Mystery mechanism on Colchester Chipmaster lathe

Mikel Levy · Nov 24, 2017

I took off the crank handle/micrometer dial assembly from the cross slide of my recently acquired Chippie because of a slight rubbing issue at one rotational position of the handle. What do I find but a very fine-toothed gear solidly mounted and affixed to the inside of the handle, driving a small pinion (as the handle is rotated), mounted on the proximal end of the leadscrew bearing bracket (please see attached photo). The pinion is not attached to any other component, so as far as I can tell, when driven by the gear it just spins on its little bronze stud, to no effect (other than producing the "rubbing" issue because of a slight wobble in the crank handle).

I have downloaded three different versions of the user manual for this lathe, and this gear and pinion are not documented in any of them (nor are the dual imperial/metric micrometer dials mentioned).

Anyway, I'm prone to missing the obvious, and maybe I can't see the forest for the trees, but can anyone tell me what the purpose of this gear and pinion is?

--Mike

Clive603 · Nov 24, 2017

Looks as if you have a dual range imperial / metric dial on the feed screw.

If so that little pinion does the rotation conversion from the native dimensions given by the dial directly driven by the feed screw to the foreign ones. Reckon yours is native imperial and foreign metric.

My Smart & Brown 1024 has similar dual dials but is native metric so the physical arrangement is bit different.

Clive

gregormarwick · Nov 24, 2017

You have a dual reading dial. One imperial, one metric. One is fixed to the handle and turns directly with the leadscrew, the other is geared down by means of that planetary gear, so that the two dials rotate against each other. That's the rubbing that you feel.

lionelt · Nov 24, 2017

Do you end up with two sources of backlash with these dual dials?

Is there wear that increases the difference between the true readings between mm and inch?

Mikel Levy · Nov 24, 2017

gregormarwick said:
You have a dual reading dial. One imperial, one metric. One is fixed to the handle and turns directly with the leadscrew, the other is geared down by means of that planetary gear, so that the two dials rotate against each other. That's the rubbing that you feel.

That's what puzzles me. There is no differential movement between the dials, they are engraved onto the same barrel. The gear and pinion are engaged regardless of which dial selected. As far as I can tell, removing the gear or pinion would not change the way the dials operate. When I get it back together, I will check to see if one rotation of the imperial dial advances the cross slide exactly .400in, and if one rotation of the metric dial advances the slide exactly 10.00mm, as indicated by the dial graduations. I must still not be getting it, because my current understanding of how this is working predicts that one rotation of the metric dial will advance the slide by .400in, not 10.00mm.

--Mike

Tyrone Shoelaces · Nov 24, 2017

Could be the gears are worn down to the point were they no longer work effectively.

Regards Tyrone.

SAG 180 · Nov 24, 2017

Doesn't the shroud over the dials need to be slid across to the measurement dial being used to engage one scale or the other?.

Danny VanVoorn · Nov 24, 2017

Mikel Levy said:
That's what puzzles me. There is no differential movement between the dials, they are engraved onto the same barrel. The gear and pinion are engaged regardless of which dial selected. As far as I can tell, removing the gear or pinion would not change the way the dials operate. When I get it back together, I will check to see if one rotation of the imperial dial advances the cross slide exactly .400in, and if one rotation of the metric dial advances the slide exactly 10.00mm, as indicated by the dial graduations. I must still not be getting it, because my current understanding of how this is working predicts that one rotation of the metric dial will advance the slide by .400in, not 10.00mm.

--Mike

Surely you have to pull or push the dials to get to the desired scale shown in the window? By doing so is when the ratio is changed.
I ordered one for the carriage feed for a new Harrison lathe I bought about 1990 at what I thought was a lot of money, when it arrived I was surprised to see it was packaged in a Gamut wrapper. In order for it to read properly it had a similar planetary gear train but only had one range to cover so I imagine the ones for cross slide travel with dual dials were really pricey. Not having the luxury of a fancy digital readout I use that dial almost every time I use that machine, saves a lot of time not fiddling with setting up an indicator or carriage stop, etc.

mark costello · Nov 24, 2017

I have the same setup and wondered what it was for. Also using the Inferial side to measure.

Clive603 · Nov 25, 2017

#2 for what Danny says about needing to push the dial in or pull it out to change from imperial to metric and vice versa. The active dial is read through the window in the shield. Nice clear system as you can't read the wrong scale but I'm not completely sold on engaging and disengaging those tiddly gears.

