Trouble with cut off operations

[goldstar31]

In the words of the late Professor C.E.M.Joad, a participant in the Programme 'Brains Trust' along with such International Worthies as Jacob Bronowski( The Ascent of Man), 'it all depends on what you mean by '*******"

Do you mean the tool in front or aft and upside. down or in forward gear or reverse?
The argument for a relatively flimsy lathe was postulated by the late George Thomas who was the winner of many awards for model machining as an addition to running his own successful engineering works both in the UK and the USA. I am gratified in the knowledge that many here have wisely ascribed to not only his designs but his long researched developments and arguments and discussions with other experts both in teaching, in industry and , of course, model engineering. One of them was 'no slouch' being involved in the Manhattan Project' If one is unaware of that 'it ended World War 2 in no uncertain manner!

By utilising a rear inverted tool post, he was able not on to solve one problem which has beset model engineers for decades. Almost every second hand Myford which I have encountered has the tell tale hacksaw marks of--- some one -who didn't choose to listen. The cost was a couple of copies of Model Engineer( Volume 142 et seq) a couple of hss blades of different thickness on the rotatable turret-- and laughingly, the scrap box.
Amongst other things, Thomas removed the constant 'loss of height; in re-=grinding blades by tilting them at at angle of 7 degrees and then sharpening the front edge of the blades att 75 degrees. To add icing on the cake, the rear inverted tool minimised the going in and out of cut with the almost inevitable snapping a relatively pricey blade( s) but went on to grind a positive 'vee' on the from cutting edge but grinding a 140 'negative' vee which successfully narrowed and rolled the swarf in diameter to again minimise a slightly off centre positioning.

It was only a kerf of an inch or so and perhaps baffled some who did not have a tool sand cutter grinder nat the time( me) and I moved aware from the Propositions of Euclid and Concepts of Pythagoras by the simple expedient of using nothing more ethereal than a worn angle grinder disk instead.

This was all in the Days of the Dodo- and I still have not replaced one of the blades from then.
As I have said earlier, I have swopped and changed the rear inverted tool post onto several varieties of lathes.

Amongst the many users, one was the editor of both George's books. He graduated in dentistry along with my late wife.
Simple- well reasonably simple applied geometry. Most of this stuff- I recalled the other night dates back to the 'real' TubalCain and worthies like Hiram Abiff , King of Tyre and and the building of the First Temple of Solomon

Regards

Norman

 

[goldstar31]

One thing I do not see here is how to set your tool height !
If you do not have a 6" aluminum scale, get one.
Steel scales work ,but tend to be slippery when used and drop into the chip tray

Rich
PS
Straight aluminum strips like .040 (1 mm) x 1/2 ( 12mm) x6 (150mm) work can be substituted

GHT also made a rather pretty little tool height gauge -with two knives one for ordinary lathe tools and t'other for the upside down variety. vis . Model Engineers Workshop Manual

If you want to retain a SHARP edge, don't use steel strips

Cheers

N

 

[accelo]

"Frankly I don’t see any reason why one should be better than the normal tool post location"

It works better because Gravity is your friend if the tool is upside down.
I have a Myford and it uses the upside down rear mounted cutoff tool.
It works beautifully for two reasons;
It bypasses the top side as it is mounted on the cross slide directly and the above mentioned gravity assistance.
I have other lathes of similar weight and nothing out preforms the Myford for parting.
Another features are the tool is always set on center and it doesn't require resetting every time.
It is also keyed to the cross slide so it is automatically parallel.
If your lathe has provisions so a rear mounted cutoff tool can be incorporated, I highly recommend it.
Rick

 

[dazz]

One thing I do not see here is how to set your tool height !
If you do not have a 6" aluminum scale, get one.
Steel scales work ,but tend to be slippery when used and drop into the chip tray
This works for any Lathe Tool
Place the scale between the workpiece and the toolbit and gently bring the tool into contact capturing the scale.
If the scale leans into the work ( away from you) , you are too high. So lower the tool
When the scale is vertical, you are on center !
If the work piece is not available, use the tailstock barrel for setup, but a turned surface is more accurate.

Last suggestion from an oldtime machinist ---Tool height becomes more critical as diameter decreases !
Chuck up a 1/4" (6mm) dowel and do the above setup and you will see how sensitive you can be

Recommendations to set tool cut edges high are a function of the front relief angle -beware !

Rich
PS
Straight aluminum strips like .040 (1 mm) x 1/2 ( 12mm) x6 (150mm) work can be substituted

Hi
You could make a gauge for your lathe.
http://denfordata.com/bb/viewtopic.php?f=14&t=705
Dazz

 

[tornitore45]

The reason for a rear tool post is that old and/or hobby lathes have a threaded spindle making it dangerous to cut in reverse with such a heavy torque imposed by a parting tool.
With a locked chuck capable to drive in reverse and placing the tool upside down one can duplicate the same advantages of a rear tool post with inverted tool set in the regular tool holder and running forward.

