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dr pepper
04-11-2010, 06:34 AM
I have a brand new cap start induction lawnmower motor, I'd like to build a more meaty bench grinder, this motor has the right speed and power.
Theres a 16mm shaft out one end, I can make a stub shaft with a boss and headed washer to clamp the wheel using a lh thread bolt, the other end of the motor is sealed, however just behind the cap is the shaft, drilling a hole in the cap will make space for another shaft at the other end.
How would I go about joing a stub shaft to this, i was wondering if I could just make a stub shaft with a rh thread and drill/tap the end of the motor shaft and screw it on.

Your Old Dog
04-11-2010, 07:30 AM
I'm not the machinist I'd like to be so I would just file a flat on the motors shaft and turn a (as in one) collar to fit over it and the shaft I'd like to drive and then put some small set screws in it. I don't have the skills to thread a motor shaft without botching the job up :D Good luck and let us see your grinder when you're done.

gda
04-11-2010, 07:40 AM
I would not attempt to drill and tap your hole with the motor together. The chances of getting no runout from the tapped hole are small. You may want to take it apart and do your hole and than turn the stub true, or along with the tapped hole make a locating counterbore.

The shaft may also be induction hardened. Lots of chances to mess it up.

John Stevenson
04-11-2010, 07:42 AM
Rob,
Can you post a picture of the rotor, chances are i have done similar at some point.

Bill Pace
04-11-2010, 09:59 AM
That sounds like something Sir John could do in his sleep - show him a picture and he'll prolly get you started.

dr pepper
04-11-2010, 10:07 AM
Ok will do when I'm home.
Of course I will do this with the rotor out on the bench, or in the machine.
The step sounds like a good idea without turning, however if I've got it right machining a slightly larger register in both motor and stub shaft sounds like a secure and straight way of doing it.
One thing that I've just thought of is that a mower motor is meant to start up quickly, grinders tend to be shaded pole motors and slowly crank up, the mower motor will go bump and away, I wonder if the shock would be dangerous with the grindwheels.
Maybe I could replace the start cap with a smaller one, or build one of the simple soft start cicruits I've seen on the web.

lakeside53
04-11-2010, 11:15 AM
One way - Bore the motor shaft, make an extension shaft as a light interference fit, knurl the insert end slightly, press into the motor with red loctite, true it (if required) in a lathe.

If you have room, an external (larger) extension may be easier.

Willy
04-11-2010, 12:22 PM
If the adding the shaft to the other end of the motor is not possible, and you want to utilize the motor, you do have another option.
Just use the motor to turn a double ended arbor mounted in a set of pillow block bearings. Not quite as compact as a motor with a double ended shaft, but with a little thought in mounting and fabrication you will be close.

digr
04-11-2010, 08:10 PM
I have done this in the past by pressing out the shaft and making a new one or adding on by welding and press it back into the armature.

dr pepper
04-12-2010, 03:13 AM
Pressing the shaft out, sheesh dont think I'm that brave.
Sounds like the effective way is to screw/interference fit a slightly larger shaft and then turn down to make true.
I'd like to make a whole new one, maybe I'll try a practice on a knacked motor, I have one lying about.

Evan
04-12-2010, 04:13 AM
Drill a hole in the end ot the shaft and tap it for an appropriate size socket head cap screw. The hole doesn't need to be perfectly concentric. You can probably get it pretty close though by chucking up the motor by the OD of the housing in the lathe and bumping it until the free end is in the right place.

Lock the spindle and hook up the motor to power and run it in order to drill the shaft. Then you can chuck up on the other end of the shaft and hold the stub end with a centre in the tails stock.

Turn the end of the shaft down by about 1/8th of the normal OD and then put on a slight taper of a couple of degrees. Use a small boring bar tool to do this. Remove the motor and chuck up the stub shaft. Drill it to the same size as the small end of the taper you just put on the motor shaft. Without changing the angle of the tool turn out the hole you just drilled to a matching taper by running in reverse and cutting on the back inside of the hole to make a precisely matching taper. Drill a clearance hole for the SHCS and fasten the stub shaft to the motor shaft using some loctite.

