Rear mounting parting-off tools

[Tony AParticipant @tonya]

I have read, on a number of occasions in articles in MEW, parting-off using a rear mounted tool.   Would someone like to comment on the advantage of using this technique over the front mounting of the tool.   I use a QCTP so swapping tools is not a pain.

[Tony AParticipant @tonya]

[Michael Cox 1Participant @michaelcox1]

Hi Tony,

There are two main advantages to a rear mounted parting off tool. Firstly, with a conventional front mounted tool, if it digs in the tool tends to go under the work piece and is wedged between the workpiece and the topslide because the lathe is very rigid and this tends to make it dig in deeper. If the same happens on a tool mounted in a rear toolpost then the tool tends to ride up on top of the toolpost and the toolpost is deflected away and hence the tool tends to “undig” itself.

The second advantage of the rear mounted toolpost is that the chips tend to fall away rather than collect in the parting groove so there is much less likelihood of the tool jamming in the slot due to chips wedging between the tool  and the groove.

If you have a flange mounted chuck and can reverse the drive then the same result can be had by mounting a parting tool upside down on the front side of the lathe. If the chuck is mounted on a screwed mandril nose piece this cannot be done because the chuck will tend to unscrew.

I have fitted a rear toolpost to my Asian minilathe and the difference in parting performance is amazing.

Regards

Mike

[Tony AParticipant @tonya]

Hi Michael

 

Thanks for the answer.   It is so obvious, why didn’t I think of that.   Now to make a rear mounting toolpost.

Regards

Tony

[chris stephensParticipant @chrisstephens63393]

Hi Tony,

If you are going to make a rear tool post for parting, I would strongly recommend that you read GHT on the subject. The most important bit to learn from him is not the “vee” cut in the top surface, or bottom if rear mounted, but the 7 degree of top rake. This rake makes parting off a pleasure and makes sharpening a doddle.

The “vee” is really only needed if the parting tool has to be horizontal not “raked”

If I were to offer a bit of advice on parting, it would be use as narrow a blade as you can get away with and have as little of the blade protruding as you need. This does mean you might need more than one parting off tool, but it is worth it. There is no point straining to part off 8mm Dia stock with a 1/8″ wide blade, if you can use a 1/16″ one. The narrow blade, with rake and a drop feed of cutting fluid (I use CT90), will really help. 

I don’t have to remind you to have the blade EXACTL:Y at right angles to the work, do I?

chris stephens

PS I do all my parting from the front and can’t remember the last time I had a dig in. Having said that, I predict I shall have one tonight . One should never tempt the workshop Gremlins, but Hey, I’ve a good day so far.

[mgjParticipant @mgj]

I know thats the conventional wisdom, but in fact if a tool is mounted a touch high on the front, or  a touch low at the back the tendency to dig deeper is just the same, because either way its being dragged into the work.

 

I struggled with a  couple of rear toolposts and used blades of 1/16. 3/32 and 1/8, and they were all just as darned bad as each other. (flood with coolant, cut wide slots, steep rake angles shallow rake anges and so on, front toolpost or back.) I could never, in 20 years, reach this nirvana that others seemed to, of being able to part off at high speeds and feeds.

 

UNTILL I bought a Kit Q cut from Greenwoods. And then looked at the geometry of that Sandvik tip, and then bifurcated with the Dremel the cutting face of ordinary HSS blades. The difference seems to be that the chip is actually rolled in on itself, so coolant gets in there and it never jams in the groove. (Nemett in ME recommended the same thing if I recall)Also with the commercial tipped blades, the carrier is significantly narrower than the tip so the back clearance is built in.  Not that that is essential – one of my tipped tools is actually designed for 3mm tips, though I am using 2.5mm wide tip, and the clearance is minimal.

 

Since then, tipped or HSS blades, I have never had a jam up, and the days of inching in, slow speeds and extra wide slots are gone. 600rpm, front toolpost with the Dickson type holders and feed in – and the bar parts off.2 1/2″ diameter bar, single deep slot, lots of coolant, straight in, mirror finish. (TE eccentrics) I’ve parted off 3/12″ bar in a single slice, but that wasn’t an interupted cut of course.

