How far to be off tool centre before a turned finish becomes poor?

Title kind of says it, by how many millimetres / tenths / hundredths does your lathe tooltip need to be off exact centre before the turned finish starts to degrade?

At the moment I just have a piece aluminium sheet offcut to boost my tool height, and its probably off by about half a millimetre. My finish could definitely be better, so will better centring help? I have some shim on the way to try but I just want to get a feel for expected results!

Oh for one of those posh expensive QCTP's with the adjustable dial ....

Cheers!

Mark, what a good idea, thanks!

A lot will depend on the diameter of the work, 1mm below ctr is not such a problem on a 150mm piece of work but it is if you are turning a 3mm pin.

Probably safer to be slightly low, if you are having finish problems then some care in getting the tool height right will eliminate this as a potential souce of trouble. As there are many variables that can influence finish then the more you can eliminate helps you to zero in on what is not working.

Mike

I do not have a QCTP,nor do I want one.I do have a few homemade tool posts and a collection of shims from 1/8" down to drink can thickness.There is no shortage of material to use for shims,and they work [and they,re cheap !}

Dont get too excited about QCTP,s,a lot of good work was done before they ever came along.

Buy a can of beer,drink it,then cut up the can !

Cheers !

Larry.

I use old "credit cards" which are about .7 mm thick.

Like Jason says it depends on the work diameter. I normally set the tool height by reference to a live centre in the tailstock, so not that accurate. Probably 5-10 thou; but I'm often turning parts from an inch upwards. If it's important, I'll face off and tweak the screws in the 4-way toolpost until the pip disappears. I'd expect to get finishes in the order of 1-3 microns Ra.

On the repetition lathe I just tweak the tool locking bolt until the pip disappears, or it looks about right if I'm turning an external diameter.

Generally I don't get too hung up on tool height, except for parting tools, where I take a bit more care. Even then I don't sweat the last few thou.

Andrew

when turning O/dia with tool below centre front clearance will increase and rake decrease so the tool will cut, When facing below centre will leave a pip at the centre. With the tool above centre,front tool clearance will be zero or less and the tool will rub and not cut,rake will also be reduced,the detrimental effect will be serous or failure at small diameters and the effect decrease as diameter increases. when facing with a tool above centre the tool will try to ride over the centre pip. Some years ago there was a letter in the model engineer about the writers experience when working at a company in Manchester and no doubt his experience was with a lot heavier engineering,it was stated that the turners when trying to get good finishes on diameters on tougher steels would set the lathe tools above centre height and halved the cutting speed ,Curious ,I tried it on some EN 8 bar a couple of inches in diameter and it worked though the tool must only be set a small amount above centre and plenty of lubricant. With model engineer sized work regarding boring I always set the tool slightly above centre to ensure there is a bit extra clearance as it is all too easy to loose all clearance

Thanks all for the responses, a lot to chew on there. I went back and gave it another try and added some support from a dead centre, fiddled about with the tool angle and adjusted speeds and feeds and got some OK finishes. Certainly not mirror finish but I am just using a bog standard cheapo turning tool.

I will have a play around with the various shimming methods suggested and see if that helps. I've also come to the conclusion that one or two of my insert tools are wrecked.

Further to the shimming issue sections of old / broken hacksaw blades with the teeth ground off are a pretty useful size.

I often feel the term shims is misleading as it implies a stack of thin layers. In my block tool post using days I generally had thick and thin "spacers" under the tool with shims for final adjustment. Spacers usually chosen from the range 1/4", 3/16" and 1/8".

Probably worth figuring out a way to measure tool tip height off the machine so you can pre-select the shim stock needed from your stock go known thickness shims and shallow spacers. Block on a flat base with a hinged finger arranged to be level when resting on a tool tip with the right number of shims underneath to bring it up to centre height would work. Cheap short travel dial gauge to measure height of finger so you can work out what shims needed. I as going to make something like that using the old Unique pressed tin lever type gauge I've had lying about pretty much forever. If the finger is reasonably long a ruler would be accurate enough but the device gets unwieldy. Which is actually what I started thinking about. Mr Posh puts a big round elephants foot end on a dial gauge in the usual sort of post stand and calls it good. Effective but spendy.

