Lathe facing convex or concave

Yes Mr Stevenson, I've been bitten with that particular coefficient in the past and now, depending what the job is I sometimes leave it in the chuck overnight and take the finishing cut the day after when all has settled down to an even temparature. It's surprising how the tenths diminish by taking a finishing cut on a workpiece that's a bit warm and then measuring it once it's cooled down.

Rick

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Yes Leylands were a great firm in their day

HI Niko & Guys
If you faced a large Diameter using cross slide . and it faced a slight angle . running a dti across the face using
the same cross slide. surly it would follow the same path ie zero . ?
Nobby

Graham, never thought I would find myself agreeing with a toolmaker about tolerances but there you go, perhaps you toolmaker types are evolving into designers?

Mark

PS. have to agree about AEC, I have a fondness for the Mercury.

Jason, as Graham mentioned the convention is to face a disc and measure along a diameter. If you really wanted to "map" the deviation you could use slips on a ref. plane as you would check a straight edge (put two slips with a suitable height difference at each end and then measure the position/height along the span).

Mark

Hi All

So The Cavaliers and roundheads are at it again....

Anyway.... If you want to check the accuracy of the saddle in relation to the plane at 90 degrees to the spindle axis, all you need is a known to be accurate straight edge long enough to be near the maximum swing of your lathe and for ease of use a 4 jaw chuck and a dial indicator.

Lightly grip the straight edge in the 4 jaw, (use soft but firm packers to protect your straight edge) then adjust the straight edge to touch and deflect a dial indicator tip set on the saddle at the centreline of the spindle. touching near one end of the test straight edge. Keep adjusting the straight edge until it deflects the indicator the same amount (Exactly) when touching near both ends of the 180deg rotated straight edge. At this point do not move the saddle or cross slide.

Then set the straight edge flat and level (Be careful not to move or knock it) and by traversing the cross slide with the indicator sliding along the straight edge you can check for any lateral movement errors of the cross slide. Any concavity or convexity will show up, You may find the slope is not linear if the lathe is old, the travel may wander sideways.

As mentioned in a previous post in this thread you have to first (know) the spindle axis is exactly in line with the bed axis. Do this with a test bar as previously described.

Both this and the axis test are interrelated. Yes this is art as well as science.

Regards
John

Well I probably see it wrong being on mainland Europe, but if you face a large diameter and than run a DTI across the whole diameter it will show any deviation from flat. The first halve from where you have started the cut to the centre will measure of course and hopefully zero. But if you pass the centre and move the crosslide with DTI to the other side of the faced part it will show (+) double the devation if the part is convex and (-) halve the deviation if the part is faced concave. Niko.

Edited By speelwerk on 24/03/2014 13:13:32

It'll depend upon which side you run the clock, Nobby.

Clocking the front side should give a zero reading if the tool was sharp and also took a full cut, but if you run the clock on the back (other side of the axis), it should show double the actual error.

Martin.

Quite so, if you only went to the centre; but if you went right across the face, the DTI would register twice the error when you reached the other side.

Gary

Hi Gary Martin & Guys

That,s fine I should have given it more thought it,s my age !!!! I will try it tomorrow Tue 25th . on my S7 , Mk1
Nobby

Hi All

Not many lathes can traverse the cross slide much past the centre line. Using a dial indicator on a freshly the turned face should show minimal or zero error. It is just replacing the tool. and replicating any error

That is why the straight edge method is normally used for testing, as long as you know it is straight it is a reference independent of the lathe itself, it is very easy to set it up to be at exactly 90 degrees to the spindle axis.

It allows you to test the entire travel of the cross slide for both straightness, and if it faces at exactly 90 degrees, or faces concave or convex.

With a sufficiently large disk you are prepared to sacrifice you can face it then check for concave or convex with gauge blocks or feelers against a straight edge applied across that face. A similar result to the straight edge method above but a lot slower and difficult to do accurately.

What I find interesting when doing any testing is to graph the results with excel or graph paper. When doing a test write down the results every say 5mm then graph them. If you think you are going to get a straight line with this facing test you are in for a surprise, unless the lathe is in very good condition and from a good maker you are more likely to see the line wavering due to errors in the dovetails or guide rails under the cross slide. You really need an indicator that can resolve .0001 inches for this type of testing.

Regards
John

Well, since the first halve of the faced part is zero till the centre line you can turn the crosslide as far out as possible, set the dail near the centre at zero and than move it across the back part. Niko.

If the lathe faces convex you get twice the error plus, halve minus if it faces concave. Another problem is to get a good quality steel rule, I use them for checking straithnes of musical box combs. When I bought several of them of different length and asked the question what the accuracy was, they where not able to tell me. Niko

You can clearly read that I have never checked it myself, it should be of course plus twice for convex and minus twice for concave. Niko.

Hi All

Twice the error?

Not really.....You are stopping in the middle of the piece if you are cutting a test disk. The error is simply the error along the radius.

Regards
John

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Edited By Gary Wooding on 25/03/2014 07:56:48

Edited By Gary Wooding on 25/03/2014 07:57:57