Parting made easy

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Paul C

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Many may already know this but I tried a new way ( for me anyway) of parting off a part.
I remembered seeing a YouTube video of someone parting off by having the parting tool upside down and then running the lathe spindle in reverse so I decided to give it a try.
What a difference! Cut thru smoothly, no chatter, no digging in.
This will definitely be my preferred method.
 

mcostello

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My preferred way also, I made the best of both worlds and got an insert parting tool. the insert tool works better and upside down makes it even better than that.
 

goldstar31

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Old stuff! I have an ancient rear inverted parting tool holder to the design of George H Thomas from originally Volume 142 of Model Engineer and published again in the Model Engineers Workshop Manual and the kit( for those who don't own a scrap box:oops:) from Hemingwaykits in the UK.

My setup is rather pretty because the removed swarf forms into neat little rolls which drop from the tool as if by magic.

Of course, I spent a lot of time- boring and drilling castings for the Quorn tool and cutter grinder.
For 7 old English £10 notes, I took delivery of another almost complete set of Mark One castings- to have another senior moment or twoo_O

Eventually, I will be able to take things-- in turns. A pun I will.:rolleyes:

Well if someone can do it in Italian- why not in English?
 

BaronJ

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Hi Paul,

If you have a rear tool post with the parting tool upside down, the lathe runs in the normal direction. However if you are running the lathe in reverse with the parting tool upside down in a front toolpost, and you have a screw on chuck, be very careful not to have a dig in. While the risk is low, the chuck could unscrew !
 

goldstar31

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Adding a tad to John B's very sensible reply, running in reverse can lift a saddle------ and we know what happens---------- or perhaps we don't.

I was given a Myford Super 7 top-slide where the spigot had torn out of the saddle. Then, if anyone reads Screwcutting in the Lathe by Cleeve, there is a splendid photograph of a steel boring table- which Myford made as a one off after pulling through tee slots. No, it's in Model Engineer and not his book!
 

bazmak

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You can only part off running in reverse if the chuck is a bolt on type
Rear toolpost with tool upside down is the prefered setup depending on the lathe/job
it can stay in situ without changing setup
 

Paul C

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Mine is a grizzly with bolt on chuck so no worries there.
 

abby

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Parting off with the lathe running in reverse and the tool upside down does hit a problem with tool centre height . My Boxford - a Southbend clone made in the UK - has a fourway toolpost that takes a 12mm square tool obviously if the tool is upside down then it is 12mm below centre and there is not enough room for packing.
To overcome this I made this simple parting blade holder.




It fits into the toolpost like a normal 12mm square tool and saves loads of messing . I would prefer a rear toolpost but my cross-slide has no provision for mounting one and the slotted cross-slide is rather expensive to justify re-placement.
 

kvom

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My lathe has no room for a rear mount tool unfortunately.
 

goldstar31

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[
My lathe has no room for a rear mount tool unfortunately.
For those in a similar situation, might I suggest trying a subtable.
Amongst other advantages, I can transfer tooling from my Myford lathes onto my Sieg C4- and vice versa.

Cheers

Norman
 

CFLBob

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Sherline's cutoff holder is designed to be mounted rear mount with the blade upside down. The motor only runs one direction on those micro lathes.

What has always puzzled me is why it would work better rear mounted. Since the work is rotating, the cutting angle seems to be exactly the same. The only difference that I see is that when front mounted, the forces push the cutter down onto the cross slide and when rear mounted, it acts to lift it off the cross slide. Here's a crude sketch.
CutGeometry.jpg

For a similar reason, I don't see why you'd want to run the work in reverse. If the work touches on the centerline it just pushes the tool holder down instead of pulling it up.

Edit to add: since I taught myself how to turn stuff on the Sherline, cutting off with the tool on the back of the work is how I learned and how I approached it when I got the bigger lathe.
 

Richard Carlstedt

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Bob, Nice sketch and fits perfectly with my comments about parting off, so i will refer to it.

