121.6 Threads Per Inch

[rgsparber]

After watching that amazing video on the James Webb mirror drivers, I got thinking about simple, fine movement devices.

Many years ago, I learned, in Guy Lautard' s Bedside Reader, how to work one thread pitch against another to get a finer thread. For a compact unit, the key is to select threads that a very close in pitch yet have different enough diameters so one can fit inside the other.

Now, I am positive this will be offensive to some, but I have no problem looking over Metric and Imperial threads as one spectrum. What if I took a 1/4-28 threaded rod and tapped it 4 mm - 0.7? Looking at the diameters, there is enough metal to do this.

28 TPI is a pitch of 0.0357 inches. 0.7 mm pitch is 0.0275 inches. This means that one revolution screws in the 1/4-28 by 0.0357 inches while it unscrews the 4 mm - 07 by 0.0275 inches. The net effect is a change of .0082 inches per revolution which is 122.6 threads per inch.

Picture the 1/4-28 partially threaded into a block. I hold the head of the 4 mm - 0.7 screw. As I turn the 1/4-28 into the block, it unscrews from the 4 mm - 0.7 screw. Since the Imperial thread is coarser, the head of my metric screw moves towards the block.

This would be useful as the fine adjustment on my finger Dial Test Indictor which can resolve 0.0001 inches. That would be 4 degrees of rotation.

Rick

[mklotz]

There is a program on my website, DIFFTHRD, that does exactly what you describe. You tell it (via a data file) what metric and inferial threads you can produce with your equipment and then input the effective differential thread pitch you desire. It calculates the combination of your available pitches that will come closest to your desired pitch.

It also has the ability to calculate parameters for a typical arrangement of a differential thread implementation but I won't go into detail on that here.

A sample output from the program is shown below.

My device to simplify the measurement of bores...

Simplifying the measurement of bores

uses a differential thread fine motion arrangement to achieve the equivalent of a 160 tpi thread.


---------------------------------------------

DIFFERENTIAL THREAD CALCULATIONS


Number of data items read = 64

Desired effective pitch of differential thread [100 tpi] ?

Of available threads, best match to 100.000 tpi is:
Coarse thread = 22.000 tpi = 1.155 mm/thrd
Fine thread = 28.222 tpi = 0.900 mm/thrd
with an effective pitch of 99.786 tpi

Pitch of coarse thread [22 tpi] ? Pitch of fine thread [28.222222222 tpi] ?
Thickness of coarse (fixed) nut [0.375 in] ?
Thickness of fine (movable) nut [0.25 in] ?
Desired motion of movable nut [0.25 in] ?

Effective pitch = 99.786 tpi
Motion for one revolution = 0.01002 in
Total turns to obtain desired motion = 24.946
Minimum length of coarse thread needed = 1.509 in
Minimum length of fine thread needed = 1.134 in
Maximum distance between nuts = 1.134 in
Minimum distance between nuts = 0.884 in

[rgsparber]

Marv,

How do you deal with backlash? Or is the force of the finger on the DTI enough?

Rick