Finding Centre Height

[ZigFireParticipant @zigfire]

Hello,

Did a search for this subject with no joy.

My question is what are peoples opinions on the best way to find centre height on a lathe for tool setting?

 

Any tips or tricks greatly appreciated.

 

Cheers

Michael

[ZigFireParticipant @zigfire]

[TerrydParticipant @terryd72465]

 

Hi Michael,

 

In the morse taper spindle of your lathe headstock you can use a NEW fixed centre (after removing the chuck) and the point will be on the centre line of the lathe but you really do need a really sharp fixed centre but they’re not expensive.  You could then make yourself a fixed gauge such as the one from George Thomas’ book – ‘The Model Engineers Workshop manual’ – available from most good libraries.  This gauge is rather like a height gauge but is fixed at your lathe centre height as measured from the bed or other datum and your tools can be set to it.

 

The upper stylus is for setting from the lathe bed and can also be used for marking the centre of bars accurately while in the chuck, the lower stylus is for marking from the cross slide.  Simples

 

Terry

 

Edited By Terryd on 20/07/2010 08:09:22

[ZigFireParticipant @zigfire]

Hey Terry,

 

Now that is simple. Way to go, thanks very much. With my peepers getting worse with age this will help heaps.

 

Cheers

Michael

[Gordon WParticipant @gordonw]

I just use a bar, turned to exact length, well 2 bars, one on cross slide, and one on top slide. Easy to see, and also can be felt with thumb nail. This may not be accurate enough for threading or very small dias.

[Cornish JackParticipant @cornishjack]

Simplest and fastest – pinch a small ruler between the tool point and the workpiece (circular) – if the tool is on centre height the rule will be vertical. Easy to see/check even slight  angular discrepancies,easy to adjust.

How I hate those $£**^!? meerkats

 

[Keith LongParticipant @keithlong89920]

Hi Michael

Another (very old) trick to use is to lightly nip a ruler or similar between the tool tip and the o.d. of the job so that the ruler stands upright. You can then sight (along the axis of the work) the edge of the ruler against a known vertical line or a set square sitting on the lathe slide/bed or where ever is convenient. When the ruler is vertical the tool tip is on centre. Surprisingly accurate and the sensitivity improves with reducing work diameter.

Keith

Cornish Jack – You just beat me to it – need to learn to type quicker!!

 

 

Edited By Keith Long on 20/07/2010 10:40:40

[Sam StonesParticipant @samstones42903]

Hi Michael,

I may attract an outburst from some ME members when I explain my method of finding centre height.

Like Topsy, it just growed.

First, I have to say a few things which will actually take me longer to write than it will for you to set the tool height.

1. My eyesight is not the best either, requiring a 4″ binocular magnifier to handle most of my small work, especially when making 1mm diameter dowels for the skeleton clock.

2. I’ve had almost 60 years using lathes of various kinds, mostly my Myford ML7 which happened to take 5/16″ square HSS tool bits. I made a four-way tool post to accommodate these tool bits. [It’s probably a Myford standard anyway.]

As new, they were close to spot-on-centre unless a top rake was introduced and then trimmed/ground to keep the tool sharp. Most of my fine work was/is carried out with no top rake, so that centre height remains constant. I’m sensing a shudder from the initiated, but let me say that I’m also familiar with Mohr’s Circle and cutting theory from my HNC studies way back.

3. Even though I had a four-way tool post on the Myford, and an (upside-down) rear tool post for parting off, I adopted a `one-tool- suits-all’ for general facing and surfacing, and even parting off. The latter was a little bit time consuming in operation but often saved time in setting up. More shudders!?

So how do I find centre?

With the tool set as close as I can, I begin to face off the end of a piece of bar until coming close to centre. (The bar may even be the start of a workpiece.)

Depending upon how high or low the tool tip sits, this action leaves a pip or similar mark which, without exception, will be at dead centre. I can use this pip/mark to set the tool height as required.

Unlike the `height-gauge’ shown by Terry which will allow you to not only see but `feel’ the correct height, my method does require close-up inspection.

For a few quid, (I’m not sure of the price), you might also choose to buy yourself a `centre and edge finder’ normally used in the milling machine to pick up hole centres and edges. I’ve never used one in the lathe, but I don’t see why not.

Have fun,

Sam

[ZigFireParticipant @zigfire]

As usual thanks for all the great feedback, help and tips guys.

 

Cheers

Michael

[chris stephensParticipant @chrisstephens63393]

Hi Guys,

I have a QC tool holder, which is of course adjustable for height and I wanted some means of setting tools to the right height. In a wonderful second hand shop, now  unfortuneatly no more, I found a Boxford comparator, like a dial gauge but with a centre zero, and I thought this would be a good basis for a height gauge. With this comparator on a stand and sitting on the cross slide I could set the tool but what is “centre height”?

