Gallery, Projects and General > Project Logs
DTI Challenge 2022
vtsteam:
I took it apart again and checked out the pinion with calipers. It actually doesn't seem worn to me, it's simply reduced at the top and then splined. I think also I may not have pressed the pointer on far enough to keep the pinion gear from having end play.
The bronze bushing on the clock face does have a fair amount of wear, however..... (photo below) or at least it isn't a close sliding fit on the pinion shaft. A much easier job to replace if I decide to in future.
I also felt that I may not have properly lubed the plunger rack and the plunger rack guide -- which is a flat slotted brass piece (photo below). A rod slides in the slot to keep the plunger from rotating. When I tried those parts in place while the mechanism was out of the case and the pinion removed (no gears meshing with the rack) I could hear a slight squeek and feel a little resistance to moving the plunger.
I had previously tried putting a tiny amount of oil on the guide rod that slides in the brass guide. This also carries a spring and it's very fiddly to screw into the plunger, twist and then engage in the slide. When I did that previously, I don't believe the oil actually made it to the guide brass by the time it was installed.
I also hadn't understood earlier how to engage or disengage the plunger rack from the mechanism gears. I now realize that you can rotate the assembled mechanism inside the case before fastening it down to disengage or engage the rack. I thought the adjustment was only to center the pointer spindles in the face, but it has a mechanical assembly purpose as well.
This made assembling the plunger much easier this time, and I also was able to oil the slotted brass guide instead of the guide rod.
Here are some pics of the indicator showing the parts I mentioned above:
vtsteam:
Reassembly went better all around, and this time, the indicator does return to zero most of the time, though it's a little lazy if you move it less than .002 from zero. It then returns to about .0004. Otherwise it zeroes well. I think it's probably usable, assuming you want readings anywhere else.
The range is to 90 thou. Technically it should be 100 thou, and I could unscrew the zero adjust screw to add another 10, but with the wear and spring strength as it is, that reduces return force, and makes zero less certain.
Ideally the pinion bushing should be replaced, and maybe a stronger (or shorter) return spring would make zeroing perfect, and give the full range, but I think it should be good to try as is.
Another interesting fact, only two of the case holes have threaded bosses under, so only two screws can be fitted -- which was probably the original configuration.
Here's the indicator after cleaning, and oiling, polishing the lens and reassembly.
awemawson:
Bob will be proud of you Steve, it's a great improvement on what you presented us with initially :thumbup:
vtsteam:
I hope so Andrew. :beer:
But if I did anything wrong or could have been improved, I hope he lets me know!
jiihoo:
Thank you Steve for this writeup, very interesting. And the link to Doubleboost videos where Bob teaches how to fix a DI/DTI was pure gold!
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