Author Topic: Finding tool centre height on a lathe  (Read 43622 times)

bogstandard

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Finding tool centre height on a lathe
« on: August 25, 2009, 04:15:08 AM »
This post is from a request I had from a member, to show how I find the centre height for my tooling.

This isn't a definitive post, and I hope that other people will show their own way of achieving the same thing, then everyone will be able to find one that suits them.

I go on and on in my posts about tool height, and always having it on correct centre. The first thing you should get your head around when starting to use a lathe is tool clearances and getting the tool exactly on centre.

But I will state now, that getting it on centre and getting it to cut right can mean two different things. You first have to know the limitations of your machine. A very large and rigid machine will not flex as much as a small benchtop hobby lathe, also one fitted with a QCTP will flex a little more than one with a fixed toolpost. On big machines, you will find it doesn't really make that much difference and will take great big cuts with no problems. Now when you come to small lathes, you will have to find out just how much the tooltip does flex, because you can allow for it in some little way.
You might find that getting the tip exactly on centre, then raising it a couple of thou will compensate a lot for the flex. If I am setting up for deep grooving or parting, even on my rigid machine, I set the tooltip a couple of thou high to allow for the flex of the tool. So when pressure of the cut is put on, the tool flexes downwards, and ends up near as damnit on centre height.
I can show you how I get centre, but it will have to be up to yourselves, thru practice, to find if you need to compensate for tool flex.

Another point on this post is that I use a QCTP, as I find it a lot more convenient for myself to do so, so I can't show you how to do it by shimming using a fixed toolpost. But for those who have to shim, if you can follow this post, it will give you pointers on what to aim for.

I won't go into all the theories that go with cutting metal, as to how the metal is turned into a plasticised material and peeled off --- blah blah blah. All you need to know is that when the tool is at the correct height for your machine, you will get very smooth and low pressure cuts.

This upmarket C-o-C shows what happens in the three stages of tool height.

The first is when it is too high. As you can see, the cutting edge isn't being used, but rubbing just below the edge. So you give it a bit more pressure, to try to get it to cut, the tool is forced down, the tip hits the material and starts to cut, but at a much increased depth and pressure, tool tip usually smashes off and buggers up the job. A high tool tip is a definite NO-NO, except in the above mentioned circumstances.

The second one is when it is too low. Usually the tool will cut, but you are taking a risk, because if it digs in, the tool will be dragged down and under the part you are trying to cut, usually ending up with a ruined job and brown trousers. But on the other hand, you use a low set tool to start to find your centre height, so if you are careful, and don't take large cuts, you can get away with it.

The third pic needs no explanation.




So to cut thru all the BS and waffle, lets get down to it.

I use an old trick of a ruler or small flat plate strip to find basic centre height. Other people use a centre in the tailstock, specially made height gauges etc, all have the same outcome. They give you BASIC centre height.

This is how my way works.

You trap a ruler gently between the tooltip and the outside of the job.

If the ruler ends up like this shot, the tool is too high.




This one shows it as being too low.




And this one is good enough for me for a quickie roughing job.
But for most people, they would accept that for true centre height, and use it as is.

Please excuse the bent ruler.




So now onto how I get my tooling spot on. Just remember, high set tools have no room in the cutting regime, so basically there are only two types, low set and spot on (except for what I have mentioned previously).

Because I am very lucky (or foolhardy) I can have each of my tools mounted into it's own tool holder, so once I set the original centre height, all my other tools were set to the same height, as I will be showing, so I don't need to go thru this process every time. I had to lose the centre height on one of my tools just to take these photos.

So first off, I got a razor sharp tool. A blunt one is no good, as you come up to centre height, it is liable to 'knock' the centre pip off rather than shave it off.

I took a facing cut with the tool set slightly low.




Then by gradually raising the tool for each successive cut, you will get a tiny 'pip' formed. If you get a magnifying glass and look closely, you can just see it.




Eventually, you will shave that pip into nothing, and your tool will be spot on height.

Don't touch anything, because I will show you how to make a quick and easy height setting gauge, so that you can set all your other tooling to the same height.




