Stefan strikes again! Rotary broach DIY

Once again Stefan Gotteswinter cooks up something marvelous for the small shop. This time for a rotary broach for machining internal hexes for allen wrenches, squares for sockets and carriage bolts. It employs a fixed length broach cutter design so an adjustable offset is not needed.

The video is in three parts starting here:

Compact rotary broach - Part 1 - YouTube

Those of you unfamiliar with Stefan's machine shop YouTube video offerings might watch a few of his series.

recently started watching his channel and very impressed so far. he finds the line between way too much info, ie, oxtool, and too hobby like, ie, this old tony.

Forrest Addy said:

Once again Stefan Gotteswinter cooks up something marvelous for the small shop. This time for a rotary broach for machining internal hexes for allen wrenches, squares for sockets and carriage bolts. It employs a fixed length broach cutter design so an adjustable offset is not needed.

The video is in three parts starting here:

Compact rotary broach - Part 1 - YouTube

Those of you unfamiliar with Stefan's machine shop YouTube video offerings might watch a few of his series.

Click to expand...

That looks pretty useful, but he starts from a Hemmingway kit.

Here's one that was done by Doc Nickel of a DIY project from scratch.

http://www.docsmachine.com/projects/rotarybroach/rb01.html

Don't want to rain on anybody's parade, but rotary broaches have been covered in the model engineering press for quite a few years.

Here's one.

Oops. Typed too slowly.

With Stefan's videos it is often not what he is making but how he approaches the work that is the lesson. Very well done.
Both links above start with some form of kit. This one Rotary broaching - mikesworkshop starts with raw materials only.

One of the key things I learned is the existence of Mechanical and Metal Trades Handbook by Europa Lehrmittel. More affordable and imo more useful than Machinery's Handbook for day-day use.

Couldn't the one degree offset be put into the shank/bore instead of the body?

bikebuilder said:

Couldn't the one degree offset be put into the shank/bore instead of the body?

Click to expand...

Yes, that is pretty much what Doc Nickel did at the link I posted above. I believe he used Frank Ford's method of making a broach tool using a dremel. Clever fella that Frank Ford is...

Doc basically took a turret tool and disassembled it and ground the plate for the offset. As it turns it wobbles as it should, since the offset is built into the tool, as it would be on a rotary broach.

Awesome, I've been wanting to build a rotary broach.

traditional-tools said:

Yes, that is pretty much what Doc Nickel did at the link I posted above. I believe he used Frank Ford's method of making a broach tool using a dremel. Clever fella that Frank Ford is...

Doc basically took a turret tool and disassembled it and ground the plate for the offset. As it turns it wobbles as it should, since the offset is built into the tool, as it would be on a rotary broach.

Click to expand...

'

Careful, the 1 degree angled broach axis has to intersect the work axis ant the center of the cutting edge section.

Look again where Stefan went through the broach axis alignment procedure about 3:10 of part 3

Forrest Addy said:

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Careful, the 1 degree angled broach axis has to intersect the work axis ant the center of the cutting edge section.

Look again where Stefan went through the broach axis alignment procedure about 3:10 of part 3

Click to expand...

Forrest,

Surely wasn't meaning to dis Stefan in any way, and even pondered that kit from Hemmingway as it looks like a decent piece of kit.

However on the website it say you can do it on a perpendicular mill using feeler gauges to offset the 1 degree.

I'm not trying to split hairs, but are you saying that method is more accurate, or is a better solution than using a sine gauge with a surface grinder like Doc did ? I thought Doc's solution was pretty clever also, using a turret tool. Both seem to work, so no problem either way.

I have one question about your statement, in that the mention the 1 degree offset has to intersect at the center of the cutting edge, and I agree in the perfect world. But couldn't the intersect be off center and still be able to cut, as long as it could wobble ? Wouldn't produce all corners at the same depth unless it was intersecting in the center, though.

For that matter, the 1 degree is kind of arbitrary, and if it was a slightly different angle it would still work. But that would have been two question had I asked you about it, as I said one.

Kudos to the machinist who figured out how to make a wobble cut, very clever.

Cheers,
Alan

Sorry, I didn't ,mean to get over technical. In some instances (as when starting without an entering champher) centering of the broach makes for trouble-free starts.

Remember this tool is quite flexible. While the tool can accommodate some misalignment, this also means an off center start is very possible without an entering champher. If a champher is not allowed for some reason an allowance can be left to be faced off after broaching.leaving crisp edges.

As long as the cutting face axis of the broach is concentric with the axis of the hole to be broached, clean starts will be assured. However you design the tool with with an adjustable slide, a bent axis, a flexture, whatever, all that matters is the broach gets a concentric start on the pilot hole.

BTW it the pilot hole is a trifle over-size the broach becomes far easier to drive. Stefan had difficulty driving the 6 mm internal hex which is apparently the practical capacity. If the pilot hole was say 6.4 mm the broach would be easier to drive and there would still be sufficient corner to ensure positive engagement with no loss of torque capacity. These little adjustments may not be allowed but when the option is open it's good to know with a 1.07 x inscribed circle (for a hex) overbore you can broach 6 mm internal hex with comparative ease and 8 mm if need be.

There are also tricks like cupping the face of the broach cutter to form a positive rake, increasing the rotary broach's nutation angle (at time of build), etc. Broaching is still machining and whatever works to aid metal cutting for turning or milling is applicable to broaching..

Forrest Addy said:

There are also tricks like cupping the face of the broach cutter to form a positive rake, increasing the rotary broach's nutation angle (at time of build), etc. Broaching is still machining and whatever works to aid metal cutting for turning or milling is applicable to broaching..

Click to expand...

Yes, this is exactly what Frank Ford did by using a dremel to dish out the end, creating that very effect, where the corners were raised, presumably to allow the corners to rock/wobble easier.