Cross slide graduations?

I've just got myself a colchester chipmaster and I'm just trying to get my head round the graduations on the cross slide (I've not used a lathe since the late 80's) One full revolution moves it 5mm so 10mm off the diameter. The cross slide major graduations are numbered to 100 so that would be 0.1mm per graduation. But they actually go up in units of two so there are actually only 50 graduations so that is 0.2mm per graduation

In between the major graduation there are 5 minor graduations so that would be 0.04mm.

These numbers dont seem that 'sensible' to me. Have I got it all wrong?

So the marked markings on the crosslide indicate amount off the diameter then. Or am I missing something.

regards Martin

That seems quite a coarse feed screw for a lathe that size, I would have thought about half that. 2.5mm pitch or 10tpi would be more likely and what a quick google shows

Is it definately a metric machine? Have you measured movement with a DTI as an imperial one would move 5.08mm for the same high pitch screw.

If it is a metric machine then they are more likely to have a scale that shows diameter so 10mm divided by 100 divisions would be 0.1mm per div off diameter. But if imperial machine with 10tpi screw 1 division would equal 0.001" as they are more usually graduated for radius

Edited By JasonB on 29/08/2021 07:06:36

Edited By JasonB on 29/08/2021 07:09:53

I've not put a DTI on it yet but I will do. The machine is the Continental version so I assumed it to be metric but perhaps not.

If you have a four jaw chuck, you are going to need a DTI, and a magnetic base.

Preferably both Plunger and Finger types, with graduations in which ever system of units your machine uses.

Until you do get these things, load a piece of bar and take a skim off it,

Do NOT withdraw the tool!

Measure the diameter. in Metric and Imperial units

Set the Cross Slide dial to zero.

Advance the Cross Slide by one division and take a cut.

Remeasure.

The difference should tell you what the divisions on the dial are. (As you say you should have removed the equivalent of two divisions, if the dial is graduated for radius, or if diameter that is what your measurements will show.

Howard

Your numbers are correct. You will soon train your brain to live with it. If not, you can make your own dial graduated in a way that suits you better.

The imperial Chipmaster has 200 marks around its circumference, each representing 0.001" movement (i.e. 0.002" off diameter). This gives a feedscrew pitch of 5tpi, comparable in coarseness to the metric machine's 5mm pitch.

The metric one has 250 marks around its circumference, so they are a little closer together than those on the imperial machine. Advancing one mark moves the slide 0.02mm. This gives a resolution of about 8/10 of a thou', comparable to the imperial one whose resolution is 1 thou'.

Metric lathe cross slides are usually calibrated in mm off the diameter.

So your feedscrew is 2.5mm pitch. This corresponds to data available on the web by a quick Google search.

Please give a reference to your data source.

My data source is in the garage. When I hold a ruler between the back of the dial and the cross-slide, one turn of the wheel advances the ruler from 10mm to 15mm.

For any lathe like the Chipmaster where the manufacturer offered both imperial and metric versions, the resolution of their dials will be comparable.

Please think through the implications of what you have said above. If the pitch were 2.5mm, the information supplied by the original poster tells us that there are 250 graduations around the circumference. Your theory would mean that each graduation moved the slide 0.01mm or about 4/10 of a thou' in old money. How many imperial lathes are graduated this finely? If you want to split thousandths on an imperial lathe, you go between the dial marks.

I have owned 3 metric lathes including my current S7 - though on that one the Xslide uses a 5mm pitch ballscrew as it's CNC converted. Its original leadscrew however is 2mm pitch and the dial calibrated 0 - 40 (though of course at 40 it goes back to zero). So dial indicates mm off diameter. Previous dual-dial metric lathe was the same for metric. I've also used other metric lathes including a Myford 254 which was the same.

**LINK**

The link above is to a previous thread here. More than one post covering Colchester lathes and saying that they are calibrated as "diameter".

The OP said 100 graduations, so with a 5 mm pitch Leadscrew, each division would correspond to 0.050 mm, or about 0.002" in old money. To my mind, this suggests that each graduation would mean 0.05 mm off the diameter .

Howard

.

