Best way to enlarge an existing shaft keyway???

[ex-Gooserider]

Not exactly an engine building question, but definitely a modification challenge....

I am in the process of building an adaptive electric skateboard for a wheelchair using friend. He has supplied many of the parts, I mostly need to design and build the brackets that hold everything together.

I have found one big problem with the parts I was given, IMHO because of lousy workmanship on the part of the original supplier - who is not terribly helpful.... :wall:

The motor is an electric 'outrunner' style - the sort often used on BIG RC model planes. The shaft on it has a keyway, that is intended to match the keyway on the supplied pulley.

The problem is that the supplied key is 3mm^2 X 16mm, which is (I'm told) one of the standard metric sizes. It fits perfectly in the pulley, but the keyway in the motor shaft is only about 2.85mm X 14.5mm, and the key won't fit in the shaft at all.

The supplier's "answer" is to grind the key to fit, but aside from not wanting to be making a non-standard key that will be a pain to replace if it is ever lost, that would mean having a poor fit in the pulley....

Seems to me like the better solution is to try and enlarge the keyway in the shaft to the CORRECT size, so that stock keys can be used....

It would be easy enough to fix the pulley by going at it with some needle files, but the best way to deal with the shaft is less obvious...

The shaft can be removed from the motor without much hassle, and I have access to a Sharp mill (Bridgeport clone) with 2-axis CNC / DRO function. My thought is to clamp the shaft in the vice w/ a V-block after trying to get the existing keyway as vertical as possible, and then try to mill it wider...

Any better / alternative ideas?

Any suggestions about the best way to get the existing keyway vertical that are more precise than the old 'Mark 1 eyeball'?

Also my smallest mill bit is 3mm. I know that ideally one should cut slots with a bit that is narrower so as to only be cutting one side at a time, but if I just drill down with the bit every couple of mm, and then run it back and forth to clean up, will that be good enough?

Thanks,
Ex-Gooserider

 

[Cogsy]

Doesn't help with the 'non-standard' key problem, but you could mill the key down for the section that runs in the shaft rather than grind the whole side. You'd probably have to do both sides and you'd end up with a 'stepped' custom key but it would be a nice fit in the shaft and pulley. Wouldn't take overly long to machine and would be easier than enlarging the keyway in the shaft IMHO.

 

[ex-Gooserider]

I thought of doing that, and aside from the issue of a custom key (I have an objection on principle to doing custom parts if an off-the-shelf part will do, as it makes for future service pain) the challenge comes up of simply doing the 'work holding' on a part that small... Unless I do some custom jig fixturing I don't see how I'm going to securely hold the key in the mill vice with enough clearance to get just the key. Assuming I need to mill down 1.5mm on the sides, plus 0.5mm clearance to not mill the vice, leaves me with just 1mm in the vice, which doesn't seem to me like enough to be safe...

ART

Doesn't help with the 'non-standard' key problem, but you could mill the key down for the section that runs in the shaft rather than grind the whole side. You'd probably have to do both sides and you'd end up with a 'stepped' custom key but it would be a nice fit in the shaft and pulley. Wouldn't take overly long to machine and would be easier than enlarging the keyway in the shaft IMHO.

 

[Cogsy]

Ahh, I see your dilemma. I assumed that you just had a length of keysteel to make a key from rather than a key to modify. If you did have a length of it I think it could be done, but modifying an existing key would be very difficult.

 

[Hopper]

You could do the slotting the old fashioned way. Mount the motor spindle in the lathe with the end supported by tailstock center. Set the existing groove toward the toolpost in the horizontal position and lock the spindle by engaging back gear etc.

Mount a 3mm wide parting or grooving tool sideways in the tool holder.
Use this as a slotting tool, running the carriage back and forth by the handwheel to get a shaper effect.
Use the cross slide to feed the tool in to the correct depth a few thou at a time as you go.

 

[Blogwitch]

Very easy to get the shaft square to the cut.

Just get a bit of well fitting STRAIGHT and FLAT plate and stick it with superglue into the slot.

Mount the motor shaft into vice using a couple of parallels or V-block to get it level with the vice.

Clock up and down and gently move the rotation about until you get zero deflection.

Use the plate sticking up to then find centre of the shaft by using an edge finder.

Once vice has been tightened up, just knock out the bit of plate.

Put whatever width of cutter you want in the spindle chuck and just gently cut your new slot, full depth and plenty of lube, you will not be removing a lot of metal so you should get away with it.

I think you are thinking too much and thus making the job too difficult for yourself, doing it this way shouldn't take more than half an hour.


John