Parting Off MEW225

[MuzzerParticipant @muzzer]

Alan – that seems to be cutting very nicely. Great to see you aren't pussy-footing about there! I suspect a decent power feed is half the answer. Your machine is evidently nice and rigid, which must be a satisfying result – is it your own design?

Here's mine in action again.

When I have enough beer tokens I plan to get a Multifix-type toolholder, as the Dickson system isn't as rigid as you might hope. Then I should be able to crank the speed up a bit further.

Murray

Ah – I see it's your Stepperhead. I must try to keep up….

Edited By Muzzer on 06/02/2015 14:36:51

[Alan JacksonParticipant @alanjackson47790]

Muzzer- That is also a nice rigid lathe, I have an old Colchester Chipmaster which is much the same machine, its a pleasure to operate.

Alan

[Neil WyattModerator @neilwyatt]

Hi Alan,

I was going to ask if it was used with the lathe in reverse.

A front mounted tool would just be a mirror image, and would effectively be like one of the parting tools that has a 'leg' (rather than an arm) that braces on the cross-slide.

Neil

[Kiwi BlokeParticipant @kiwibloke62605]

Our Editor has written:

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.

and:

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

This is engineering. It is science, and therefore obeys immutable physical laws. It is not a matter of personal belief – however fervently held – nor magic or superstition. It's not mysterious, and it's not worth trying to re-invent the wheel. Disagree with science at your peril. The phenomena are understood. The misinformation contained in the original article and in this thread does no-one a service. If you want to know the truth, just read George Thomas' articles, starting in Model Engineer 5 March 1976, V 142, p 228 (was it really that long ago?) and re-printed in his book, The Model Engineer's Workshop Manual (TEE Publishing, 1992).

[blowlampParticipant @blowlamp]

If you have a lathe in reasonable condition, then as I have stated already, the only cause of parting off woes is due to swarf building up in the groove and getting wedged between tool and workpiece – this is why the problem exacerbates with deeper cuts and why lock ups never happen in the early stages of parting off.

Flood coolant and inverted cutting tools help by removing the swarf from the groove as it is produced, but a properly designed tool is the key to it all.

 

 

Martin.

Edited By blowlamp on 07/02/2015 01:34:34

[IDPParticipant @idp]

Chaps,

Perhaps I’m a bit late in this debate but I have observed that successful parting off requires the blade to be at right angles to the work piece. Might seem basic but an off square blade will jam, the cut is interrupted and we then have a dig in. How many actually check this very important feature.

Regards IDP

[Michael GilliganParticipant @michaelgilligan61133]

Posted by IDP on 07/02/2015 06:09:08:

How many actually check this very important feature.

.

My 'last minute sanity-check' is: [with the lathe switched OFF] run the parting blade gently to the face of the chuck, and look at the gap. Simple, and surprisingly effective.

MichaelG.

[S.D.L.Participant @s-d-l]

In a previous job I volunteered to part off some 316 stst M24 bolts that were too long. I was actually designing things but there was no one free to do it and I had a Factory Acceptance Test the next day and knew that the Resident Engineer coming would reject them for too many threads sticking through.

So onto the Colchester Master front Dickinson post add a Sandvick parting tool. Feeding by hand and Bang tip gone. see Mick and get another tip second bolt, bang another tip gone. Mick says lets see what your doing. digs out the rear tool post and fits on with sandvik tool moved to back and up side down. puts it at about 400rpm and cuts with power feed.

I did the rest of the bolts in less time than I did the fist two. Micks view always part under power feed.

Fast forward to nowadays got a Colchester student MkII in the workshop (Garage) so big by Myford standards. It came with original Dickinson T2 quick change post, using a Sandvik parting blade it parted Brass and aluminum under power fine EN was ok some of the time, stainless was always a disaster. I was looking for s rear toolpost but eventually I saw that the tool holders pulled out and twisted when there was a jam and fired back into place. I replaced this with a Chinese version of the Multifix type. I got the money back by putting the Dickinson and holders on e-bay and the Multifix that is half the size of the Dickinson will part 2" 316 StSt as if its butter sat on the front.

