Parting Off MEW225

[Bob Brown 1Participant @bobbrown1]

Andrew

May be a different material will give different results as brass is a relatively easy material to machine.

Bob

[Chris TriceParticipant @christrice43267]

Drilling brass illustrates the point. All the time the cut is light, the forces drawings the tool in are light and all other parts of the system (friction of the slide, mass etc) are sufficient to keep the tool where we want it but if the forces suddenly increase, you get the drill equivalent of a dig in. Brass is particularly bad but it does illustrate that there is a force drawing the tool in in cases where the top rake at the cutting edge is more than modest or the material we're cutting is 'hungry'.

[Chris TriceParticipant @christrice43267]

I've suddenly had a thought while thinking about the drill scenario. Would reducing the angle of the front rake to just a degree or two help prevent dig ins when parting off. The tool height would need setting very carefully but would the front of the tool effectively rub the immediately preceding cut piece of material stopping the blade from being drawn in? Would you still get the suggestion of a snatch but no actual dig in as the material continues to revolve? Just posing the question for consideration.

[Anonymous]

Posted by Bob Brown 1 on 22/02/2015 09:32:43:

May be a different material will give different results as brass is a relatively easy material to machine

And the answer is………with the top slide unconstrained by the gib strip the parting tool got over half way through a 1-3/8" bar of steel before the top slide tipped and the insert broke. No chatter, vibration or drama, just a 'bink' and a broken insert. It is interesting that the insert pretty much split in two, like separating a couple of slices of bread. Given that the support slot in the blade is V-shaped I assume that the increase in load due to the top slide tipping, and hence sudden increase in DOC, was enough to cleave the insert. The lathe itself didn't even cough.

I find drilling brass is ok up to about 8mm, after that the drill tends to get pulled in. It is all about rake angle. The helix angle on jobber drills is on the order of 30º-40º. If I need a bigger hole that 8mm in a hurry I use a slotdrill as a flat bottomed drill. The helix angle on the slot drill is more like 20º and they don't snatch or get pulled in.

Andrew

[CabengParticipant @cabeng]

I've drilled 12 x 11mm holes (6 in brass, 6 in free-cutting bronze) x 15mm deep this evening, with a Dormer A002 drill. No dig ins.

I only seem to get dig ins when either breaking through into fresh air, or opening up a hole with a bigger drill. Blind holes (as per today) don't cause difficulty. Is that the same for Andrew & Chris, and anyone else?

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by Cabeng on 23/02/2015 00:08:39:

I only seem to get dig ins when either breaking through into fresh air, or opening up a hole with a bigger drill. Blind holes (as per today) don't cause difficulty. Is that the same for Andrew & Chris, and anyone else?

Yes. I suspect that the force required to push the chisel point through overcomes any tendency for the rake angle to pull the drill in.

I just use a slipstone to remove the rake angle for the first 0.5 mm or so and the snatching problem goes away. Put a dab of yellow paint on the drill and keep it for brass.

Russell.

[Anonymous]

I would concur with Russell, when drilling with no pilot hole then the chisel point probably counteracts the 'pull-in' force on the flutes. The problem seems worse with opening up rather break through. Bronze seems worse than brass?

I'm disinclined to modify drills just for brass; I don't do that much machining in brass. If I did, then I'd invest in the 'proper' slow spiral drills rather than fudging it with slot drills.

Andrew

[Chris TriceParticipant @christrice43267]

Yes, I agree. Opening up an existing pilot hole is when it happens the most and bronze is worse. It was the drill chisel point offering resistance that made me think if reducing the front rake of a parting off tool would stop the tip being dragged into the work to any significant depth if it started to dig in by limiting the depth of cut that could physically be taken with each revolution of the workpiece?

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Chris Trice on 24/02/2015 02:13:08:

… It was the drill chisel point offering resistance that made me think if reducing the front rake of a parting off tool would stop the tip being dragged into the work to any significant depth if it started to dig in by limiting the depth of cut that could physically be taken with each revolution of the workpiece?

.

That's an interesting idea, Chris; but I think you may be trying to 'mask the symptoms' rather than 'curing the disease'.

There are many grades of Brass and Bronze [some of which behave better than others] but generally; I use zero or negative top rake for all turning operations on Brass, and have never experienced any drag-in effect when parting.

