Subject: Should this be brass? (9" SB Lathe part)

From: [email-masked] (Chris Kantarjiev)
Date: 3 May 1996 17:48:40 GMT

Finally got the compound on my 9" SB apart last night; apparently
there has been a change in the size standard for pin wrenches. What
you buy now for a 1" radius wrench comes with a 3/16" diameter pin;
the holes in the lathe are 5/32". Ed Richley came to the rescue by
providing a backup lathe and a four jaw chuck; we mounted the wrench
in a fixture and cut the pin to size. (We briefly contemplated drilling
the holes out to size, but this seemed like more fun. Now I'll probably
remove the finish on the wrench, fill in the cast '3/16' and repaint...)

Anyway, the compound was still hard to disassemble, because the compound
rest nut was held in the compound rest swivel very tightly. There isn't
enough clearance to drive it "in". There was a threaded hole, though,
that allowed it to be pulled "out", which we did.

I wondered how I was going to get this back in and aligned properly; it's
a press fit. Ed thinks that it should be loose, and is just gunked up; 
alignment is done by threading on the compound rest screw, and then kept
by tightening down the set screw.

That could be. As I proceeded to clean this part, I really had to wonder...
why is this brass? It's marked up enough and fits poorly enough that I
have started to think that it may be a replacement part - yes, it has
an Acme thread in it, but the ends are very poorly finished. Seems like
brass is a poor choice for this application because it will wear quickly.

Does anyone know, offhand, if this part should be brass? Maybe Dino (the
former owner of this lathe) made up a replacement from available stock,
instead of buying a proper cast iron replacement?

Thanks.

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From: [email-masked] (Randolph Lee)
Date: Fri, 03 May 1996 22:31:14 -0400

In article <[email-masked]>, [email-masked]
(Chris Kantarjiev) wrote:

snip
> that allowed it to be pulled "out", which we did.
snip
> brass is a poor choice for this application because it will wear quickly.
snip

they are made of brass just so they will wear before the lead or X screws
do... it is much easyer and a lot cheaper to replace the nut than the
screw...

if abrasives are let in to the nut (via tool post grinding or the like or
even a lot of cast iron work)  then the nut will act as a lap... and lead
screw wear will result... but keep things clean and one screw on an engine
lathe can last through quite a few sets of nuts...

very high tol lathe (like a schaublin instrument makers lathe) builders
will not sell just the nut... you have to get a matched net and screw set
to hold the as built tol levels... (the instructions on resetting the
spindle bearings on a Schublin for example, require you to work to + or-
.001mm or about .00005 inch levels) 

~R

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From: [email-masked]
Date: 4 May 1996 16:15:19 GMT

>instead of buying a proper cast iron replacement?

I suspect that the feed-screw nut you talk about is probably 
made of *bronze*, which is a much tougher material than brass.
I think SB used it because it has superior lubrication qualities than
steel or cast iron - that is, the mating part, being made of steel,
would rather run on a different material than steel.  Also, it
is cheaper to replace the nut rather then the feedscrew, so
a slightly softer material like bronze would be sacrificial in a sense.
The nut would wear faster than the steel feedscrew.

Although, I usually see that the wear in parts like this are a result
of lapping - that is, abrasive junk gets on the threads, and wears
both parts equally.

If the threads are *real* sloppy, you could make a new bronze nut.
I did that with my Hardinge milling machine table screw, and it
worked out pretty well.  The compound on a lathe is usually not
called on for repeatability or accuracy, though.  I would be more
concerned about the crossslide screw on the carriage.

Good luck on your project!

===========================================================

 1959 BMW R50                                           Jim Rozen              
 1969 BMW R60US                                (JRR0 @ watson.ibm.com)                 
 1975 BMW R75/6    (The "new" one)        

===========================================================

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From: wayne weedon <[email-masked]>
Date: Sat, 04 May 96 09:19:28 GMT

In article <[email-masked]>

> instead of buying a proper cast iron replacement?

 It's probably Phospher Bronze which is the correct material for a
leadscrew nut, and can also look like brass.  Is there not backlash
adjustment for it?

-- 
wayne weedon

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From: [email-masked] (Jonathan M. Elson)
Date: 11 May 1996 05:50:28 GMT

W. Kaiser ([email-masked]) wrote:
: Jonathan M. Elson ([email-masked]) wrote:
: > 
: > Oh NO!  Never use a cast iron part on cast iron or steel!  They make a 
: > terrible bearing.  

: I seem to remember reading somewhere that cast iron makes a fine bearing 
: for steel.  

This may be true - it depends on load, speed, number of revolutions of
life, etc.  A leadscrew nut is a WHOLE different ballgame than a low-speed
bearing under light load, however.

: I'll look for a bronze insert, but I think the bearing that holds up the 
: tail end of the lead screw of my South Bend is plain cast iron.

I think my Atlas uses a similar setup, but the bracket is most likely that
awful Zamak that most of the smaller pieces of the Atlas are made of.  It
IS a fine bearing material, however.