 

To find a true centre height I  turned down the end of a piece of 3/4″ by 3″ stainless to true 18mm by 1″. I then milled the first 1/2″ to “exactly” 9mm, in the manner of a D bit. If this piece is mounted in the lathe to run true on the 18mm portion, and the flat is set horizontal, it must perforce be on centre height. I then set the comparator to read Zero on this flat. It is much easier to set a height by watching a needle move than using a fixed gauge like the picture a few post above.

 

I can hear you lot saying something like “overkill” and you would most likely be right, but in my defence, I had the access to the stuff and I was bored one afternoon. 

 Let us remember that the first  “E” in SMEE is for experimental, so I experimented.

 

If I don’t need to set tools to exactly the right height, ie when I am not using geometry critical carbide tips, I use the thin strip caught between work and tool method, rough and ready but good enough for simple purposes.

chriStephens 

 

[Gone AwayParticipant @goneaway]

Following on from Chris’s comment you can also use a “half-centre” if you have one. Or buy one – they’re quite handy at the right time.

[chris stephensParticipant @chrisstephens63393]

Hi Sid,

Indeed they are quite handy but they are not usually “half” centres. They generally are slightly more than half or less depending on which way you look at it. They need to be such to prevent them from acting like a “toolmakers reamer” and cutting an ever deeper “centre”.

chriStephens 

[Sam StonesParticipant @samstones42903]

Hi Michael,

Here’s one which will fall into the category of `grandmas and eggs’, and which is already generating in my mind, “What have I started here?”.

However, I have to ask “How important and how accurate is setting the cutting tool to the centre of the workpiece?” Do I hear growls of resentment?

I must also preempt and simplify my comments by suggesting that we consider a basic lathe setup for surfacing, ie. reducing the diameter of a short length of bar projecting unsupported (cantilevered) from the chuck.

To start with, in every situation when metal is being removed by the `cutting’ process (which is not really cutting anyway), stresses act upon every part of the machine, the tool, and the workpiece. These stresses translate into strain (deflection/movement) of every component involved. They will also to some degree, take up any slack in the machine.

Clearly, the stresses generated will be in opposite directions, starting at the point of contact between the cutting tool and the workpiece. For simplicity and my own limitations, the actual direction of the resultant force is a combination of three forces generated at the cutting tool tip. In other words, there is the (main) downward force on the tool, there is a sideways force mainly from the feed, and a `backward’ force pushing the tool away from the workpiece. By definition, there will be an equal and opposite resultant force in the workpiece.

For all sorts of reasons, I believe it is important to note here that, relative to the cutting forces, the deflection of the projecting bar (at its tip), will be proportional to the cube of the projecting length, and to the fourth power of the diameter. Similarly, the overhang of the cutting tool (also cantilevered), will add to the deflection in proportion to both the length of tool overhang cubed, and the vertical `thickness’ of the cutting tool cubed. Don’t be surprised if the workpiece is somewhat thicker at its tip than close to the chuck.

With these factors in mind, it is easy to see that the question of centre-height accuracy is controversial.

So, relative to the style and sharpness of the cutting tool, the depth of cut, the surface speed of the workpiece, the feed rate of the cutter, the material being cut, and the presence (or otherwise) of lubricant/coolant, (there could be others), we have a situation where the workpiece will lift by some amount, and the tool will dip by some other amount.

And by how much?

Suffice to say that IF the tool is always too high under any conditions, the front face of the tool will rub against the workpiece. The converse is perhaps preferable and, as a result of the above comments, will take place regardless.

Anyway, keep your `cutters’ sharp gentlemen!

Sam

[chris stephensParticipant @chrisstephens63393]

Hi Sam,

You could add to your list the diameter of the workpiece. It should be obvious to all that centre height is more important for small diameters and less so for larger diameters. If one should need proof think about facing the end of a bar, a tool set a bit too high or low will cut perfectly well till you get to near the centre, when there will be left a “nub” .

chriStephens 

[TerrydParticipant @terryd72465]

hi Michael,

 

Me again I just used to check the height by putting the sharp centre in the tailstock and swing the toolpost round so the tool could touch it and then adjust.  Extremely quick, simple and accurate.  I also used to use shims and kept each set with it’s tool so I didn’t have to check again,

 

When facing, if tool is too low it produces a ‘pip’ which is a cylinder.  If too high the tool will leave a hillock (or ‘mound’) shaped ‘pip’. Hope that makes sense.

 

I always set the tool on centre, it takes little time and it doesn’t matter about forces involved or diameter of work etc.  Rather a fraction low than high.

 

Regards

 

Terry

Edited By Terryd on 21/07/2010 04:14:40

[Ian S CParticipant @iansc]

Hi  folks, the way I do it is, When I first got my lathe I put a center in the mandrel and used it to scribe a line across the blade of my 6″ square, used that ever since.Ian S C

[Sam StonesParticipant @samstones42903]

Chris wrote :-

Hi Sam,

You could add to your list the diameter of the workpiece. It should be obvious to all that centre height is more important for small diameters and less so for larger diameters. If one should need proof think about facing the end of a bar, a tool set a bit too high or low will cut perfectly well till you get to near the centre, when there will be left a “nub”.

chriStephens

Hi Chris,

You’re spot on with regards to the nub, the pip or the remaining tit. There was just a hint about this in my post yesterday, timed 20/07/2010 10:49:52 (ie. third paragraph from the end). I wouldn’t agree with your comment about the tool being a bit too high `cutting perfectly well’ unless of course the resultant forces `bend’ everything into line. See my last paragraph in my posting timed 21/07/2010 02:22:56

As for diameter, you are correct again. It didn’t make the grade because, as you say – it is so obvious.

