imperial to metric keyway slot enlarging/broaching

[mattnj]

hi,
can anyone help with the following, or point me to a workshop that can...

3/4" bore coupler (19.05mm) currently has a 3/16" (4.76mm) keyway slot down through its length, about 100mm.

I need to get the keyway slot enlarged to a 6mm keyway which is on the shaft.

Or I suppose the alternative would be to source a 6mm keyway with a step at the top to 4.76mm.

Like everything, would be a doddle to a person with the right tool!

Thanks

 

[Rum Run]

Modifying the key is the easiest thing by far: can be done with a file if necessary or a milling machine if available.

Opening up the keyway in the coupling could be done with a single tooth tool in a lathe, with the coupling held in the chuck and stationary, running the tool back and forth with the apron wheel. This is commonly done in model engineering but could be laborious given the size.

I'd modify the key personally, but this is assuming the loads are acceptable for the 3/16" keyway. Which I don't know of course.

 

[mattnj]

Modifying the key is the easiest thing by far: can be done with a file if necessary or a milling machine if available.

Opening up the keyway in the coupling could be done with a single tooth tool in a lathe, with the coupling held in the chuck and stationary, running the tool back and forth with the apron wheel. This is commonly done in model engineering but could be laborious given the size.

I'd modify the key personally, but this is assuming the loads are acceptable for the 3/16" keyway. Which I don't know of course.

thanks, I have a decent size lathe...whats the tool i'm looking for?

 

[earlybird]

Basically like a boring-bar with a 6mm wide cutting edge, used like a shaping tool as Rum Rum indicates. TBH however, for a 4" long cut along a 3/4" bore, presumably in steel;- my advice would be to forget it. Life's too short and success would be unlikely IMHO!
Ebay will provide a length of 6mm sq. key-steel for £2-£3. Some-one with a milling machine and vice could soon put a step in a 4" length.
(Milling can also be done in a suitably equipped lathe).
Alternatively, broaching kits are available, too expensive for DIY, but you might find a local general machine-shop so equipped who could broach a 6mm keyway in the coupling.

 

[Daydream believer]

Put the 2 parts together and then with a drill bit large enough diameter to cover all 4 corners of the 2 grooves ( you need not cover all corners you could just make diameter match distance from flat to flat ie 6mm plus 3/16) drill down them. The drill should follow the grooves naturally leaving a round hole. You could do it with one drill bit then another slightly larger so hole size matches the bit diameter tighter
Then instead of a square woodruff key turn up a piece of silver steel, or use the top part of the drill bit you used, as the key
The key does not have to be square it is there to provide a resistence to shearing force
It really depends on the load to the unit and whether it is uni directional or if reverse is applied regularly thus introducing play. If parts are case hardened this may make drilling harder

 

[Deleted member 36384]

If it was me, I would be trying to machine the keyway in the coupling to 6 mm. Phone around machine shops and ask if anyone cuts keyways with a 'linear broaching machine'. Assuming you don't have a weird sized coupling the bushing that allows the broach to cut the slot should be available. It would take less that 5 minutes to machine if the shop has the bushing and broach, so cost wise it should be low.

For example http://www.bandhgears.co.uk/keyway.php