Colchester Master 6_1/2" Square head - Metric / Imperial Hand Wheels

[Howie Dines]

Hi all, I just wanted to share an observation about my 6_1/2" master. (Mch. No. F 3/73617X .. 1972)

I'm in the process of making the missing parts for a taper attachment for the lathe. One item is the cross slide screw which is a dual start LH acme thread and appears to be 10TPI.

But this then got me thinking, is it actually metric or imperial as I have the sliding hand wheels to move from Metric to Imperial. Also on the headstock I have the lever to switch between MM & Inch for thread cutting.



Inch dial shows 0.400" per turn (Diameter) 0.200" in actual slide travel from dial
mm dial shows 10mm per turn (Diameter) 5mm in actual slide travel from dial

So which is actually correct?

I don't yet have a DRO, but it was easily sorted by measuring the real movement for 30 turns of the hand wheel.
152.63mm which would calculate to 6.0090"

152.63/5mm = 30.526 turns
6.009"/0.200 = 30.045 turns.

Therefore the screw must be imperial.

The Imperial scale handwheel is true and the mm scale is really out by 0.08mm (3 throu) per turn on the diameter of the part.

Conclusion:- Until I fit a DRO, carry on physically measuring when getting close to final size.

All the best, Howie

 

[Clive603]

Something wrong with your dials if one scale is out. On British dual reading dials both imperial and metric should be equally accurate as there are conversion gears inside. Ordinary outside gear on the screw drive shaft, inside ring gear on the scale and a planet gear to transfer the drive.

The sliding sleeve on the Colchester type physically shifts the planet gear. Alternatively, as on my Smart & Brown 1025, the gear sizing arrangements are such that the plants gear can mesh with two ring gears, one for each scale, whose tooth counts differ sufficiently to accurately convert between the two standards.

See here:- https://www.circuitousroot.com/arti...ials/top-slide/dual-dial-cartridge/index.html , for details of the Colchester (Gamet) dial operation and mechanics.

Clive

 

[johnmontrose]

The sliding sleeve on the Colchester type physically shifts the planet gear.

Inside the mechanism, it is a 125t gear and a 127t gear (pitch shifted onto a 126t pitch circle) with a small double width idler gear. I am not sure that planet gear is the correct term. The logic is driver-idler-driven.

Gamet made two versions (you can find the patents on Espacenet). The early one had all external gears. The later version had the 127t and 125t as internal gears, leading to an smaller diameter overall form factor for the same function (because the idler could go 'inside' rather than having to go 'outside').

To answer the OP's question, the leadscrew will be imperial. You can dismantle the dual-dial to prove this to yourself - the imperial scale will be fixed to the leadscrew. The metric scale is driven off the imperial scale and is in that sense the 'follower' not the 'leader'.

Another way of saying this is if you removed the idler gear, you can only turn the screw by turning the imperial scale. The metric scale would just spin freely on its shaft.

A second way of verifying this is to look in the parts manual - if there were a metric leadscrew made for the TT attachment, it would be in there as a separate part number.

If the dials are not reading correctly, dismantle, clean and examine. Hopefully all the teeth are good.

 

[Limy Sami]

If the dials are not reading correctly, dismantle, clean and examine. Hopefully all the teeth are good.

If they're not, be prepared for some serious sticker shock.

 

[john.k]

This was very costly optional extra on these lathes .....so its surprising so many have the geared drive............I have an odd little spanish capstan lathe with the same idea on both handwheels.

 

[Clive603]

johnmontrose
Just looked more closely at the strip down which confirms that its a dual width idler pinion.
Apologies for passing on incorrect information. Was told years ago by "one ought to know" that there was a gear change when moving the sleeve.

Compared to the dual dial set up on my 1025 the Colchester arrangement is exceedingly complicated with lots more bits. Mine is pretty much just a drive gear, two scales with internal gears, a double width intermediate pinion on its shaft and a longitudinal thumb screw to lock things down when zero has been set.

Clive

 

[Howie Dines]

Thanks guys for replying.
I've just had a play and yep you are very correct. When I slide the scale to mm, it only takes approx 29 1/2 turns to get to the 150mm of direct travel.
So the dials are correct
I never realised that.
My planetary gear is a smaller plastic one compared to those in the link.

When I strip and clean the one on the compound slide I'll add pictures.

Howie

 

[john.k]

I have the gearing on mine,but I dont think I have ever used it............ive always used a simple mental conversion of 1/10mm is four thou.......close enough for anything I do.

 

[johnmontrose]

This was very costly optional extra on these lathes

I think the manufacturing process must have become more efficient because the higher end manual lathes from places that cannot be mentioned here seem to have dual dials almost as standard (look at the AL-960B from Hare & Forbes in your town).

I have not quite worked out if the modern lathes could have a metric leadscrew with the imperial dial driven off it (i.e. would or could the Gamet dial work in reverse, as was the question in the first post of this thread).

Given that the gears are under no load, 3D printing would be an effective way to make them these days.

It would be possible to adapt the principle to make a saddle handwheel dial. With the idler gear having two different tooth counts, you could make a/b x c/d gearing between the (likely circular pitch) rack and handwheel. That would allow you to have a scale that measures a sensible number.
.

 

[Clive603]

The dial on my 1024 is vastly simpler with far fewer components than the Colchester type so, presumably much less costly to make.

Metric native machine with imperial secondary dial so the system works both ways.

Most of the parts in this picture. Inner gear is on the feedscrew so not shown



Gear and subsidiary (imperial) dial. One of the feedscrew thrust bearings in the middle.



Clive