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Parting off with a 5/6" toolpost

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William Ayerst17/11/2020 09:48:08
259 forum posts

Good evening gents, I've been playing around with my lathe in order to get familiar with how it works and what I can do with it, and I've realised I've got a bit of a quandary.

I'm using the Myford toolpost, which accepts 5/16" bits.

Using an 8mm knife or chamfer tool is fine, but when I try to use an 8mm parting bit the work jumps around, because 8mm brings the tool above the centre line.

I have some 5/16" carbide bits, but they don't include a parting off tool, only a very wide grooving tool, which I can use in a pinch, I guess.

I've asked Father Christmas for a grinder and some 5/16" HSS blanks, so presumably I can make my own parting off tool, but I see many people with blade-type tools and I wonder if that might be a good idea? I'm not sure where to start with regard to finding one that would fit at the correct centre-height on the toolpost.

Would this work?

Many thanks!

Dave Halford17/11/2020 10:16:50
1820 forum posts
19 photos

RDG claim it fits the post, however the blade is already 5/16 without the holder height.

Nigel McBurney 117/11/2020 10:20:00
947 forum posts
3 photos

Why not just grind off some material from the top face of the 8mm tool,no tool will cut correctly if set above centre,a tool below centre will cut as front clearance is increased and top rake is reduced.A blade type tool saves a lot of time as it only needs grinding on the front face,though a hand ground parting tool from hss toolbit was very common years ago,it does have the advantage of side clearance on both sides of the tool which can help with difficult work.

SillyOldDuffer17/11/2020 10:31:19
7714 forum posts
1705 photos

⁵⁄₁₆" is the maximum tool capacity of the tool-post, and 8mm being slightly larger may be too much.

Although the discrepancy shows up most with parting-off, which is I think is by far the most difficult basic lathe operation, it's likely your other 8mm tools are too high as well.

The cutting point needs to be on or just below the centre of the lathe's spindle axis. Even slightly too high causes the tool to rub rather than cut; it may look as it it's OK, but not really - the tool will quickly go blunt.

I started lathe work assuming that cutters of the same size just plugged into the tool-post. Not so. Safer to read ⁵⁄₁₆" as UP TO ⁵⁄₁₆" and select smaller shanks, which are shimmed up to put the tool point at right height.

My lathe will accommodate 12mm tools, which I use for rough work. I actually prefer 10mm shanks most of the time, and have 6mm shanked tools for fine work. Hefty shanks bend less than small ones, which is good, but they take up more space, which is bad. Much depends on the type of work being done.

Parting off is notoriously difficult and I recommend learning ordinary turning and facing first to get a 'feel' for the machine. Parting off with a small lathe is fussy squared. Tool at correct height, bend and flex minimised by locking all unused slides and reducing overhang, careful lubrication and swarf removal, and above all a steady but not too fast feed rate. Parting off is easier on a big lathe with a hefty rear tool-post and power traverse. Hesitant owners of small manual lathes will have a torrid time, especially if the lathe isn't snug.

Don't be put off by early failures; one problem is parting is counter-intuitive in that the correct response to trouble is usually to go in harder rather than back-off. Except when the tool digs in!



Edited By SillyOldDuffer on 17/11/2020 10:32:24

William Ayerst17/11/2020 10:32:11
259 forum posts

I don't yet have a grinder, so I guess for now I'll be creating more chips with the brazed carbide grooving tool, and then make them myself with HSS. Cheers both!

peak417/11/2020 10:47:06
1545 forum posts
165 photos

I would look towards obtaining or making a rear parting off toolpost.
It uses an inverted parting tool, and will make your like a lot less fraught.

Whilst looking for a net photo to illustrate my point, I came across This Thread on here.


John Olsen17/11/2020 10:47:29
1216 forum posts
92 photos
1 articles

A bench grinder would be a really good thing to ask Santa for, or save up for one if you can. Carbide is all very well, but with HSS you can grind special tools that are not available in carbide.

One thing that helps with parting is using the narrowest tool that is feasible for the job. With my Myford ML7, I find that a tool about 3/32 wide, or about 2.4mm if you prefer, is about as wide as it is comfortable to use. Younger and more rigid machines can use wider tools, making it also possible to part off larger diameter stock. On my Unimat, I have done a lot of parting with really narrow tools, about 1.5mm or a 1/16" being quite useful on that. Such a tool is off course only useful to about half an inch diameter, but that is plenty on such a small machine.


SillyOldDuffer17/11/2020 10:53:48
7714 forum posts
1705 photos
Posted by William Ayerst on 17/11/2020 10:32:11:

I don't yet have a grinder, so I guess for now I'll be creating more chips with the brazed carbide grooving tool, and then make them myself with HSS. Cheers both!

For what it's worth I have a low opinion of brazed carbide tools. They have their purpose and are cheap, but in my workshop they combine the disadvantages of both HSS and Carbide Inserts, without having their merits either.

  • Booby trap learned on this forum; new sets of carbide brazed tools may not be sharp! They need touching up on a wheel.
  • The main disadvantage of HSS is the need to buy a grinding wheel and learn how to use it.
  • Ordinary grey wheels required to grind HSS are too soft - carbide needs a special Green Wheel.
  • Grinding loses the reset convenience of carbide inserts. Inserts can be swapped without disturbing the tool-holder, and pre-shimmed to go straight into the tool-post. HSS always requires a certain amount of adjustment, which is why folk like Quick Change Tool Posts.
  • Carbide works best pushed hard at high-speed, which may not be possible on a hobby lathe.

