I understand that metric lathes are always calibrated in diameter, odd though that your imperial one is too! All 3 metric lathes I've owned were / are diameter calibrated. Harrison lathes being aimed mainly at a professional market, where a shop is likely to have a milling machine as well, the main use for a tee slotted cross slide would be for boring where you wouldn't use the feed calibrations. So most of the time it would be useful to have diameter calibrations as the main use would be turning perhaps?

Personally I've always been perfectly happy with diameter calibration as I've never had to make the change from imperial to metric.

If you want a groove of known depth from a known diameter, just turn to the new diameter required??

Agree with Spurry that its a case of what you are used to. My big argument against diameter reading dials is that machines so fitted only ever, so far as I've seen, have them on the cross slide. The top side being calibrated directly in travel (i.e. radius) so you have two different scales on the same machine. Problem can get worse if you have more than one lathe tho'.

My Metric Smart & Brown 1024 has direct distance reading dual metric – imperial dials on both cross and topside. Very nice until you read or set the wrong scale. It also has an accurate saddle travel reading dial. Metric only unfortunately. My habit is to set the dials so the final cut is made on zero. Gets all the maths and figuring out of the way before you start cutting. With 2.5 mm and 100 thou per rev on the dials one turn out is easily seen. High end industrial accuracy and repeatability makes life so much easier. Given a sharp tool the cut is what it says to a tenth of a thou or better.

My other lathe is a Pratt & Whitney Model B 12 x 30. That has a diameter reading dial calibrated 0-250 in thousandths of an inch on the cross slide and direct travel ( i.e. radius) reading dial calibrated 0-200 in thousandths of an inch on the top-slide. So you have to keep your brain engaged when using both slides. Not a great lover of the diameter reading cross-slide dial for normal turning but had a job this week involving facing out to a diameter leaving a ridge which sowed the direct reading to a advantage. Quite relaxing to watch the dial tool smoothly down to the designated zero for 5" diameter with the wartime built machine purring away at around 100 rpm. P&W gears are quiet. Colchester it aint. Being a four shaft lathe it was easy to drop the drive with less than a thou error using the apron controls for either the forward / reverse single tooth dog clutch or the main drive clutch. With conventional gears for speed setting and screwed star wheel engaged clutches for the power feeds too its had to imagine a bigger contrast to the clutchless, two shaft, 1024 VSL with its motor driven push button infinitely variable speed drive. Anyone used to a typical model engineers lathe could get straight on with the 1024. The P&W would take more of an adjustment.

200 increments on a diameter reading imperial dial is fine. Lets you work to a thou easily. But calibration by decimals of an inch is an abomination. Thous and no decimal points is far nicer. I know there is no escaping decimals with a metric dial but I still don't approve.

Edited By Clive Foster on 29/01/2016 22:26:47

My imperial lathe (Harrison M300) is as described; diameter reading on the cross slide and direct reading on the top slide. It seems logical to me and I've never had a problem with it.

While my engine build is mostly imperial all of my commercial work is metric. For metric work I use metric micrometers and do the 40 thou equals 1mm conversion in my head. If I need accurate metric dimensions off a given diameter I touch off the tool on the work (with a fag paper) and zero the cross slide dial. I then work out the change in diameter and do the conversion to imperial.

Like the meerkat says – simples.

Andrew

My 63 L5 is metric and has a radius reading crosslide dial , ive had it 30 yrs but if i had the choice i would have preferred a dia reading dial as it makes things a bit simpler i suppose at some point i could make a dia dial , i have made my own tailstock dial and though the thread in that is imperial but its still great for drilling to exact depths

As you have a DRO why don't you just set that to read in radius rather than diameter.

As for the groove just bring your parting tool upto the work as its rotating, when it touches simply zero the DRO and then feed in the required amount. You can use incremental if you don't want to loose the absolute.

I have a 10tpi screw and same divisions as you on my 280 and have no issues with it reading dia.

I can't see how it would be a problem to turn a diameter using the DRO, then change the tool to a groove tool, "touch off"" on the OD, reset the DRO x value to zero, then cut your groove to the required dia. It's standard practice everywhere I have seen.

"Changing tools loses the DRO calibration " Well it does BUT,

as I mentioned above, that's why you "touch off", meaning you touch the new tool to the spinning work so as to take off nearly nothing, you'll see it when it happens, and then set your DRO accordingly. Same thing on a milling machine. It's been done this way for ages. The real advantage is the fact that with glass scales, backlash in the screws a non-issue because the glass scales read the absolute position.

Many DROs (such as my Shumatech, have diameter and radius mode)