Need a thread mic

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hobgobbln
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Need a thread mic

Post by hobgobbln »

Is there a thread mic with v's on both anvils? I am doing small runs of very short threaded disks and would like to get away from thread wires for ease and to speed things up.

I was looking at the Starrett 575M series and noticed they have a v on one side and a "conical spindle" on the other. Using thread wires, as long as I'm pretty close to perfectly centered the wires will be parallel and the reading should be accurate. With the point on the spindle I would have to sweep the mic to find the center but inside of a curved thread. I don't see that being easy.

Thanks,
Griz
Richard_W
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Re: Need a thread mic

Post by Richard_W »

I have never seen one and I am having difficulty understanding why you would want one? When you measure a thread with a thread mic, the V goes over the thread on one side and the other side there is the groove side of the thread. To have two V sides you would need the spindle to not rotate and the V groove would have to be off set on the spindle side to match the lead of the thread. That would be a very expensive thread mic!

My first thought was that you are trying to measure a tapered thread, but then that would only get you close and you would still have error in your measurement with a double V thread mic and a standard thread mic would be in order. In a production environment thread mic's are much faster than thread wires. Thread rings are more common in a production shop.

If you are having difficulty with thread wires and do lots of a particular thread, then a set of "GO" and "NO GO" thread rings may be the thing to get. For me thread wires have served me well for more that 40 years. Also for short runs on a CNC we use thread wires at work and thread rings for repeat runs. Due to the cost of thread rings. The beauty of thread wires is you know how much material you have to go and a thread ring is try and re-cut until the ring fits.

Richard W.
hobgobbln
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Re: Need a thread mic

Post by hobgobbln »

I worded my initial post wrong. I meant opposing V's ie; a male 60 degree V on one side and a female 60 degree V on the other. Same idea as how the teeth on two gears mesh together.

I would think a thread mic with a cone on the spindle would have to be in the absolute dead center to get an accurate measurement. Thread wires have been much more forgiving with position in my limited experience. I took a couple pictures of extreme examples.
left.jpg
centered.jpg
right.jpg
None of the threads I have to make are common so I would have to have GO/NO-GO gauges custom made. Right now I am making 6 different pieces in small batches and the occasional special one off. I really don't think I want to pay for all those.

I'm not having trouble with the thread wires, its just tedious and slow. After I set up the machine to thread I check the first 5 pieces to make sure they're right. After that I check every 5. I don't trust my little machines which is why I check so often. That being said, I am able to hold +- .0005 once I'm set up. I don't need that high of precision, but I am new and trying to establish a good work ethic so accurate measurements are important to me.

If there is no such animal, so be it. I really hate to shell out a couple hundred bucks on such a specialty tool anyway but if it will make my life a bit easier, money well spent.

Griz
gcarsen
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Re: Need a thread mic

Post by gcarsen »

Thread Mics are very handy, but get expensive very quickly!! the set you are looking at will only work on a small range of pitches. they make sets that have quite a few interchangeable anvils to measure specific pitch ranges. for a quick example google "Fowler 52-219-091" for an example. this style is far better in my opinion. they have charts for the individual pitches, which anvils to use and the measurements that you work too.
I have used them many times, and like them.
Grant
Richard_W
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Re: Need a thread mic

Post by Richard_W »

You are looking for one of these like Grant mentioned.

http://www.ebay.com/itm/0-1-SCREW-THREA ... 519992f22b
hobgobbln
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Re: Need a thread mic

Post by hobgobbln »

That is exactly what I was looking for! Thank you

Griz
Torch
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Re: Need a thread mic

Post by Torch »

How does the accuracy of that thread mic compare to measuring over wires? I only see 3 v-shaped anvils to cover that entire range. I would have thought the pitch spacing of the v would be specific to the thread pitch. I gather from the images that the "fixed" end is adjustable. Do you set it against a standard, or against the anvils themselves?

Oh, nevermind the last question. Just found this youtube video of setting/using that particular model:
https://www.youtube.com/watch?v=uOQ0mAWd84g



(just remembering all the times I dropped a thread wire, which then bounced into the chip pan...)
Torch
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Re: Need a thread mic

Post by Torch »

I finally broke down and bought one of the Shars thread mics referenced by Richard, and played with it for the first time today. Thought I'd give a little review for others contemplating one.

Right off out of the box I noticed a touch of roughness in the micrometer mechanism, near the fully closed position. I wound the thimble out of the barrel, cleaned the threads and now it's smooth so there must have been a bit of dust/dirt in the thread. The ratchet mechanism is light and very smooth, maybe because the ratchet mechanism is actually part of the large diameter knurl, not the small diameter knob on the end. The end knob is fixed, unlike what I'm used to so it took a minute to get used to that.

The v-anvils and spindle points are packaged in an oily bag, so they had to be wiped down and sorted. They are laser-etched with their corresponding pitch ranges (in metric and inch) but the print is very tiny. Good eyes or a good magnifying glass is required to read them. It's actually easier to just compare the size with the others and sort them that way.

Inserting them into their respective bores, I noticed they are a very precise fit. They tend to spring back slightly due to air pressure. You have to keep your finger on them for a few seconds until the air pressure bleeds off to make sure they are fully seated, and they require a firm grip to remove, exiting the bore with a "pop". Setting to 0 is a bit fiddly but once there, it returns to exactly 0 every time (using the ratchet). Removing then replacing the same set, it still returned exactly to 0, however 0 differs between sets, and must be reset when you change pitches.

One spindle point was too tight to slide easily into it's bore. All the other anvils and points were exactly the same diameter, to the tenth, but for some reason the 28 - 44 tpi point was almost 4 tenths larger. I carefully removed the clip and took 2 tenths off with sandpaper in the lathe, which was sufficient for it to slide nicely into the spindle bore. That was not as easy as it sounds -- those suckers are well hardened and I dulled a surprising quantity of sandpaper in the process! And now I have to remember that the one with no etching is the 28 - 44 tpi size...

