Rear mounting a parting tool ... if you can

Thanks for the info @mikey ! Now that I have converted my Craftsman/Atlas 12" to the MLA A-11 cross slide( coupled with a solid plinth tool post was a quantum leap forward in rigidity ) I'm considering getting MLA's rear mounted tool post for parting and knurling. Just trying to wrap my head around the physics as to why a rear mounted tool post makes for more rigid parting, than the standard way? I've read too many accounts as to its effectiveness, so I'm confident that it in fact works.........just trying to understand why?
 
As Mikey educated me on this awhile back on the front side the parting blade is forced down and into the work whereas on the back side the parting blade is forced up and away from the work. That's why parting on the back side works so much better than parting on the front side.
 
mickri said:
As Mikey educated me on this awhile back on the front side the parting blade is forced down and into the work whereas on the back side the parting blade is forced up and away from the work. That's why parting on the back side works so much better than parting on the front side.
Click to expand...
This doesn't make sense to me but I'm an idiot. If your parting tool is on center then it will be pushed down and away from the work. I believe that the benefit is that the back mount is more rigid the compound. Lathes are designed to have the force pushed down on the carriage, not pulled up by cutting forces. I made a solid toolpost and my parting woes mostly went away (9x19 and 10 x 22 lathes). I am probably wrong though (see the first sentence of the post).
 
Think of a rocking motion with the fulcrum being the base of the tool post. As the blade is forced down there is a upward force at the outside end. This causes the tool post to lean into the work forcing the blade into the work. The opposite happens when parting on the back side. Here the blade is forced up which forces the outside end down. Now the tool post is leaning away from the work pulling the blade away from the work. We are only talking about a few thousands but that is enough movement to cause the problems with front side parting.
 
mickri said:
Think of a rocking motion with the fulcrum being the base of the tool post. As the blade is forced down there is a upward force at the outside end. This causes the tool post to lean into the work forcing the blade into the work. The opposite happens when parting on the back side. Here the blade is forced up which forces the outside end down. Now the tool post is leaning away from the work pulling the blade away from the work. We are only talking about a few thousands but that is enough movement to cause the problems with front side parting.
Click to expand...
Now it makes sense. Thanks for the clarification.
 
akjeff said:
Thanks for the info @mikey ! Now that I have converted my Craftsman/Atlas 12" to the MLA A-11 cross slide( coupled with a solid plinth tool post was a quantum leap forward in rigidity ) I'm considering getting MLA's rear mounted tool post for parting and knurling. Just trying to wrap my head around the physics as to why a rear mounted tool post makes for more rigid parting, than the standard way? I've read too many accounts as to its effectiveness, so I'm confident that it in fact works.........just trying to understand why?
Click to expand...

How a rear mounted parting tool increases rigidity has been debated for many years, sometimes quite energetically. Even Hemmingway, the tool supply guys in the UK, don't know for sure. My take is in agreement with Mickri and I'll try to explain why I think of it this way.

Over the years, I have learned that my intuition is pretty good but it is also sometimes pretty wrong so I've learned to test things instead of assume that my intuition is correct. The difficulty for me is that I don't have the test equipment I should be using to test something definitively. That is, I don't own strain gauges. Instead, I have dial indicators so that is what I used.

Now, my intuition tells me that a rear mounted tool post with an upside down cutting tool acts as a fulcrum. Since the tool post is locked down solidly to the rear of the cross slide and since the tip of the tool is being pushed up as it contacts the rotating mass of the work, forces will be transferred to the rear of the cross slide. This motion takes up any clearance between the cross slide and the bed of the saddle and forces are thereby transferred down into the structure of the lathe.

In contrast, when parting from the front, the tip of the tool is lifted up, thereby increasing any clearances there might be between the cross slide and underlying saddle. On some lathes, like my Emco that has near zero play in these structures or big lathes that have enough rigidity to resist these upward forces, parting from the front is a non-issue. However, on lighter lathes or lathes with some wear in the cross slide to saddle interface, there may be enough movement to cause instability in the tool and that results in chatter as the front of the cross slide lifts with the cutting forces.

To test this, I used a Brown & Sharpe 0.0001" dial indicator to see if I could detect movement in the front and rear of the cross slide of my Sherline lathe at the moment the parting tool touched the work piece. I found a very tiny amount of positive movement of the needle in the front and a tiny negative, almost imperceptible, movement in the rear, respectively. My Sherline lathe is very tight despite its age but this is what I saw. Again, this is using the wrong equipment to test this scenario properly but it did confirm my intuition about what is happening, at least enough to satisfy me.

There are all kinds of reasons why this is not a valid experiment but you know what? None of this really matters because I, and many, many others have experienced the same thing. Rgardless of how it works, rear mounted parting tools do work and they work well. I have yet to see a report of someone who has had a negative impression of this tool. I swear that if I had doubts about mounting a tool like this I would NOT take such a strong position on it.

Anyway, that's my take on it. Hope it helps.
 
Mikey, you are always so generous in sharing your wealth of knowledge and experience and are also excellent at articulating it. Thank you so very much.
this leaves me much to think about. Parting was a frightful experience on my old 10” Rockwell lathe. My 15“ “South Bend” lathe from the ‘70s parts very well conveniently, enough so that I most often do parting under power feed.
as a very young man and more than a half-century ago, I worked after school at a screw machine shop and sometimes operated a small turret lathe with a lever action cross-slide that did the last parting operation when pulling the lever to move the cross-slide towards the operator side (basically just what you are suggesting).
i presently have a challenge I haven’t solved which would be perfect for that arrangement where facing could happen conventionally and parting could be done “on the back stroke”.

My apologies for being a thread “derailer”. unfortunately my brain flies off on tangents all the time and enjoys it!
 
You must log in or register to reply here.
Back
Top