What is the advantage?
Gravity clearing the chip is not the reason, one may cite the cutting oil washes away as a disadvantage and would be just as wrong.
Picture the tool/tool-post sitting on the cross slide as an inverted L hinged at the bottom. The downward cutting force flexes the entire geometry pulling the tool to dig in, the more it digs the higher the force, the bigger the dig. The structure flex and the flex raise until either the tool breaks or snaps back after it finally swallows the big bite, the latter case repeats originating chatter.
Now flip the tool upside down an run in reverse and try picturing what happens.
Things are easier now, the cutting force pushes the tool away until the chip load balance the resisting force of the flexing tool-post assembly.

If it can be of any consolation I went from disastrous results, bone jarring chatter and white knuckle holding of the wheel to easy, relaxing, uneventful, predictable parting by just learning to set up, grinding the tool and selecting the right speed and feed. Like swimming, is not something one can entirely learn in books; one must pay attention, sense the feedback from the machine, build experience and learn.

 

[Charles Lamont]

A tool height gauge is the most useful little tool I have ever made. Here is the idea: http://www.hemingwaykits.com/acatalog/Centre_Height_Gauge.html

Wizard69, you say you don't see why a rear toolpost should be an improvement, and then get pretty close to explaining it yourself! The argument goes like this: a tendency to dig in causes deflection (some, no matter how stiff the machine). Part of that defelection is in the form of a rotation about the mounting. At the front toolpost the deflection is down and forward into the workpiece, whereas with a rear toolpost the deflection is up and backwards, out of the work. If you have trouble visualizing this, imagine a tower crane. In the first case a dig-in is a self-reinforcing feedback system, while at the rear it tends to be self correcting. (I didn't notice the above post by tornitore45 before posting this)

 

[ALEX1952]

Tomitor45 says to reverse the spindle and turn tool upside down, this will result in the tool just rubbing, if the tool is to be turned upside down the rotation is not changed. We used this method on short runs of components using form tools on a centre lathe, set so the saddle did not have to move with a plunge cut for the form and then part off to length on the same setting ,we would also use long bar stock as in capstan turning and part of without reversing which for production purpose is quicker, no stop start etc.
I think the one big advantage of rear mounting is the tool, once set, which for most of us professional and amateur is a bit of a pain in the rear to get right, is a bit of a black art which none of us seem to agree on. I have been in the game 50 yrs and like to consider myself a good engineer/machinist and will avoid parting like the plague.

 

[goldstar31]

Alex- thank you for your comments but I'm not a professional anything although I was brought up in the blacksmith's shop from the age of 3 or just able to look at what my Dad did at the other side of the anvil.
Does that make 86 or 87 years--- Heaven forbid. I WAS a bean counter but that was 35 years ago.
There is is no Black Magic in parting off though there probably is in my survival after very early retirement.

What t is the ability to find a few pounds/dollars to buy the book. I'm on my second one and the ability to push preconceived notions to one side and to read what someone with miles more ability than I- and get on with an open mind to create what is a relatively simple bit of machining. The Great LBSC ( Curly Lawrence) wrote about the 'Words and Music' about his locomotive building. And well he might.

You will note that I studied 'history' and have concluded that there isn't much new anyway. There wasn't much education in those grim days of war in Britain and I read whatever was to hand. Would you believe that the father of the Railways couldn't read or write. His cottage was on the other side of the river to where Dad worked.
The mystical railway gauge of 4 feet 8 and a half inches is nothing to do with the Brits- it is the width of the Roman chariot ruts that were worn on the Roman Wall. Who preceded it all seems to be a guessing game.
Incidentally, I'm still teaching or mentoring as a wend my stumbling way into my 90th year. With one eye almost gone and the other desperately in need of another 'shot', I'm still struggling to improve my knowledge( between tots of whisky)
Get a hold of GHT in the Public Library or wherever and put a quick end to the so called horrors of parting off.
So my kindest regards for the future-- and do some thing far more demanding


Cheers

Norman - but not from the Norman Conquest

 

[fcheslop]

Had to part four bits of round 65mm dia alloy today and using Geo Thomas rear tool post it was a walk in the park unlike the butt clenching memories pre rear tool post days
Buy the book and a whole world of well designed tools will be available including his tool setting gauge and most canbe made for very little cost and in some cases little effort
I know it may seem old fashioned to make youre own tooling as you go but the lessons learned and the knowledge gained is pure gold
Using his boring tools in a modified form I can bore down to 1mm all from some bits of mild steel and a few bits of drill rod/silver steel total cost probable less than £10 for the small set and about £20 for the larger tool post set up that bores from 6mm up
Just my two bobs worth
Good luck and cheers

 

[RonW]

Hi Dazz,
Ironically I have an identical square to the one you show. It must be more than 60 years old having been bought form my mother's shop in Northern Rhodesia, now Zambia, while I was in the model making phase of my life. I will have to dig it out and use it as you suggest. Never thought of that. Thanks,
Ron W

 

[goldstar31]

I've been trying to assist in finding Mini-Bonelle drawings etc which is a far more difficult tool to make than than the modest little GHT rear parting tool.
My mind did wander off almost blinded with hullabaloo about ' Much a Do about Nothing- or not a lot'
GHT DID publish two smaller books himself called 'The Universal Pillar Tool' and 'Dividing and Graduating' which were after his death, joined and considerably expanded in a companion book entitled 'Workshop Techniques'
Amongst other pursuits, I made the UPT with the Mark2 castings after obtaining a quite useful fabricated and welded one which was almost given to me- for few coins. Again, not having the temperament to make and tolerate the construction of an electrifying dividing head, I made up a GHT Small Dividing Head with- seemingly the ability to do wondrous things with a one hole division plate but with two Acme threads. The boring(?) was done with the delightful GHT little precision boring head albeit from the main book.

