Chinese Vials
Re: Chinese Vials
Quite some years ago I bought an old Starrett 199 at auction for about nothing (I think it was something about $10) with clearly broken vial, but otherwise pristine. I forget where I got the vial, but it was from a noted high precision vial maker, and talking to one of their guys about the details of refitting my 199 I was able to get a "just out of spec" vail for also near nothing. I've got the sheet on it somewhere, but even though it was out of spec, it was only a tiny bit, and way more sensitive than my 98s, which themselves are far better than I really need. Of course that would not be the case if I needed to quantify the error noted within the factory spec, but I didn't care. And as I said, I've got the spec somewhere, so technically I could use that to provide some pretty good confidence quantification if I ever did care that much (in no possible world). Anyway, they are bedded using plaster in a sort of semi-floated support that is adjusted by screws. Never used it, never even came close to needing it, but it sets and gathers dust among other such excesses. I've still got the remains of the box of plaster I bought for that job, I just ran across it a few weeks back.
Russ
Master Floor Sweeper
Master Floor Sweeper
Re: Chinese Vials
Here is first candidate for a level base. It's all I could find laying around. It looks like some of the Starrett levels mount the tube in an aluminum cylinder with a cutout window, or is that SS?
Whatever I mount the tube to, I will mill out a pocket so the assembly is flush with the top. I was thinking of the mounting and adjustment - some way to have a screw come in from the side rather than the top - maybe an eccentric?
I think the next one I may make using a piece of just one side of the disk and lay it flat with stand-offs, more like a Starrett 98.
Whatever I mount the tube to, I will mill out a pocket so the assembly is flush with the top. I was thinking of the mounting and adjustment - some way to have a screw come in from the side rather than the top - maybe an eccentric?
I think the next one I may make using a piece of just one side of the disk and lay it flat with stand-offs, more like a Starrett 98.
Standards are so important that everyone must have their own...
To measure is to know - Lord Kelvin
Disclaimer: I'm just a guy with a few machines...
To measure is to know - Lord Kelvin
Disclaimer: I'm just a guy with a few machines...
Re: Chinese Vials
OK, I will use a differential screw that is simply a tight fit to the base and the vial carrier. I think threads 88 and 92 TPI would work. Looks like less than 5 tenths differential per turn, which would result in about 5 divisions on the vial.
Standards are so important that everyone must have their own...
To measure is to know - Lord Kelvin
Disclaimer: I'm just a guy with a few machines...
To measure is to know - Lord Kelvin
Disclaimer: I'm just a guy with a few machines...
Re: Chinese Vials
The Starrett has a convex / concave washer pair at one end and the adjustment screw at the other end.
Steve
Steve
Re: Chinese Vials
I have two ideas for the mounting adjustment.
The first is with a differential screw - two screws pressed into a thumb-nut, which will be slightly recessed from the walls. I am having some difficulties with this idea as I'm sure I would me limited by what kind of thread I'll be able to make in that scrap iron. If I used common 28/32 TPI threads, it looks like ~8° rotation of the nut will result in one division on the vial.
The second is an eccentric. I bet if I took a screw and tried my darnedest to turn the end concentric and smooth for a very slight press into the vial carrier, I bet I would just the right amount of eccentricity. It looks like one thou offset would give me the full adjustment range. It's far easier to implement, but not as fancy.
The first is with a differential screw - two screws pressed into a thumb-nut, which will be slightly recessed from the walls. I am having some difficulties with this idea as I'm sure I would me limited by what kind of thread I'll be able to make in that scrap iron. If I used common 28/32 TPI threads, it looks like ~8° rotation of the nut will result in one division on the vial.
The second is an eccentric. I bet if I took a screw and tried my darnedest to turn the end concentric and smooth for a very slight press into the vial carrier, I bet I would just the right amount of eccentricity. It looks like one thou offset would give me the full adjustment range. It's far easier to implement, but not as fancy.
Standards are so important that everyone must have their own...
To measure is to know - Lord Kelvin
Disclaimer: I'm just a guy with a few machines...
To measure is to know - Lord Kelvin
Disclaimer: I'm just a guy with a few machines...
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Re: Chinese Vials
Another approach might be a screw with a conical tip. The carrier would ride on the side of the cone so that it went up as the screw went in. A hold down spring would be needed, of course. The lift per turn would be the screw lead times the tangent of the half-angle of the cone. If my back-of-the-envelope calculation is correct 1 degree and 40 tpi would give you about .0004" lift per turn.
Re: Chinese Vials
That was a thought too, since I have some leftover needle valves from my RC hobby model plane engines. I actually first envisioned a sliding ramp - it's dead easy for me to make two almost parallel surfaces.
1/8" rod that comes to a 1 degree conical point would be over 11" long - for perspective. My needle valves would not work. I'm sure I can turn a very slight taper without any set-up at all (I know I just need a short length of taper).
1/8" rod that comes to a 1 degree conical point would be over 11" long - for perspective. My needle valves would not work. I'm sure I can turn a very slight taper without any set-up at all (I know I just need a short length of taper).
Standards are so important that everyone must have their own...
To measure is to know - Lord Kelvin
Disclaimer: I'm just a guy with a few machines...
To measure is to know - Lord Kelvin
Disclaimer: I'm just a guy with a few machines...
Re: Chinese Vials
Hey all, I never did update my topic here. I came across another level topic and that reminded me of this. What I ended up doing was using the segment of brake rotor. I tapped the brake rotor with an M5-0.9 thread and the vial carrier 1/4-28 on both sides, then I made differential screws such that both threads start at the same time in the base and vial carrier from the top. This ratio should give me a differential of 0.00028" per turn. I made the screws from Delrin and they are a bit finicky because my tap/die set is Chinese carbon steel so they bind a little on the rough thread, but it works. Calibration always seems to end up floating about a division one way or the other as it relaxes. I lapped in the base on my larger surface plate. I may return to it with finer paper and make the screws out of steel, but the dies are not very good.
The vial is graduated in 4 seconds per line. The end result is that a 30 degree turn of a screw moves the bubble one line since my screw distance is 4.625" (that was determined by the location of the rotor cooling fins). The base is 8" long, so the bubble should move one division with a difference of 0.000155" end-to-end.
The vial is graduated in 4 seconds per line. The end result is that a 30 degree turn of a screw moves the bubble one line since my screw distance is 4.625" (that was determined by the location of the rotor cooling fins). The base is 8" long, so the bubble should move one division with a difference of 0.000155" end-to-end.
Standards are so important that everyone must have their own...
To measure is to know - Lord Kelvin
Disclaimer: I'm just a guy with a few machines...
To measure is to know - Lord Kelvin
Disclaimer: I'm just a guy with a few machines...
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- Posts: 1852
- Joined: Tue Dec 06, 2016 4:05 pm
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Re: Chinese Vials
The thermal coefficient of expansion of Delrin is about ten times that of cast iron. Delrin has low water absorption but you still might get a half a thou or so change in length as humidity changes. It is also not immune to creep, though it's better in that respect than most plastics.
I'd use steel.
I'd use steel.
- liveaboard
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Re: Chinese Vials
Finally someone found a use for an old brake rotor!
Great work.
Great work.
Re: Chinese Vials
Yeah, I wanted to see it work first before struggling with steel. And then I was worried about corrosion with dissimilar metals. I was hoping that having identical screws on both sides that any expansion would be more or less equal. The whole assembly is symmetrical. It is tight and the screws have that stick-slip feel, not like a Swiss watch at all.John Hasler wrote: ↑Thu Sep 03, 2020 1:15 pm The thermal coefficient of expansion of Delrin is about ten times that of cast iron. Delrin has low water absorption but you still might get a half a thou or so change in length as humidity changes. It is also not immune to creep, though it's better in that respect than most plastics.
I'd use steel.
I have another idea for a brake rotor - I want to make it into a worm wheel for a telescope mount drive. I was going to inlay a brass ring and use the poor-man's hobbing method with a 1/2-13 spiral tap.
Standards are so important that everyone must have their own...
To measure is to know - Lord Kelvin
Disclaimer: I'm just a guy with a few machines...
To measure is to know - Lord Kelvin
Disclaimer: I'm just a guy with a few machines...
- tornitore45
- Posts: 2078
- Joined: Tue Apr 18, 2006 12:24 am
- Location: USA Texas, Austin
Re: Chinese Vials
Having the holder bottom flat is nice but not necessary. One can have a 3 point base with the lone point as a screw to zero the vial on the lathe bed near the headstock. Then move the level at the tail stock end and see if the bubble moved moved.
Mauro Gaetano
in Austin TX
in Austin TX