My take is many tool posts are not as rigid as we think and power feed is the key with a tool that rolls the chips or swarf so that it is narrower than the groove being cut.

Steve

[Anonymous]

Posted by S.D.L. on 07/02/2015 10:35:45:

My take is many tool posts are not as rigid as we think…….

Which is precisely why my Dickson toolpost is still sitting on the shelf above the lathe. I haven't absolutely decided not to fit it, so I'm not quite ready to sell it, but I am pretty sure it will never get used.

Another reason to stick with my existing toolpost is that it has a radially slotted disc underneath which mates with a similar disc on the top slide – like a dog clutch. So it is easy to set the toolpost at an angle for, say, chamfering, and then set it back orthogonal, with no measurement needed.

Andrew

[MuzzerParticipant @muzzer]

+1 Steve

As I said above, I intend to get a Multifix clone to replace the Dickson, as it's clearly the next weakest link on my Bantam.

[Peter G. ShawParticipant @peterg-shaw75338]

Chaps,

Perhaps I’m a bit late in this debate but I have observed that successful parting off requires the blade to be at right angles to the work piece. Might seem basic but an off square blade will jam, the cut is interrupted and we then have a dig in. How many actually check this very important feature.

Regards IDP

When I use my front mounted parting off tool, I actually use a magnifying glass and a set of magnifying clip on spectacles to get the cutting edge as near parallel to the work as I can.

Peter G. Shaw

[Neil WyattModerator @neilwyatt]

Hi Kiwi Bloke,

I have read the George Thomas articles several times. To my recollection he offers no rigorous explanation of the superiority of a a rear-mounted parting tool though he offers much sage advice on the process itself. But all of it is empirical, none of it experimental.

I stand by what I said, I still have to see a convincing explanation.

Neil

[HopperParticipant @hopper]

Posted by Peter G. Shaw on 08/02/2015 11:35:16:

When I use my front mounted parting off tool, I actually use a magnifying glass and a set of magnifying clip on spectacles to get the cutting edge as near parallel to the work as I can.

Peter G. Shaw

I usually bring the side of the parting blade up to the front face of the stationary chuck to make sure it is square on. If I am doing something particularly critical, I might even use a dial indicator to make sure the parting blade is square to the lathe axis.

And, ISTR in either one of GHThomas or LH Sparey's books, they say that an inverted rear toolpost parting blade places the load on the lower half of the headstock bearings, which are set in the main headstock casting and very solid. Using a front parting tool in the conventional manner puts the load on the top half of the bearing, ie the less solid top bearing cap, or relatively thin casting above the mandrel. This flexes and allows more chatter than the rear post.

Personally I reckon the swarf falling out of the groove is reason enough to use the inverted rear blade. Works a treat on my 1930s Drummond M type lathe which is in far from new condition.

[Chris TriceParticipant @christrice43267]

I have no proof of this but instinctively I think the reason for more dig in's with a front mounted post than a rear mounted post is partly due to slop in the feed screw. With a front mounted post, the toolpost and blade are being deflected down and into the work therefore there's a tendency for the work to climb over the tip and drag it in with no force from the feedscrew to stop it unless you have an extremely good fitting feedscrew and nut. The smaller the diameter gets, the more of a problem this becomes. With a rear tool post, the forces are flexing the blade away from the work and hard up against the feedscrew pulling it in at all times. Probably no one factor is solely responsible but a number of factors muddy the waters when looking for a single cause. So ideally, a rear mounted tool in a toolpost that tends to flex away from the work, preferable tipped that produces narrower swarf than the width of the slot and with plenty of cutting lubricant should do away with parting off woes.