I just so happened to have the old stand that my previous mill backstop was made out of, but you could use almost anything as long as it stands up on it's own, and is high enough. An old engineers square would be perfect, in fact, for the price of them, it would pay you to buy a new one just to make this setting jig.
I blued it up so that it shows what I am about to do.
A lot of people have made jigs that sit on the base, and you raise the tool until it touches the underside of a little arm that is set to height. They used to be good, but with the advent of a lot of tipped tooling, they can now not be used, I will explain a little later.




So with the tool at centre height, the gauge sitting on the cross slide, very gently scribe a line onto the height gauge using the tool.




I will just go on walkabouts now to try to explain a principle.

In the machining world, you will hear a saying, 'split the line'.
It means that when you have blued up a part and marked it out, if you machine to the line, then actually go a bit further and split the line in half, you will be almost or exact to marked size. A line drawn by say a height gauge, spring dividers or even my lathe tool will be approximately 0.002" (0.04mm) wide. So if I can get to split the line, I should be within 0.001" (0.02mm). So basically, spot on. The human eye is very adept at seeing things like this, and even someone with bad eyesight can usually see when a line is split.

So getting back to it, I can put a new tool into position, and by raising or lowering it until the line is split, you will be spot on height.
I couldn't get down to eye level with the camera to show that the top of the cutting edge is exactly in line. So a bit of hoptical dillusion is shown, and you will have to take my word for it.




Now to get back to height gauges and my comment about the ones that you raise the tool up to.

This is one of my tipped boring tools, and as you can see, it has negative top rake. A lot of this type of tooling is being used by home machinists nowadays. So if you try to bring the tool up to height, it is very difficult to get it set correctly with the old type of setting gauge.




But with the line splitting method, you have no problems seeing when you are correct.




I cheat as well.

I don't use a height setting gauge, that is just for you lot.

I just grab my known correct height tool, and scribe a line on the end of a bit of bar I am doing the job on, and set any new tooling to that.



I'm sorry that it is such a long winded post, but I hope it explained how I get my tooling to the right height, and maybe gave you a few pointers.

I do hope that people will comment, and also post how they do their thing. There is always more than one way in a cat skinning exercise, and if anyone can show me an easier way to achieve the same thing, you can bet your bottom dollar, I will take it on.


Bogs
« Last Edit: August 25, 2009, 05:59:42 AM by bogstandard »

Offline arnoldb

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Re: Finding tool centre height on a lathe
« Reply #1 on: August 25, 2009, 05:56:17 AM »
Thanks for the write-up Bogs - excellent as usual.

One comment I would like to make regarding using the rule to find height: The thinner the stock one uses with the rule, the bigger the deviation will be on the rule relative to center height, so it becomes more accurate on thinner stock to set "center " height (as long as the stock is already concentric to center line!).  Another "gotcha" I found with the rule was that deepish engraving markings on it with thin stock can have an affect, so I use an old 0.5mm feeler guage.

Cheers, Arnold

bogstandard

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Re: Finding tool centre height on a lathe
« Reply #2 on: August 25, 2009, 05:58:41 AM »
Thanks for that Arnold, just the sort of comments we are after.


John

Offline ozzie46

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Re: Finding tool centre height on a lathe
« Reply #3 on: August 25, 2009, 07:05:23 AM »


 THanks John, 

    Had not heard of "split the line" before but it makes sense.
    I have used the ruler ,tailstock and creeping up on facing cuts methods.   Still parting off on steel is a pain on my 7x.
I run lathe in reverse with tool upside down and its still scary.


 Ron

bogstandard

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Re: Finding tool centre height on a lathe
« Reply #4 on: August 25, 2009, 07:43:25 AM »
That is the problem I described Ron, about tool (in fact the whole machine) flex.

Things like parting off on these small range of machines can be a real nightmare for the user.

All you can really do, is make sure all gibs are tightish, a good quality lube on the slideways, and lock everything up before parting.

All I could suggest for your setup is to lower the tool by a few thou, to see if the tool will flex into a more centralised position.

I have recently been given a tipped parting tool, A Q-Cut, and it really does seem to work. I normally don't have trouble parting, but sometimes with some grades of stainless, it becomes a sphincter twitching exercise, but I have found with this tool, it cuts thru like a hot knife in butter, and now I can part off in my usual way, under power.

