Motor Coupler Part 2 - Fabrication

The last post I told you about the coupler I designed.  This next section is how it was made.  As an engineer, I have a lot of experience putting lines on a paper.  Turning that into a real object is an entirely different thing.

The only way to make this kind of part is on a lathe.  My friend, Larry, graciously let me use his lathe and provided all the advice and help I needed to get it done.  Here's a picture of the setup he has, plus a reminder of what the part looks like that we're making.

The first tidbit of info is that stainless steel is easier to machine than low carbon steel and easier to get a good, smooth, surface finish.  The same goes for aluminum.  I didn't want to make my part from aluminum for a couple reasons, and had already bought a big block of cold rolled low carbon steel.  Oh well.

We started by making a dummy shaft that's the same size as the motor shaft that the coupler fits on.  Since the motor weighs 150 lb and is pretty big, it's a lot easier to test fit this piece instead of the motor.  The final pass put the diameter within 0.0002" of the motor diameter.  If I accidentally oversize the hole that the motor shaft goes in by just a little bit, it'll be game over and try again, which will be tough to swallow since it's the last dimension we'll cut.

The remainder of the 3 hour first day was spent cutting my 4 inch diameter by 9 inch long bar of steel in half, aligning it to minimize runout, and machining a center on one end.  A lot of that time was spent just thinking of how small we could cut it and still be able to machine all the surfaces in the right order.  Here's a picture of what the part started out as after being cut in half, though this is actually the half we didn't use.

Ok so the plan of attack is to rough cut all the dimensions, but leave a little extra material for later.  The reason for this is after extended periods of heavy cutting, it's possible that the part will move in the chuck.  If you do a precision cut of one surface and the part shifts, you will no longer have the concentricity or perpendicularity that you need.  If you do all the finishing cuts, which are lighter loading on the part, you will have a much better result.

The next day we spent another 3 hours machining just one feature.  You can see the 1.675 diameter on the right side of dimensioned picture (above), I spent the whole evening knocking down that diameter.  Instead of being 0.600 inches long it’s more like 2.5 inches long.  We had to make it that long so the chuck could hold it well and still fit a tool between the chuck and the .00000 surface in the next setup. 

The next day we met, we spent another 4 hours and made a lot more progress.  The part was flipped around so the chuck is holding onto the 1.625 diameter.  First I faced off the end (left side in the picture), then hogged out the 1.875 diameter on the left side.  Third we bored out the 0.875 diameter.  It was surprisingly quick to make a ¾ inch hole 2.5 inches deep in a piece of steel.  Now that 80% of the material was gone it was time to retighten the chuck and make some final cuts.  The 0.875 diameter was opened up.  Then the .00000 face was skimmed off.  Then the 3.740 diameter was cut.  I had the flywheel there, so we were able to check that it was a tight fit.  And that’s where we stopped for the night.

Finally we're to the last critical dimension, which is the hole that the motor shaft fits into.  You can see me cutting that dimension in this picture using a boring tool.  Being in inner dimension, it is tough to get a good surface finish and tough to check the diameter accurately.  You can use a micrometer, and unless you have some really specialized tools you can only use calipers to measure down to 0.001" accuracy.  That's where the gauge pin I made earlier helps out.  Once the hole is nearly large enough, you run the tool in repeatedly without changing the radial position.  This skims off more and more of the high spots on each pass and the hole will slowly get bigger.  This happens because the cutter is somewhat V shaped, so it leave small grooves.  On the outside dimensions I used a flat tool for the final cuts, so you don't see much of this behavior.  After taking a long time to close in on the final dimension, the gauge pin is very tight, but fits.  Here you can see the final part and a few pictures of it installed on the motor and the flywheel.  The next step will be to have the keyway broached at a machine shop.

Motor Coupler Part 1 - Design

This post really shows off my engineering side.  All day long I help design parts and create drawings for them.  This project is an opportunity for me to do all that plus figure out how to actually make the part.  This sort of project always gives me a deeper appreciation for the people who bring form to things I look at all day on a computer screen.  As a warning to all of you, this could get boring.  Then again, if you're like me this could be pretty interesting...or maybe still boring.  Enough said.

The electric motor has to somehow transfer power to the transmission.  Some people choose to set up this connection clutchless.  In this case, the flywheel, pressure plate, and clutch disk are removed and a single coupler attaches to the motor shaft and the transmission shaft.  Benefits to this system include simplicity and low rotating mass (i.e. more apparent torque for acceleration).  The downfall here is that changing between gears could be slow.  To other option, of course, is to retain functionality of the clutch, in which case you need a coupler that attaches the motor shaft to the flywheel.  The benefit here are that you shift as quickly as you would have been able to with the gas engine.  The downside is that the coupler design and fabrication is much more challenging and you have higher rotating mass (i.e. some of the torque that the motor produces is used accelerating the mass of the rotating components and not as much is available to accelerate the car).  Based on my friend's experience, I decided to incorporate the clutch.  And here it is.

In this next picture you can see how it fits in with the motor and transmission.  The motor is on the left.  You can see the coupler attached to the shaft and the flywheel attached to the coupler.  You can also see the transmission (or in my car's case the midshaft) that is piloted to the coupler.  This shaft must be supported with a pilot bearing so that it does not flex when you disengage the clutch, which could cause all kinds of problems.

So we have a couple important features here.  First, the ID of the coupler must be a very close fit with the motor shaft or it will cause vibration.  To improve the fit, I've added a slot in the coupler and a bolt that will clamp it to the motor shaft for a slop free fit.  Second, the face that the flywheel mounts to must be perpendicular to the motor shaft.  If it isn't, the flywheel will wobble causing bad wear on the clutch parts.  Third, the OD of the coupler pilots to the flywheel.  This must be a very tight fit or the flywheel will be unbalanced.  And fourth, the pilot bushing must be concentric with the motor shaft so that the transmission shaft is concentric with motor shaft.  Still with me?  Here are all the dimensions I used.

You can pay someone to build this for you for $500-600, but part of the fun for me is to figure out how to do this yourself.  Luckily I have some very generous friends who have the equipment needed to make it and were willing to help me out.  I have to give a big thanks to Larry for supplying me with around 12 hours of time on his lathe and just as much time in advice.

So now that we've got a design, it's time to start cutting chips.  I went to the local metal store and picked up a 4 inch diameter bar of cold rolled (CR) steel.  I got 9 inches figuring that would be enough to have two tries at getting it right.  I probably spent at least and hour discussing this machining process with my friend to make sure everything turns out the way I want it.  Considering how long this is going to take, this long conversation was well worth it.  After all this discussion we cut it down to about 4.25 inches long.  The shorter the piece, the less problem you'll have with wobbling, vibrating, etc.  Plus, even at that length, the starting piece weighs over 15 lb.

Next up, how to make it...