Finding Tool Center Height¶
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.
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.