 

You don’t HAVE to build a rear tolpost. I have a proper Myford one, and an Ivan Law designed one. You can have both free gratis and for nothing if they;ll fit your lathe. They really are that useful (and have gathered dust for 4 years!!) But if you want them, by all means have them, but when you fnd out how useless they are, please don’t post them back.

 

I suspect that the success of back parting toolposts has a lot more to do with chip clearance than geometry. Operating from the front toolpopsts should in fact be more rigid because all is being pushed down onto the bed. At the back it’s all being lifted and any slack will be magnified. And the chip clearance issue has been very effectively addressed by the modern tip shape  – these tips just fold it up, roll it up and spit it out

 

 

The only thing I do  is of course is to make sure that the toolposts is genuinely square to the work.

 

Glanze from Chronos also do a very nice tipped blade. In some ways its more versatile because it is a blade in a clamp type holder so you can keep it short for every day, and extend it when you need it. The Kit Q cut is fixed length, (but with less forward overhang)  but its Sandvik tips cut more freely despite being .5mm wider.

 

 

[chris stephensParticipant @chrisstephens63393]

Hi Guys,

Forgot to mention that the rear tool post in an excellent place for an upside down chamfer tool. I keep one in my rear compound slide all the time, yes, I have two compound slides, doesn’t everyone? Had meant to put a parting tool in it as well but it did not come up to my expectations.

chris stephens

[mgjParticipant @mgj]

Chris – as a matter of interest, are you using Quick change toolholders? If one is not, then I can see a use for a back toolpost, despite the problems of spiking both work and self with extraneous pre-set tools. (Depends on size of lathe I suppose, and I daresay one gets used to it too)

 

With any of the quick change types it’s the work of a couple of seconds to change a tool which “arrives” all set at centre height, so the back toolpost is probably much less useful.  – in my case my 60deg chamfer tool is also the bevelled face parting tool used to make sure whatever is being chopped off on the right comes off cleanly..

 

 

Edited By meyrick griffith-jones on 09/10/2009 21:53:32

[chris stephensParticipant @chrisstephens63393]

Good Evening Meyrick,

If I am the right “Chris” this time…..

I sure do use a quick change holder, over a dozen of them (not bragging, just stating the facts) and still not enough, three for parting tools, three for boring bars etc etc, well you get the idea. 

In the Myford though, I have stuck with the four way tool post, and make everything to be on centre for easy changes. I have got a Myford rear post, but not fitted anything in it yet. 

 I have only just started using this lathe in earnest, since junking the phase converter and fitting a VFD. Now a “pleasure” to use, but I have not made many “specials” for it yet. When I make tools for the Bantam, I now think about making them fit the Myford too.

Does this answer the question?

chris stephens

[John HaineParticipant @johnhaine32865]

I have a KitQcut and a rear-mount parting tool on my Super 7, the latter being one of the commercial versions bought from Kirjeng.  I have made a little scoop in the “top” face of the blade, using a diamond needle file, which seems to help make the swarf curl in on itself away from the edges of the cut.  I find both tools excellent and normally part off at the lowest non-bgc speed with slow power feed and a steady flow of cutting oil from a brush.  Never had a dig-in (fingers crossed for next time!). 

 

The KitQcut gets used when I want to use the topslide to accurately set the length I’m parting off, otherwise I normally

 

On a previous lathe, on the other hand, a **** 220 which had rather inadequate roller bearings in the headstock, I never ever got an HSS tool to part properly, and only the KitQcut would do it. 

 

John.

[mgjParticipant @mgj]

Chris – only you can say…….

 

I tried these rear tool posts in the Myford during my 20 year parting nightmare…. but I did find, compared with a larger lather, the RTPs do make it all a bit cramped.

 

John, you experience mirrors mine. I used a Dremel to cut that scoop and use the tool in the front toolpost. Result is the same.  I have collected up quite a few of the Myford/Dickson type tool holders for hte QC system, over the years – and I’m like Chris. You never seem to have enough, but that’s why I’m all front toolpost..

