Pitch Diameter Measurement

Floating Carriage Micrometer.

Thanks.

I was unaware of this method.

Back in the mid 70's we used a three wire method. 3 wires of known diameter. Sized such that when they were the centerline of the wire would match the pitch diameter.

2 were placed on one side of the male thread and 1 placed on the opposite side.

This was for tapered oil well threads but would work for sharp tapered male threads also.

Of course, you might be measuring female threads, straight threads, acme threads.....

Please give us more info!

Is this for personal use, prototyping, quality control, backward engineering???

Must confess, I'm learning something new here. Checked out this 3 wire method, and it appears to be Machinerys Handbooks favorite. They provide a lot of formula, tables and methods of use. The topic is 'measuring screw threads'.

Thanks.

I am looking for standard instrument available for three wire measurement. I could not find any, in Mitutoyo or so,

It seems I need to modify standard vernier to one usable for three wire...

Regards

There is no modification. Once the 3 wires are in position one uses a standard measuring tool to measure the linear distance, and thus the diameter. If using a vernier caliper your accuracy will be the accuracy of the instrument plus one half of the total of the wires diametrical tolerance. An outside micrometer is a vernier instrument that is best suited to these measurements easily offering accuracy of .0001" -.0002".

I guess where I am confused is the idea that something must be modified.

Standard threads may be quickly inspected by use of:

go/not (ring) gages.
thread mics.
3 wire.
to some degree OC.

I prefer to use thread mics. If they are not available then 3 wire.

This is assuming we are talking about external (class A) threads. I do not want to underestimate the ability of the OC when in the hands of a qualified and experienced qc technician. However this method should not be used where a quality plan specifies otherwise. Typically 3 wire or thread mics are the way to go. But I have measured using an OC while waiting for a set of odd wires to come in and found the accuracy to be equivalent yet the efficiency was completely nonexistent and my quality plan would not allow for other than a ref only analysis when using the OC.

Mitutoy "dial caliper" #505-623 6" is what I have used for years to measure bottle and screw cap thread diameters. I have my original set from 15yrs ago, and they were old then.

You are not measuring the pitch diameter if all you are using is a 6" dial caliper.

Here....is a link for my favorite tool

and here is the image of a Mitty interchangeable thread mic.

Different measurements for different industries.

my caliper has blades allowing me to measure the "T" and "E" dimensions of a bottle neck. it is necessary to know the inner diameter of the neck, or in your case the shaft, as well as the outer diameter, the edge of the threads.

from your image it looks that the "basic pitch diameter" is the mean measurement.

with combinations of metal, glass, and/or plastic it is important to have all measurements usually to .005". it is neccessary to understand the amount of interference between closure and package to gage adequate application torque and resulting removal torque after the materials relax over about 24hrs.

Merry Christmas to All.

I'll take your word for it. The pitch diameter is a very controlled feature in many applications. ⁺/_- .0002 is not out of the norm. Again it depends on environment and/or application.

Merry Holidays to All.

.005" is standard for Plastic closures. There is material shrink that has to be accounted for with the finished parts. Color/additives, heat, cycle are all integral to holding tolerances. Lazy Line techs will bump colorant up to meet min requirements so they dont have to pay attention, and they will increase line speeds to "look good" on through-put. All of this plays havoc with control of the finished part quality.

-unless you are in plastic packaging manufacturing, youre right you will take my word for it.

-isnt the internet a nice place to play. We can all be ass-holes when we choose to be, right Rumble-Fish

Happy New Year

I will explain the process as under:

As I could understand, Pitch circle dimeter of threads is related mostly in case of worms. If I am right, and if the grove depth is standard, you can measure the outer dimeter of the worm and deduct the adendum to get the pitch dimeter.

In case you want to physically inspect the correctness of PCD of the worm, The three pin method can be applied. The pin dimeter should be equal to the circular pitch according to the DP/Module cutter used. Better, put the two pins on a plane surface and the third at the top(Vertex) and measure the height.

In case you want to measure PCD for other thread forms, feed details.