rotary broaching tool question.

I was over at docs machine website and found an interseting project he has done.

Making a rotary broach and cutter

I was wondering if a rotary broach can be used to create this form of id shape.



Thanks for reading.

Not sure of the size you are looking for but Greenlee makes SlugBuster punches in that shape for BNC bulkhead connectors.

Rick

I have a couple Slater rotary broaches, and make the punches myself.

I have thought about the shape you illustrate, and am pretty sure it can be done. What may cause some thinking/set up considerations is that the center hole for the punch needs to be centered on the form. IOW, the hole you need to drill, needs to be offset from the centerline of what would be a complete circle instead of a D shape.

So making the parts might be more conducive to working on a mill, say, rather than a lathe. Or use an offset collet or 4 jaw in the chuck.

If you've never done it before, pay attention to what Doc notes about centering the "focus" (my term) of the wobble at the end of the punch or it will give you no end of consternation as to why it won't cut easily and smoothly. Though both terms are relative.

You can also "wobble broach" with a ball bearing center in a lathe, center stuck in the center of the punch, and the TS off set to whatever gives ~1° over the working length of the punch (for a 1-1/2° taper each side on the punch itself). I wouldn't try it the first time with my best/high dollar center, though.

Point being, whether you use a factory tool or make the tools yourself, it takes a _lot_ of pressure to cut shapes with a lot of material removal (as that D shape would be) once they start getting over maybe 1/4" - 5/16" dia. Slater and Somma both publish charts of pressure required for some common shapes. The process is also a little like tapping : Use a large chamfer to center and start the punch. Then, if you can use a clearance hole that is even a couple/few thousandths larger than right on size, the pressure will go way down and speed of production up.

smt

boaterri said:

Not sure of the size you are looking for but Greenlee makes SlugBuster punches in that shape for BNC bulkhead connectors.

Rick

Click to expand...

Think more like all the pesky plastic twist knobs that fracture, I break them off stuff constantly, I'm running out of small vise grips in the shop and I think I would like to get into/understand rotary broaching. I have a pretty good supply of busted hss cutters,

stephen thomas said:

I have a couple Slater rotary broaches, and make the punches myself.

I have thought about the shape you illustrate, and am pretty sure it can be done. What may cause some thinking/set up considerations is that the center hole for the punch needs to be centered on the form. IOW, the hole you need to drill, needs to be offset from the centerline of what would be a complete circle instead of a D shape.

So making the parts might be more conducive to working on a mill, say, rather than a lathe. Or use an offset collet or 4 jaw in the chuck.

If you've never done it before, pay attention to what Doc notes about centering the "focus" (my term) of the wobble at the end of the punch or it will give you no end of consternation as to why it won't cut easily and smoothly. Though both terms are relative.


smt

Click to expand...

I thank you for the very informative write up.

I have a feeling that I am going to either buy or make one for the milling machine instead of the lathe because of the fact that the offset hole is going to take more material more evenly.

I looked at the Slater broaches they are really nice. A bit too nice unless I was able to make them pay or found one at a yard sale.

That being said,has anyone posted any rotary broach plans for a mill?

That being said,has anyone posted any rotary broach plans for a mill?

Click to expand...

Its the same tool.

In the lathe the broach holder goes in the turret and the part rotates in the spindle.
In the mill, the broach holder goes in the spindle & rotates, and the part is held in the vise/collet chuck/etc on the table.

smt

stephen thomas said:

Its the same tool.

In the lathe the broach holder goes in the turret and the part rotates in the spindle.
In the mill, the broach holder goes in the spindle & rotates, and the part is held in the vise/collet chuck/etc on the table.

smt

Click to expand...

I got it now.

Rotary broaching for full forms usually works well up to .025" material removal per side. We make custom 'D' shape rotary broaches for customers all the time. To determine your pre-drill, offset the major by .050". This will likely leave a small radius in the middle of the flat. You may be able to use a smaller pilot hole, but this will depend on material, depth etc.

Not to discourage you from trying rotary broaching but if your immediate need is to replace plastic knobs with something more durable a round hole with a couple of short setscrews on one side works just fine on D shaped shafts.

