Need to broach a longer keyway

[great white]

You may have seen my two other recent posts on making an adapter to mate and 8x8 Argo transmission to a chain drive motorcycle engine.

I’ve got most of it sorted out, but I need to broach a keyway and I’m a little stuck on how to do this one.

The adapter is mild steel. The transmission input shaft is 1”, slip fit and has a keyway.

I need to broach a keyway into the bore in the adapter to match the transmission input shaft keyway.

I’ve never broached a keyway before, but I have watched it being done. I’ve “read up” on it and watched several videos on it. Doesnt seem overly difficult, just takes a while and a lot of back and forth cuts.

The problem is; most broaches seem to only be in the 1 3/8’s range for length and I need to broach a keyway a good 3” long.

It will be slow going no matter how I go, as I will be doing it on my lathe, which means doing it “handraulically” on the cross slide carriage.

Any tips or ideas on how I can broach a keyway that long?

I’ve seen “Mr Tubalcain” do it on youtube using a cut-off blade, which might be an option except that I don’t think I can get the length I need with such a small bore (ie: 1” bore limits how far in I can fit tooling).

I have thought of broaching from one end and then flipping the piece over (ie un-mount it, flip and put back in the chuck), but that seems risky at best of ending up with two broaches that don’t line up/match...

 

[T Bredehoft]

Many of us (with industrial experience) would opt for a broach using an arbor press, the broach itself is long enough to cut 3" long with no problem, one pass. Without the broach and arbor press, (I don't have either) your lathe is probably the best bet. If you can lock your spindle, you can power feed the cutter.

 

[great white]

Many of us (with industrial experience) would opt for a broach using an arbor press, the broach itself is long enough to cut 3" long with no problem, one pass. Without the broach and arbor press, (I don't have either) your lathe is probably the best bet. If you can lock your spindle, you can power feed the cutter.

Hmm, never though of that. The atlas does have a locking pin on the bull gear, but I’m not sure if it can be used to power feed the Carriage like that. Better go consult the MOLO to see if its and option.

Come to think of it, I do have a couple long HSS blanks in my box that I may be able to turn into a broach tool of approriate size....

 

[benmychree]

With broaches, generally you can broach a keyway in a part that is the length of the broach plug; in the case of a 1" bore, that is 2 1/4", trying to broach longer than that risks breaking the broach. Cutting the keyway in the lathe should be no big problem for a 3" length, the boring bar could be as big as perhaps 3/4" in diameter, plenty stiff. The tool needs to be the exact size of the keyway to be cut, it cannot be expected to be adjusted side to side to make an accurate width, it needs a couple degrees side clearance at most, front clearance 3-4 degrees and back rake about 5-6 degrees. Before I retired, I had a Mitts & Merrill keyseater that could make from 1/8 - 1 1/4" keyseats 12" through the part with a single tooth cutter.

 

[JimDawson]

I did this one on my lathe, it's only about 1.5'' deep x 22mm dia, but if the bar was longer it could have been any depth. Just grind up a key width HSS bit. Cross drill a bit sized hole in the end of a bar that will fit the bore, and secure the bit with a grub screw. Set up in the lathe and and you too can become a shaper.

Takes a few minutes to do, but is easy

 

[derf]

Many of us (with industrial experience) would opt for a broach using an arbor press, the broach itself is long enough to cut 3" long with no problem, one pass. Without the broach and arbor press, (I don't have either) your lathe is probably the best bet. If you can lock your spindle, you can power feed the cutter.

If you lock the spindle you don't have any power feed....

 

[benmychree]

It would be possible to power feed with the spindle locked if the gearing between it and the feed train were disconnected and a suitable power source were introduced, but is it worth possibly damaging your lathe if something hangs up?

 

[kd4gij]

With broaches, generally you can broach a keyway in a part that is the length of the broach plug; in the case of a 1" bore, that is 2 1/4", trying to broach longer than that risks breaking the broach. Cutting the keyway in the lathe should be no big problem for a 3" length, the boring bar could be as big as perhaps 3/4" in diameter, plenty stiff. The tool needs to be the exact size of the keyway to be cut, it cannot be expected to be adjusted side to side to make an accurate width, it needs a couple degrees side clearance at most, front clearance 3-4 degrees and back rake about 5-6 degrees. Before I retired, I had a Mitts & Merrill keyseater that could make from 1/8 - 1 1/4" keyseats 12" through the part with a single tooth cutter.

You can make longer plugs for longer bores. I broach 1/4" key way in 1" bore 6" long all the time.

 

[Cadillac]

You can make yourself a boring bar to line bore the keyway. Use asbig a bar you can fit in bore and put a hss bit perpendicular to bar. One end in chuck other supported by tailstock. Mount your part on the cross slide use the carriage to traverse the part, lock the spindle. All manual.

 

[jcp]

I had a Mitts & Merrill keyseater that could make from 1/8 - 1 1/4" keyseats 12" through the part with a single tooth cutter.

We had a Davis. It used broaches up to 3/4" or single tooth up to 2". Very handy.