Michael, from all the responses both detailed and simple, it looks like you have lots to choose from. Clearly, it will be what’s best for your needs. The little extras (like the `power’ laws I mentioned), are a bonus for anyone interested.

Best regards to you all.

Sam

[BogstandardParticipant @bogstandard]

Because of the types of cutting tools on the market today, especially tipped boring bars, which usually have a negative top rake, the height gauge is now no longer accurate enough for those types of tools.

 

Like Ian above, ‘splitting the line’ setting can easily get you to within 0.002″ or even closer.

 

Here is a post I did about the issue. I used the facing off method to get my final centre height, but Ian’s method is again very acceptable. Once you have made your setting piece, that is it, no more messing about.

 

http://madmodder.net/index.php?topic=1661.0

 

You could use a piece of tube squared off at the ends for your setting gauge.

 

 

Bogs

 

 

[ZigFireParticipant @zigfire]

Hi Guys,

 

Once again thanks for all the great tips. I now have a few ways to try out and find a consistent relatively easy method .

 

Cheers

Michael

[John Wood1Participant @johnwood1]

Hi all

 

My Wife reckons I often over complicate a problem by making it too technical, the trouble is, quite often she proves to be correct !!  I reckon the above subject can fall into this ‘trap’ as I too have tried many methods of centre finding when I started out and now have quite a collection of height guages, finders, sharp centres etc. etc., most of which don’t get used at all these days.

 

I find that simply turning a piece of round bar and adjusting accordingly depending on the pip serves me very well, I do have to take a bit more trouble if setting a tool to turn a very small diameter but the method still remains the same. If you need an indicator then Ian’s method of scribing a line from the mandrel across the blade of a 6″ square and using that as a guage is so simple it can’t really fail, it’s simple, efficient and takes up no room on the shelf – “keep it simple” says ‘er indoors.

 

Cheers, John

[Sub MandrelParticipant @submandrel]

| Hi  folks, the way I do it is, When I first got my lathe I put a center in the mandrel and used | it to scribe a line across the blade of my 6″ square, used that ever since.Ian S C

Don’t spoil the fun Ian!

Neil

[John OlsenParticipant @johnolsen79199]

Hi Guys, I suggest you refrain from trying the face off and check the pip method if you are using carbide inserts. It is liable to wipe the edge off your insert if the height is too low. Don’t ask…. actually most of the tips I have done in have been from doing this without remembering, eg the job need facing so you wind the tool into the middle, it turns out to be a smidgeon low, exit one insert.

 

John

[Neil LickfoldParticipant @neillickfold44316]

This is what I do to find approximate centre height when only using a boring bar when the stock piece already had a hole in it. I mark tge outside, then wipe a edge from something that is held vertical. It leaves a mark on the side. You could wipe a known tool on t outside too, but does leave a mark that may not be wanted.

[Chris MateParticipant @chrismate31303]

Variable:12mm indexble set and HSS tools
I made a height guage with a tip, but as other stated the forces are not always the same effect, so I find I use the ruler method most and if not a hole in centre I fine tune it with QCTP adjust..

Fixed:10mm set
However I bought a set of 10mm indexable cutters, and in this case they are all perfectly indexed in the sense their tips allways at the same height, unlike the 12mm set I bought. So in this case I made a fixed toolpost and when any of these 10mm tools are fitted they always on centre. I use these for cutting smaller things, like needle felting holders I made holding various number of needles. These are not heavy cuts so I never had a not on centre situation here, even if carbide tips are replaced.

Something to think about:Turning towards you.
If tip is slightly high and you plunge the tool ito the turning stock, it may just cut and take the bite.
If its way high, the tip may dig in and something has to give as workpiece turns over towards you…?

If it on centre in can only go down and rubb.,,

If its too low, it may rub more than cut.

Boring bar you may want to go up a bit sometimes, rather than downbelow centre a bit.

[DMBParticipant @dmb]

Ever since the QCTP was fitted, I simply do a light facing cut, watching carefully near the centre for a small pip to form. Experience has taught me some idea of how much to twiddle the adjusting screw to achieve perfect centre height cutting. Usually only 2 attempts and I'm there. Very quick and easy. Swivelling the toolpost around to do the centering with just about any old turning tool and then reposition for turning. In all other centering needs, the old trick of nipping thin metal or steel rule, gives same result. Many a situation on the mill with various types of cutters can be dealt with in the same way. Never seen the need for special setting tools, just seems like unnecessary complication of a simple exercise.

John

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