Maybe you can get something similar to it, I know the far east is now starting to copy them.

http://www.greenwood-tools.co.uk/shopscr23.html

A far eastern copy, about the middle of the page

http://www.chronos.ltd.uk/acatalog/Chronos_Catalogue_Parting_Tools_82.html


John

Offline NickG

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Re: Finding tool centre height on a lathe
« Reply #5 on: August 25, 2009, 09:04:53 AM »
Some great ideas in there that I will definitely be using! Must get QCTP too!

Cheers Bogs.
Location: County Durham (North East England)

Offline John Stevenson

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Re: Finding tool centre height on a lathe
« Reply #6 on: August 25, 2009, 09:27:07 AM »
I usually squint at it, mumble a few curses, invoke ear of bat, tongue of newt and set it  by eye.

Anything requiring  a spot on centre job usually requires sacrificing a virgin but doe to the shortage around Nottingham I'm still squinting.

John S
John Stevenson

bogstandard

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Re: Finding tool centre height on a lathe
« Reply #7 on: August 25, 2009, 09:53:18 AM »
John,

The old mumbo jumbo works for me sometimes, but when you have to describe something, I daren't write down all the quick fixes, as you get the magic circle on your back in double quick time, and the safety nannies even quicker.


John

Offline NickG

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Re: Finding tool centre height on a lathe
« Reply #8 on: August 25, 2009, 10:51:59 AM »
John,

Sorry for  :offtopic: , but why does the boring bar have a -ve top rake?

Thanks,

Nick
Location: County Durham (North East England)

Offline Spen50

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Re: Finding tool centre height on a lathe
« Reply #9 on: August 25, 2009, 11:48:31 AM »
I made a little tool for that too.

bogstandard

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Re: Finding tool centre height on a lathe
« Reply #10 on: August 25, 2009, 12:09:32 PM »
Nick,

I have absolutely no idea. I suppose these replaceable tip people came up with an idea that works, and that is how it is. A few normal turning tools are like that as well. Maybe one of the tip experts can let us in on the secret.

All my boring bars with changeable tips are like that. Luckily, 90% of my tipped tooling use the same cheap tip, so I don't argue, it does the job just fine when set to centre height, so I use them.

John

Offline Stilldrillin

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Re: Finding tool centre height on a lathe
« Reply #11 on: August 25, 2009, 12:55:59 PM »
The negative raked tips have a chipbreaker groove around them.

The chipbreaker`s cutting edge meets the metal at the correct angle.......  :thumbup:

Usually!

David D
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Still modifying bits of metal... Occasionally, making an improvement!

Offline ozzie46

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Re: Finding tool centre height on a lathe
« Reply #12 on: August 25, 2009, 01:31:12 PM »


  Thanks John, I'll Look into it on this side of the pond.


 Ron

Offline 28ten

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Re: Finding tool centre height on a lathe
« Reply #13 on: August 25, 2009, 02:10:40 PM »
Slightly  :offtopic: I have been thinking of trying one of those parting tools, as my attempts at parting on the mini lathe have been less that satisfactory. I removed the QCTP as that was flexing something 'orrible.
I like the last bit about using a known tool  :thumbup: something to remember when I get a decent QCTP
If it ain't broke, i'll fix it until it is.

bogstandard

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Re: Finding tool centre height on a lathe
« Reply #14 on: August 25, 2009, 02:17:11 PM »
David,

The boring bars I use actually use the same tips as my normal tools, which are at the normal rake angle, and as you stated, do have a chipbreaker groove.
Why they set the boring bars down like that I have no idea, when they work perfectly well at normal angles. There must be a reason for it, but ??????????????????


John


Offline spuddevans

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Re: Finding tool centre height on a lathe
« Reply #15 on: August 25, 2009, 02:45:31 PM »
There must be a reason for it, but ??????????????????

The only thing I can think of is this, would the steeper angle cause the chips to break more than create long stringers that, if boring in a confined space, may get tangled up?? maybe the angle helps to make smaller chips and so clear a bit better?  :scratch:

I dont know if that's the reason, just what came into my tiny mind as I read through the thread.   :scratch:


Tim
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bogstandard

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Re: Finding tool centre height on a lathe
« Reply #16 on: August 25, 2009, 03:02:46 PM »
Tim,

 :offtopic: Having at last got the second brain cell jump started, I had another look at the end view of the boring tool.