With impish glee, I made a 360 degree attachment for my Quorn using a borrowed GHT headstock dividing thing for the Myford - and Jack Radford's division cutter thing only to find that I have the only Quorn with 359 set of divisions.

How, why, what and when but I have a unique tool and cutter grinder!

Norman

 

[tornitore45]

[QUOTET]omitor45 says to reverse the spindle and turn tool upside down, this will result in the tool just rubbing, if the tool is to be turned upside down the rotation is not changed.[/QUOTE]

Nothing could be more wrong that this.

 

[ALEX1952]

tomitor45 I have just re-read your post several times to find out where we are at odds, I now think I understand your method. Are you using the front tool post, rotation in reverse and tool inverted which I agree will cut, good bad or indifferent on which I can't comment, however at the end of Para 2 you say "and running forward" which I find very confusing, am I correct in my understanding of Para 2. and if so please explain "and running forward.
Regards Alex

 

[tornitore45]

With a locked chuck capable to drive in reverse and placing the tool upside down one can duplicate the same advantages of a rear tool post with inverted tool set in the regular tool holder and running forward.

"With a locked chuck capable to drive in reverse and placing the tool upside down"
AND actually running in REVERSE

one can DUPLICATE the same advantages of a rear tool post with inverted tool set in the (regular) replace "regular" with Rear tool holder and running forward.

My bad, hope this clears it.

The whole point is the reason to mess with rear tool holder is because the chuck unscrews if one tries to run in reverse BUT if one CAN run in reverse just flip all your parting tools and remember to part in reverse.

 

[ALEX1952]

I understood the unscrewing bi,t it was the tool post and direction that was making me think I was more daft than I actually am. As an aside on the subject of screwed chucks I have seen some Chinese type machines with set screws to lock the chuck on the nose which I think is a suspect method if for no other reason should it slip you are probably going to Kna*****r the nose. As an apprentice I was taught on Colchester Student lathes which has a keyed nose with a large screwed collar to lock in place, I have seen this undo when the brake was used and the chuck bounce up the ways, probably not done up tight enough but it happened causing major distress to yhe apprentice.

 

[goldstar31]

Why unscrew a chuck when the lathe can perfectly well cut off parts whilst running in forward drive?
Which is what George Thomas understood-- having had a long history off using threaded spindles as well as other machine tools in his works. I doubt that few , if any has been the principal/owner of your own workplaces.

Perhaps a moderator would lock this as it is not getting anywhere .

 

[tornitore45]

There are reasons to part in reverse if BOTH conditions are met
1) The machine has a chuck mounting method that allows it
2) For any reason that is not really important the tool holding system is not very rigid and can not be improved.

Those conditions are rarely seen together in one machine because most industrial grade (not hobby toys) are very rigid and generally have moved away from a threaded spindles.

Still if one has a rigid machine and the chuck does not unscrew there is no harm in parting in reverse.
Saddle lift considerations are in order if one plans to hog down with a heavy hand.

 

[Richard Carlstedt]

The main reason a rear Toolpost functions well is that the load is supported "within " the cross-slide ways.
This is important for newbies to understand
Rich

 

[Shaun free]

I am in trouble with cut off operations. I tried with a 3mm tool, then with 2mm tool (see photo). I don´t know if it the rigidity of my lathe. I´m lost. Could you help me? Even when i try to cut alluminun or brass.
troubles that i found in the operations:
- lougth noise
- it don´t cut
- i am afraid, kkk

Could you give me some tips?

Thanks a lot.

Its never a good idea to cut between centers. I always get the cut as close to the chuck as I can and part there. If you have your live center it will cause the part to bind. Also sharpen the tooling before every cut.

 

[petertha]

The main reason a rear Toolpost functions well is that the load is supported "within " the cross-slide ways.

Hmmm.. But you have substantially reduced the tool post footprint area mated to the top of the compound in order to orient the cutting tool that way.

Several people who (temporarily or permanently) replaced their compound slide with a solid base and additional tool post detente/support have noticed substantially improved rigidity in all cutting operations, including parting. I don't think there is much flex in the cross bed casting laying across the ways. Or at least relative to the top end stack-up assembly which have substantially less mass, sectional area & many more inter-contact areas: the cross slide dovetails, any lead screw float, the tool post, the tool post dovetail, the tool holder, the insert within the tool holder....