[Whilst not wishing to teach Granny] It's worth remembering that the Clockmaker's five-sided cutting broach works very well indeed, and its negative rake angle is much greater than we would normally consider for a lathe tool.

MichaelG.

[Chris TriceParticipant @christrice43267]

I prefer to think of it as preventative medicine like a vaccination.

I'm not familiar with that broach or at least I don't think I am.

[JasonBModerator @jasonb]

Isn't the problem when drilling brass and bronze more to do with play in the feed screw. The tailstock tends to have a bit more play in the screw than the cross slide and the drill press really has nothing to stop the drill being pulled in.

As an example over the weekend I machined a bronze casting for a con rod, spotted and drilled through 1/4" no problem but thesubsequent 3/8" snatched straight away using the quill on the mill like a drill press. So locked the quill and used the handwheel to lower the head and it drilled perfectly without snatching and the same for the following 1/2" and 9/16" drills.

No change in rake angle just removed any means for the drill to be pulled into the work.

I was given some watchmakers broaches but have yet to find a need for them

Edited By JasonB on 24/02/2015 12:38:44

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Chris Trice on 24/02/2015 12:27:53:

I prefer to think of it as preventative medicine like a vaccination.

I'm not familiar with that broach or at least I don't think I am.

.

Chris,

I will grant you the vaccination

The Clockmaker's [or Watchmaker's in smaller sizes] broach is a very traditional hand-tool; slightly tapered over its length, and pentagonal in cross-section. The five cutting edges therefore have pronounced negative rake, but [especially on hard leaded engraving brass] they cut very sweetly. They are used for sizing the holes in Clock plates.

MichaelG.

.

Edit: See 522 in the engraving, here.

Edited By Michael Gilligan on 24/02/2015 13:04:48

[CabengParticipant @cabeng]

Having gratefully taken repossession of The Good Book Sandvik, done some revision, and had a think about how to put this one together …. these sketches of what happens at the cutting region are based on two in the Sandvik book:

I-6: The chip is sheared off the parent metal as the work passes the tool, and flows across the rake face. The angle of the shear plane depends on the material, rake angle, feed, depth of cut and speed. Between the deformed chip and the rake face is the flow zone, where the speed of the material varies from near zero at the rake face to chip speed – material in the flow zone is therefore subject to shear, so there’s some force being exerted by the chip as it moves along above the flow zone, which in turn gets passed onto the tool.

I-25: Indicates the distribution of compressive stress in the work piece, which combine to produce the F, the force on the tool. Obviously, the magnitude and direction of F again depend on material, rake angle, feed, depth of cut and speed.

Once those parameters are set (e.g. parting steel, 2mm wide tip, 0.002”/rev feed, 6 degree rake angle, 1000 rpm), the magnitude and direction of the force on the tool is fixed, and will not change during cutting. So if the tool starts off cutting ok, it will continue in that vein, and will not have any tendency to dig-in part way through the cut. Full stop!

The major influence on the force and its direction are the side and top rake angles on the tool, for parting we’re only interested in the top rake. Increasing the rake angle reduces the force significantly, Sandvik quote a reduction of 1% – 1.5% for every degree more positive. Increasing the top rake also reduces the radial component, and although I can’t find any figures of % reduction/degree, it is obvious that it could eventually disappear altogether – this would be the point at which a tool would develop a tendency to dig in. But that is not going to happen during parting/turning operations, because the tool’s rake angle is fixed.

The example of drill grabbing when breaking through or opening out a hole is a different situation, and does not provide a valid comparison with what would happen during parting. It is, however, an excellent example of the influence of rake angle!

The rake angle of a conventional twist drill varies from centre to lip, from zero or near-zero degrees at the web, to helix angle at the lip, so say from 0 to 30 degrees. Furthermore, the point of a drill does not shear away the metal in its path, as a turning or parting tool does, but forces it out of the way by plastic flow. Which is why, if you stop the drill at the right moment, you get a raised pip on the underside of the work.

So you continue to break through, when one of two things can happen:

1) The flutes of the drill engage with the bits of uncut but deformed metal around the periphery, and the helix drags the drill in, or:

2) The gradually increasing rake angle reduces the tangential and radial forces to a level at which the drill can dig in.

Or a combination of the above.

