Broaching tool.

[NeoTech]

A friend of mine came by and asked if and how a broaching tool could be made.

He wanted to "index" a outside ring onto a inside cylindrical part. The inside part and be drilled with apropriate sized holes easy enough and the excess turned off in the lathe.

But the inside matching pattern gets a bit trickier.. So i thought this up, a piece of 40mm steel will get slots milled into its outside diameter with a fullradius mill, and then tapered in the lathe to a 1 degree taper. Then "teeth" will be put into the piece (maybe they are upside down). and the contraption get case hardened. The rings will be made out of aluminium so i imagine this could be forced through them with my 10 ton shoppress and is sharp enough it will cut the inside pattern he want.

[awemawson]

The clever bit about making broaches is to ensure you set the taper such that the gullet of the tooth can easily accommodate the curled up bit of swarf removed by the tooth. If you don't it will jam. Conventionally the last several teeth (4 or 5) are at finished size. I made a very similar one many years ago to broach the spline fitting handles of a Mk1 Colchester Student I was rebuilding. I think it's still kicking around in a box of bits somewhere!

[NeoTech]

Ah, so the "tooth" depth, will need to be deeper or have something that can discard swarf somehow.. coz in my design case there it will get stuck i imagine..

[sparky961]

I too have a project coming up that I'm likely going to need a custom broach for.  Any hints on the best order of operations?  Do you make the circular cuts first, or the linear ones?

[Jasonb]

A lot will depend on how long the item that needs broaching is, if its say 5mm thick there won't be an issue but if it 50mm thick then you have a lot more swarf to get rid of.

I'd also add a parallel section to lead the broach in true that is a good fit on the initial hole. A typical 1/8" broach only removes about 0.002 per tooth so you may want to recalculate your taper.

Having seen some poor results on MEM recently using case hardened steel I would suggest silversteel (drill rod) and harden it

J

[awemawson]

In an ideal world you need to form the broach, harden it, then grind it to finished size and sharpness. In reality you probably don't have the kit to do it, so you need to protect the cutting edges whilst hardening. Assuming you don't have a salt bath available <G> probably the easiest way is to dip it in a foundry wash coat as used on tongs and stirrers. Something like Zircofluid 1219 from John Winter & Co :

 http://www.johnwinter.co.uk/foundry-products/mould-a-core-coatings

The fine cutting edges will oxidise very quickly when heated to hardening temperatures if uncoated. Remember when plunging in oil to cool, it must be held vertically or it will bend like a banana.

 

[NeoTech]

Sparky yes i plan to turn it between centers for a uniform size, then put in the dividing head and cut the straight flutes with a full radius endmill (5mm), and after that back to the lathe and do a interupted cut with a crazy sharp tool for the taper. taking off a little at a time..

but its prob. something wrong right there so.. *waiting for knowledge to fall into thread* or getting whacked with a stick. 

[tom osselton]

I also want to do some broaching in the future and was planning to build this as I don't have a press http://modelengineeringwebsite.com/Broach.html
http://modelengineeringwebsite.com/make_broaches.html

[NeoTech]

Thats looks kinda usefull though.. i have a 10 ton hydraulic press though so thinking of making a guide sleeve for the tool. but need to recalculate all the angles on that thing.