This takes about twice as long to explain as it takes to do.

dr pepper
04-12-2010, 07:51 AM
I read that a dozen times, and got the basics, my simple thinking head doesnt quite follow.
You say drill the hole in the end of the motor shaft by clamping the motor to the apron, then use the motors power to bash the hole.
Then turn a register on the end of the motor shaft, I'd have to strip to motor and remove the rotor for this as the non drive end shaft is flush with its bearing, but thats no problem, then turn the slight taper after that.
Then turn the pre prepped stub shaft taper with the same setup that did the taper on the end of the motor shaft, ok but why reverse.
So I'd have a stub shaft with a screw thread and a tapered register, and a motor with the opposite tapered register, screw it in till its solid.

dr pepper
04-12-2010, 08:04 AM
Its ok I got it now, grey matters kicked in.
Thats a really good procedure.
Only thing is my machine is only a 7 by 20, the motor would swing in the gap, but non of my chucks will hold the motor housing.
Your idea with the taper sounds good though.
Not having a steady will make boring a taper hard, esp when my cguck will not even hold the rotor, and its a bit iffy holding the drive end of the shaft.

Evan
04-12-2010, 08:06 AM
Clamp the motor in the chuck by the outer case and bump it till it is aligned for drilling.

If the shaft doesn't protrude from the bearing then you will have to remove the armature for subsequent operations.

In that case make the stub shaft slightly oversize so it can be machined after fitting to precise concentricity and bearing fit.

The reason for running in reverse is that a boring bar tool can be used to machine the external taper normally and then without disturbing it the internal taper on the far inside of the bore by running in reverse. This makes an identical taper. Make the inside bore taper slightly longer and then machne small amounts from the end that meets the register until it is a few thou short. The screw will pull it up and lock it. Loctite is only needed to hold the screw.


Cross post.

With care you should be able to hold by the shaft in order to drill a centre hole in the short end. Use very low rpm.

Circlip
04-12-2010, 08:42 AM
'L's teeth, we're tigh--- FRUGAL I Yorkshire but even I bought a bench grinder.

Other way is as stated, using it to drive a countershaft like in older designs with pillo*k blocks. With suitable pully ratio, you could even drive heavy wheels safely.

Regards Ian.

digr
04-12-2010, 08:48 AM
Pressing the shaft out, sheesh dont think I'm that brave.
Sounds like the effective way is to screw/interference fit a slightly larger shaft and then turn down to make true.
I'd like to make a whole new one, maybe I'll try a practice on a knacked motor, I have one lying about.
Try it out, take your old motor apart and press the shaft out, they come right out on any motor I have tried it on. I am sure that at motor factory if they needed a special shaft they wouldn't add on to a old one.

Circlip
04-12-2010, 08:54 AM
Nowt like integrity DrPepper, thats the integrity of a ONE piece shaft.

Never played with a gyroscope?????

Regards Ian.

dr pepper
04-12-2010, 10:57 AM
Theres ally bits stuck out on the ends of the rotor, for counterweights to fit to for balancing, I'd squash these presing it out, unless I mnake a little jig to press against.

Grinders with 1kw motors are really expensive,more than my beer budget.

Evan
04-12-2010, 12:40 PM
It will hold. I have done exactly what I explained on a small starter motor that I used to drive my electric bike and it never loosened. Don't underestimate the holding ability of a locking taper, especially when it is clamped. The socket head cap screw gives it back a lot more strength than the missing material gave it.

digr
04-12-2010, 03:16 PM
Theres ally bits stuck out on the ends of the rotor, for counterweights to fit to for balancing, I'd squash these pressing it out, unless I make a little jig to press against.

Grinders with 1kw motors are really expensive,more than my beer budget.
You usually have to make some kind of fixture. Don't try it if you think you are going to ruin your motor, just telling you what has worked for me in the past The biggest motor was a two horse and the smallest was a small fan motor.

dr pepper
04-13-2010, 08:50 AM
Actually I've done a similar thing come to think of it, I pressed out the shaft of a car alty and replaced the windings with magnets to make a wind powered generator.