Micrometer Screw

Off to the bar so I've done my bit!

So you're exchanging the action on one inclined plain for action on a much more shallow and longer inclined plain... You don't climb as fast, but in return, it's much easier to climb.

A gold sovereign for the first "real" engineer to find one in use in some actual device!!!

This screw mechanism is based on two screw pairs each with different pitch such that the slider moves a distance proportional to the pitch difference. This device is used in precision measurement instruments. It is called a differential screw thingy.

For another gold sovereign I'll show you the instruments its used in.

Principle is simple but never seen this type of instrument - it will be like a screw jack- with a reduction gear mechanism -

Suppose the top screw (the larger, hollow one) is held in a guide - then one rotation of it will pull in the smaller screw by the pitch difference between them and that reduction ratio will generate the equivalent extra torque

Looks like the screw out of a ceramic tool holder. Could not google a picture.

Regards JD.

hi to all again .... i am surpized a bit the so called guru,s on this forum are making fun of it or are confused about it i dont know wht the west define a status of a guru ,but we in india a guru is consider to be a wise men who help others ... i would like to inform, all my brothers -n- elders of a piece of history about this ,i got this thing in a book called mechanical movement by someone called HENERY T.BROWN EDITOR oF AMERICAN ARTISAN published in new york in 1871 around 138 years ago. the day i saw this i am confused about it this thing,this looks very amazing but how do this work i can understand as both the screws are in opposite direction and diff. pitch so how does one moves the another if this technology was a avaliable around so long why couldn,t we see anything like this around i have talked to many people about it but no one heard to such thing ,even there is no such thing on the web itst its amazing a forgotten technology.. this it not only one drawing in it the book contain many such amazing mechanical things.. hope that no war of words will start with my this post but something positive will come out by grace of god........ ......................................................................................... all i had is hope and HOPE....that keeps me going.....

Considering a screw is really an inclined plane, but this sort of work like a lever and fulcrum. I don't think I need to explain that.

The threads are in opposite direction so that tightening one end loosens the other and visa versa. If they were in the same direction, you would get the pull would you.

Make any sense?

The status of "Guru" on this forum has nothing to do with "west," south, north or east. One only has to post enough messages to obtain the status. Whether or not good answers play any role in it or not I am not sure but "me thinks not." Often good answer status is awarded mostly to those that get there first only to find out later the dissertation was merely a brain fart.

Welcome trailblazer11.

I, and I beleive we, all enjoy seeing the out of the ordinary. I am a "Guru" because CR4 says I am - not because I say so.

I have seen similar somewhere before but can not for the life of me recall where. I am a machinist (was) by trade and think that it was somewhere in that environment.

I want to say it was on the mainscrew (leadscrew) of a movable plate for an inspection microscope......just can't quite recall.

Also, I am a novice collector of rare and out of print tech manuals and the such. I look forward to researching the title you have provided.

And again, welcome.

"I have seen similar somewhere before but can not for the life of me recall where."

Tap handle (used for threading holes) has differential screw arrangement, towards holding the shank of taps.

This is not intended directly at you, TexasCharley.

Jeez! I think we all better lighten up about this "guru" thing. Life's too short to get all balled up about this type of crap! All it takes in one asteroid to make this whole issue completely ridiculous! That or the coronary you're going to have by stressing over things like this... I'm guessing this isn't the only trivial stuff they stress over in their lives.

Trailblazer11-- This device is more commonly referred to as a:

DIFFERENTIAL SCREW

1. A compound screw by which a motion is produced equal to the difference of the motions of the component screws.Source: Webster's Revised Unabridged Dictionary (1913)

This is one of the mechanical contrivances you can keep in your bag of engineering tricks for an entire career and never find an actual use for. It always seems to be that the utility of this device for linear is greatly limited by its large length relative to the machine where you want to achieve the function it offers.

But that's just me, maybe.

The one good application that comes to mind is in the type of milling vise that moves both jaws together or apart the same amount by rotating one screw. Since the difference of motions is exactly zero in this case one is able to set up to machine a feature at the exact center of a workpiece, regardless of its width, without needing to reset the milling machine table for each in a succession of separate workpieces.

