Bakafish
Cast Iron
- Joined
- Feb 21, 2022
- Location
- Tokyo Japan
I picked up a pair of used (110-502-10 0-13mm 0.5 micron graduated) differential screw micrometer heads and servicing them was challenging enough that I thought I should document it here for others. If you aren't aware of what a differential screw is, it is an arrangement that leverages the difference in pitch of two different threads to allow extremely fine movement.
Mitutoyo's finest spindle thread pitch is 0.10mm on the blue band Series 148, they provide 100 microns of travel per turn, but by using a differential thread these special differential micrometers move a scant 25 microns per turn of the fine adjustment thimble (light blue) which are clearly marked at 1/2 micron intervals.
I made this model and cutaway to better visualize what's going on in there (sorry to the colorblind people, Fusion360 doesn't make it easy to select these colors.) The turquoise main spindle is a traditional 0.5mm pitch micrometer, it allows the unit to cover 13mm of travel and sets the base measurement. The blue 'fine adjust' barrel is connected to the orange differential nut with 2 set screws. Rotating the fine adjust causes the differential nut to screw into one thread while screwing out of the other, the difference in pitch causes the green and pink sections to either get closer or further apart depending on the rotation direction. These two parts are kept clocked to each other by a small v-groove cut into the barrel of the pink part. There are TWO tiny set screws, one that rides in that groove the second one locking it in place. This is very tricky to adjust as they are tiny tiny screws, and they need to engage the groove closely enough to prevent any rotational slop, but loose enough to ensure smooth linear travel of the fine adjustment.
The turquoise main spindle just unscrews like a normal micrometer head. Before disassembly, screw the fine thimble forward to the minimum limit, then remove the two stacked set screws on the green shank and count the number of rotations that are required to unscrew the green rear section from the orange differential nut so you can reassemble it correctly later. The two set screws in the fine thimble don't really need to be touched, you should be able to clean and lubricate the differential nut and threads with those parts still attached. If you do disassemble these parts, you may have some difficulty getting it properly oriented, so try to mark its location before removing (ink through the set screw holes one at a time.) Finally unscrew the fine thimble from the pink front section and clean and lube the threads. There are the standard split threads and tension rings to tighten up the nuts, but I've never had to adjust these on any of my units.
Because these units are really two micrometers siamesed together, they are really intended to be used where you are trying to do small movements within the range of the fine feed. Basically the course feed gets you to a location, then you treat that as zero and just use the fine to make small movements at that location. The error will stack otherwise. I will need to do some testing with them to know how useful they ultimately are, but they were cheap and unusual and needed some TLC so very much my kind of thing.
Mitutoyo's finest spindle thread pitch is 0.10mm on the blue band Series 148, they provide 100 microns of travel per turn, but by using a differential thread these special differential micrometers move a scant 25 microns per turn of the fine adjustment thimble (light blue) which are clearly marked at 1/2 micron intervals.
I made this model and cutaway to better visualize what's going on in there (sorry to the colorblind people, Fusion360 doesn't make it easy to select these colors.) The turquoise main spindle is a traditional 0.5mm pitch micrometer, it allows the unit to cover 13mm of travel and sets the base measurement. The blue 'fine adjust' barrel is connected to the orange differential nut with 2 set screws. Rotating the fine adjust causes the differential nut to screw into one thread while screwing out of the other, the difference in pitch causes the green and pink sections to either get closer or further apart depending on the rotation direction. These two parts are kept clocked to each other by a small v-groove cut into the barrel of the pink part. There are TWO tiny set screws, one that rides in that groove the second one locking it in place. This is very tricky to adjust as they are tiny tiny screws, and they need to engage the groove closely enough to prevent any rotational slop, but loose enough to ensure smooth linear travel of the fine adjustment.
The turquoise main spindle just unscrews like a normal micrometer head. Before disassembly, screw the fine thimble forward to the minimum limit, then remove the two stacked set screws on the green shank and count the number of rotations that are required to unscrew the green rear section from the orange differential nut so you can reassemble it correctly later. The two set screws in the fine thimble don't really need to be touched, you should be able to clean and lubricate the differential nut and threads with those parts still attached. If you do disassemble these parts, you may have some difficulty getting it properly oriented, so try to mark its location before removing (ink through the set screw holes one at a time.) Finally unscrew the fine thimble from the pink front section and clean and lube the threads. There are the standard split threads and tension rings to tighten up the nuts, but I've never had to adjust these on any of my units.
Because these units are really two micrometers siamesed together, they are really intended to be used where you are trying to do small movements within the range of the fine feed. Basically the course feed gets you to a location, then you treat that as zero and just use the fine to make small movements at that location. The error will stack otherwise. I will need to do some testing with them to know how useful they ultimately are, but they were cheap and unusual and needed some TLC so very much my kind of thing.
