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Cross slide dial calibrations - opinions sought.

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Andy Stopford14/05/2020 19:24:47
59 forum posts
1 photos

I bought a CJ18A mini-lathe a few months back. It's not at all bad for the price, but there are aspects where there is room for improvement.

One of these is the maddening 0.025 mm divisions on the dials, presumably a 'close enough' approximation for those who would rather work in thousandths of an inch. I prefer to work in metric and being rubbish at mental arithmetic, they do my head in - I have to constantly refer to a cheat sheet or use an increasingly oily calculator to work out what cut to put on.

So, fine, I'll make some new dials with proper 0.02 divisions, but whilst I'm about it, how about doubling them up for the cross slide so that the dial shows the reduction in diameter rather than radius, and save myself even more mental arithmetic.

I know some lathes have this arrangement but I've never used one, and wondered if anyone has any opinions on whether this is a good idea?

Pete Rimmer14/05/2020 19:30:54
872 forum posts
54 photos

Yep, it's a great idea. Putting even-depth lines on is reasonably simple. Stamping numbers not so much, unless you can etch or cnc engrave them.

HOWARDT14/05/2020 19:57:39
653 forum posts
15 photos

While 0.02 would be easier than 0.025 to figure, I find that I can still divide by 0.025 to get the number of graduations needed to move. It’s just another job to add to the list of modifications before we can buy the ideal machine we couldn’t afford to get

Andy Stopford14/05/2020 20:00:07
59 forum posts
1 photos

Thanks Pete. I'm using this method to stamp the numbers:


It's a bit hard to see in the photo, but the number stamp is laid in the toolholder with the clamping screws just touching it and pushed by finger up against the rear face, then a tap with a hammer and it looks acceptably neat.

OK, I confess the taps with the hammer weren't as hard as they should have been, so after a clean-up skim they're a bit faint, so this one is going on the top slide. Which is good, because it gave me time to think of posting this thread.

And yes, that really is a dodgy amount of jaw to be protruding from the chuck...

JasonB14/05/2020 20:15:26
19525 forum posts
2133 photos
1 articles

What is the actual pitch of the leadscrew? 0.025mm divs would suit a 2.5mm pitch but you would be left with an equally hard to keep track of 125 divisions if changing to 0.02mm.

Changing to 0.05 off dia sounds like the best option provided it's not actually an imperial 10tpi leadscrew that has just had metric handwheels added.

Andy Stopford14/05/2020 21:09:28
59 forum posts
1 photos

No, it's genuine 1mm pitch metric fortunately

John Haine14/05/2020 21:21:05
3527 forum posts
194 photos

My understanding was always that metric lathes had their cross slide calibrated in mm off the diameter, imperial in inches off the radius (i.e. actual tool infeed). All the metric lathes I have owned/used follow this convention.

Andrew Johnston14/05/2020 21:29:52
5824 forum posts
662 photos

My imperial lathe is calibrated in thou off the diameter. I wouldn't have it any other way. One measures diameter so it makes sense to be able to directly set the remaining material to be removed.


speelwerk14/05/2020 21:38:44
377 forum posts
1 photos
Posted by John Haine on 14/05/2020 21:21:05:

My understanding was always that metric lathes had their cross slide calibrated in mm off the diameter, imperial in inches off the radius (i.e. actual tool infeed). All the metric lathes I have owned/used follow this convention.

I remember the first lathes I stood behind (Hembrug AI and a Schaublin) were mm of the radius getting me seriously confused behind a Weiler which did mm of the diameter. Niko.

IanT14/05/2020 21:40:35
1746 forum posts
164 photos

Sorry to differ - I prefer my dials to be direct reading. Up to me to decide what I'm trying to do with them.



Pete Rimmer14/05/2020 21:58:46
872 forum posts
54 photos

I also prefer diameter-reading dials. One of my favourite tricks is replacing 5tpi 2-start Colchester cross slide screws with 10TPi single-start. The thread dimensions are exactly the same except half the pitch/helix angle, turning the direct-reading dial into a diameter-reading one.

Paul Lousick15/05/2020 08:02:17
1642 forum posts
613 photos

Add a digital readout.

Even modify a digital caliper and you can switch between metric and imperial. A cheaper setup will not solve the radius / diameter problem but will be much easier to read.


Dennis D15/05/2020 08:31:17
72 forum posts
2 photos

+1 for Pauls suggestion. I use a modified digital tyre depth gauge on my cross slide, Search Poor Mans DRO for plenty of different types.

Just spotted Stuarts post further down the page

Edited By Dennis D on 15/05/2020 08:36:49

Stuart Smith 515/05/2020 08:54:28
158 forum posts
26 photos

See my other post on fitting a cheap DRO system using the TouchDRO app and digital calipers.