As far as opening up a pre-drilled hole is concerned, obviously 1) above does not apply at the start of the hole, so grabbing is then down to 2) on its own. Of course, it’s not just brass that does this, steel can do it as well, as anyone who has tried to open up a hole in steel with an electric hand drill can attest. But it certainly is more prevalent with brass.

As a further example of rake effects, we need only look at how to avoid drills grabbing in brass – stone a flat on the lip, so zero rake all the way across the drill radius, or use a slow helix drill, with typically 13 degrees helix angle, so that the rake doesn’t go ‘over the top’ at the lips.

As far as Chris’s adventure with an aluminium tip is concerned, again it’s down to inappropriate rake angles. An aluminium tip has a top rake of 20 – 25 degrees, so cutting force down by something between 20% and 37% and radial force very much reduced, perhaps gone altogether. Additionally, there will have been some side rake that further reduces the radial component, and possibly an additional effect of the entering angle, if it wasn’t 90 degrees.

So I hope the foregoing has now convinced you that:

1) The reasons for drills grabbing in brass are readily explained, and relate basically to the rake angle at the lip of standard helix drills.

2) Comparing what happens when drills grab in brass with parting operations is not valid.

3) A correctly formed (i.e. with sensible and realistic top rake) and set up parting tool does not have any tendency to ‘dig in’.

[Mark CParticipant @markc]

I doubt if your statement "Once those parameters are set (e.g. parting steel, 2mm wide tip, 0.002”/rev feed, 6 degree rake angle, 1000 rpm), the magnitude and direction of the force on the tool is fixed, and will not change during cutting. So if the tool starts off cutting ok, it will continue in that vein, and will not have any tendency to dig-in part way through the cut. Full stop!" is correct, as the diameter changes the loads will also change.

Here in lies the problem – non of this is "static" and any one piece of the jigsaw will change the outcome.

If parting was as easy as getting the tool geometry correct and setting the speed/feed, everyone would be able to get it to work without any drama – the reality is that they can't and it does cause problems.

The original thread was about the reason for using a rear tool post (I vaguely recall) and most people find the rear post a more forgiving arrangement than the front. Some of the reason for this can possibly be seen in the force vector diagrams I posted that show how the tool will deform "into" the work rather than away from it but this all assumes a true rigid support – that ain't happening any time soon!

Mark

[Neil WyattModerator @neilwyatt]

>"Once those parameters are set (e.g. parting steel, 2mm wide tip, 0.002”/rev feed, 6 degree rake angle, 1000 rpm), the magnitude and direction of the force on the tool is fixed, and will not change during cutting. So if the tool starts off cutting ok, it will continue in that vein, and will not have any tendency to dig-in part way through the cut. Full stop!"

It also assumes some sort of automatic feed with no variation, not humble finger power.

Neil

[Mark CParticipant @markc]

Neil,

No variation is the exact opposite of the requirement – to maintain consistent cutting you really need CNC control or a very skilled operator

Mark

[Martin KyteParticipant @martinkyte99762]

Hi Cabeng

So we need to examine what happens when the depth of cut varies.

From your diagrams increasing the depth of cut will alter the radial and tangential forces shown in I-25. As long as these two forces can be made to remain in an approximately constant relationship the angle of F should be insensitive to changes in cutting depth. In order for this to happen the radial force should increase at a rate equal to or slightly greater than the tangential force. for a given top rake the rte of change of radial force with depth of cut should depend on the front rake which increases the area of contact and the sharpness or radius of the tool tip (in the vertical plane). So intuitively razor sharp tools with lots of front rake are asking for trouble.

Answers on a postcard please.

regards Martin

[Ian S CParticipant @iansc]

Can't find them now, but I had some pamphlets from Sandvic, and they quoted hp/kilowatts as an important part of cutting in the lathe, for parting off a 100 mm bar, can't remember the speed(1000+rpm), coolant in L per minute, and something like 20 hp on the lathe. Enough coolant for the steel bar to remain at room temperature.

I was at the time looking for something relevant to my situation, although I was quite impressed with what could be done in a manufacturing situation.

Ian S C

[blowlampParticipant @blowlamp]

I think most people on this forum will be using GTN style parting inserts.
Not having done any exhaustive research, but I believe this style is only available with negative rake geometry – you have to move to something like the GFN or Sandvik system before the freer cutting positive rake inserts become an option.

Martin.

[MuzzerParticipant @muzzer]

Yes, my Korloy inserts are GTN style too.

