Readers may recall I am re-doing my '24 Speedster transmission after drum failures. See http://www.mtfca.com/discus/messages/257047/302817.html I am re-using my good low-speed hub and riveting on a good used drum. A skilled machinist friend is doing the actual work and has the drum secured to the hub. So far so good.
However, run-out on the drum surface is 0.009" (0.004-5 on the radius). While most members caution against turning the drums (the more meat the better) I am wondering if this 0.009" run out is too much. Should we take a light cut off the surface to true things up?
The friction surface is slightly dished in the middle, with an edge diameter of 7.483" and a centre/valley diameter of 7.478".
I'm heading over to my friend's shop in an hour or so and hope to have informed guidance for him. Thanks in advance.
If you do anything to the surface of the drum use a crankshaft grinder rather than a lathe. The results are a polished surface that is true.
Chris,
The fact that the friction surface runs out .009 is not so bad in itself. Balance aside, that shouldn't hurt the function of the drum, especially for low speed. A brake drum may make for choppy braking. The imbalance it may create is worth considering but, consider also that the rest of the surfaces on this drum are irregular cast finishes. Meaning, even if there were zero runout on the band surface, you may still have a large out-of-balance situation due to the rough cast nature of the drum. Rather than target the runout alone, check the unit to see just how out of balance it really is. For all we know, the runout may actually improve the balance!
.009" is magnificent. Don't turn the drum down. The drum will crack sooner if you remove any metal from it. It will crack less soon if you just put it together.
NO!
Don't turn the drum. It would have to be much worse than .010 out to make a significant difference to a model T.
Drive carefully, and enjoy, W2
Royce and Wayne are CORRECT. There are so many turned drums that are paper thin and then they blame it on Kevlar
I meant to say that there are so many turned paper thin drums that CRACK and then they blame it on Kevlar.
I have turned Hundreds of Drums on a lathe, never a broken one, and Kevlar bands since the came out.
A used drum is never true to start with. Most are out of round, low in the middle, and High on the out sides.
If there is a groove in the middle, don't machine that out, and if it is to deep, replace the Drum, but I have never had to.
Always use a expanding Mandrill in side the fresh reamed bushing, on centers.
Slow speed, and either use a drum deadener, or if you get small Harmonics in the drum, it has no ill effects.
Last spin the drum in the lathe, and using Emery, smooth the Band surface until it is smooth to the touch. It takes about 2 to 3 Minutes.
Normally you will only take off about .010, to .025 thousandths wall thickness.
Thanks everyone for your comments. We decided to just polish the surface a bit more and didn't take anything off the diameter.
For those that don't have a mandril but do have a set tru lathe chuck you can grab on the gear sleeve and center on the ID of the newly fitted bushing. Same result. Harmonics or chatter is a small issue but the use of a succeedingly fine files and sandpaper gets you a fairly good finish. I've never experienced a cracked drum resulting from turning them down but then I don't run Kevlar.
All that said, small variations like what you have aren't enough to worry about. I'd be more concerned about balance.
You can center the bushing with a Dial Indicator in one spot, but the length of the bushing will still be off. If you don't have an Expanding Mandrill, you would have to use say cones, or tapers to center both ends of the bushing, or when assembled, the center, and the fresh cut out side, will be off.
Drums are not balanced from the Factory, they are a long way off, we balance when recut.