Hi,

Is the keyway 3 mm deep or 3mm wide?

Good luck………

Hello where are you roughly? I have a press and access to a 5mm broach. I am east of  cambridge if this helps

Paul

I’ve dome 3/16″ keyways in steel using the lathe so your 5mm in that nonferrous should be easier. Just make a simple holder and grind up a tool, the existing 3mm keyway will help guide the tool.

Iailing that an imported 5mm broack and homemade guide ban be pressed through if you have a reasonably sized bench drill.

Does raise the question if using a 5mm keyway does it also need to be the correct depth for a 5×5 key or are you just going to widen the 3mm and not go the extra 1mm deep?

The second method in the video might be best suited. Racking the stationary cutting tool bit up and down by the milling machines vertical feed lever is much easier than racking a lathe carriage back and forth by the handwheel. The latter can get very tedious very quickly.

What a splendid idea Charles.

Effectively answers the too much strain on a small lathe when using the saddle feed issue and simple enough to be worth making for “the one and only keyway I shall cut in my life” job. Of course once you have got it …..

That said I’m unconvinced by the strain on the carriage argument. Yes a keyway is a wide and relatively heavy cut if you go straight at it. However if, as I do, a starter hole is drilled half in the job and half in a plugged bore just a touch smaller diameter than the key width most of the heavy cutting is already done so the stain is seriously reduced. Only the last bit of the end corners being full width. A little bit of attention to the corners with a file helps there. I routinely drill the starter hole before boring.

If the OP is worried about stress a bit of filing to remove a useful percentage of the metal to be cut isn’t too difficult. Alternatively a narrower tool could be used and one side at a time cut. If working in the lathe use a 3 mm tool and shim up 1 mm to do the top side and down by 1 mm to do teh bottom.

Clive

The first keyway I cut I used the racking the method and ended up with a lot of play in my Myford saddle (a very poor guide arrangement for resisting this sort of abuse !). For my second go I removed the topslide leadscrew and rigged up a handle and link similar to Charles Lamont’s above but anchored to the saddle and reciprocating the topslide. Arrangements with adjustable tool height are useful here. It’s still hard work and I wish I had come across Clive’s method above of easing the job at the time.

Why not make a stepped key, 5mm on one side and 3mm on the other.
Only thing modified then is a small piece of square key stock, and it’s a simple job to make it

Dave

Go for a stepped key.

Tony

With the smaller one going on first a stepped key won’t work unless the shaft keyway extends to the end of the shaft so it can be pushed in from the end.

But they do not have to “share the same key” they can share the same keyway in the shaft but have separate keys.

Asuming the shaft and nut have 5mm keyways just use a 5×5 square for those. When it comes to the gear make an inverted “T” shaped key with a 5mm bottom to fit the shaft and a 3mm top to fit the gear.

EDIT posted between your two, so may as well just enlarge the keyway.