Thread micrometer anvil question?

bcall2043

bcall2043

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I need to cut a male 2 ¼-6 thread (spindle thread plug gage) and have been looking at the options for measuring the thread. One option is to purchase a dedicated 2 to 3 inch diameter thread micrometer. This is not a likely choice for me due to the cost but the look has raised some questions. The information provided by web sites that sell this kind of tool leave me with the question of the number of anvils you would expect to be supplied with this size thread micrometer. Some say 4 anvil sets, some say anvils are separate option, and some just say the quantity supplied varies with micrometer range. I looked at an old incomplete B&S 3 to 4 inch diameter range set and there are slots in the box for 7 anvils covering 2.5 to 64 threads-per-inch. I don’t know if all seven were originally supplied for that micrometer since only one set of anvils was in the box. Another old B&S thread micrometer set with the range of 1 to 2 inch diameter had 7 anvils in the box covering the range of 2.5 to 64threads per inch.

The question is what is normal thread-per-inch that a 2-3 inch diameter thread micrometer would be expected to measure and have anvils for? Thanks in advance for any input on this question.

Benny
The Orphanage Never Closes
 
Don't know right off the top of my head, Benny. Not without looking it up. Why not just use wires?

I never really cared for thread mics, myself. I have some triangle attachments for mics that are supposed to be used for that, but with the helix angle of that thread, I don't know it they would work. Too much offset between one side and the other.
 
Tony Wells said:
.............. Why not just use wires?

I never really cared for thread mics, myself. I have some triangle attachments for mics that are supposed to be used for that, but with the helix angle of that thread, I don't know it they would work. Too much offset between one side and the other.
Click to expand...

Tony,
I have been reading about available methods to measure threads in an effort to learn how. Wires are probably the option I will use. I have purchased a cheap set of wires but have not gotten them out and tried to measure any threads yet. I kind of worried about the spread of the wires on 6 pitch thread and the micrometer anvil size but have not measured any 2-3 anvils yet. The only threads I have ever cut was with a tap, die, or on a lathe (cut-and-fit) using a mating part. I never have used a measuring tool to cut threads to a measured size.

Benny
The Orphanage Never Closes
 
The thing about either thread mics, or triangle attachments is that you really need a standard to use as a comparator to the measured thread. Learning to successfully cut to print is an important task, and every machinist should learn how.

On wires, there are three normally used, and a formula is used to calculate an "over-wire" measurement for the high and low limits. On a very coarse thread, with the pitch so great that a mic anvil will not bridge two wires on one side, I have, through calculation and direct measurement of the actual major diameter arrived at a measurement target range using only a single wire. Much easier to balance than all three, to be sure.

You have the wire formula, and the proper high and low dimensions for that thread? You could (and probably should) measure the existing spindle threads, since you want a good fit, without being too loose, for sure. And if this is to be used as a "stand-in" for that particular spindle, it will serve well to make attachments for that lathe. By measuring the spindle, you can see what the actual PD is. Thread mics and triangles bear on the entire flank, rather than at the pitch line, as wires do. And therein lies a trap or two.
 
You can "cheat" when the anvil will not hit the two wires correctly. It was taught to me many years ago. Use a gage block of known value to bridge the two wires. The trick is to get a a bridge that is an even value, say .200, that is easily subtracted. Take the measurement and subtract the known value from it. Now you have your measurement.

"Billy G"
 
Last edited by a moderator:
bcall,

In all honesty, I've only measured threads once in my life and that's when I had to make a Whitmore (or is it Whitworth?) bolt for an antique Lister diesel engine that was in the 3/8 or 7/16 diameter range. They are cut (if memory serves) at 55 degrees and not 60. Aside from that one time, I've always been able to get by with a quality "sawtooth" thread gage. Some of them are pure novelties while others are fairly precise.

In that particular case, I was very careful to shape the tool bit properly which is a much easier task than measuring threads...

... Just tossing that out there.

Is this for your chuck spindle?

Ray
 
I will throw in my 2c about thread measurement. I have only done it with wires. The trick there is that it would be easier with 3 or 4 hands, as you have to keep 3 wires in place, move the mic into position, and then turn the thimble...

I found that a piece of packing foam (the white stuff is what I used) helped a lot. Jab the wires into the foam at "about" the diameter you are measuring. 2 wires on top, 1 on bottom. The 2 wires on top should close together. Once you have that, position the wires over the threads using the foam as a handle. Once you get the mic on the top wires, the pressure will hold them in place, as the foam is pretty light.
 
Tony Wells said:
The thing about either thread mics, or triangle attachments is that you really need a standard to use as a comparator to the measured thread…………………

You have the wire formula, and the proper high and low dimensions for that thread? You could (and probably should) measure the existing spindle threads……….. And therein lies a trap or two.
Click to expand...

Tony,
Thanks, for your input.

Your right I will be using the lathe spindle as my standard to make my “male thread gage” that I will need to make the internal treaded part.
Today I measured the anvils of the two micrometers I have at hand and below it the result:

  1. Brown & Sharpe 2 to 3 inch micrometer 0.2495"
  2. Fowler 2 to 3 inch micrometer 0.2565"

Used the chart below to select wire diameter size of 0.092”:
Wire Dia Select 086.jpg
The spindle thread size of 2 ¼ - 6 thread per inch would make the thread pitch, P, equal 0.166 inch if my math is correct.

The following chart from the wire kit shows the relationship of the wires, threads, and thread pitch:
Three Wire Method 088.jpg
The distance over the two wires would be the same as thread pitch with the third wire sitting somewhere in between the other two. It would appear that my concern about the micrometer anvils not being able to fit over the three wires at the same time is a non-problem.

As long as I make the measurement over wires of the lathe spindle and the measurement over wires of male thread gage the same (as close as I can). I should not have to worry about a real thread dimension right? I would only be using the micrometer as a comparator.

Think I will be going with and using thread wires for the first time.

Everyone,
Thanks for the help.

Benny
The Orphanage Never Closes

Wire Dia Select 086.jpg Three Wire Method 088.jpg
 
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