Rotary Broaching for dummies, and on the cheap?

J

joe_m

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On another thread ( http://www.hobby-machinist.com/show...ice-Needed-for-One-Off-Part?p=61518#post61518 ) someone was looking for help with a one-off broaching operation. He mentioned watching videos and still being confused.

Me too. But to make it worse, I've actually got 18 broaches - squares, rectangles, and hexes - that I found in a chest years ago. Unfortunately I haven't found anything resembling a holder.
I've watched a few videos showing broaching on a lathe and, if I understand the process correctly it goes something like this:

1. The broach sits in a broach holder on the tailstock or tool holder. (I'm pretty sure)
2. The broach holder holds the broach at a very slight angle to the work. (I'm halfway certain.)
3. The work, which is held in the lathe chuck or faceplate, has a starter hold drilled to get rid of the excess metal and give the broach room to enter. (I'm somewhat sure)
3. The broach holder holds the broach but is free spinning. (I'm vaguely sure.)
4. The lathe is turned on, the work spins, the broach is moved into the workpiece. (I'm pretty sure, you can't broach air.)
5. When the broach hits the work everything spins in unison and a square, rectangular, or hexagon shape hole magically appears in the work. (just a wild a$$ guess.)

Of course I'm talking about blind broaches like on the end of a hex bolt, not a through hole.
So how far off am I on the process, and more importantly is there a shop-made option for those $300 broach holders that are for sale on ebay?
thanks
Joe
 
hi joe
everything you said was correct as far as what i have read i was interested in this myself and went to the manufacturers sites to find out their data and specs. I plan to try to make a holder for my tailstock. the rotational wobble is 1* so i figure on building it the best i can with the equipment i have trying for no wobble and i'll probably achieve the one degree using my 3 jaw chuck. what i'm not sure about is the load and vibration on the spindle bearings so i'll try this on my 9x20 hf lathe it could use better bearing anyway
not having a buck chuck may be of some use anyway lol
steve
 
Joe,
Perhaps I can add a couple items to your excellent dissertation:

1. The broach sits in a broach holder on the tailstock or tool holder. (I'm pretty sure) correct
2. The broach holder holds the broach at a very slight angle to the work. (I'm halfway certain.) correct
2a. The broach is usually held at 1 degree; the center of the form is always kept on center with the pilot hole.
2b. Most commercially available rotary broaches have a 1.25 degree back-taper so they will not crash the part.
3. The work, which is held in the lathe chuck or faceplate, has a starter hole drilled to get rid of the excess metal and give the broach room to enter. (I'm somewhat sure) correct
3a. For best results, the pilot hole should have a chamfer. This helps the chips start curling up easier.
3b. The pilot hole needs to be deeper than desired broaching depth to make room for chips.
3. The broach holder holds the broach but is free spinning. (I'm vaguely sure.) mostly correct- [the broach is held in a spindle in the broach holder which is spinning freely]
4. The lathe is turned on, the work spins, the broach is moved into the workpiece. (I'm pretty sure, you can't broach air.) correct
5. When the broach hits the work everything spins in unison and a square, rectangular, or hexagon shape hole magically appears in the work. (just a wild a$$ guess.) mostly correct
5a. When the workpiece makes contact with the broach, it touches the closest corner. One corner is closer because the broach is on an angle. As the workpiece drives the free turning broach, the two turn together. Eventually every corner becomes the closest corner until a complete revolution is made. As the broach is fed into the hole, every corners is cut little by little until the full form is complete.
5b. The machinist should take credit for being magical, not the hole.

Warning: If you have it in your head now, stop reading. This may hurt your brain: the tools also work in a mill. In the mill, the tool holder is turning, and the part remains stationary. There's a YouTube video that shows it in a Bridgeport.

Hope that helps!
The new guy
 
Thanks Broach, it's a bit clearer now.

Couple of more questions:
1. If everything is spinning together, then how does the edge cut? Is it just the amount of pressure imparted by the feed?
2. If the bit turns in the mill, which I can actually picture, then that would be the easiest broach holder to make right? It'd just be anything that would or could hold the broach at 1 degree from perpendicular, like a boring bar holder?
 
Also, those holders seem extremely expensive for hobby use.
On Travers, I saw them ranging from $1500 to $3K
 
That's different of course. All of a sudden it becomes affordable.
Thanks
 
re: joe_m Question #2, in a mill...

The broach can't be held in a boring bar holder at 1 degree, because then it would just drill a hole. In a mill, the broach doesn't spin, but is pivoted at the center of the form, at a 1 degree angle. (much like making a dot by pivoting your pencil) The body of the broach holder is turning which makes this all possible. As the pivoting broach is fed through the stationary part, the form is produced.

Hope that helps.
 
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