Clive.

Edited By Clive Foster on 14/12/2018 22:54:21

when needing to shim tools i find formica swatches cut into strips, copper slate strip from the builders merchant,credit cards,the cards from WH Smiths they give with reciepts and steel shim down to 2th serve well.I always get my tools to centre height apart frm boring bars which I keep a little above to accomodate deflection.

Frank

A little bit below centre height can actually be helpful on small flexible lathes, or worn lathes. The tool does not dig in if "grabs" but is deflected away from the job.

One of the most useful tools in and around a lathe is a centre height tool or gauge.

So much faster & precise than any other guess work inc tailstock "that'll do" centre.

Base either off the bed ways or cross slide.

Make a nice one in less than a day, dunno why more people don't use them / have them.

These theoretical values are all very well in an ideal world. Surfacing is not the same as sliding. The suface cutting speed while sliding is constant on a cylinder for any one cut; for surfacing the same value reduces to zero as one approaches the centre point.

Young’s modulus, Hooke’s Law and other mathematical calculations are required to precisely affix a value for deflection of a cutter at any time during a cut - sliding or surfacing.

The likely cause for the poor finish can be attributed to any one of several reasons, depending on the user’s knowledge or guesswork.

It is most likely due to deflection of some kind. Along the surface or oscillations up and down while cutting.

These are often caused, in practice, by the cutting tip geometry altering in either a random or cyclic manner. Or both at the same time.

Tool quality and machine rigidity are two main variables - the others are represented in tables which offer speeds for different materials (workpiece and cutting edge), which are good practical targets or limits of sensible operation.

Hobbyists generally use small cutting tools in small, less rigid lathes, and sometimes are forced to use quite extended cutter overhang. These factors will alter that tool setting by a variable small amount. If the cutter is resonating, as it cuts, it will leave a less than ideal surface finish. Chatter is the phenomonon noticed, when the resonance becomes very obvious to the operator.

My advice to the OP is to minimise any possibilities of tool deflection, adjust the tool for optimum position and try at different speeds and feeds (so resonance can be avoided, or at least minimised).

That means avoiding cheap tools or holders (they bend more), use appropriate sharp cutters or inserts, ensure inserts are held securely in position on the holder (if used), shorten cutter overhang to a minimum - not just tool extension from the toolpost, but also to maintain the cutting point as close to the centre line of the carriage and toolpost, tighten all possible opportunities of unwanted movement in all planes (including the axis for the cutting operation over the desired range of movement). This may mean restricting the use of the slide at the extremes of travel.

There is little point in setting the cutter carefully at precisely centre height if the tool is immediately deflected downwards when making a cut. How many lock the compound slide and cross feed gib when making a sliding cut, or lock down the carriage and compound for a facing cut? Agreed, centre height is the ideal - but don’t be fooled into thinking a static setting is going to be the same as when actually making a cut with a non-rigid set up. It may be only the standard starting point for any particular machine and direction of cut.

I expect the OP’s operational shortcomings are actually a combination of factors, not simply - or only - cutter height. A ‘posh’ but poor QCTP could make things worse if it is excessive tool-overhang which is causing the grief! Lantern-type single tool holders certainly helped the modellers, of long gone days, to minimise some of the potential pitfalls while turning.

First thing to do could be to inspect the suface finish and see if it can be determined whether the roughness is down to oscilations around the circumference or along the surface. If neither of these, it could well be the cutter height (that typical eight degree magical nominal cutter-clearance angle in any or all planes).

If you’ve read all this, you may realise that surface finish shortcomings may be caused by any one (or more) of several, possibly small, sources - not just the tool height setting, which is just one of the possible causes.

