truing pressure plates
 

I've got a set of pressure plates I was going bring into machine shop at school to true up, but I got some questions.
1. can you just reface them on a lathe or should a person face them and grind them for a nice finish?

2. how much can you take off and still be safe? (these have some pitting on them)

3. when you guys put them in a lathe, did you put them in a 4 jaw or 3 jaw chuck? would it matter?

Thanks guys

1. They'll be fine as-turned on the lathe.
2. Any more than about 30-35 thou off and they get too thin in my opinion.
3. What matters most is that the axis of the driveshaft is perpendicular to the surface of the clutch plate. To do this, I made a mandrel with a slight taper that I could press the clutch plate onto and chuck in the lathe, using a center in the tailstock on the other end. This has always worked well for me.

Fellas, is it fair to say that sufficient cleaning up can be had by surfacing on a large disc sander? If the face is clean, it will eventually seat in and find it's own way if it is a little off. That is what we did (and were recommended to do) when restoring our Original (maybe your setup is different?).

Unless you are an experienced machinist, I would think you might get into more trouble using a lathe than just sticking with the simple alternative first off.

Like Matt said, if you're not taking the time to ensure the face is perpendicular to the bore, it's all a waste of time and there isn't a lot of extra material to work with if you mess it up.

Just my 2 Cdn cents...

You want a flat surface. It's next to impossible to do that with a disc or belt sander. A sander will leave high and low spots. Whoever told you it's ok to use a belt sander told you wrong or was too lazy or cheap to do the job right.

I chuck up on the collar that the pin goes thru. I use a 3 jaw chuck. I do not use a tailstock to center the pressure plate.

I first trued up a pressure plate when I was in high school and had pretty minimal lathe experience. With the tool set up correctly, simply facing a part with no other machining is among the easiest things to do on a lathe. This is not rocket science by any means.

As Merk said, you want a flat surface perpendicular to the axis of the shaft...you'll get neither of those things with a belt sander.

I've done some plates on a pice of glass in a figure 8 as you would do a head just to clean them up, but with pittng the best way is a lathe,a disc sander always wants to take more off one side than the other, and would take forever.

A good lathe finish will be just fine, You do not want to put them on a surface grinder to put a smooth (finish grinder) finish on them. You want them to have some grab so they work properly.

I do mine like Merk and chuck up on the center hub. I did my 100 and 126 clutches that way.

I'll play the devil's advocate......a woodworker will do final prep of a chisel or planer knife using fine grit sandpaper that is taped to the machined surface of his table saw or even a piece of glass, so it is super-flat. Our tiny 'O' pressure plates easily fit on our 12" stationary disc sander (not a belt or palm sander which I agree would be nearly impossible to keep flat) and with a gentle hand, came out just fine. All we were doing was taking a few thou off of rust and grime. And, we also 'kissed' the clutch material as well to take the shine off of it. Our clutch works flawlessly, even with double the horse power and original spring, it does not slip.

Again, the bore on the pressure plates isn't even near exact enough to ride perpendicular on the shaft, so it might deflect a few thou from being perfectly perpendicular to find it's way.

I agree the lathe is the best option, so long as you have the knowledge.

'Super flat'...perhaps, but not perpendicular to the shaft. You are extremely lucky to have only needed to remove a couple thou from the pressure plates to clean them up. Most I have trued up needed 20-30 thou removed due mostly to a bunch of runout that caused them to wear so unevenly in the first place. I've junked half a dozen at least because they had 40-50 thou of runout. I've also had a few from low-hour tractors that looked great but had a lot of runout, too, and would have worn unevenly as well. You could probably get away with less accuracy on the alignment of the rear plate because it slides on the shaft and will self-center to some degree, but not the forward one.

I've rebuilt several clutches and I know what works and what doesn't. When I had just gotten into this and didn't know any better, I did what you are describing...the clutch would just barely hold in 3rd with no load. Does your clutch 'work flawlessly' just driving around or under heavy load? If all you've tried is the former, I'd certainly hope it would work with no load on it. Pull a plow in some hard ground and you'll change your mind. A completely new, stock clutch is pretty iffy above about 14 hp or so, and will slip very easily in 2nd or 3rd (particularly 3rd) under load with 18. BTDT. You need all the help you can get from the pressure plates.