Parting Off MEW225

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by Vic on 19/02/2015 10:32:34:

What about this type of blade, the chipbreaker?

http://chronos.ltd.uk/acatalog/New–Clamp-Type-Parting-Tool-s-with-Chipbreaker-Blade–5–cobalt-.html

[IMG]http://www.chronos.ltd.uk/acatalog/392685BLADE2.jpg[/IMG]

Yes, the v groove along the top (as recommended by GHT) should tend to reduce the width of the swarf.

Russell.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by blowlamp on 19/02/2015 11:59:08:

Maybe it would help to compare a standard HSS parallel (Eclipse style) parting blade to a handsaw without any 'set' on its teeth. < etc. >

.

Martin,

Are you aware that the Eclipse blades are tapered from top to bottom [i.e. Dovetail section] … it's only slight, but very important to their effectiveness.

MichaelG.

[blowlampParticipant @blowlamp]

Posted by Michael Gilligan on 19/02/2015 12:22:16:

Posted by blowlamp on 19/02/2015 11:59:08:

Maybe it would help to compare a standard HSS parallel (Eclipse style) parting blade to a handsaw without any 'set' on its teeth. < etc. >

.

Martin,

Are you aware that the Eclipse blades are tapered from top to bottom [i.e. Dovetail section] … it's only slight, but very important to their effectiveness.

MichaelG.

But not from end to end (when viewed from above) are they?

Martin

[Michael GilliganParticipant @michaelgilligan61133]

Posted by blowlamp on 19/02/2015 12:39:44:

Posted by Michael Gilligan on 19/02/2015 12:22:16:

Posted by blowlamp on 19/02/2015 11:59:08:

Maybe it would help to compare a standard HSS parallel (Eclipse style) parting blade to a handsaw without any 'set' on its teeth. < etc. >

.

Martin,

Are you aware that the Eclipse blades are tapered from top to bottom [i.e. Dovetail section] … it's only slight, but very important to their effectiveness.

MichaelG.

But not from end to end (when viewed from above) are they?

Martin

.

No … but [arguably] neither is a saw blade.

MichaelG.

[JasonBModerator @jasonb]

Michael the set of teh teeth makes them wider than the body of teh blade, just like a 2mm insert will be held in a 1.5mm holder so its only the very edge that is full width.

I think the saw blade comparrison suffers due to the movement from being hand held, don't seem to get an issue with slitting saws that have no relief or very little if hollow ground.

Edited By JasonB on 19/02/2015 13:19:52

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 19/02/2015 13:18:59:

Michael the set of teh teeth makes them wider than the body of teh blade, just like a 2mm insert will be held in a 1.5mm holder so its only the very edge that is full width.

.

Jason,

That's equally true of the dovetail section Eclipse blade … which is the point I was making to Martin.

The only substantial difference is that the parting blade is cutting on both tips, whereas Martin's handsaw cuts alternately.

MichaelG.

[JasonBModerator @jasonb]

Posted by Michael Gilligan on 19/02/2015 12:45:23:

But not from end to end (when viewed from above) are they?

Martin

.

No … but [arguably] neither is a saw blade.

MichaelG.

Michael you are looking at them from different angles when looking at an eclipse blade from above you need to look at teh saw from the end towards the handle not from above.

Oh and don't forget that a good quality handsaw is hollow ground, Think my Disston is.

[JasonBModerator @jasonb]

Imagine this is looking down onto the top of the parting blade as it makes teh cut, there will be no clearance either side with an Eclipse blade which is the point martin is making.

[Bob Brown 1Participant @bobbrown1]

There will be a side clearance if you add back rake.

Bob

[blowlampParticipant @blowlamp]

Posted by JasonB on 19/02/2015 13:18:59:

Michael the set of teh teeth makes them wider than the body of teh blade, just like a 2mm insert will be held in a 1.5mm holder so its only the very edge that is full width.

I think the saw blade comparrison suffers due to the movement from being hand held, don't seem to get an issue with slitting saws that have no relief or very little if hollow ground.

Edited By JasonB on 19/02/2015 13:19:52

Don't forget that a slitting saw tends to carry the chips to the outside of the slot by its rotation and as you say they are hollow ground which is bound to help prevent binding.

Martin.

[Mark CParticipant @markc]

Got to running some models, the inserted tool is from Sanvik and is meant for simulation. The spring tool is 3 mm face width and roughly the size you might use on a 5 inch lathe. I used 240 lbs tip force and you can see how the displacement vectors differ…..

Mark

[Neil WyattModerator @neilwyatt]

Now that's interesting!

Neil

[Anonymous]

I must be mis-interpreting the results, as they seem to show that the Sandvik tool deflects a lot more than the swan neck? Which is not what I would have expected.

Andrew

[Mark CParticipant @markc]

And that is just the tool Neil. The rest of the simulation will have to wait for time availability on the machine (it takes up a lot of resources) and I have not finished preparing the simulation yet. To my mind, you can start to see why a front tool is always prone to problems on the lighter machines and why the rear tool inherently cuts more predictably/stable.

Mark

[Mark CParticipant @markc]

Andrew, It is exactly what I expected – the swan neck puts considerably more material in the loaded CSA compared to the inserted tool. It also ensures the deflection is always away from the work due to the effective hinge point high up.

Mark

[Anonymous]

CSA?

The numbers are a bit fuzzy, but it seems like the Sandvik tool deflects about 0.3mm but the swan neck only 0.003mm? It may well be what you expected, but it isn't what I expected, so you'll have to be patient and explain it in words of one syllable to a dummy.