Clive, I have the tool height written in indelible ink on the face of the cross-slide, measured from that face to the tip of a centre mounted in the headstock and checked that it runs true. When i set up a tool, i just sit it in the toolholder, lightly tighten the screws, grab the vernier caliper, sit the end on the tip of the tool and drop the depth gauge to the face of the topslide then i instantly know how much to shim it. Works almost every time, just have to remember to check once the shims are in and the screws tightened fully. Sometimes it needs a little tweak due to the pressure of the screws compressing the air gaps between the shims. Beauty of this is that you are measuring from the cutting edge of the tool regardless of the size of the tool stock.

.

... and therein lies the wisdom of the approach that Andrew suggested.

i.e. With a facing cut, adjust the tool height such that no pip is produced.

MichaelG.

I set my tool to centre height with a steel rule on the bed and mark one eyeball. Tool height is 4". It is rare that I have no centre hole so I don't generally have centre pips to judge by. Seems to work fine. I believe the trick on larger lathes in industry was to set a tool to the correct height then use it to scribe a line on the tailstock barrel for future use.

Martin C

Lot's of good advice already, and I particularly agree with NDIY that other factors are more likely to be the cause. It's even possible the material is the problem - the last lump of cast-iron I machined was so coarse grained that a mirror finish was impossible, even by polishing.

Having the tool too high is to be avoided at all costs. But, provided the diameter of the work is large compared with the error in tool height, setting the tool low doesn't seem to matter much, it may even help. (Being a millimetre low on a 20mm diameter rod wouldn't matter, but being a millimetre low on a 4mm diameter rod would be disastrous.)

When the tool is cutting at centre height, the amount of metal above the cut is minimised because the job curves away from the tool tip. Dropping the tool height increases the amount of metal above the cut, which I guess will make the metal slightly more likely to tear while consuming more power than is absolutely necessary. Grossly reducing the tool height will tend to lift the work, perhaps bending it, or even wedging the tool underneath.

It turns out that it is relatively easy to set a tool to centre height. Thank goodness the geometry doesn't require us to calculate and set complicated tool positions before a lathe will cut properly! And because the tool height is standard, the other parameters like rake and relief can all be calculated relative to it.

However, although I don't think Ross is having finish problems solely due to tool height, it's well worth shimming the tool correctly. Generally when setting up a lathe it's best to eliminate everything that might cause a problem. Improve rigidity by minimising overhangs or adding support. Experiment to find the optimum cutting speed, depth of cut and feed rate. Use the right cutting tool and make sure it's in good condition. It may be necessary to lubricate. Know that some materials are poo whatever is done! Make sure swarf doesn't get trapped - mincing previously cut metal against the job can be ruinous. Also, make due allowance for the operator - practice and experience make a big difference.

Dave

Agreed, but if you associate the centre height with surface finish, without taking other things into consieration (as well as, or instead of), you may be decidedly disappointed with the correlation.

I adjust for no pip on a facing cut, but that may well not prevent a poor slidiing finish if the tool is deflected downwards by a serious amount or chatters or flexes sideways or the cutting edge is not keen or

Even a facing cut finish can be very different from the circumference to the centre pip. If operating with a long overhang or a flimsy tool shank, the cutting forces being far greater at high suface cutting speed near to the circumference than at the centre can make differences in finish apparent; the liklihood of passing through a ‘resonant patch’ while surfacing is greater - and may show as different patterning across the face, even if chattering is not picked up by the operator.

Without the operator considering all possibilities for poor surface finish, they will likely miss the real cause, or combination of causes.

Andrew can likely rely on the ‘centre pip’ method rather more reliably than those with less-than-rigid equipment. He can, if he wishes, improve his surface finishes by using other methods as appropriate.

It is often the case of looking at the whole wood, not just one single tree, for a solution to a problem, is it not?