[Anonymous]

Posted by Chris Trice on 08/02/2015 13:26:44:

I have no proof of this but instinctively I think the reason for more dig in's with a front mounted post than a rear mounted post is partly due to slop in the feed screw. With a front mounted post, the toolpost and blade are being deflected down and into the work therefore there's a tendency for the work to climb over the tip and drag it in with no force from the feedscrew to stop it unless you have an extremely good fitting feedscrew and nut. The smaller the diameter gets, the more of a problem this becomes. With a rear tool post, the forces are flexing the blade away from the work and hard up against the feedscrew pulling it in at all times.

I don't follow that?

If I assume a front mounted tool and start feeding the cross slide towards the lathe centreline, and then push on the cross slide it moves even closer to the centreline by the amount of play in the screw. So far so good. If I now assume a rear mounted tool and start moving the cross slide towards the lathe centreline (in the opposite direction to before) what happens when I pull on the cross slide, ie, in towards the work? On my lathe it moves closer to the lathe centreline by the amount of play in the screw.

Surely the rotating work is always trying to drag the tool inwards towards the centreline, irrespective of where the tool is?

Andrew

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by Andrew Johnston on 08/02/2015 15:24:42:

Surely the rotating work is always trying to drag the tool inwards towards the centreline, irrespective of where the tool is?

Hence G.H.T's recommendation to "b. Use a top-rake less than that normally usedon a turning tool for the same material"

Neil, can you explain your meaning of the difference between "empirical" and "experimental"? Wikipedia tells me that empirical evidence is "source of knowledge acquired by means of observation or experimentation."

Russell.

[JasonBModerator @jasonb]

Whats really needed is a scissor type tool with two cutting edges that move in on opposite sides of the work at the same time so all forces will cancel each other out, mounted directly on the lathe axis.

Looks like I may have come up with a contender for the "JS Challenge Cup" already

[thomas oliver 2Participant @thomasoliver2]

My lecturer at Tech College was well esteemed and he had an explanation for the advantage of rear parting. In normal front parting the reaction forces are against usual less sold casting above the front bearing, In the rear parting mode, the reaction is downward into the more solid casting of the headstock which reduces vibration and chatter.

[Gordon WParticipant @gordonw]

.How about fitting the fixed steady with blades instead of brass fingers ? I do not intend to patent this.

[thomas oliver 2Participant @thomasoliver2]

Sorry – I see the point has been covered, except that all said lecturers statements were backed up by experimentation and measurement. One such that I performed mysellf was to prove the validity of the 118deg. angle gorund on twist drills. This turned out to be the angle whiich gave penetration with the least downward force.

[John Stevenson 1Participant @johnstevenson1]

Posted by Kiwi Bloke 1 on 07/02/2015 00:23:45:If you want to know the truth, just read George Thomas' articles, starting in Model Engineer 5 March 1976, V 142, p 228 (was it really that long ago?) and re-printed in his book, The Model Engineer's Workshop Manual (TEE Publishing, 1992).

 

.

 

Presumably this is the truth according to George Thomas and not possibly the real truth ?

 

It's like every other post in this thread just an opinion, in this case his.

Edited By John Stevenson on 08/02/2015 17:00:45

[mickParticipant @mick65121]

Suffice it to say that all production lathes, capstans and Auto's have rear mounted parting blades, this is for two very good reasons, the first being it won't foul or hit any other tooling while being indexed, the second and more importantly is that after a hundred years or so its been proved beyond any doubt that its where it works best.

[JasonBModerator @jasonb]

Is it not because its quicker to move the cross slide one way to perform one opperation and then the otherway to part off, so no need to index the tooling?

[Neil WyattModerator @neilwyatt]

Every argument for a rear mounted toolpost must also apply to normal turning.

So, side from visibility, why don't we mount all tools at the back?

Neil

Russell – you have me hoist by my own petard. What I meant(said Humpty Dumpty) is that GHT's opinions were just based on observation and he had no (detailed) underlying theory of why things are as they are.

[blowlampParticipant @blowlamp]

We need someone with a rear-mounted parting tool to temporarily invert their lathe to see if it starts to act like a front-mounted tool.

Martin.

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