Michael,

I am inclined to agree with you, to me it looks like a captive leadscrew that moves a close fitting nut

Regards

Brian

Could it be a worm gear driving a rack Michael?

Regards,

IanT

Well, one should really speak of a "worm and wheel" for the conventional " worm drive", so there's no reason why one shouldn't use the "worm" terminology for a linear drive, thought of course we would normally call the "rack" a "nut"! I can't imagine the logic behind a 2.9mm pitch which doesn't seem to correspond to anything sensible in inch or metric.

Maybe, but it does say Type "English", so I wonder if some of the measurements in metric are nearest imperial equivalent. The knob seems to be knurled, so I guess for manual fine adjustment, rather than a precision movement in terms of exact degrees of rotation per linear distance, since there's no angular scale.

2.54/2.9 = a shade over .875, hence 10 turns of the knob would be near enough 7/8"

Edited By peak4 on 15/04/2018 11:13:31

Edited By peak4 on 15/04/2018 11:14:16

Wormscrew and racknut,which I think is the correct terminology, is hardy perennial in alternative to leadscrew drive systems.

One of the optical lab equipment stage makers had a section in its catalogue explaining the relative advantages of micrometer, leadscrew and racknut drives as they made all three types. Unfortunately I left that catalogue behind when made redundant so don't have the details or even recall the firm. Old so certainly binned by the twit with a Phd who took over. MIght just as well have taken it with me. My guess-recollection is that it was said to give better stability and higher axial load capability than a comparably accurate leadscrew. Especially in vertical applications. Probably the only compact type with acceptable performance if you have to use things with a vertical push up, i.e. knob at bottom, arrangement. Bad practice but sometimes inevitable.

Uncalibrated movement probably isn't an issue for that type of device. Although pretty much all my really precise gear had calibrations, good ones too, it frequently surprised me how rarely I exploited the calibrations. Usually worked by educated twist of the knob to zero out fringes, maximise detector response or best image quality.

Victoria used a similar system for the Y axis drive on some of their P, U and V series machines. Arranged to have pretty much 100% wormscrew and racknut engagement through the whole travel. Presumably the theory was that uneven wear would be minimised. Didn't seem to have worked out like that on the one I had to help make a new racknut for. About a foot of phosphor bronze bar cleaned up to a precise rectangle to match the slot in the casting with about 1/3 radius of a thread cut on the business side. Went banana in about 6 directions when unclamped! We got it straight. Eventually!

Clive.

here is a worm rack, very similar to what you find on a 4-jaw chuck jaw. can't think why they woudl choose an odd pitch

I spent a while considering it may be some complex calculation that allows you to measure gradients using newton's rings.

Then close look at the device shows it is not graduated, so the pitch cannot be critical. Perhaps the pitch is just what turned out to be eitehr convenient to use or convenient to manufacture?