Scottl said:

Not to discourage you from trying rotary broaching but if your immediate need is to replace plastic knobs with something more durable a round hole with a couple of short setscrews on one side works just fine on D shaped shafts.

Click to expand...

Nice idea Scott. The rotary broach could also be used as a punch in soft materials and just pecked in for a small number of pieces.

Old post but a great link to just what I needed. Doc's Machine shop bookmarked.
Thanks Alonzo!
have fun
i_r_

i_r_machinist said:

Old post but a great link to just what I needed. Doc's Machine shop bookmarked.
Thanks Alonzo!
have fun
i_r_

Click to expand...

What shape are you going after? I'm about to try out a 3/8" square in Stressproof.

Your 3/8 square should be .530 across corners. Start with something between a 7/16 and 1/2 drill and see how much pressure it will require. Full form will require a lot of pressure, turret lathe sized pressure. Your form will have a helix. This can be minimized by reversing spindle rotation at 1/2 depth.

i_r_machinist said:

Old post but a great link to just what I needed. Doc's Machine shop bookmarked.

Click to expand...

-It's still woefully incomplete, even all these years later. But I am still working on it.

The turret lathe's done and in production, the Springfield only just got completed last Holiday season (there's a full thread in the "Antiques" section) the Nichols got re-rebuilt since this thread started and is now a damn-near-new machine, and I just finished a light refresh of a Hardinge HSL, which isn't even on that site yet. (There's a thread in the Hardinge/Bridgeport section if you're interested.)

I still use that original broach tool on occasion, although I've had to make new punches once or twice. I did also eventually pick up a commercial unit, as the one I made had a 1" shank. I picked one up with a 3/4" shank so I could use it in the mill.

Doc.

I have seen vidieos of broaching square holes by first drilling small holes in the corners then a larger center hole. I suppose you could rough out most of the hole on a vertical mill with small end mills for the corners.
Bill D

Cole2534 said:

What shape are you going after? I'm about to try out a 3/8" square in Stressproof.

Click to expand...

Four diferent sizes under 3/8" square in drill rod. I'm going to design the broaches around a 1/2" ground dowel pin. I'll post a pic after I'm finished.
have fun
i_r_

Dowels are case hardened. You may like the result better if you use a used end mill. I have also used drill blanks.

Your 3/8 square should be .530 across corners. Start with something between a 7/16 and 1/2 drill and see how much pressure it will require. Full form will require a lot of pressure, turret lathe sized pressure. Your form will have a helix.

Click to expand...

I have not seen this in depths up to 3x width across flats, though many references cite it.
Keep the back taper to 1/4 or 1/2 degree over the tool inclination helps minimize. (1-1/2 deg max total back taper for tool run in 1 deg offset holder). I have broached up to 1/2" square in 360 brass, up to 3/8" in steel.

This can be minimized by reversing spindle rotation at 1/2 depth.

Click to expand...

This seems to be true.

Don't run the rpms too slow.

Strongly agree - use busted DE mills for stock for punches. Most likely to have near full hardening "everywhere". Dowels have somewhat soft centers, are not HSS.

smt

somehow i thought the broach worked by the corners digging in. so the more corners the better. yes/no?

somehow i thought the broach worked by the corners digging in. so the more corners the better. yes/no?

Click to expand...

No.

It's the (very small) rocking/slicing/"sawing" effect of the edges.

However, forms with more points tend to have to remove smaller area/volume/chip thickness so can tend to require lower pressure for same aprox area of opening. But that is a wide rash generalization. Squares take more pressure than hexes for similar ratio entry hole diameter to side width , but rectangles take more yet and rectangle ratio between side-to-end is rather limited for a single entry hole.

As others note, the bigger the entry hole the better because it improves "chip" clearance and DOC.

Per your theory, shallow splines tend to run well, but i would not automatically predict that 12 splines would run with less pressure than a hex, e.g.

Really, in crude terms, it gets back to the maximum DOC/chip thickness required at any point in the form.
Sharpness angle matters, but as with any cutting process is a trade off between ease of cutting and durability.

smt