Not quite, Howard … What he wrote was:

The cross slide major graduations are numbered to 100 so that would be 0.1mm per graduation. But they actually go up in units of two so there are actually only 50 graduations

MichaelG.

That is true, but what is not correct is your statement that "your feedscrew is 2.5mm pitch".

The original poster has said that one turn of the wheel moves the cross-slide 5mm. I have measured the same thing on my own machine this morning. Are we both mistaken? Are we both lying?

In effect, you are trying to claim the cross-slide pitches of imperial and metric Chipmasters differ by a factor of two. This is not sustainable. Even without detailed knowledge of the machine, consider what would have to change in the power cross-feed gearbox mechanism were this to be so and its linked effect on the longitudinal feeds.

The Chipmaster was made in both imperial and metric versions and it is more economic for the manufacturer to make as many parts of the two machines as close to each other as possible: all leadscrews same diameter and thread form, just different pitches, apron gears, handwheel and rack similar, gearbox layout, size and shape the same just with different internals.

The two machines need to offer a comparable range of speeds, thread pitches and feed rates. Changing something like a leadscrew pitch by a factor of two makes doing this difficult and expensive.

Be careful with a quick google search, reading further the couple of refs are 9/16" x 2.5mm then if you click the link it says double start 5mm lead, I made that mistake this morning.

Edited By JasonB on 29/08/2021 10:05:50

So, if the Leadscrew has a 5 mm LEAD, 50 divisions on the handwheel will give a 0.1 mm increment per division.

If this is DIAMETER, the tool will actually advance by 0.05 mm which seems reasonable distance, since smaller increments can be estimated between the marked divisions.

The real check is to measure before and after.taking cuts.

Howard

Blimey, thanks for all the input. Not sure what to do with it all as I cant fully follow some of it. I think as advised I need to measure the actual movement with a DTI.

Colin, I don't think you should need a DTI, there's a 2:1 difference and you should see that with a rule! If you nip the rule in the tool post and improvise a pointer in the chuck - a nail would do - position the rule so the pointer is on a fiducial mark, zero the dial, then wind it in one rev. It should read 5mm or 2.5mm depending on the lead.

Some extracts from the thread I linked to earlier:

Muzzer:
"The Colchester Bantam also reads directly in terms of diameter reduction. Once I realised it is fitted with metric leadscrews and dials on the cross slide and topslide (but has an imperial main leadscrew and gearbox!), I was able to see that the dials indicate direct 0.1mm (major) and 0.02mm (minor) increments. I was confused for quite a while by the mixed units, although it seems to be a common situation on Bantams, many of which were supplied to schools.

I measure the actual diameter with a digital caliper and dial in the required final cut directly. Works as intended.
..and later..
...My Bantam cross slide dial measures directly in mm diameter increments, so I can now dial in the required additional cut directly. If I'm feeling paranoid, I can take several cuts and confirm that I am approaching the final diameter from above by using the calipers after each cut.

Murray"

John Stevenson:
"All my lathes are diameter mode.

So are all my drawings

So are all my DRO's

Which then leads me to ask if you have a radius dial on a machine do you program the DRO to also read in radius ?
and

After working on more lathes over the years that I can shake a stick at a few things I have noticed but not exclusive.

The bigger the lathe the more chance it's in diameter mode.

European lathes tend to do diameter more than small lathes or American ones.

Lathes like the Myford probably went radius as it's hard to do diameter dials on small machines due to size restraints and dropping the pitch from 10 to 20 means it wears out quicker."

So some Colchester lathes do work in diameter mode and it may be that different versions were available. If they hoped to export metric lathes then diameter mode would be more attractive.

You do get used to the peculiar choice of cross slide dial increments. It starts well enough - one turn is 10mm on the diameter, and whole millimetres are numbered 10, 20 etc. Then it goes wrong with each millimetre divided into five major then five minor graduations, presumably somehow conceived to be as near as possible to traditional British units. After a bit it is OK, and does keep you on your toes!

It may be related to the power cross feed which is half the rate of the along axis. This may have dictated the two start cross lead screw, whereas a single start would be more satisfactory for precise setting, allow better dial increments, but would need a higher geared apron power feed.