 

I reckon that kit Q cut is great. Actually I was offered second hand a standard Sandvik blade for peanuts, which takes the same tips. Idea was to drill a couple of holes in it, and mount it on a bar- same sort of arrangement as the KQC, and in theory cheaper than buying a Sandvik holder for the blade. This is when I found that the Sandvik steel was tough. But seriously tough. All the way through. I think RHA steel armour isn’t as tough. Having blunted the drill and resharpened it twice, I ended up having to buy a solid cobalt one. The system now works very well on the 6″ lathe, but for the convenience, I think the proper holder might have been better!

Edited By meyrick griffith-jones on 10/10/2009 11:35:58

[Graham PorcasParticipant @grahamporcas42677]

Tony

Don’t hesitate, if you have problems parting off then go and buy, borrow or make a rear tool post.

For years I struggled and usually resorted to the hacksaw until my father bought a Myford rear post and I had a go. It was a total revelation and since then I have had no problems, the hacksaw will never go anywhere near the lathe again.

I usually use carbide tips but also use HSS for specials. It takes seconds to bolt in place and as I leave the tool set at centre height no setting is required. I can part off 2″BMS in my pre cross feed Super 7 in minutes. Adequate cooling/lubrication is essential and I use a bottle mounted on a shelf above the lathe to give a constant trickle rather than the usual brush, this also helps with crud removal and leaves hands free for controlling the feed. Getting he feed rate right is not difficult and is best done by ear, the bacon sizzling sound is most satisfying.

Like Meyrick I am sceptical about the conventional wisdom explanation of not digging in but really don’t care as it just works, the really strange thing is that I now have now fear of parting off using the front tool post and this also now works, can anyone explain that?

Graham

 

[FunnyturnParticipant @funnyturn]

If you put lathe in reverse and turn parting tool the other way up, does this have the same effect as rear mounting? Seems the geometry would be the same!

 

Brian

[David Clark 13Participant @davidclark13]

Hi There

Would work the same but only do this if your chuck is bolted on and can’t unscrew.

regards David

 

 

[keithmartParticipant @keithmart]

Hi

I have a ‘conquest’ lathe and this is exactly what I do. It works fine.

Of course the chuck is bolted on on this lathe

regards

keith

Leeds UK

[David HaythornthwaiteParticipant @davidhaythornthwaite90386]

I Have tried them all for parting off on my Myford Super ML7B. Normal parting tool at the front often dug in. Rear Tool Post improved things greatly. Made GHT rear blade type tool post worked fine. Kit Q cut at front – even better. Finally fitted rear mounted QCTP from RDS tools and mounted kit Q cut upside down on a QC holder. Absolutely magic. Can part off washers under cross slide power with no problems. However it required me to make a stepped holder for the Q cut in order to get the blade height down to centre height.

QCTP at the back stops the rear blade getting  in the way of work and knuckles. Also allows chamfer tool to be fitted.

Hope this helps.

David

 

[CirclipParticipant @circlip]

Must have been due to living a sheltered life, but all the rear mounted toolposts of note that I have observed have had the tools mounted upside down therefore NOT neccesitating running the spindle in reverse.

 

 It’s then a very basic geometrical fact that any “Dig in ” pushes the tool AWAY from the rotation.

 

  Wonder if you’d made the RTP before shelling out lots of dosh David if HSS would have worked for you too?

 

  And in the words of a great man, it would take a F***wit to forget the rear post is there and get trapped on it.

 

  Regards  Ian FW Extrodinaire.

[David Clark 13Participant @davidclark13]

Hi There

I think that parting off with the tool in the rear post means that when parting off, the cross slide lifts up into the dovetail which makes it much more rigid.

regards David

[Michael Cox 1Participant @michaelcox1]

Interesting comment David and I have seen the same mentioned before. However, I cannot see that the fact that the cross slide lifts up into the dovetail is any more rigid than when the cross slide is pushed down onto the solid surface under the cross-slide.

