Plus or Minus

Just been having a discussion with a friend and the subject of DRO's on lathes came up the question if say you have set zero and you move your cross slide in towards center your DRO should give a minus reading likewise if you move the saddle towards to chuck this again a minus reading. I suggested it was a personal prefrance as to which way the cross slide and saddle should be moved to give a minus reading what do the forum members think ?

H

Hi Harry,

I'd have thought it's irrelevant to be honest. We know we are moving along the work piece or removing material, it's how far or how much that is being removed which is more important than the direction. If you covered up the +- sign with a bit of tape I doubt you'd notice any difference in using the lathe and DRO.

Ed.

I think of it this way....

If I am taking a cut after zeroing the dro, then as I move the crosslide to the centre, I am effectively reducing the diameter of the work, so I'd say the convention as stated stands....logically thinking...

But others may have a different approach/opinion

Edited By John Rudd on 17/02/2019 16:46:12

The convention with CNC is that the Z reading, the longitudinal axis, decreases towards the headstock; while the X axis, the cross slide, decreases towards the spindle centreline. Normally one sets the tool so that the actual cutting diameter is indicated on the DRO.

If you have aDRO that can accomodate tool offsets, then this is a logical way to work: you reference the lathe with some reference tool, or other way of "homing", so that when you dial the tool number into the DRO it will indicate the actual diameter that tool will cut.

One way to handle Z is then to zero the Z axis on the very end of the bar sticking out from the chuck, and then all your measurements on the part are negative.

You don't have to do it this way but it is a sort of standard.

You could set it the same way as the handwheels which are plus as you take a cut, seems to have worked for years.

Though it would seem better with a DRO to work the opposite way slowly reducing the reading as the cuts are taken off and if you go past the desired size you will be reading negative for under size.

Edited By JasonB on 17/02/2019 17:18:17

You can reverse the direction a DRO axis reads in so it's entirely up to the owner. I have mine set so as the tool moves towards the centre, the number decreases (along with the diameter of what you're machining). I tend to zero from the tailstock so as I cut towards the chuck, the number increases but that's just me, however, on my milling machine, sending the table to the left increases the number.

On my lathe the dro for the cross slide is set to show the diameter of the workpiece and i I don’t pay any attention to the negative or positive on it or on the carriage dro or for the dro on my mill either .

There's absolutely no reason why one way should be better than another, just check when moving to a new machine.

Neil

The CNC convention John gave is based on the idea that if you forget the minus in front of a number the tool will move in a safe direction, doesn't quite make sense when boring or cutting away from the chuck though.

Martin C

Actually no, I believe it's derived from viewing the lathe as a milling machine tipped over on its left side and then its back. The milling conventions follow from coordinate geometry, (x,y) origin at nearest left, height z zero when tool touches table and increasing upwards.  This allows the same basic controller system to be used for both types of machine.  It also results in some oddities like the conventional lathe spindle direction being labelled as clockwise, because it is, viewed from the other side of the headstock!

Edited By John Haine on 18/02/2019 07:47:40

Just be thankful you haven’t got a robot with multiple coordinate systems

Mike