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Setting up rear parting tool properly

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William Ayerst21/07/2021 15:24:27
259 forum posts

Dear all,

I'm using a rear toolpost parting tool and it's generally very pleasing to use - but one thing I've noticed is that occasionally I will part off some work and notice that one of the faces is very slightly convex.

To me, this indicates that the tool is crooked in the toolpost, or the toolpost itself is crooked. Now, I'm using a toolpost which has a very positive key onto the t-slots of my cross slide, so I think it's less likely to be that, and more likely to be the way the tool is held.

The toolpost has a boat shaped cutout on the bottom, onto which sits a boat with grooves ontop. The tool sits on the boat, and then there's another plate which sits (seems to sit?) between that and the adjustment screws.

However I seem to secure the HSS tool, when clamping down it always seems to twist - either away from perpendicular, or rotating around its axis.

Clearly, I am doing something wrong here!

Rod Renshaw21/07/2021 15:47:46
301 forum posts
2 photos

There was a recent thread on this issue recently, see "Parting off Problem" which seemed to cover all aspects. Also I can't make entire sense of the photos. Is the OP using the wrong end of the parting off blade or is the blade sloping "down" towards the work when it should be either level or sloping"up"? Everything is upside down on rear toolposts, of course,


Tony Pratt 121/07/2021 16:25:29
1648 forum posts
8 photos

The first picture looks 'wrong' to me, the tool boat should be nearly horizontal, maybe like that to get the centre height right? As it is the 2 clamping screws are going to have edge contact only & will not be very secure. More detailed advice from others will follow I'm sure.


William Ayerst21/07/2021 17:00:09
259 forum posts

Rod, this is a REAR toolpost, so the cutting edge is facing downwards. I guess the angle achieved to reach the centre line is increasing the rake, though?

The tool boat is a Myford one (the new Myford, not original) and the tool is 8mm - if the tool is placed on the boat and the boat is flat/horizontal, then it's well below the centre-line of the lathe. Maybe that's what the extra packing piece (currently ontop of the tool) is for?

Edited By William Ayerst on 21/07/2021 17:00:51

JasonB21/07/2021 17:10:23
21315 forum posts
2419 photos
1 articles

Certainly looks wrong to me and I don't even use arear tool post, the tool should be horizontal or the whole thing pointing downwards slightly towards the work. That way you don't need to grind that nasty notch into the tool, it's left flat and the incline of thetool gives you a few degrees of positive rake.

Rod Renshaw21/07/2021 17:24:58
301 forum posts
2 photos


Yes I get that, but the slope determines the rake which in turn has a big effect on the quality of the cut. Have you tried packing the blade to get it near center height without needing that slope? You have try to picture the set up as "seen" by the work, which is not easy to visualise at first. Good luck.


SillyOldDuffer21/07/2021 17:25:43
7482 forum posts
1657 photos

Hmmm, got to say I don't care for those boat adjusters. Sworn at rather than sworn by! They're prone to wander, fiddly to set to height and mess with the rake.

Wandering will be worse used that way round because the cutting forces lift the boat and bear on the two fixing screws, which are also canted where they press on the spacer, yuk. If the lathe is run in reverse, with the tool the right way up, the cutting forces press down and the post is more rigid. Alas, Myford lathes aren't top dog for rear parting because the chuck is likely to unscrew when they run in reverse.

I'd dump the boat in favour of a flat rear tool post which is shimmed to height. The plain post eliminates a lot of problems, even used upside down. (I expect to be told boats are easy when you know how. Even if it's my fault I don't trust 'em.)

Parting off tends to wander at the best of times. The tool has to be straight and stiff.


Howard Lewis21/07/2021 17:42:26
5237 forum posts
13 photos

The tool has a top rake ground into it and is then mounted in a toolpost which provides further rake..

I would suggest minimising rake, and just setting the tool on the centreline, with minimal overhang to maximise rigidity.

If the face is concave, or more likely convex, the blade is flexing, which suggests that the end of the end of the blade has not been ground absolutely square, and possibly the feed rate is a little too high..

I used to grind the end of mu blade at an angle, to avoid a pip on the work, but found that the angle caused the blade to flex, and because the swarf produced is wider, a greater tendency to jam in the groove.



Alan Jackson21/07/2021 17:55:05
231 forum posts
115 photos

This is much more rigid than the boat toolholder. The base block has a shallow slope enabling the cutting tool height to be adjusted.


rear toolpost2.jpg

duncan webster21/07/2021 17:59:22
3456 forum posts
63 photos

The face of the tool holder shown in the top picture should be facing the chuck, so it looks as if it is set up for the lathe going in 'forward'. This confirmed by the second photo. No chance of the chuck unscrewing, but the tool is canted the wrong way, hence the need for the notch. I think this is simply the wrong tool holder for the job. Boat tool holders have a very poor reputation anyway

not done it yet21/07/2021 22:51:40
6279 forum posts
20 photos


The rear tool post shown by. A J is a far preferable set up. Mine,I think, is even better. It is of fixed height - made that way for my particular lathe, I expect. I can remove or replace the whole caboodle, without fear of altered cutter height.

With the blade always held parallel to the cross slide, any extension (or retraction) for parting different diameters is a simple affair. I always keep the extension to a minimum. Nor does sharpening the blade alter the cutting height.

I originally purchased a QCTP with several tool holders (supposedly better value by an amount, but not in hindsight). I wish I had not because the parting tool holder was at an angle - just a PITA in use! I’ve since modded that particular tool holder for a different purpose.

As an aside, I’ve never used the push-type knurler, either, which was also bought at the same time - as part of a set of tools to fit that QCTP). I was caught out twice in the same day, but won’t make the same mistake again.

Trumpet / Flugel21/07/2021 22:52:06
6 forum posts

Hi William. I might be wrong but I think your tool is marked as an "8mm external grooving" tool. This is for making external grooves of a modest depth. A more usual parting tool blade is shown in Alan Jackson's photograph. Trying to part off with your tool may be a challenge too far.



John Baron22/07/2021 07:57:16
487 forum posts
189 photos

Hi Guys,

This is a picture of my "Norman" patent rear tool holder with a 12mm X 2mm blade in a holder. It is horizontal and set exactly at centre height. The tool holder can be used on the front tool post as well.


Howard Lewis22/07/2021 08:35:10
5237 forum posts
13 photos


One of the advantages of using a rear toolpost is that the swarf falls away downwards, and is less likely to jam in the cut, to cause problems..

The consensus seems to be:

Mount a proper parting blade, with minimal overhang, square to the lathe axis, the front clearance ground square to the blade, with Zero top rake and at centre height, in a rigid toolpost.

The set up that you show satisfies very few of those requirements.

Like S O D, I dislike boat type tool holders; they are a compromise with ease of use as the priority, with rigidity and correct tool setting a long way down the list of attributes.

If you want to see a rigid rear toolpost, take another look at Alan Jackson's, or at MEW 306

The very least that you need to do is to set the upper face of the boat to horizontal (Preferably dowel / clamp in that position )

Get a proper parting blade, don't grind in any top rake, clamp it square, shim it to centre height, with minimal overhang, and then try again. Don't let the tool rub, just keep up a steady gentle feed.

Do those things work? Lots on here will tell you that they do.

Some of us with Power Cross Feed even part off under power!

Let us know how you get on.


William Ayerst23/07/2021 09:43:31
259 forum posts

I re-set the grooving tool with the packing piece between the boat and the tool, instead of ontop of the tool - and it was almost bang-on centre. It is still flexing a bit - for example if I part out halfway then I can see the tool generate a little swarf on re-insertion.

I have a parting blade tool and holder (the kind that is inserted into a normal toolpost, rather than integrated into the toolpost) but the tip of the blade is way over the centre height, so I've had to grind down the top of the tool so it's got a chance in hell of cutting. In this view, it looks like I've inadvertently got some positive rake...

It looks like I need to fabricate myself a dedicated 'blade'-type rear toolpost holder like Alan or John? I can't seem to find anywhere to buy one!

JasonB23/07/2021 10:06:03
21315 forum posts
2419 photos
1 articles

William, as your tool bit in the second photo is parallel along it's length it will not suit any degree of positive rake as the tool will become wider the more it is fed into the work which is not good. It needs to be ground flat or with slight positive top rake.

Better still the tool holder can be modified by milling some material off the bottom of the bit that fits into the tool post so the whole thing is lower.