It looks to be because of the angle of the side face of the tip. By angleing it downwards, it allows a smaller diameter hole to be bored because of the clearance angle.

John  :offtopic:

Offline spuddevans

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Re: Finding tool centre height on a lathe
« Reply #17 on: August 25, 2009, 04:02:04 PM »
Ahh, that makes more sense than what I wrote.

Tim
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Offline Stilldrillin

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Re: Finding tool centre height on a lathe
« Reply #18 on: August 26, 2009, 02:15:26 AM »
Tim,

 :offtopic: Having at last got the second brain cell jump started, I had another look at the end view of the boring tool.

It looks to be because of the angle of the side face of the tip. By angleing it downwards, it allows a smaller diameter hole to be bored because of the clearance angle.

John  :offtopic:

My thoughts, exactly.......  :thumbup:
David.

Still drilling holes... Sometimes, in the right place!

Still modifying bits of metal... Occasionally, making an improvement!

Offline No1_sonuk

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Re: Finding tool centre height on a lathe
« Reply #19 on: August 26, 2009, 03:58:23 AM »
I tend to use a centre in the tail stock.

WRT the boring bar:
One book I read suggested raising the bar above centre, then angling it to give clearance in small holes.  I assume that's why boring bars are mostly cylindrical in shape, and why the boring bar holder for my QCTP has a V-groove.
Indexable bars probably just have the flats milled at a convenient angle for the minimum hole size.

Offline NickG

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Re: Finding tool centre height on a lathe
« Reply #20 on: August 26, 2009, 05:10:15 AM »
I use the tail stock too but think will change to one of these more accurate methods.

 :offtopic: 28ten, your trouble parting off is likely to be a general lack of rigidity with the small lathe etc. People have had good results though when changing the bearings for taper roller bearings. Isn't there some rear tool post mod for the mini lathe too?

Nick
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bogstandard

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Re: Finding tool centre height on a lathe
« Reply #21 on: August 26, 2009, 05:31:17 AM »
Quote
Indexable bars probably just have the flats milled at a convenient angle for the minimum hole size.

My indexables have flats machined on them to put the tip at that angle.

But getting back to the point of the posting, that is why the scribed line is used, rather than the fixture that you raise the tooling up to. The scribed line can cope with any eventuality, including tools that are mounted upside down, left hand, right hand, WHY.


Bogs

Offline NickG

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Re: Finding tool centre height on a lathe
« Reply #22 on: August 26, 2009, 06:31:44 AM »
The scribed line is definitely what I'll be doing when I get the QCTP.

I guess I could even do it before that, just use one of the angled HSS holders with a tool bit I only use for scribing the line. I would have to make sure I clamp it totally flat each time though.
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Offline AndyB

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Re: Finding tool centre height on a lathe
« Reply #23 on: September 01, 2010, 04:14:43 PM »
Hi all,

thanks for the great idea Bogs; as always, I am in awe of the simplistic way you teach...and the amount I learn from you (I am a novice to all this)!

The way I set to centre height is by using a sharp centre set in the tailstock and lining my cutting edge/point to that. I can back up my eyesight with feel with a finger.

Maybe it is a bit easier on old Drummond lathes like that...
Waveney Valley, Suffolk/Norfolk Border

Offline krv3000

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Re: Finding tool centre height on a lathe
« Reply #24 on: September 20, 2010, 06:04:21 AM »
HI all ? parting off i never have aproblem keep the cut wel lubrecated ie for s/s and steel and the feed at a stedy rate       regards bob

Offline Pete.

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Re: Finding tool centre height on a lathe
« Reply #25 on: December 19, 2010, 11:55:28 AM »
On my lathe, a previous owner has scribed the center height on the side of the tailstock ram, so all you need to do is slide the tailstock up and set the height of the tool using that mark.

Offline Bogstandard

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Re: Finding tool centre height on a lathe
« Reply #26 on: December 19, 2010, 12:41:40 PM »
Pete,

On a lot of early lathes, that is how they came from the factory, purely for that purpose.

