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Rear mounted tool post

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chris stephens03/10/2010 13:28:19
1049 forum posts
1 photos
Hi Guys,
There is one good reason why Andy's rear tool post would work better and that is that it is directly mounted to the cross slide.  Without the extra set of dovetails in the top slide, to add slop, the toolpost must be more rigid and therefore a more consistent feed can be achieved. This does not make the rear toolpost better of itself, but it does overcome certain  inadequacies in a poor lathe.
Certainly on the SMEE stand at Ally Pally last January I found parting off a little trying on a Mini-Lathe, you could see the top slide moving down quite markedly as the tool tried to dig in, a rear tool post might have helped, but on the other hand a few hours spent "finesse-ing" the gibs would have cured the problem too, and this  work would have helped normal turning as well!
chriStephens 
AndyB03/10/2010 15:48:49
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167 forum posts
7 photos
Doh!
 
Thanks Chris, of course it is!
 
John Olsen03/10/2010 22:57:38
1294 forum posts
108 photos
1 articles
A couple of points from above that I would like to emphasise.
 
First the narrow parting blade. I only had a Unimat 3 for many years, and had a lot of success with a parting off blade about 1mm wide. This was ground from a piece of 1/4 inch HSS and could go in just over a quarter of an inch. Very successful, lasted many years, and made me wonder what the fuss was about. Of couse the Unimat is only a tiny machine, you would not be wanting to ask it to part off two inch diameter 4140 bar. Mind you, within their reasonable size range, these small machines are actually quite rigid.
 
Then with the Myford (ML7) ...I applied the lesson and used the narrowest parting blades I could find, with pretty good success until the last few months. Then I discovered that the headstock bearing had developed a bit of play. I had not had any trouble turning things accurately to diameter, but I noticed that I was having trouble parting off, and realised that the job seemed to be trying to ride up on the tool. So checked the bearings, took some shims out, and it is all behaving much better now. So rigidity is important, reasonably enough.
 
 
I have used a rear tool post in the Myford, it didn't seem to generally be any better than the quick change holder in the front. However the rear tool post I have has a disadvantage, the tool is set on a rake which is nice since it avoids having to grind the top to get rake. But there is no other centre height adjustment on the holder, so to get the  correct centre height you have to accept a given overhang. I think this is a bit too long on mine, so will reconstruct the head to combine an adjustable centre height with the built in rake feature. I will probably make another head with zero rake for brass etc.
 
 
I have a tipped parting tool for the Myford, however the tip is wider than I consider acceptable. I think about 3/32 or 2.5mm is about the maximum for an ML7. A Super 7, with its better bearings, should probably be able to cope with more.
 
regards
John

Edited By John Olsen on 03/10/2010 22:59:26

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