole, heat up an appropriate square bar and push it through the hole.

I think those shafts are 1/8' anyway.

 

[kd0afk]

I was actually talking about the worm that turns the gear. The cheaper machines are one piece shafts mounted in two hooks pressed from a plate but the better ones are mounted in posts that are blazed to a plate. The worm needs to thread on to the square shaft. Im probably over thinking the size issue.

 

[MachineTom]

So the worm threads on to a square shaft, assume a external square, no broach needed, file ir mill square. If the hole is blind then a rotary broach is used, many articles on this site here and on web on making a rotary broach.

 

[abby]

You can broach as small as you are able to make the broach , square or hexagon.
I make box spanners as small as 12 BA using a piece of hexagon allen key fully hardened and ground square across the end , pushed using the lathe tailstock.
Drill a suitably sized hole to start and the broach cuts with no effort , quicker and cheaper than trying to find one.
If broaching a blind hole make sure it is deep enough to accomodate the swarf produced.

 

[GWRdriver]

kb,
What abby said, you can make them as small as the eye can see, . . . but one thing you need to be aware of is that rarely is a square broached hole fully square. The pilot hole is usually drilled a bit oversize so that the broach only removes, or notches, the four corners. Some manufactured square holes (sleeves for instance) are fully square, but typically a hand broach does not attempt to remove all the material within the hole that isn't a square.
If you were to make a broach (easily done) it would be beneficial in this case, where gears are to run together, to form a pilot or guide on the broach which would keep the broach centered as it proceeded through the pilot hole. Otherwise the broach might drift or skew and the gears would probably end up having a tight spot in the running fit.