> There is a drawing error on Fig 5 in that the parting tool should be inverted and to the right of the workpiece, but hey-ho these happen.

That's how the drawing was supplied and it illustrates the measurement the author made.

The differences could be down to lathe type or the individual setup. I am not convinced the place where the toolpost is attached to the slide is critical – if the toolpost and slide are very rigid and effectively act as one lump of metal. Reg may have a less rigid setup, and that may be why it works for him.

I think the real question with rear toolposts (which obviously do work) is that in use they try and lift the rear of the cross slide off the bed. This puts pressure on a normally no-critical bearing surface and also they apply a large twisting moment at the front guide acting to separate the main bearing surfaces, the opposite of the normal action. The lower front bearing surface has to take the largest load of all,.

Should be a recipe for disaster, but it isn't – why?

A second thought – surely in normal turning the forces on the tool are the same as on a front mounted parting tool…

I am sure the parting debate will run and run!

Neil

Edited By Neil Wyatt on 20/01/2015 11:05:10

I don't think there is any great mystery. It must be simply down to the design of the actual machine and the severity of the load. If you take a very light cut with a razor sharp tool you will barely load the machine whereas if you take a full cut with an indexable tool, you are going to want the saddle to be fully constrained against being lifted by the cutting forces.

Never owned a Myford but it seems they can tolerate moderate upward loads in typical ME useage. I've looked at the design of my Bantam and there is no obvious means of constraint. Interestingly, the Colchester rear toolpost is designed for operation of the machine in both directions although I haven't bothered getting one.

Personally, I don't feel the need for a dedicated rear toolpost, as I can swap tools quickly easily with the QCTP and keep the machining area clear of projecting tools – surely that is its primary benefit? However, I often machine parts with the tool behind the centre line (usually a boring bar), with the machine running in reverse. TBH, it seems to me that if you are going to make use of a rear-mounted tool, you may as well run it in reverse and keep the tool loads in the direction the machine was designed for.

Murray

.

Aahh you mean a finishing cut ?

Wish I had a fiver for every time there's a thread on parting! Using a rear mounted parting tool there's no need to reverse the motion as the tool is inverted, you only reverse when using an inverted parting tool in the front tool holder and yes this could spin the chuck off under some circumstances.

I have a sub-Gibraltar toolpost for my mini-lathe that fits on the t-slotted slide. A simple cube of meehanite that has a twin-screw t-nut to attach it and takes the standard toolpost on top. It's very good but annoyingly I got the height slightly different so I have to use different shims for each tool :-/

Neil

Neil assuming you made the bit of iron too small then why not cut a shim the full size of the block and fit it under the toolpost so you don't have to use two sets of shim for each tool?

J

PS it would have looked better painted yellow

> Nice T-slotted topslide for the mini-lathe Neil.

> Any more details? – Enquiring minds need to know!

Yes – if you want to keep your sanity, don't do it. I chewed it out of a block of CI making the bottom to match they existing slide (plus recess so it can come further back over the index) and then three t-slots in the top.

Would be an easy job on a mill, but I used a taig/peatol vertical slide…

Jason

I hate unsecured shims, actually with most tools it takes a 1/16" shim so it's not a big issue.

The green is good old Humbrol Brunswick green I use for various home-made accessories I do have a full tine of (old formula) yellow smoothrite that matches the lathe and one day I will do a full repaint!

More pics here

Neil

Edited By Neil Wyatt on 22/01/2015 09:40:26

I was asking for help re parting-off only a couple of years ago, but here is what I have found, – My lathe has 3/4 hp motor but not really enough power for parting steel with an insert tool. HSS tool ground with side clearance and 2mm wide or less work well. But cannot have top rake ground or the clearance is lost. I ground a bit more front clearance and tilt the cutter to get some top rake. Like IanSC I take an extra side cut to widen the groove when needed. To me it seems obvious that the more joints they are in the set-up the less rigid it will be. I have been looking for a bit of heavy steel to make a one piece toolholder and not use the top-slide. I don't have slots on the cross-slide, just 2 x 6mm bolts to hold top-slide or toolpost, so need a flange for mounting,hence steel so it can be welded up solid. Do not use shims if possible, one thick one if must, just an extra 2 joints. Slow speed, slower than normal, just to keep down vibration which is difficult to stop once started.

Chester DB8, older belt drive ( which I like ) 8" dia. max. The toolbit is 3mm wide I think, can't get at it now. No roof and 2" ice. 400 revs ok at 1" dia maybe but 200 better ie less vibration. A "V" section tool is easier on steel. I 'm not into mass production tho'. Every thing is better if all gibs etc. are tight but not always very practical.

I can't believe I'm reading the confusion in this thread… The late, lamented George Thomas wrote pretty much all anyone could ever need to know about parting off (and many other topics) in his many comprehensive Model Engineer articles and his books. Perhaps it's time for a re-run?

Merryweather's article is BAD and MISLEADING (yes, I'm shouting!). It may not be his fault – perhaps it has been mangled by editing or production errors, but it doesn't belong in anything with aspirations to be an authoritative journal. As printed, his explanation is faulty. What determines in which direction a tool is deflected under cutting loads is the relationship of the direction of the resultant force on the tool to a line joining the cutting tip to the centre about which the tool and its holder, toolpost, etc. can rotate. (By rotate, I mean move by flexing. It will certainly flex – nothing is infinitely rigid, and Myfords certainly ain't!). A rear parting tool, inverted, with the lathe running in the 'normal' direction is successful at avoiding dig-ins because, as it is loaded, it tends to be deflected up and away from its point of contact, thereby relieving the forces upon it. Just think how it would move if its toolpost were hinged to the cross-slide. Similarly, but the opposite, for parting-off with a tool in the 'normal' toolpost.

The article is not clear. Merryweather's Fig. 5 bears no relationship to the photos of his toolholder set-up. I THINK he must part off with his lathe running in reverse: there seems to be chip build-up on top of the tool (photo 3). This, of course, isn't a good idea for lathes with screw-on chucks… If this is correct, photo 3 suggests that the tool and holder will be deflected clockwise, pivoting about its mounting to the cross-slide, and therefore relieving the cut, as load is applied. If this is the case, the set-up could be reversed, with the tool-holder mounted 'in front' of the spindle, and the lathe run in the 'normal' direction. It is, in fact, more-or-less a re-invention of the 'spring tool' (look it up), although rather less springy….

Another reason given for the success of rear parting-off tool-holders, especially on non-rigid Myfords, is because the tool loads lift the cross-slide, minimizing dovetail clearance, and tighten-up the whole shebang, reducing chatter likelihood. This doesn't apply to inverted-vee beds, of course.

Incedently..if tool crabs sideways towards headstock. .that would. Improve situation in the FRONT mounted tool..
Which is the situation for those who have crosslides that are set for concave facing…

So I don’t know what conclusion that leads one to…
…