Woodruff key (slop) ?

G

graham-xrf

H-M Supporter - Gold Member
H-M Supporter Gold Member
Joined
May 27, 2016
Messages
3,477
Ever since the experience of a not-quite-tight cotter pin on my school bike pedal crank, I have always known that any spare space in a place where steel is supposed to locate can lead to very rapid deterioration. Now I have an unknown in my South Bend lathe spindle. I will be posting a picture, and some measurements, but I would like to establish what the assembly norm should be. From measuring on them, the key reasonably corresponds Woodruff No. 606, nominally 3/16" x 3/4".

There are two spindles, one for the SB-9C and the other for the SB-9A. Both appear to have identical dimensions, down to quite fine limits.
The key in the 9C spindle just drops in, and I can squeeze a 0.002" feeler gauge in beside it. The key looks OK. I can see it's original machining marks.

Somebody please tell me it's not OK to put it back into the re-assembly like that?
 
You could loctite it in and let it set up before assembly. It should fit tightly or it will continue to wear and get even looser.
 
Measure the thickness of the key. That will tell you if its the key or the key seat. If the key measures out correctly then the slot is big. Your choice to run it, patch it, or change spindles.
 
Yeah, I've always wanted woodruff keys to be press fit. It could easily be a newish key that someone put in there after a previous one got beat up. I agree with loctite. Other options are to make an oversized key, or cut the slot to the next key size.
 
I have a feeling he won't be able to cut the slot. Not unless he has a carbide woodruff cutter. That spindle is hard..

I'm wondering if you can find a metric key which is slightly bigger, and fit the lower, then fit the bull gear..

I just don't know if loctite will handle 2 thou wiggle room.. I suspect it might, but would feel better myself with a mechanical fit.
just my 2 cents.
 
I would measure the width of the key and see if it's the key or keyway. If it's the key it's an easy fix. I really don't know how much of a problem it will present though...

It's your machine and you can do what you want. But, if it was mine, I wouldn't worry about it. There's most likely going to be clearance in the keyway cut into the gear as well. There has to be some clearance for assembly. You'll be running the lathe most of the time, and maybe even all the time, in one direction, so there won't be much back and forth play anyways....

If you are concerned, use some Loctite as John suggested. Two thousands clearance isn't that much and there are Loctites made for slightly larger gaps. Check the Machinery Handbook. Yes, tight fit is best, but I don't really think you are much out of limits if at all. My guess is that the key is on the small side.

I don't really see it as much of an issue. I have two South Bend lathes with this setup and wouldn't personally worry about it myself.

But, do whatever makes you feel good.

Ted
 
Mitch Alsup said:
Can you get some 4140 and make your own key to fit.
Fit the spindle end as a plier-press-fit in the spindle, and just tighter then slip-fit on the gear.
Click to expand...
I would be entirely OK with that - except you may have deduced, given that I am looking at two spindles, and two keys, I am in the machinist equivalent of "snookered". :)

I have found 1/16" x 3/4" Woodruffs in a 10-pack from eBay for £5.50 per pack, apparently steel to BS46 specification.
I don't need 10 of them, but the thing I notice is that most other offerings were like £4.40 each!
Apparently BS46 is "key steel", described as " an unalloyed medium carbon steel, much stronger than ordinary mild steel, but more difficult to machine. It bends or deforms considerably before failing. This steel is used where the greater strength is required, but less costly than a full alloy steel".

Indeed, if I were making it, I would use 4140, or similar, and I would make it a "plier-press!" fit.
I love the phrase. It guess it must be like a interference fit, but only in millionths. In any case, I would have it so, instead of using Loctite. For all sorts of reasons, a key is something you may want to get out again. I don't think the glue should be a gap-filler!

When the keys arrive, I may still face the sloppy fit dilemma. The tolerances in "Machinerys Handbook" show possible ranges from pretty sloppy to full interference fit. If that happens, I will have to take exceptional action. I don't mind removing metal to a fit. We can't be "adding metal".
 
You must log in or register to reply here.
Back
Top