Parting off seems to be the biggest bugger to many home shop guys. There are many reasons
for problems, and most relate to slop or play in the gib surfaces. There is also some spindle related issues.
Looking at Bob's sketch on the left side , you will notice that when doing regular lathe turning, the spindle tries to climb over the tool bit which is resistant to turning and the resulting Radial Force on the Spindle Bearings takes place at the ONE O'CLOCK position. So your spindle head-stock bearings get the most force and resultant "wear" at this location. Please note that your Spindle bearings are made to handle Two Forces, Radial (Plunge) and Axial ( right to left).
Cutting Off is a Pure radial force and besides that, the cutter width (!) is much larger than normal, so spindle looseness plays a big roll.
Again, looking at Bob's sketch, but on the right side, we see the resultant force on the spindle bearings occurs at the Seven O'CLOCK position which is generally wear free (and smoother) and allows parting off a bit better

Now we all know that Parting off should be done as close to the chuck as possible to prevent a crash, but keep in mind the following.
1. The Parting Tool must be sharp
2. The Parting Tool must be square to the axis ( use your chuck face during setup, and hold it flush with the tool)
3. The Parting tool should only stick out from the "tool holder" equal to about the radius of the work piece + 1/8"- It needs support and Watch over hang beyond the Tool Slide !!
4. Lock your carriage and your tool slide ! ( But not the cross-slide )
5. Be generous with cutting lube !
6. You are told to not use the Tail Stock for support, but you can....IF you remove it when you are get down to .5 00 to .600" diameter and have a lighter feed rate
7. Part by using your Power Cross-slide Feed !. Yes it works because it is smoother than hand turning a crank- When using your hand feed , you tend to go from One O'Clock to Five O'Clock on the hand crank much faster and that results in a heavier feed rate ! (crash time)
8. Lastly , If the cut is deep, try to step over during the process to give chip clearance and then return to you normal cut. On a 10" Lathe and a 1.5 " diameter stock for example, make it a point to widen the groove on the scrap side when you are 1/2 way through. ( 3/4") and don't forget to relock the cross-slide
Have Fun
Rich
 

BaronJ

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Hi all,
I said that I wouldn't do this !

But anyway here are a couple of pictures of my "Norman patent" rear tool post. Its fully adjustable for hight and interchangeable with the front one. It also takes 1/2" inch tool bits.

31-07-2019x002.JPG 31-07-2019x003.JPG
That post is 35 mm in diameter. The bright cap screw is used to adjust the hight.
The tool block is secured by a split collet using an M6 cap screw to clamp it.
 

CFLBob

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Really good, Rich.

Just one thing I'm not seeing in my head.

Looking at Bob's sketch on the left side , you will notice that when doing regular lathe turning, the spindle tries to climb over the tool bit which is resistant to turning and the resulting Radial Force on the Spindle Bearings takes place at the ONE O'CLOCK position. So your spindle head-stock bearings get the most force and resultant "wear" at this location. Please note that your Spindle bearings are made to handle Two Forces, Radial (Plunge) and Axial ( right to left).
Cutting Off is a Pure radial force and besides that, the cutter width (!) is much larger than normal, so spindle looseness plays a big roll.
Again, looking at Bob's sketch, but on the right side, we see the resultant force on the spindle bearings occurs at the Seven O'CLOCK position which is generally wear free (and smoother) and allows parting off a bit better
I've had work climb up onto the tool like you talk about, when cutting using the geometry on the left. In the right view, the spindle has the same forces on it because of rotational symmetry but tries to "climb under" or dive under the tool bit. Either way, the work is usually damaged and the cutoff blade snaps off.

When you talk about the spindle bearings getting the most force and wear at the 1:00 position, I think the only wear that would happen would be in the outer race itself because that's the only part that's fixed. The inner race and the balls or pins are rotating with the shaft, so they're constantly moving in and out of that area.

The difference for the other side, at the 7:00 position, is just that since we use that feed direction much less often, the outer race doesn't get loaded as often as the it does on the 1:00 side, right?
 

goldstar31

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Frankly, I don't think that the spindle forces have anything to do with it. After all, the are designed to cope with cutting forces both axially and radially- and arguably with heavier loading than is imposed with a narrow parting tool.

Where the problem lies is a tool going in and out of cut. That is from worn or badly set up saddle and boring table and in some cases, Heaven forbid, adding a top slide.

Me I cannot recall breaking a blade in 'years' although my George Thomas parting tool has lived on several creaky Myfords, an old ptrwar Zyto, a 918/920 and currently either on my Myford Super 7B or a Sieg C4.
Apart from only grinding the front- with a lick on my grinder.