I was surprised when I took delivery of the "H01" grade parting inserts for my Korloy parting tool recently. These are uncoated inserts which are specifically for parting light alloys.I was expecting noticeable top rake but in fact they have negligible (zero?) top rake and when put alongside the coated steel cutting inserts, the angles looked pretty much the same. I haven't had a chance to try them yet but you might surmise that they will require more force to push them into the work. That's really not an issue in the scheme of things. I'm suspecting that swarf formation and evacuation is regarded as more critical.

Not the best photo in the world by the time I've zoomed and cropped it but you can just about see how they compare. The aluminium cutting H01 insert is the uncoated one near the front.They appear to have been the same shape until the top face was ground flat on the H01. Doesn't appear to have changed the rake angle much but the swarf will probably form differently after it has come away from the cutting edge.

Murray

[Chris TriceParticipant @christrice43267]

I still think it's about the tool tipping forward into the work as the load on it suddenly takes it past the point it can shear the metal away which dramatically tilts it in even further into the work and it digs in. To return again to the toolholder in the article that started this thread off, if it works, we'll know, or at least be nearer a conclusion.

Edited By Chris Trice on 27/02/2015 15:45:49

[blowlampParticipant @blowlamp]

Has anyone taken into account deflections in the spindle, chuck and workpiece, which usually have a fair bit of unsupported overhang and can bend visibly during a cut?

Martin.

[Mark CParticipant @markc]

Martin,

I did think about trying to model the head stock and chuck etc. but there are so many variables and it would be so complex (static analysis would be pointless) that I didn't bother in the end. I did think about the work deflection and it is just another bit of the "system" which was what I meant when I was talking about having to take everything into account. Even the bed will be moving/deflecting and it all ads up.

Mark

[CabengParticipant @cabeng]

To the charge of not giving sufficient consideration to variation in cutting speed as diameter reduces, I can only plead guilty. But easily fixxed. As the cutting speed reduces, the force on the tip increases, but as Martin has pointed out, the radial and vertical components should remain in similar proprtions. Hence the direction of F should remain more or less unchanged, hence the tendency to resist dig-in would remain unchanged. My last line therefore remains unchanged.

Neil: if a model engineer can't feed with reasonable constancy, and can't feel what's happening in the cut so that he can vary the feed, then he should be locked in his shed and not allowed out until he's learnt how to do it. It's called developing the skill.

Mark C: If parting was as easy as getting the tool geometry correct and setting the speed/feed, everyone would be able to get it to work without any drama

Well, once things like sorting out the machine adjustments, chuck, etc., have been attended to then yes, you've got it in one, it is easy. This easy:

As he was using the DRO to cut blanks off to length, perhaps this fits your suggestion for CNC – Child Numerical Control! Or just a very skilled operator?

Back to Martin.. on a postcard? Now, now, don't be silly, I don't do postcards! But you raise an important point for those using HSS, yes, high rake, sharp tool, is asking for trouble. Combine that with setting above centre height so that rake increases as depth increases, and it gets worse. Then sharp corners between the cutting edge and the flank – on an inserted tip there's always a small radius at these points, which influences the radial outward force – increasing this radius increases the radial force. So with sharp corners on an HSS blade there won't be as much outward force to resist dig-in. Maybe it would be worth stoning a small radius here? But make sure either side is the same, or the different radii would tend to push the tip to one side and cause other problems.

Blowlamp & Muzzer – I've used Iscar GFN1.6 and GFN2 since I was introduced to them 20+ years ago. Positive 6 degree rake.

[Mark CParticipant @markc]

So are you suggesting that you would be happy to let your youngster loose with a lump of 3" bar?

It is relatively easy to get the end of a 5/8 bar by almost any means – I have used turning tools before now to butcher small diameter bar but you can't do that on bigger stuff once the cut gets over square (as deep as it is wide) you are into a different game and the deeper it gets the bigger the problems can be, which is what most people will have in their mind when they think about parting problems (I stand to be corrected on that).

As to locking people in purgatory until they develop the skill – why bother when a rear post or spring tool clearly fixes the problem.

Also, as the force increases the deflection of the tool/work interface changes (increases) and the relative angles due to tool tip height become more critical (perhaps I am the only person that has ever managed to "drag" a tool under a small diameter part?).

Mark

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