5mm Broach

[Paul Alciatore]

No, I have only used a broach once for some gears and did not break it. But a 5mm square piece of hardened steel with two inches of teeth both creating the need for force while also weakening that 5mm square just does not seem a formula for success, IMHO. I would definitely search for or make a broach that can be pulled instead of the more traditional way of being pushed through.

If any others here have experience with this kind of job, I will gladly yield to their superior knowledge. But until then, I would approach this job with a lot of caution.

And I still wonder if the end mill approach might not be better. Just with a technique that is modified from what I outlined above. Perhaps drill a couple of 1/16" holes through the hub with their centers on the centers of arcs in my drawing. Probably need to do them from both sides, meeting in the middle. Then use a solid carbide end mill to enlarge those two holes to the 1/8" diameter. That will remove a lot of the metal without any need for sideways milling. Plunge the end mill in again offset from each of those holes by about 1/4 the mill's diameter so you are eating away an arc about 1/64" deep each time. Work your way toward the center of the bore doing this until you are left with just a few hills between the plunges go remove. Finally, mill sideways, perhaps 1/2" deep at a time, following the path I outlined in the drawing above. That's a bunch of steps, but he doesn't want to buy a broach for this one-off.

Sounds like you have some experience in the area of broaching.
Did the broach give any warning before breaking or shattering?

[Paul Alciatore]

If I understand it properly, I think I like this idea. Lets see, the bar is sized to fit the bore of the hub so holding is not a problem and it can be pushed through with an arbor or hydraulic press. The cutter could be a HSS drill stock with flats ground on the top and two sides, making it cut 5mm wide. I would leave a small part of it's length the full round of the drill rod for my adjustment set screw to press against. I would forget angling the hole and use a zero rake angle. I would make that hole round at the diameter of the drill rod. And I would use a set screw on the back side to allow adjustment of the depth of the cutter in increments of a few thousandths for each pass. A set screw in the end of the bar would press against the flat top of the cutter, aligning it's cutting edge perpendicular to the axis of the rod and holding it in place after adjusting the depth of cut.

This would require multiple cuts for a full keyway, but for a one-off that is doable. And you could make different tools for different keyway widths.

This has the potential of creating a shop made set of bars and tools that could be partially interchangeable so you could make keyways of different widths in different diameter bores. The depth of the keyways would also be easily adjustable.

Just spit-balling.

I've used Mikey's lathe method a few times for slots and other shapes. Even "broached" a hex into a hole with a special cutter that I made which has 120° V between the two cutting edges. It sounds slow and painful but in reality it goes pretty quickly and easily.

Cutting on the push is great if the bore diameter of your 2" long hole is big enough to permit a stiff enough bar. If not then I'd go with the idea of cutting on the pull stroke.

Such a bar is not that hard to make if a couple of shortcuts are considered. For example I feel like the option shown here would be easy and quick to make. In fact I think I'll do it this way for making up a couple of slotting bars in two sides to suit my shaper. Click on the pic for a bigger view.

[SVS]

A couple of shaper “poke” tools. This is nothing new folks.

[flathead4]

I once made an arbor bearing for my horizontal mill. It's a little over 2" long and has an internal keyway. I cut the keyway as described above on my 9" South Bend lathe with a boring bar and bit if ground HSS. It literally took me all day running the carriage back and forth, and left me with a blister. A few weeks ago I had another need to broach a keyway and just went ahead bought a broach. In my limited experience, they "key" is keeping the broach straight while applying pressure. If it gets the slightest cocked, relieve the pressure, straighten it up and start again. If you it get bent, it could break. I had no problems.

[JRouche]

I read some of this squabble you folks have, Love it because you worked it out and I learned something

[olf20]

I think we have decided on modified key and use a inch broach that I have.
Will post results when done. Thanks for all the replies. As usual I have
increased my knowledge on the art of broaching.
olf20 / Bob

[BCRider]

To be fair the manual shaping on my lathe for a 5mm wide cut would certainly require no more than 0.001 to at most 0.002 DOC. And that's on my heavier 12x36 machine. On a little SB9 I can see trying to do something like this might be just over the top for it. It's a bit of a grunt even on my bigger and stiffer machine.

But as someone with no keyway broaches at all and that would not use them enough to justify the price it's pretty much my only option. At least until I make the pokey holders for the shaper.....

[Randy]

Here's the cover of the very first HSM magazine I ever got. This fellow made a broach with a single cutting tooth, and a wedge to advance it a thou or so for successive passes. That supports the cut better than a cantilever bar, making large L/D ratio keyways possible.

I believe Clickspring has a YT video on HSM made broaches, but he works mostly in brass, so his method might not be suitable for steel. Worth a look, however, since his videos are a joy to watch regardless.

Lastly, I'll point out (if no one else has already) that a 3/16" keyway is almost the same as 5mm, and way easier to do than 1/4".

[lynnl]

Here's the cover of the very first HSM magazine I ever got. This fellow made a broach with a single cutting tooth, and a wedge to advance it a thou or so for successive passes. That supports the cut better than a cantilever bar, making large L/D ratio keyways possible.

That would be Phillip Duclos. It's also included in his "Shop Wisdom ..." book.

[mikey553]

Just recently I had to cut a 3/16" wide keyway in aluminum hub with .625" bore and 1.2" long. I did it on my mill using a 1/2" diameter bar and a self made HSS insert. It took maybe 20 minutes of cutting and was very easy to do.

Many years ago I have cut a 2.5 mm wide keyway in steel hub with 9 mm bore about 16 mm long. At that time I did not have anything better than a 7" mini lathe, so that is what I used for cutting. The tool was a small bar with a brazed carbide insert. It took multiple strokes of the carriage, pushed manually by hand. It was easy to do and did not take a long time either. I still have this hub on the spindle of my little drill press.

The point is you can use what you have and still achieve decent results.

[Paul Alciatore]

I suspect that "we" may have individually "decided" on different things.

I think we have decided on modified key and use a inch broach that I have.
Will post results when done. Thanks for all the replies. As usual I have
increased my knowledge on the art of broaching.
olf20 / Bob

[JohnMartin]

Given an inch size set of keyway broaches and the need for a 5mm keyway, I’d first broach it with the 3/16” broach and then enlarge to the 5mm with a boring bar in the lathe. Not too much additional metal to cut, and should be very easy on the lathe and the operator. I can remember - years ago - cutting keyways of about that size from scratch using a 6” Atlas lathe. No broaching or drilling ahead of time. Slow going, but doable, if you do it right - which means a sharp cutter (resharpening when necessary), small feed, several spring cuts after every new feed, and cutting oil. I recall seeing somewhere that someone had attached a lever to move the carriage, to eliminate needing to crank the carriage handwheel. Not a bad idea.

[J Tiers]

The one and only keyway I have done with the "lathe as shaper" method was a 5 mm keyway. I ground a cutter, and did it 100% on the lathe.

It is the one and only because when I was done, I swore an oath never to do that again.

Doing it on an Atlas 6" is more than I would have. I did it with a Logan 10" and thought that was lathe abuse.

[plunger]

I recall seeing somewhere that someone had attached a lever to move the carriage, to eliminate needing to crank the carriage handwheel. Not a bad idea.

I have made a lever for my topslide of my lathe. I simply put a block on the carriage and attached a bar to it that linked to a lever. It is relatively painless doing keyways using this method and is quite quick. .

https://www.youtube.com/shorts/REaIOLe413Y