I had a couple of pieces of 1/4" fine thread rod next to the lathe, so I used those as my test pieces, to compare the results against thread wires. The first thing I noticed was that hobgobbln is right -- a male v spindle would be quicker and easier than sweeping the point. The anvils and points rotate freely in their bores so I'm sure a male V shaped spindle insert would align with the thread helix angle, no problem. The 28 - 44 tpi female v anvil insert has to be thin enough to fit a 44 tpi thread, so it can slip between the 28 tpi crests instead of fitting over the crests. So it is necessary to hold the mic on an angle where you can see the v, making it difficult to see when the point is centered, so it's all by feel (at least in that size). It took some practice to become reasonably proficient.

As I practiced, I noticed that my measurements got a bit bigger over time. I checked 0, and sure enough, it was now 0.0002" oversize. I'm not sure if that was because the anvil/point inserts seated in more firmly over time, or because my hand had warmed up the frame in the cold garage. It was about 5°C in there today and I did not have the heat on. I re-zeroed things and they stayed put thereafter.

I dug out some additional shiny new 1/4" - 28 bolts for additional practice. The advertised accuracy of the mic is 0.00016". Compared to thread wires, the mic consistently read 1 to 2 tenths larger. Considering the wires and the inserts each cover a range of pitches, there's bound to be some averaging going on in their respective charts so either or both may be wrong. And since the pitch diameter for this thread has a range of 0.005", and that the different bolts I measured varied by up to 0.002" by either method, that's close enough for government work.

Is the mic faster than wires? Well, maybe with more practice, but to be honest, I have had enough practice with the thread wires now that I can probably get the actual measurement as fast or faster using wires. However, the mic does have the advantage of reading directly, so it is not necessary to calculate the pitch diameter from the measurement over wires, and dropping a wire in the chip pan can really slow the whole process down, so the potential is there. I'll reserve judgement until I've actually cut a few threads using the mic.

Would I recommend it to someone else? It does seem to be a decent quality instrument (abovementioned tight fit of the point notwithstanding) that does what it claims to do. However, the mic is limited to measuring 60° threads of 1" or smaller diameter. The thread wires cover a broader range of profiles (eg: Acme, 55°) and sizes for about half the price. If you cut a lot of threads within the envelope of the mic, then it's probably worthwhile. If you just cut the occasional thread, especially oddball profiles, then the wires cheaper and more versatile.
Richard_W
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Re: Need a thread mic

Post by Richard_W »

Torch wrote: so it is not necessary to calculate the pitch diameter from the measurement over wires, and dropping a wire in the chip pan can really slow the whole process down, so the potential is there.
What you do is use a small piece of tape attached to one end of the thread wires. This acts as a flag for when you drop it into the chip pan. You can see the thread wire almost instantly. I tape two wires together and the third just get a small piece attached to it. Masking tape works well and I suppose you can get some bright colored tape to do the same thing. Works especially well with a chip conveyer on a CNC lathe.

Richard W.
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Harold_V
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Re: Need a thread mic

Post by Harold_V »

Richard_W wrote: What you do is use a small piece of tape attached to one end of the thread wires. This acts as a flag for when you drop it into the chip pan. You can see the thread wire almost instantly.
That's a pretty good idea, especially if you're using fine wires.
I tape two wires together and the third just get a small piece attached to it. Masking tape works well and I suppose you can get some bright colored tape to do the same thing. Works especially well with a chip conveyer on a CNC lathe.
I tend to discourage that idea, and I've experienced the reason why it isn't acceptable. With very fine threads, the taped wire isn't much of a problem, although it still can be.

What happens is the spacing of the pair of wires isn't exactly correct, so the wires see the thread at an angle to one another. That prevents full seating of the wire, yielding an oversized reading. Wires are best used unrestrained, so if you must use something to pair the wires, make sure they are free to move at both ends (like attaching to a string or light cable, not rigid in the least). Do NOT plug them in to a rubber eraser.

I don't mean to belabor this point, but it can spell the difference between good and bad threads, so an entire run of parts can be scrapped because the error repeats consistently.

So then, if one must flag the wires with tape, do so with each wire individual of the others. The bright colored tape suggested is a very good idea, as I've also spent a great deal of my time looking for small wires in a not so clean chip pan.

Incidentally, if you read the instructions that come with precision thread wires, you are told they must NOT be restrained by anything (like tape) when in use. Once you've made some scrap, the reason becomes quite obvious.

Harold

Edit:
Precision thread wires are not the same wires the home shop tends to buy and use, an example being the popular PeeDee wires. Precision wires are just that---wires ground and lapped to five places, with finished ends, and are VERY expensive. A typical set (one pitch), today, would most likely cost in the neighborhood of $80. A high price for six inches of wire. That, in no way, is to suggest that the sets of thread wires used by the home shop are not acceptable. They are, and are far better than using a nut, feeding by calculated depth, or just plain guessing.
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Torch
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Re: Need a thread mic

Post by Torch »

Richard_W wrote:What you do is use a small piece of tape attached to one end of the thread wires. This acts as a flag for when you drop it into the chip pan. Richard W.
Oh sure, NOW you tell me... :lol:

I'll have to try that.
hobgobbln
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Re: Need a thread mic

Post by hobgobbln »

Thanks for the info Torch.

My bandsaw decided to implode so the thread mic funding went towards replacing it. One of the Fowler mics that gcarsen mentioned is on the top of my now mile long "need to buy" list. I'll add an update here when I do.

Griz
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