So how only that? Well, the Thomas design calls for a built in 7 degrees- and 140 degrees on the front cutting edge. So there is none of this bother of grinding lots of metal off. WhatIS done is to grind a 140 degree female kerf along the 'top' of the blade( which is underneath, of course) and a male front of 140 degrees again inclusive.

This enables the swarf not only to drop being upside down cutting but the swarf is forced into lovely little curled up rolls.
Really, it has taken longer to type than part off a bit of 2" round mild steel.

Quite bluntly, I wonder what the fuss is all about, I made the thing- as I said and I broke the thin blade in the turret and simply carried on regardless ever since.

There is a caveat- and I make no apology, I like so many members of the old brigade followed the carefully researched writings of Thomas. One of my even more aged friends- he is now 92, set off and seemingly made ALL the Thomas Accessories including twin Acme thread headstock dividing gear( which will poo poo a lot of present correspondence.

That's me- others can adopt what they wish

Norman
 

fcheslop

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Im another happy user of the Geo Thomas rear tool post although mine is cantilevered as I dont have the long super 7 cross slide
Around 4 years ago I bough a new style of blade or new to me its has a T section when looking at the front with a half round ground along the top cutting edge.The idea is that the swarf forms into a radiused curl that is smaller than the width of the cut to prevent jamming in the cut .
This set up will part 3inch mild EN!A or the likes with no probs all day long
The blade came from Chronos in the UK but I cannot find a reference at the moment
The rear tool post can be made with extra heads and its very handy when batch working
I also pinched Geo Thomas's boring tool design but instead of making tools to go into a standard style holder made the tool from a solid block that take his eccentric collet idea and it greatly improves finish and the amount of material you can remove without chatter or its associated problem . With this toolpost I can bore from 3/32 to lathes max depending on the boring bar fitted.
Like most I had nothing but tears n fears when parting or forming deep grooves until I took the time to make the tool post
cheers
 

RM-MN

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The real reason that parting with the tool upside down on the back side is that every moving part of the lathe must have some slack in it. If you tighten the gibs until there is no slack, you can't move the cross slide or compound so you loosen them just a little. When you cut on the front side the forces caused by the cutting action tend to pull the tool down and the slack makes the toolpost tip forward, making the tool dig in more until it pops free (chatter) or breaks (crash). By turning the tool upside down and running it on the back side the rotation is such that the tilting of the toolpost moves the tool away from the cut which relieves the forces and it continues to cut.
 

goldstar31

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What a brilliant idea making the boring tool holder SOLID and the use of eccentric collets which allow movement in height but also controlling the amount to which the boring tool itself extends thus minimise the amount a bar needs to project.
Of course Thomas was on the scene gathering cups and prizes before these 'Johnny come lately' people appeared with their U tubes or whatever they are- thinking that they have found the Holy Grail or invented the wheel.
I as a non whatever they wish to describe themselves , slavishly follow Thomas, Chaddock, Tom Walshaw as Tubal Cain and the enigmatic 'Martin Cleeve' who could part 3" round faster on a ML7 than anyone else with a proper mechanical hacksaw. Again, I loved his dividing and graduating with things like a strip of paper and Pi and cut gears with a lathe tool ground at both ends. Incidentally, I have his swing tool He then got excited and put TWO on his boring table. Now Hemingwaykits have converted it for screwcutting- with, as you remark- and eccentric pivot.



Norman
 

Richard Carlstedt

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Really good, Rich.

Just one thing I'm not seeing in my head.
.............................................
The difference for the other side, at the 7:00 position, is just that since we use that feed direction much less often, the outer race doesn't get loaded as often as the it does on the 1:00 side, right?
Absolutely Bob

There is one other VERY important reason not mentioned
Rear tool post parting tools are mounted in the "Center" of the cross-slide ( Stability +)
Which means the cutting load is vertical (Upwards ) and the dove tail counteracts it EVENLY.
but normally many Parting tools are held on the (front) Left side of the Cross-slide and some extend beyond the support (Vertically !) of the cross-slide
This means cocking or rotating (CCW under load) of the Cross-slide OR Tool-slide and failure !
So on front mounted Parting Tools ,moving the tool holder to right...towards the center line of the cross-slide if possible will help immensely towards success. Remember to have that Tool-slide retracted for 100 % support

On a side note, I met George Thomas in January 1977 at the ME Show when I was in London.
What a superb gentleman and craftsman and I was generally speechless most of the time during our discussion.
Rich
 

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