Andrew

[Mark CParticipant @markc]

Andrew,

CSA = Cross sectional area = area of tool steel stressed.

The tip of the insert is predicted to deflect 0.14mm but it is a little misleading as the insert is smaller than the swan neck tool at a face width of 2 mm and the supporting seat is 1.6 mm wide.

Could you see the direction of the movement indicated by the displacement vector arrows? I think the 240 lbs force is much higher than you would see in normal use by a hobby engineer (and probably in an industrial setting also?)

Mark

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 19/02/2015 13:34:52:

Imagine this is looking down onto the top of the parting blade as it makes teh cut, there will be no clearance either side with an Eclipse blade which is the point martin is making.

.

Excellent illustrations, Jason … But they do nothing to change my opinion.

I cannot see, and have never found, any reason to put a 'front to rear' taper on an Eclipse blade … In fact, I believe it would be seriously counter-productive.

As the blade is designed, there is a 'top to bottom' taper which provides the equivalent of the 'set' on a saw-blade [remember that the cutting action when parting is effectively vertically upward with respect to the lathe bed]. … If the blade is cutting cleanly then the sides should [if anything] just provide a scraper effect; fine-finishing the two surfaces of the work, and sweeping the swarf upward.

Now: If we [foolishly] add a taper from front to rear, it brings no benefit, but adds two risks:

1. It weakens the blade
2. It provides a pair of gaps which could trap swarf.

Why would we choose to introduce those risks, when the blade, as designed, works so well ?

MichaelG.

[Mark CParticipant @markc]

Michael,

I agree with Jason here, given there are very few machines that face perpendicular to the machine axis, without any side clearance as you describe the tool will rub on one side or the other generating heat. I have always "foolishly" ground some relief on the back of a parting blade (which is why I went over to inserted parting as deep parting gets expensive when you need to start again with the grinding). Perhaps the inserts are also in need of the support area being full width all the way back to stabilise them in use?

Most of the turners I have known all grind a small chip former to try and collapse the chip into a narrow section as the inserts do and aim for a chip breaker arrangement to break the chips up (although I can't think of many places left that still regularly use HSS parting blades).

Also, when you grind the back rake on the top of the blade (which is tapered) you automatically generate clearance at the sides as per Jason's illustrations?

Mark

[Michael GilliganParticipant @michaelgilligan61133]

O.K. … I give up

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by blowlamp on 16/02/2015 11:09:56:

I can't think of any commercially available tool that uses the Swan-neck principle to circumvent parting off problems.

.

Martin,

Just for the record …

The Raglan S24 may [historically] fit the description.

MichaelG.

[WALLACEParticipant @wallace]

I’ve seen a commercial one years ago on a stall at the MEE at Olympia. Definitely swan necked and also odd that it used a disc of ( presumably ) HSS with a segment cut out of it – not the usual long blade.

I was tempted but the thought of sourcing replacement cutters put me off…

W.

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by Michael Gilligan on 19/02/2015 12:22:16:

Are you aware that the Eclipse blades are tapered from top to bottom [i.e. Dovetail section] … it's only slight, but very important to their effectiveness.

Yes, and some of the cheap Chinese holders are poorly made so that the blade is not held vertically. I have had one that held the blade such that the clearance on one side was negative!

Could be another cause for parting woes for beginners.

Russell

[Chris TriceParticipant @christrice43267]

That would give parting woes to anyone. I've also had a couple like that and they've put me off most blade holders of that type. Even if they hold the blade upright, they often distort the blade as the locking nut is pulled tight. I almost exclusively use the type with a replaceable tip now or grind up a special micro parting tool from HSS square for miniature brass work.

[Anonymous]

I managed to do a couple of experiments this evening. First, on the question of 'pull in' I did some trials with a 1-1/4" bar of brass (CZ121 I think) and a brand new 0.4mm corner radius CCGT insert for aluminium. All tests were done with a spindle speed of 800rpm. I tried parallel turning at 4, 8 and 12 thou per rev, with a depth of cut of 50 thou. In none of the tests did the saddle get pulled in. I then tried facing off, again with a depth of cut of 50 thou and feedrates of 4, 8 and 10 thou per rev. My cross slide is quite loose, I can easily move it back and forth by hand, but at no time did it pull in. This was the case even at the end of the facing cut, the tool probably being 8 thou or so below centre height. So for me at least pull in with brass doesn't appear to be a problem when turning. Drilling of course is another matter.

I also did some parting tests on the same bar of brass. In all cases spindle speed was 370rpm and feedrate was 6 thou per rev. The first test was with the top slide locked, and the gib screws done up tight. There was no problem, as one might expect. The second test was with the top slide unlocked and the gib screws set so that the top slide moved nicely, with being loose. Again there was no problem parting off. The third test was done with the gib screws wound all the way out, so that they weren't doing anything. I measured the lift on the back of the top slide as about 25 thou. The parting off was fine, with no lift of the top slide until the last 1/16" or so of the diameter, when the remaining pip normally fractures anyway. The top slide lift, such as it was, was without drama. If anything the parting tool was probably a bit below centre.

While I wouldn't plan to part off with the top slide loose, I'm not convinced that the top slide necessarily plays much part (yes, I know it's a pun) in the parting off saga. Chatter is quite a high frequency effect, many times the spindle speed. I think that chatter is generally caused by an interaction between the tool and work. The toolpost and top slide may add a little to the effect if they are not particularly rigid, but I don't think they are a prime cause.

Andrew

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