Ok, I lied earlier. I said my triple start leadscrew was one turn per inch- turns out that one is .375 inches per turn. If I mark out some hand crank discs with one mark every .1 inch, and wanted each mark to represent three thou, then that's 125 marks around the circumference. That's also 12.5 inches around the disc, and the disc becomes 3.98 or so in diameter. I'd like a math check- is that correct?
I could essentially take a tape measure that's laid out in tenths of an inch, make a loop out of 12.5 inches of it and put it on the outside of the disc. Each mark represents 3 thou, each inch represents 30 thou. I can cut each mark in thirds by eye to arrive at a resolution of one thou. Four turns of the hand crank would be a motion of 1-1/2 inches. Doesn't seem too horrible, does it?
This leadscrew is clean, straight, and accurate over distance as checked by a tape measure. So far it's the best thing I have to use on my layout table. I also have a few zero-play nuts for it, plus the roller thrust bearing pairs which I can pre-load to control end play. I like the fact that I could move the carriage fairly rapidly with relatively few turns of the hand crank, yet still be able to resolve down to a thou with some visual interpretation. I don't like the fact that each turn of the screw is .375- I'd rather it be .5, or .25, but it is what it is. If I lay a measuring strip on my slides, I should find it fairly easy to get distances laid out without letting the magic smoke escape from between my ears.
Going back to the markings around the hand crank discs- I've suggested .1 inch between marks. I could go with the standard tape measure divisions of eighths and sixteenths, but then I'm stuck with that and I'd have to interpret sixteenths into decimal. Then my head would explode.
I'm sure many of you will have been rolling your eyes by now, but I'll accept any of your thoughts.
A 1/8th marked tape would probably be a lot easier to find than a 1/10th one. If you made each 1/8 equal to .004, then a 1/16 would be .002 and a 1/32 would be .001. No exploding head, no math, just go by the marks.
Also, no matter what divisions you end up with you can always make up a vernier suitable for those divisions to get accurate smaller increments.
I had forgotten about the vernier- I could do that-
Hmm- if 1/32 = .001, then 375 would have to be evenly dividable by 32 in order to get a scale that repeats properly. It doesn't. I would have to provide either 125 marks (3 thou per mark), 75 marks (5 thou per mark), or a full 375 marks (1 thou per mark) to get a repeating scale. 25 marks would give me 15 thou per mark, and 15 marks would be 25 thou per mark.
The only one I can see that makes sense is 25 groups of 5 marks, which is 125 marks, or 3 thou per mark. I could certainly make a vernier to read off each thou from, I like that idea.
Whatever mark spacing I use, it has to be nicely dividable by 375. The marks could be 1/16 apart, that's no problem- but I couldn't use them if they are notated as fractions of an inch around the dial. I couldn't look at the one inch mark, for example, and know how far that is- yes, it would be 32 thou, but say I want to hit a dimension of 4.2 inches- tell me, how many turns of the dial do I need, and what number do I stop on-
Come to think of it, even the 3 thou per mark is going to be hard to keep track of. In my example (4.2 inches) I'd have to crank in 11 turns (getting me to 4.125) then figure the difference, which would be .075, then divide that by 3, coming to 25, then crank in 25 more marks. Or crank 10 turns, giving 3.75, then figure the difference to make up to 4.2- so that would be .45 which is another 150 marks. Or crank 12 turns to make it 4.5 inches, then subtract .3, which is 100 marks- Yep, there's some heat developing in the cranium
Why not just print a scale with whatever divisions work instead of trying to force it to work with arbitrary divivisons on a tape measure. An inkjet printer must be more accurate than most tape measures.
Sure, I could print a scale and it would work. I've done this before for both 50 and 100 divisions per rotation. But those work with 20 and 10 turns per inch, respectively. My main concern here is whether I can mentally deal with the fact that one rotation with this lead screw is not .1 inch, or some other 'neat' value- it's .375, or 2.66666 turns per inch.