There is a problem with these early lathes. As the tailstock wears, they get what is called a droop snoot, where the ram starts to hang low the further it protrudes from the main casting, so if you set your tooling with the ram even partially extended, it would be set too low.
The other problem with the tailstock marking system is that most times, the ram can rotate a little in it's housing, so rendering the height marking useless, it could be almost anywhere.

Problems, problems problems  :bang: :doh:


Bogs


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Offline Pete.

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Re: Finding tool centre height on a lathe
« Reply #27 on: December 19, 2010, 02:00:40 PM »
Gotcha John, I never thought of that.

Easily remedied by marking the nose or side of the tailstock housing rather than the ram, much less versatile though!

Offline AndyB

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Re: Finding tool centre height on a lathe
« Reply #28 on: December 19, 2010, 02:34:14 PM »
Oops! :doh:
I'm with Pete there. I don't extend the tailstock though, so maybe that's why it works...well it does for me.

Thanks John

Andy
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Offline Bogstandard

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Re: Finding tool centre height on a lathe
« Reply #29 on: December 19, 2010, 05:36:17 PM »
Pete & Andy, or anyone else who uses the tailstock method.

I am certainly not trying to say your are wrong in using that method, it has stood the test of time. I was just trying to make you aware that a problem could occur and you need to check if either of the two problems are present on your lathe before you use it.

My last lathe was an old atlas, and before I repaired it, it had those two problems in abundance.

http://www.homemodelenginemachinist.com/index.php?topic=2114.0


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Offline Jonny

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Re: Finding tool centre height on a lathe
« Reply #30 on: December 19, 2010, 05:39:18 PM »
The scribe type methods only get you some where near unless happen to drop lucky.
Cant beat taking a face cut and altering height to remove pip.

Offline Bogstandard

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Re: Finding tool centre height on a lathe
« Reply #31 on: December 19, 2010, 06:50:59 PM »
A bit of a back against the wall statement there Jonny.

What about tooling that can't be used in a facing off scenario?


Bogs
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Offline Stilldrillin

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Re: Finding tool centre height on a lathe
« Reply #32 on: December 20, 2010, 02:18:32 AM »
Most of the tooling I use can be used for facing.

So, I have an (originally), 2" length, of 1/2" dia brass. With a "centre" point turned on one end.

Hold in chuck. Set tool to "centre" height.

Reverse the bar in chuck jaws. Then take a small proving pass across the end face......  :thumbup:





Unless there's already a part finished component held in the chuck.......  :doh:

David D
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Offline Jonny

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Re: Finding tool centre height on a lathe
« Reply #33 on: December 20, 2010, 07:28:44 AM »
Not exactly Bogs i honestly cannot think of any type of tooling where you have to do that way.
May be a knurling tool.
Any type of tool that cuts be it boring bars inc internal thread cutting, grooving can be used to face off, same with cut off and grooving tooling, why settle for second best.
 
Quite agree Dave but i dont bother with a point.
Will also show some weaknesses in setup, whether that be tool flex or beds and slides.

Offline Bogstandard

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Re: Finding tool centre height on a lathe
« Reply #34 on: December 20, 2010, 12:55:05 PM »
You are quite right, if you put the tooling into a position where you can use it to face off.

Unfortunately, you then have to move the tooling to a position where you need to use it in it's correct mode of operation.

The advantage of a scribed line technique is that you set the tool to the correct height and then it doesn't need to be touched, as it is done in it's working position.

Rather than arguing over it, you do it your way, I will do it mine, and I will waste not one bit of metal by continually facing off.


Bogs

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Offline BillTodd

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Re: Finding tool centre height on a lathe
« Reply #35 on: December 20, 2010, 01:07:07 PM »
Make yourself one of Bogs' tubular squares then cut a groove in it at the right height.

http://madmodder.net/index.php?topic=2175.msg42514#msg42514

Bill
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Offline wanabemachinist

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Re: Finding tool centre height on a lathe
« Reply #36 on: December 22, 2010, 01:26:14 AM »
i know the answer to the boring bars having a negative angle position and still be considered a positive .

BECAUSE! :clap: