Parting Off MEW225

[Chris TriceParticipant @christrice43267]

Kiwi, I think you're being a bit over zealous in your criticism. Let's forget side forces for the time being. They're only relevant if the front edge of the tool is angled. Lets assume a tipped parting blade like a Glanze is being used. In the case of a regular front mounted tool, the forces acting down on the blade tip tend to lean the toolpost into the work. With a rear tool post, the forces tip the tool and post away from the work. The tool holder in the arcticle is shaped so that the base bolted to cross slide is in front of the tool tip so that forces applied to the tool lean the toolpost back away from the work as with a conventional rear mounted tool. I can't comment on how effective it is but the principles seem sound.

[Neil WyattModerator @neilwyatt]

My thought on the mounting point for the toolholder is that, once it's rigidly fixed to the topslide, the actual point of attachment is irrelevant.

With a front mounted tool, the front of the cross-slide is forced down onto the bed of the lathe with significant force while more modest force pushes the rear of the cross slide down. This is good and how the lathe is designed to take cutting forces

With a rear mounted tool, the topslide is being lifted up at the back of the cross-slide, and the force is shared between the top of the front of the bed and the bottom, of the rear shear. this is not how lathes are designed to work, and many will not have the rear guides set very tight. So why does this work?

I think its more simple and to do with flexibility in the toolpost causing dig-in at the front, and dig-out at the rear. Note how 'upside down' front toolholders see improved performance, even though the forces are lifting the cross slide off it's normal bearing surface!

I suspect that there are two fundamental solutions to parting problems:

1 – a tool mounted so that if it flexes, it tends to leave the work – negative feedback that will act to reduce oscillation (less chatter)

2 – more massive and rigid mounting arrangements that damp vibration (less chatter)

But my main point is that, in many cases, the actual forces on the cross slide are far from optimal and therefore they can't be the main cause of problems ass most of the effective solutions actually make the cross-slide force situation worse!

Neil

[Kiwi BlokeParticipant @kiwibloke62605]

Chris, my criticism is not driven by zeal, but by a desire not to see misleading material published. I take no pleasure in criticizing the efforts of contributors to MEW or its editor, but, sometimes, things go wrong. I'm glad you agree with my explanation of tool deflection. It's worth emphasizing that a rear-mounted tool behaves as you describe only when it is inverted and the lathe runs 'forwards'.

Merryweather's holder, as shown in the photos, will behave as you describe only if the lathe is run in reverse, which is inappropriate for lathes with screw-on chucks. The article fails to make it clear in which direction the lathe was supposed to be run. The holder could, of course, be reversed and the lathe run 'forwards' and it would behave just the same, with the same apparent advantages. However, it and the topslide would be competing for the same space, hence, I suspect, for its rear mounting.

The problem of sideways forces resulting from angled tool front faces are pretty obvious. There is a possibly more worrying sideways force and deflection problem, however. If the tool is supported by a structure to one side of its tip, it will be deflected sideways, in much the same way we have discussed. This can be a concern, particularly in quick-change toolholders, because of their 'sideways overhang'. Merryweather's holder is good in this respect: the tool tip is almost in the same vertical plane as the holder's mounting to the cross-slide. It should therefore be free from this sideways deflection.

I don't for a minute dispute that Merryweather got good results fom his set-up. Clearly, it works. The holder is quite a nice idea and I commend his originality. My criticism is of his explanation of the physics and the lack of clarity in the description of the use of the thing. I think newcomers could be misled.

[blowlampParticipant @blowlamp]

Dig-ins and deflection are brought about after the parting tool starts to bind in the groove by swarf causing a log-jam – they're the symptom not the cause.

My Mini-Lathe parts off reliably with a conventionally mounted 2mm wide insert tool because it's got good clearance at the sides and the ejected swarf exits cleanly due to being narrower than the groove.

Martin.

[Chris TriceParticipant @christrice43267]

Neill, my points were based on spring in the toolpost/tool combination. We like to think the combination is rigid but there's always a degree of spring. The swan neck cutting tool being an example. The "toolpost" in the article has the tool tip spring away from the work if mounted on the front with the lathe running in the normal direction because its base is well forward of the tool tip (forward being away from the operator).

[Martin KyteParticipant @martinkyte99762]

I think I agree with Chris.

Considering the tool tip forces it does not matter a jot where in the circumference of the work it touches, front or back as the whole thing has rotational symmetry. Just draw a force diagram and rotate the paper 180 degrees. The issue is does the deflection of the tool post increase or decrease the cut. In the case of the front mount it will lean forwards as the support is in advance of the cut (wedging) and at the back it moves away because its behind the cut (dragging). As Chris says this is how swan necked tool holders work by putting the effective spring pivot behind the cut by being less rigid than the toolpost.

regards Martin

[Speedy Builder5Participant @speedybuilder5]

I am very late in the discussion, but my 'Dig Ins' seem to be caused by backlash in the crossfeed screw. Ie, you are going along merrily when the cross slide shoots forward 0.01" or so. My remedy is to tighten the gibs a little to make the whole lot a bit 'stiffer' and the problem will usually go away. As for side deflection, my blade is about 2mm wide and it just deflects a bit giving a slightly curved end to the component when the tool is not perfectly aligned with the axis.

BobH

[blowlampParticipant @blowlamp]

The pro rear-parting argument seems contradictory to me when it's said that play is removed from the cross slide dovetails & that spindle forces are directed down to make the system more rigid, but continues by saying that a rear-mounted parting tool works because it can deflect away from the job.

If the deflection theory is true, then does it not mean that a very rigid rear-mounted parting tool wouldn't work any better that a front-mounted tool?

Martin.

[Martin KyteParticipant @martinkyte99762]

Yes

[MuzzerParticipant @muzzer]

I'm sure there are several factors at play and some setups will involve some but not so much others. I don't think it's helpful to champion one factor as the common mechanism. In the absence of any methodical, evidence-based tests nobody's going to be able to do much more than trade opinions.

Overcoming backlash or compliance by introducing springiness may have been found to be moderately successful in some cases but ideally we'd be identifying the troublesome behaviour objectively and actively – and then figuring out how to tackle it.

In may case, I started by assuming it must be possible to achieve consistent and safe parting on my machine and went about it methodically, making it achievable by minimising spring, overhang, backlash, judder etc (by using the DTI we spend so much time talking about).

Getting the speeds and feeds correct to avoid judder is another factor (helped by lubrication) that has to be tackled – any machine will judder with the right combination of speed and load, yet this is something you don't want happening when you are in the middle of a critical operation like this. If you trigger judder on a poorly set up parting tool, you are most likely asking for trouble. It's a thin line between judder and this "dig in" we speak of.

Murray

[Neil WyattModerator @neilwyatt]

Let's be fair, the diversity of opinion on the physics of parting off is enormous, and anyone offering an explanation is putting their head above the parapet. For that reason I took a light touch to editing the article as I don't want to 'tweak' someone else's views to fit my prejudices. I'm very grateful to Reg for attempting an explanation, but if anyone can come up with a better one, its a subject readers don't seem to tire of!

I don't personally agree with the explanation, but then again I think I can see flaws in any explanation I have seen presented.

I think there has been misunderstanding of Reg's fig. 5, I believe it IS drawn correctly, what he is saying (rightly or wrongly) is that you need to move the point of pressure under the work, the fact he then uses a rear toolpost with the lathe running in reverse is irrelevant – he could equally have used his toolpost the other way around with a standard tool and rotation to achieve this.

I should, however, have highlighted that the lathe is being run in reverse. Of course in this case any spring in the toolholder will act as Chris suggests, although my suspicion is that it fits in the 'rigid' class of toolholders.

Neil

> If the deflection theory is true, then does it not mean that a very rigid rear-mounted parting tool wouldn't work any better that a front-mounted tool?

Martin, my thought is that it's either/or – a tool that deflects should move away from the work-so best at the back OR a very rigid tool, which can be front or rear.

[Peter G. ShawParticipant @peterg-shaw75338]

A few years ago, I made a parting off tool to the Len Mason design.

The first attempt using an old hacksaw blade was a disaster – on the first attempt, the work very nicely rounded off the cutting corner of the blade. Ok, it probably wasn't a suitable blade.

My second attempt was to use an old half inch x 4 inch long old file in the same holder, modified to suit. The file was suitably ground down into, and don't laugh, a cutting blade about 20mm long x 1,8mm wide. It worked fine, until I realised that the blade, instead of going in straight, was making a very nice curved slot, in other words, the blade was bending. Surprisingly, it didn't break.

Version three was to reduce the blade length to 10mm, and this works fine as long as it is plunged in and made to cut, and has lots of cutting oil otherwise it chatters. This, by the way, is a normal front mounted tool. No fancy rear mountings here. If I need to part off more that 10mm (20mm diameter), I can either cut a wide slot to allow the tool to get further in, or more likely, make an initial cut as far as possible, followed by the hacksaw and facing off.

Regards,

Peter G. Shaw

[MuzzerParticipant @muzzer]

We love taking simplistic views of problems. When we talk about the tool "springing", we are actually referring to the movement of the point of the tool relative to the point on the work piece where it acts. In between them is a whole series of slides, screws, bodies, joints etc – as well as the workpiece itself. The whole system is a combination of linear (sprung) and non-linear (backlash etc) elements, often with quite different characteristics in different directions.

There have been many PhDs written about judder alone and that involves just part of the same system (a lot of judder is down to the stiffness or spring constant of that whole system).

Try applying a small force, then a large force, between these points in each of the 6 main directions and measure the resulting movements. Then imagine trying to control the path of the tool in the workpiece under significant load. On some machines this will be fraught, particularly where poorly set up.

Murray

[Martin KyteParticipant @martinkyte99762]

Well, I don't know what I'm doing wrong but my Super& seems to part off 3 inch bar OK from the front.

I only part from the back when I don't want so many tool changes.

Martin

[MuzzerParticipant @muzzer]

You're not worrying about it enough!

[Jesse Hancock 1Participant @jessehancock1]

I am with Danny on this. Neil have you thought about releasing this as a model engineering project on here?

I have been contemplating adding TEE pieces of steel to the slide (set screwed in place) so as to avoid swopping and changing the slide it'self. Any comments welcome.

Jesse

[Neil WyattModerator @neilwyatt]

> Neil have you thought about releasing this as a model engineering project on here?

I'll sleep on it

Neil

[Neil WyattModerator @neilwyatt]

Oh, go on then… just for you!

Don't try this at home…

[Martin KyteParticipant @martinkyte99762]

I should say that one thing I found to be very benificial on my Super7 was to skim flat the compound slide tool holder surface.

The lathe in question was second hand and though it was in reasonable condition had done some work in it's day. It came fitted with a four position tool turret as they all were at that time early 1990 which by it's nature had been rotated a lot. More than you would in general use with a quick change holder. The resultant wear produced a concave surface on which the tool turret was bolted. Flattening this out and using a quick change tool holder with a flat bottom made every much more rigid.

So if you have a Myford and have parting issues. It may pay you to take the slide off and check the mating surface on a surface plate with a little marking blue. You may be surprised at the result.

Regards Martin

[Ady1Participant @ady1]

The easiest system I've found has two rules

1. Use a rear toolpost

2. Use the lathes backgear

Takes my 1/3HP hobby lathe about 5-10 mins to cut through 30mm of steel bar

[Alan JacksonParticipant @alanjackson47790]

A while ago I tried out a vertical parting tool it worked ok but I must admit I did not give it exhaustive testing to destruction etc.

In order to clarify the operation of a vertical parting tool here is my rather primitive description of how I think it works.

Imagine that you are the parting tool holder whereby your two hands are held straight out from you body. You hands are gripped together and your fists are the cutting edge. You have a friend who represents the metal being cut and he (If you are a front parting tool) pushes down on your hands while you do you best to resist him pushing down. You can see that he can quite easily push your arms down. If you want to now become a rear parting tool you can turn round 180 degrees and your friend would now push your hands upwards still he can easily overcome your resistance to him pushing up. So in order to stiffen you up, say you are frozen solid or have rigor mortise and are wearing a large pair of lead diving boots to anchor you down. When your friend (or should I now say de-parting undertaker) pushes down on you hands he will not be able to move your arms down because they are rigidly fixed to your body and he have to apply more force until you tip forward on you toes. Note that as you tip forward you rotate about your toes moving your cutting tool hands deeper into the metal being cut. If you are now rotated 180 degrees to become a rear parting tool your friend (some friend) now has to apply more upward force until you tip backwards on your heels. Also note that your cutting tool hands now move away from the metal being cut as you rotate backwards on your heels. Now you have to play the part of a vertical parting tool as I am proposing, so you now can be thawed out or de- rigor mortised. So lay flat on your back and push one arm vertically upwards and clench your fist to form the cutting tool. Your friend now has to apply considerably more force to overcome your vertical arm. Your arm will be in direct compression and until your elbow or wrist give way you will have much less a problem resisting his downward cutting force. It will also not matter if you are a front or rear vertical parting tool as long as the rotating force is pushing down on your hands. You can now get up and go back to your work or whatever you were doing. I apologise for being so flippant but I hope it does explain the reasoning.

Regards

Alan

[Jon GibbsParticipant @jongibbs59756]

Alan that looks a really neat holder.

I bought a cheap rear parting toolpost for my old ML7 from Chronos and then cannibalized it to use narrower parting blades…

**LINK**

I don't know if it's because I'm a newbie who doesn't know any better but parting MS has never presented any real problems with this set-up.

I use neat cutting oil liberally applied with a brush and a steady feed by hand and haven't had to resort to back gears for anything up to 2 inch diameter in EN3B with the narrow parting blade. There's certainly no chatter unless I reduce the feed.

HTH

Jon

[MuzzerParticipant @muzzer]

You want to be careful wrestling in the workshop near all those sharp things!

About the actual tool – looks like a carbide tip brazed onto a blade – is it? And I assume the machine turns in the "normal" direction? Looks as if it could cut either direction or be swapped around in the holder.

[Howard LewisParticipant @howardlewis46836]

I made a rear toolpost for my Myford ML7, and had fewer jam ups and broken blades.

For my present lathe, I made a fourway indexing rear toolpost, to match the front one. That gives the facility to front chamfer, back chamfer and to part off. Being fairly hefty, and using a gravity fed drip of soluble oil, to the HSS blade, jam ups are rare.

Some times, in the Front toolpost, I use a replaceable 2mm tip tool. Being made with a central groove, the swarf curls up from both sides, so jam ups are rare, but not unknown.

It surprising just how fast these set ups can be run; nearly as fast turning or facing, but the rear face of the bit that falls off needs facing to clean it up.

Howard

[Alan JacksonParticipant @alanjackson47790]

Muzzer,

It is actually a HSS cutting bit brazed to a mild steel sheet. The machine is turning clockwise in this instance but I see no reason that it could be remounted for anti clockwise 'normal' rotation Here it is in action

Alan

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