Your math sounds right to me. But I also would just print a strip of paper and glue it on. Seal with clear varnish.
On the other hand, I would not use that screw. For the first revolution you will be OK. For the second revolution you can still keep up with the actual measure fairly well. After three or seven or more revolutions you are going to go nuts. You could print several scales on your paper strip:
first 0 - 375,
then 375 - 750,
then 750 - 1125,
then 1125 - 1500,
then 1500 - 1875,
then 1875 - 2250,
then 2250 - 2625,
then 2675 - 3000.
That will take you to three inches in seven turns of the hand wheel and screw. You read each scale in turn on successively rotations of the screw. You could color code them with corresponding colors on a linear scale next to the hand wheel or screw to keep track of which one to use. After three inches the pattern repeats. That's a lot of complexity. Your strip will probably be two inches wide to accommodate all those scales. And I think it will slow your work down a lot.
Personally, I would find a screw that has a more convenient pitch.
Make it fit.
Sounds like the perfect reason to get a cheap DRO, even just one axis!
As a hand-cranked table with a range of about 2 ft x 3 ft, these leadscrews are about ideal. Three cranks to go a bit over an inch is not too tedious, but 11 cranks to go that far would be. That's what my mill table has, and it is tiring to traverse several inches- but the 10 tpi leadscrew makes it easy to dial in anywhere. As you said, Paul, the 2.666- tpi leadscrew I have is going to drive me nuts at times trying to figure out where I'm at. But if I added a readout of some type, plus an inch counter (strip of tape along the axis), then I could still make fine adjustments using the hand crank markings and a vernier, still have fairly fast traverse, and still be able to track the motions easily.
Still need to do more work before I have to settle on this issue, so I'll be keeping the brain ticking-
How about changing the gear ratio? There's no rule that says the hand crank has to be directly attached to the leadscrew.
That is true- and there's also no rule that says the handwheel has to be the part doing the readout. If I gear up from the handwheel to the indicator wheel by a factor of 3.75, then 3/8 of a turn of the handwheel will turn the indicator wheel by 1 turn. It could then be marked out with 100 lines and would read .001 per mark.
So that would be what- 75 teeth on the handwheel and 20 on the indicator wheel.
If the 75 teeth were an inside gear, the 20 tooth would probably roll quite smoothly within it. Just a thought- I don't think there's room for the indicator wheel if I did it this way, but it is a way to make the gear meshing smoother.
Toothed belt would be another way to get this ratio while keeping the translation smooth and without play.
Hmm- getting to be more complex all the time. Sure would be easier to just use a 10 tpi leadscrew
Paul brought up a good point- the handwheel could be marked out with more than one scale. This makes me think that one side could be marked out in eighths, sixteenths, etc, and the other side could have a movable ring that would be marked out to read off in thou- two thou per mark probably. Then I could travel the bulk of the distances according to a fixed tape, verify the exact spot with the fractional divisions on the handwheel, then zero the ring and crank off the rest in decimal. That would work, and I'd be able to work in both systems.
Ok- let me do an example- suppose I need to move X exactly 8.394 inches. I'd crank over to the nearest fractional point, which would be 8.375, zero the dial, then crank in another 19 thou. Not too bad. Another example- let's try .467. That's .375 plus 92, so that's one full turn, zero the dial, crank in another 92. One more example- one I had today for some reason, 6.667. Crank in 6-3/8, add in .292, or crank in 6-5/8 and add 42.
This doesn't seem too bad. If I wanted to work in fractions and all I had was a 10 tpi leadscrew, I'd have to convert to decimal anyway. Of course I could refer to a chart in that case, or rely on memory and mental math skills, which is how I do it now anyway.
With any and all of this, I still would need to have a 0,0 position on the table where one corner of the workpiece would have to be positioned, otherwise I'll need to be able to zero a fractional ring on the handwheel, zero the thou ring separately on the fractional ring, then be able to zero the inch layout tapes as well. This 'problem' would be there with either leadscrew.