Alternative style has internal gears on the two rings with a piñon spanning both so the native dial, fixed to the screw, drives a free running dial in the alternative calibration. 120 - 127 ratio is close enough that a simple pinion will drive both given due adjustment to tooth forms.

Clive

Tyrone Shoelaces · Nov 25, 2017

Clive603 said:
#2 for what Danny says about needing to push the dial in or pull it out to change from imperial to metric and vice versa. The active dial is read through the window in the shield. Nice clear system as you can't read the wrong scale but I'm not completely sold on engaging and disengaging those tiddly gears.

Alternative style has internal gears on the two rings with a piñon spanning both so the native dial, fixed to the screw, drives a free running dial in the alternative calibration. 120 - 127 ratio is close enough that a simple pinion will drive both given due adjustment to tooth forms.

Clive

I'm more familar with the second style you describe.

Regards Tyrone.

Bill Fisher · Nov 25, 2017

From the Chipmaster user manual:

Cheers,
Bill

gregormarwick · Nov 25, 2017

Clive603 said:
#2 for what Danny says about needing to push the dial in or pull it out to change from imperial to metric and vice versa. The active dial is read through the window in the shield. Nice clear system as you can't read the wrong scale but I'm not completely sold on engaging and disengaging those tiddly gears.

I was going by memory from a triumph 2000 that I used to use years ago. The setup I described is presumably from something else that I have used somewhere along the line!

It is indeed how you describe it, not with two independant dials as I stated earlier. The dial barrel slides in and out on two detents with only one scale visible through the window at a time. In one position the dial barrel is locked to the leadscrew, in the other it turns independantly from the leadscrew, geared down.

Mikel Levy · Nov 25, 2017

Bill Fisher said:
From the Chipmaster user manual:
View attachment 213655
Cheers,
Bill

Bill,
Thanks so much for that page from the manual. None of my manuals have that illustration. So I gather from the drawing that what looked to me like one gear partially covered in grease (see the photo I originally posted) is actually two gears of slightly different pitch jammed together. By the way, do you happen to have a pdf of the entire manual?
--Mike

Mikel Levy · Nov 25, 2017

gregormarwick said:
I was going by memory from a triumph 2000 that I used to use years ago. The setup I described is presumably from something else that I have used somewhere along the line!

It is indeed how you describe it, not with two independant dials as I stated earlier. The dial barrel slides in and out on two detents with only one scale visible through the window at a time. In one position the dial barrel is locked to the leadscrew, in the other it turns independantly from the leadscrew, geared down.

I am still at a loss to make this work. If you look at the exploded view kindly provided by Bill Fisher above, it appears that what I took for one gear is actually two gears back-to-back. Of course I already knew that the desired dial is engaged by sliding the dial barrel forward or backward. However, regardless of which dial is engaged, one full revolution on the dial advances the slide exactly .200in (as measured by a test indicator), whereas when the metric dial is engaged the slide should be advancing only 5.00mm (.197in), so somehow the correction factor is not being applied. The compound/topslide exhibits the same behavior. Seems like there's some something simple staring me in the face and I can't see it.
--Mike

SAG 180 · Nov 25, 2017

The amount the cross slide advances never changes, it's just the metric dial rotates a tiny fraction more than the imperial dial: it's going to read 5.08mm for every 0.2" rotation. What does it look like after ten turns?.

Bill Fisher · Nov 26, 2017

Mikel Levy said:
Bill,
.... By the way, do you happen to have a pdf of the entire manual?
--Mike

Yes I do Mike - two different ones in fact: the drawing I posted came from a late 1970s version which includes a parts list. If you send a PM I will try to email it - size is about 2.3Mb so it should go through.

Cheers,
Bill

mark costello · Nov 26, 2017

Just for the public record, If You call the factory They will email You a copy, free.

Mikel Levy · Nov 27, 2017

SAG 180 said:
The amount the cross slide advances never changes, it's just the metric dial rotates a tiny fraction more than the imperial dial: it's going to read 5.08mm for every 0.2" rotation. What does it look like after ten turns?.

I will check that, thanks.
--Mike

Mikel Levy · Nov 27, 2017

Bill Fisher said:
Yes I do Mike - two different ones in fact: the drawing I posted came from a late 1970s version which includes a parts list. If you send a PM I will try to email it - size is about 2.3Mb so it should go through.

Cheers,
Bill

PM sent.
--Mike

Mystery mechanism on Colchester Chipmaster lathe

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