The main reason why a rear toolpost is more effective is, in my opinion, that it is less rigid and in the event of a “dig in” the toolpost can deflect upwards and backwards to release the dig in.

With  a front mounted toolpost, in the event of a dig in, the tool is dragged round under the workpiece and it is jambed between the the workpice and the cross-slide causing it to dig in further.

Regards

Mike

[chris stephensParticipant @chrisstephens63393]

Hi Guys,

Can some kind person point me to the PROOF, not hearsay, that a Rear mounted parting tool is any less likely to dig in than a front one.

To my mind if a rigidly mounted cutting tool  is on centre height, thus meeting the work at a horizontal diameter, any deflection would be equal whether going forward/tool upright or backward/tool upside down.

If rear mounted parting tools are superior, could it be because they are kept permanently at right angles to the work?

If more machinists kept their tools sharp and correctly mounted, I feel sure a great many dig-ins can be avoided.

chris stephens 

[John HaineParticipant @johnhaine32865]

The explanations of the better performance of the rear tool that I used to read had more to do with effects of a headstock bearing with a little radial play rather than tool deflection.  Reasoning went that cutting with a front mounted tool would cause the mandrel / work to lift and move slightly forward, suddenly increasing the effective feed depth and causing the tool to dig in.  I don’t know if this is real, but that’s what a couple of explanations I’ve seen say.  In the same lathe, a rear tool would cause the mandrel to press down harder on its bearing which means that it wouldn’t move radially.  Now that most peopla are using better lathes maybe this is no longer true…

[David HaythornthwaiteParticipant @davidhaythornthwaite90386]

Regarding Ian’s comments, yes of course all rear mounted tools are normally mounted upside down. I am using the Q cut upside down on a REAR MOUNTED QCTP hence the need to mount it lower in a QC holder. I agree with you that it is simply a matter of geometry. At the front, any dig in forces the tool downwards, but as the fulcrum for this movement is further away from the work than is the tool tip then the downward movement also forces the tool forwards into the work making the dig in worse.

With a rear mounted tool. a dig in lifts the tool and as the fulcrum is now nearer the back of the machine than is the tool tip, the lifting action is now causing deflection AWAY from the  work, releasing the dig in.

I do not agree with Chris Stephens that at centre height the effect is the same front or rear.

He asks if there is PROOF. I would say a resounding YES The proof is that it simply does work better at the rear. If you have a Colchester or something of similar rigidity, the effect is less noticable, but certainly on lighter type of lathes like Myford. It simply works better.

 

David H

[mgjParticipant @mgj]

David – with respect the geometry front and back is absolutely identical.  If you are below centre at the back or above at the front, makes no difference, you will get a dig in. Even the position of the backlash is the same.

You actually have to work quite hard to get a dig in at the front if the tool is mounted below centre. This is because if the tool starts to dig, the cut is thinned releasing the pressure – and hte greater hte back rake hte worse the problem. The opposite is true if you are high, and tthat is reversible through 180 deg to apply to a rear mounted tool.

 

That is what makes me believe that that action and fulcrums (fucra?) are not the cause of the problem, but chip clearance and lubrication is?

 

Thats how these modern bifurcated tips work. you have a very small amount of edge cutting on centre, and the rest is below, and at the front being below centre tends to push the work outwards (and there is on the cutting edges zero back rake). They also fold up the swarf allowing lubricant in and prevent jamming. 

As for effects on bigger lathes. Fortunately I haven’t been involved personally in any, but I have seen the odd one or two. (Pre tipped tools)  It doesn’t jam the lathe – agreed. It just smashes the tool, rips it clean out of its holder and keeps on going.  But it is quite noticeable.

[David HaythornthwaiteParticipant @davidhaythornthwaite90386]

OK Meyrick we shall have to agree to differ on this one. I am still convinced that the effect shown on the attached illustration has a lot to do with it. I rest my case.

David H

[Michael Cox 1Participant @michaelcox1]

David, I agree with your analysis why rear toolposts are better for parting. Your drawing is excellent.

Mike

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