Either way that grinding looks a bit rough and the edges rounded, you want the corners nice and crisp.

Rod Renshaw23/07/2021 12:11:41
301 forum posts
2 photos


Your new set up looks much more promising. But as has been said the grinding does not look good. I wonder if you need a finer grit wheel.

Arc do "Parting tool blocks" for holding blades which might suit if the dimensions are okay.

Also, your original set up with a square section tool can work okay if you get a proper HSS parting tool
(ie a parting tool shape ground on the end of a piece of square section HSS, all the ME suppliers do these) rather than the grooving tool you were using. All provided you pack it up (down?) to centre height and near horizontal. Blades are more versatile but if you only need to part off smallish stuff the square HSS parting tool works okay and might be easier to fit to your holder.

Boats are not popular, as has been said, but the one you have is more rigid than the original diecast myford offering. I think the newer ones are hardened steel so may not be easy to drill for dowels. Parting tools, once set, are often left alone for extended periods so it may be okay to just adjust the tool, packing and boat, then clamp up tight, and not fix the boat at all. You may have to experiment with these things.

Good luck!


William Ayerst23/07/2021 14:23:02
259 forum posts

Hi all,

Some more experimentation including fixing the rake on the front parting off blade:

It parted off without much fuss at all - so maybe it's as simple as that? I do see some other holders for 8mm blades but it looks very similar to what I've now got above? i.e. ebay 265202611958 or 373410105832 - is it worth getting one of that design intead or am I going to be alright?

Rod, the tool I've got is shown as a grooving tool on the etched markings, but is the same as those sold as parting off tools (from square tool steel) I can see anywhere - am I missing something? i.e. ebay 402994146273



Edited By William Ayerst on 23/07/2021 14:24:51

Howard Lewis23/07/2021 15:58:54
5237 forum posts
13 photos

Whether a grooving tool, or a parting tool, each SIDE of the tool needs to have clearance. (Which is why a parting tool narrows from top to bottom if you look at it from the front. When mounted inverted in a rear toolpost, it narrows from bottom to top )

Assuming that you intend to continue using the boat holder:

Rather than grind the top face of a parting tool to bring it down to centre height, I would grind the whole length of the bottom face. If need be make up a holder with a deep enough groove so that when the holder contacts then grinding wheel, the tool is at correct depth. In this way, any errors will have minimal effect on the angle at which the tool sits.

You could mount the Boat in the 4 jaw chuck and turn the required amount off the flat face on which the tool sits. This might be the more accurate way of lowering the tool to centre height.

How much to remove?

Since the blade is tapered, you will need to make a clamping piece with a taper to match that of the blade so that when clamped the wider, cutting face, is horizontal in BOTH planes.

The clamping screws will bear on the upper face of this clamping piece.

If you do not have milling capability, you will need to file the taper. The blade can act as your gauge.

Mount the parting blade on the boat, using your new clamping widget, (to cater for the taper ) and measure the distance between tool tip and centre height. If in doubt take off a little bit too much. You can shim UP but not DOWN.

(Although the tool is mounted inverted, the logic is still to start with the tool below centre height and shim to bring it UP to centre height, just as in a front toolpost )

Remove too little and it will still be below centre, but cannot be shimmed to bring to centre height..

Grinding a parting tool with the front face at an angle (viewed from above ) should allow parting off so that there is no pip on the parted off piece. BUT the swarf is wider than the groove being machined, and so liable to jam, particularly with large diameter workpieces as the groove deepens.. Hence the advice to grind the front square to the blade. So when presenting the blade to the wheel to grind the front clearance, keep the blade at right angles to the wheel. The radius of the wheel will provide front clearance, without it being excessive.

Excessive clearances leave less metal behind the cutting edge to conduct away the heat so that the tool heats up, softens, and wears more quickly.


Howard.  Predictive spelling!!!!!!!!!!

Edited By Howard Lewis on 23/07/2021 16:02:03

William Ayerst23/07/2021 17:30:14
259 forum posts

Hi Howard,

Both the grooving tool and parting blade have side clearance. The grooving tool in the boat is about bang on centre height with all the bolts nipped up now - thank you for the tips though, I'll file those away for future use!


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