The app allows switching between radius and diameter TouchDRO screen

Clive Foster15/05/2020 09:23:08
2529 forum posts
82 photos

Like all expressions of personal preference the debate between folk who prefer their cross slide dials to read feed and those who prefer them to read the cut taken off the diameter is inherently irresolvable.

Whats more important for the uncommitted and for those considering testing the water on the heretical side is the reason for a persons choice. If the way you do things doesn't chime with the way you think there will always be issues. (Yet another example of why neophytes should sample several gurus before deciding to follow the one who thinks the way they do.)

Moi. I'm a cut reading guy. Every machine dial except the one on my P&W Model B cross slide reads cut so its easier if they are all the same. Multiplying by 2 is easy.

Milling machines inherently work in cut so doing it different for one dial on the lathe hardly seems sensible. Especially when the topslide dial is always cut reading. Which is the one that can catch me out on the P&W. Although how much of that is because I topslide feed about every second blue moon and am not used to it. Topslide is pretty much only for setting screwcutting feeds and the zero to zero method avoids any problems there. Need brain engaged when setting the depth of cut on the cross slide to thread depth tho'. Probably the one absolute dislike.

All tooling data is in depth of cut terms so I'm already working the right way to decide appropriate speeds and feeds. Something that is becoming more important as good, reliable, insert technology slowly becomes affordable for home shop guy.

To be honest its hardly an issue for me. Helps that the P&W drives a bit different to a normal lathe anyway so I'm already primed. My normal procedure when doing anything approaching a precision job is to set the dials so last cut but 1 is going to be on zero. No thinking on the fly to finish on an odd number needed. Hafta admit that setting last cut by diameter is a smidgin easier. I'm not going to mistake it for a cut setting because the dial ahs too many numbers anyway.


IanT15/05/2020 09:55:56
1746 forum posts
164 photos

I've always called it direct reading Clive but I guess I'm a "Cut" guy too.

Liked your summary - all my machines are direct reading (Lathes, Mills and Shapers) but something not mentioned (and perhaps considered a little old fashioned these days) is that the lathe is also used for operations other than turning (drilling and milling for instance).



SillyOldDuffer15/05/2020 10:07:39
6660 forum posts
1499 photos
Posted by John Haine on 14/05/2020 21:21:05:

My understanding was always that metric lathes had their cross slide calibrated in mm off the diameter, imperial in inches off the radius (i.e. actual tool infeed). All the metric lathes I have owned/used follow this convention.

Footling about in the history of machine tools, modern lathes combining a lead-screw, slide-rest, and change-gears are due to Henry Maudslay. Around 1800 he appears to have established the British practice of working in depth of cut rather than reduction in diameter. Not daft, but neither is it ideal because diameter working is more common.

The down-side of being an engineering pioneer is it tends to embed old ways of doing things firmly in the minds of workmen, making it very difficult to change later. Turners trained on depth-of-cut are upset and bothered by diameter dials and vice-versa.

Countries that joined the industrial revolution later were free to improve on British ideas. As most lathe operations reduce diameter, it's handy to graduate dials for that, because it reduces the amount of arithmetic needed slightly and reduces errors.

So British lathes, workshops and factories preferred depth-of-cut graduations for historic reasons, while everyone else prefers diameter. But it's far from consistent: later British lathes do diameters whilst some foreign lathes do radius. Much less of a hot issue these days because most manual turning is done by intelligent chaps who can do either, rather than lines of semi-skilled workers turning in factories and likely hating it! CNC can do either or both and fitting a DRO pretty much makes the dial graduations irrelevant.


Martin Connelly15/05/2020 10:19:38
1605 forum posts
176 photos

Nice summary Dave.

Martin C

Mike Poole15/05/2020 10:44:14
2845 forum posts
67 photos

I find that a very crowded dial is more difficult to work with as the thickness of the lines begins to approach the spaces, make the dial as large as possible and then choose the intervals, the eye is very good at splitting the spaces especially if they are not too small.


old mart15/05/2020 16:54:53
2465 forum posts
169 photos

Apologies for having posted similar before, but the cross slide leadscrew and nut were very badly worn on the museums Smart & Brown model A and there was little chance of getting a new set. I saw a listing on ebay for a new leadscrew with two nuts of 1/2" X 8 ACME and enough length of thread to modify. It was unidentified, but cheap and I was lucky to get it. Unfortunately, the vendor had made a mistake in the pitch, instead of 8 tpi, it was 3mm. When I contacted the seller, he kindly refunded my money and said keep it. As finding an imperial leadscrew was unlikely, I made the metric one fit and used both full length nuts for a proper backlash setup. A new dial with 118 markings was made on the rotary table. 118 is not one of the numbers that we can index, so it had to be degrees and minutes for the markings.


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