I prefer inserts but switch to HSS when the job calls for it, usually to get better finish. Brazed carbide never gets a look in.


Clive Brown 117/11/2020 11:36:48
724 forum posts
34 photos

If you have a lathe then +1 for getting a bench grinder ASAP. I can't imagine a workshop managing without one. These days the national tool-retailing chains sell them at amazingly low prices. Not the best quality but perfectly OK for the hobby workshop.

There's lots said about insert tooling, it has it's uses, but HSS is essential for the model-maker IMHO.

William Ayerst17/11/2020 12:00:21
259 forum posts

My plan was to use HSS all the way - I would like to keep it 'period' in my workshop as I have it to the back teeth with technology on my day job - but the lack of 5/16" ready made HSS tool sets scuppered that plan until I get a grinder.

With regard to the rear toolpost I see them come up on eBay from time to time, but isn't it the same issue? I.e. I won't know what size parting tool to get to fit into one?

Ronald Morrison17/11/2020 12:21:12
72 forum posts
4 photos

You might have to do some talking with the seller and pay shipping since this offer is free shipping to the US only but this set of 5/16" tools might be what you have been looking for.

Edited By Ronald Morrison on 17/11/2020 12:21:57

SteveW17/11/2020 16:50:17
135 forum posts
11 photos

Tools that come as part of a set are very often much too wide for a hobby lathe. In my view 3mm is just about OK but less makes things a lot easier. Too thin and things get weak.


Howard Lewis17/11/2020 17:29:36
5562 forum posts
13 photos

If the cutting edge of ANY tool, (except perhaps a shear tool ) is above centre it will not behave properly.

It will merely rub, rather than cut. A tool cuts when the pressure at the line contact between tool and material is such that the workpiece shears. The flat surface of a tool above centre will not produce a shearing action. At best it will burnish, but that is not the purpose of a parting tool..

Parting off, using a rear toolpost is the better method, if a rear toolpost can be fitted and accommodated. (Not all Cross Slides provide for this, sadly ) There will now be discussions about the upward forces from an inverted tool in a rear tool post pressing the Cross Slide against the dovetails, and stabilising the whole combination.

Capstan lathes almost always used the parting off tool in a rear toolpost.

I just accept that it works far better than parting off in the front toolpost. With a screw fitting chuck, running in reverse with the tool inverted in the front toolpost is not really a safe option, unless the chuck has some means of preventing it being unscrewed by accident. This may upset the purists, but my parting tool has Zero top rake. In a rear toolpost, dig ins are fairly rare, and never catastrophic (That IS asking for trouble! ).

Rigidity of the whole lathe, and the tool holding arrangements have a great bearing on the matter.


William Ayerst18/11/2020 09:56:25
259 forum posts

Well, the height-over-centre is 0.02mm on an 8mm bit compared to a 5/16" bit - but I do understand, which is why I've got a set of 5/16" blanks and a grinder winging their way to me, as well as the 5/16" carbide bits.

As it pertains to this particular discussion about parting off tools however, it seems the two key considerations are a) buy/grind a proper height parting off tool as this is even more essential that the height is correct) and b) consider a rear toolpost.

I can find a few Myford rear toolposts on eBay with a toothed, semi-circular key underneath which presumably allows a nice adjustment for centre height. I'll go for one of those.

It seems that there's no source for the 'blade' type which would natively fit in my front four-way toolpost?

John Hinkley18/11/2020 10:23:14
1199 forum posts
393 photos


It would be a bit of a "bodge" and only a temporary fix, but, as the height difference is minimal, could you not put a thin spacer under the rear of the tool to tip the business end down a tad so that it's on or slightly below the centre. It will marginally also affect the tip clearance angle geometry but not significantly. It might get you out of your predicament until such time as your new toys arrive. After all, I believe that Myford employed a rocking "boat" tool holder to similar effect.


Mick B118/11/2020 11:43:19
2047 forum posts
117 photos
Posted by William Ayerst on 18/11/2020 09:56:25:


It seems that there's no source for the 'blade' type which would natively fit in my front four-way toolpost?

I came to the same conclusion on my Speed 10 when I replaced the rocking-type toolholder it came with, with a 4-way. I'd found the RDG HSS blade-type holder so useful that I used the vertical slide to mill one of the 4-way positions lower so as to take it.

Henry Artist18/11/2020 12:11:42
121 forum posts
46 photos

Ditch the 4-way tool post, it's just three extra ways to cut your hand. Get a Quick Change Tool Post instead.

With a QCTP it's easy to set the height of a tool and there's no more faffing about with shims. Also it would give you a bigger choice of tools that you could use on your lathe.

It can really improve your enjoyment of your lathe and quality of life in general. YMMV.

Maybe it's time to write another letter to Santa? Don't forget to ask him for extra tool holders - you can never have enough... Ever wink 2

Howard Lewis18/11/2020 12:55:25
5562 forum posts
13 photos

Storing many extra tool holders is why my lathe carries a four way toolpost at front and rear. The percentage of cuts to the hand is very small, I just move the Saddle well away, or be careful, and learn to live with what you have.


OldMetaller18/11/2020 17:24:37
196 forum posts
25 photos

I bought one of these:

Sorry it won't paste as a hyperlink.

I use it in my four-way tool post on my ML7 and it is fantastic! For bigger jobs I use RDG's rear tool post, but for the smaller stuff I make, the Arc tool is absolutely unbeatable, and inexpensive.

Best wishes,


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