I think micrometers must have been made that way although I've never seen one that I recognized as such.

Ed Weldon

The screw you refer to is commonly found on a turnbuckle, however, this is comprised simply of a right hand and identical left hand thread at each end. A compound (differential) screw is comprised of two threads of different pitches so that the motion is the difference between the pitches of the two screw threads.

I think a turnbuckle is also a differential screw, for one rotation of the screw, the nuts on either ends are pulled (or pushed) for two pitch distance, right?

Imagine one of the threads has different pitch than the other, then for each turn will yield a pull equal to the difference of pitch lengths. Let the right handed thread has a pitch of 1mm and that of left handed thread has 0.8mm pitch, then you have 0.2mm per every turn, so a micro movement.

I'm sorry, there a mistake in my projection...

In a turnbuckle, if the right handed thread has a pitch of 1mm and that of left handed thread has 0.8mm pitch, then you have 1.8mm per every turn and not 0.2mm as I wrongly said.

To achieve 0.2mm both the threads are to be of same direction.

I'm sorry, there a mistake in my projection...

In a turnbuckle, if the right handed thread has a pitch of 1mm and that of left handed thread has 0.8mm pitch, then you have 1.8mm per every turn and not 0.2mm as I wrongly said.

To achieve 0.2mm both the threads are to be of same direction.

Very Nice Ed, very well put in words, now add the Z axis and we have a team.

I have never seen this particular configuration of combining mechanical advantages in a practical application and I have seen many screws and other mechanical devices in my years as a machinist and project engineer/machine designer. I will take a crack at explaining what this is from the picture and description though. It appears to be a larger screw with a smaller screw inside of it. The larger screw has a coarser thread, while the smaller screw has a fine thread. The movement of the smaller screw in one direction would appear to move the larger screw in the opposite direction. The smaller screw would rotate many more times than larger screw to achieve one rotation of the larger screw due to the increased number of threads per inch on the smaller screw. Hence, your mechanical advantage would be "magnified." The fact that this is called a "micrometer screw" and the fact that there are finer threads on the smaller screw would lead me to think that maybe this configuration could be used in some sort of measuring instrument. My conclusion is that, because there is usually sufficient mechanical advantag in a simple screw with fine threads, there is not much need for such a device.

Regards,

Keywalker

Yes, I agree that to sufficient mechanical advantage in a simple screw with fine threads.

But when you need to achieve a micro movement against a huge force/load, the fine threads might giveaway. Square threads or knuckle threads may be preferable. Invariably squire or knuckle thread to suite huge load may call for higher pitch length, each turn (or part of a turn) may shift undesirable long movement. Will not a differential thread arrangement (under discussion) help in such requirement?

Yes, I agree that to sufficient mechanical advantage in a simple screw with fine threads.

But when you need to achieve a micro movement against a huge force/load, the fine threads might giveaway. Square threads or knuckle threads may be preferable. Invariably squire or knuckle thread to suite huge load may call for higher pitch length, each turn (or part of a turn) may shift undesirable long movement. Will not a differential thread arrangement (under discussion) help in such requirement?

There are probably some instances in which this configuration might prove desirable. I just can't remember ever seeing this before though. The turnbuckles and jacks I have seen, just not the combination of smaller screw inside a larger one with differing pitches and directions of rotation.

Looks like and Optical comparator screw to me (Starrett) which the older models have a lead in micrometer instead of a Digital Read Out. Just moves the X axis table to achive standards of 0.0001 in parts or a master glass scale.

Same principal; differrent set up. circa 1880. Model: M08 Differential Screw Mechanisms with Two Spur-Gear Pairs In model M-8, the screw is moved by a spur-gear pair at one end and by another pair attached to a keyed sliding joint shaft at the other end. The two gear pairs have teeth ratios of 62:14 and 60:15 at the screw end and rotating slider ends, respectively. The two pinion gear wheels are driven by the same crank. The screw is thus moved proportionally by the difference in rotation of the two gears. These devices were used in precision measurement equipment.

Francis Moon 2004-07-00

Reference :

• Reuleaux, Kennedy :

So response #15 wins the bean?

Yep! Too bad they're no longer legal tender.