Rockwell 11 worm rebuild question

thenrie

thenrie

Active User
Registered
Joined
Oct 7, 2013
Messages
305
Ok, to put things in the proper perspective right off, yes, I'm a rank neophyte, have no idea what the heck I'm doing...yet. Anyway...

I'm working on bringing a Delta Rockwell 11 (my first lathe) back to usable condition. I wouldn't call it a restoration, from a purists point of view, but I'm trying to repair/replace everything that needs it.

Below is a picture of the worm. The threads look good, at least to me, just by eyeball, but the bearing surface is rough. I measured it at 1.121-1.123. at various places. The major diameter of the threads is about 1.119. I'm thinking that I could take a skim off the bearing surface and smooth it up and take up the difference in the bushing when I replace it. Or, should I just not worry about it? It occurs to me that it is not a bearing surface that runs with any speed and doesn't take a lot of stress. I'm thinking that if I just replaced the bushing, I might not ever wear it out in my lifetime. What would you do?

Advice and thoughts would from experience would be appreciated.

2013-11-20_21-32-10_94.jpg

Tony

2013-11-20_21-32-10_94.jpg
 
If it's good, leave it alone. That's what I'd do. Why run the risk of making it worse? :dunno:
 
Well, that's kind of why I'm asking. Is it good? The bearing surface, as I mentioned, is pretty rough and uneven. I certainly wouldn't use it if it were a motor bearing, but in this application, is it something to worry about?
 
What is the worm for? Does it get run by a motor,or is it just hand operated to perform some function? If just used to adjust something,lubricate it and leave it alone. A good lithium grease would be good.
 
Last edited:
The worm gear runs in a bushing. It has an internal key that is keyed to the lead screw on the lathe. As the lead screw turns, the worm gear turns inside the bushing. It's a low-speed bearing, but it gets lots of mileage. It's moving whenever the lead screw is engaged, whether or not the carriage is moving. It's worn a couple thou and it measures about .001-.002 difference over the bearing length. I will have to hone a bushing to take up a little of that if I want a wobble-free fit. Like I said, I could take off maybe .002 in a finishing pass and it would still be about .001 over the major diameter of the threads, so I could still slide it into the bushing without trouble, but I'm not sure it's worth the trouble or risk of a mistake by a rank amateur.

I just had a thought. Would it be advisable to chuck it and take down the unevenness with a mill file and emery cloth, rather than trying it with a bit? It would seem to me that there would be less risk of me screwing it up.
 
Since it rotates a lot,it really needs a precision ground,accurately done surface,not a filed surface which will not be accurate,nor really round. Do you have a tool post grinder for your lathe?

To tell the truth,if you are a beginner,and want that part to last,since you can't get replacement parts,I'd recommend getting it ground at a machine shop on a cylindrical grinder. Getting a good,perfectly ground surface with a tool post grinder can be a tricky job,depending on your lathe and the quality of the tool post grinder,the wheel,and knowing how to use the grinder. For example,run the grinder for 1/2 hour before using it to warm up the bearings and get their fine tolerances to size is recommended for my Themac tool post grinder.I think they are among the finest grinders out there. Years ago I had a little Dumore,and it left a plywood "wild grain" surface due to harmonic vibrations. It looked like new,but would not leave a decent finish. The lathe was nearly new,too,and it left great finishes on its own(not finishes like you need,though).

Explain to the machinists that this part runs all the time. You need a polished surface on that shaft.

Then,you need to make the bushings to fit the somewhat smaller diameter. I used to use "Plastigauge" to check the main bearing tolerances on my 1950's car when rebuilding the engine. It is a thread of soft plastic. When you put the bushings together and tighten them on the shaft,leave a strip of the Plastigauge lengthwise on the shaft. Remove the bushings and use the included paper ruler to measure wow wide the plastic thread got squeezed. The width of the thread will give you the tolerance in thousandths. You want about .001" clearance. This allows for oil to get in there,and for the shaft to warm up. Too tight,and the shaft will bind and get galled,which is NOT wanted.

Plastigauge is still sold at auto stores,I think.
 
Last edited:
I'm familiar with plastigauge and I can't see any way to use it in this case. Plastiguage is for checking your bearing clearance on engine main and rod bearings, where you bolt the cap down on top. No way to slide it in between the worm gear and the bushing. You just have to do it by measuring with micrometers and feeling the fit.

Tony
 
I think those are only for repairing seal surfaces. After reading the installation guide, it looks to me like it would be easier just to install a new bushing in my case.
 
+1 on the farm it out. I would have them make the mating bushing also. good luck
 
You must log in or register to reply here.
Back
Top