View Full Version : Measuring Threads

I'm having a mental block this morning I guess. I want to find what the calculated pitch diameter should be, using the three wire method. I have two references that have the same formula, so I'm missing something.

With a known major diameter, and of course the known TPI and wire size for that TPI, the formula is shown as: M= D- (1.5155xP) + (3xW). So if you solve 1.5155 x P , P being 1/TPI, then simply 3x the wire size, add those two, you subtract that from the D? M should be a great number than D, so what am I missing?

Using a known measurement for a particular thread, 1.0625 x 18 TPI, the published measurement over .032 wires should be 1.071". I have done this many times, turned these threads, measured to that value over the wires, and the fit is perfect. But doing the math shown above, it doesn't work.

I'm missing something.

easymike29

03-05-2016, 10:07 AM

I think the answer is in here.

http://www.eduplace.com/math/mathsteps/4/a/

Gene

mklotz

03-05-2016, 10:38 AM

With a known major diameter, and of course the known TPI and wire size for that TPI, the formula is shown as: M= D- (1.5155xP) + (3xW). So if you solve 1.5155 x P , P being 1/TPI, then simply 3x the wire size, add those two, you subtract that from the D?

By adding 1.5155*P and 3*W before subtracting you're actually subtracting 3*W from D. The only thing you should be subtracting from D is the 1.5155*P.

And, yes, easymike's advice is germane; you need to understand the order of operations in algebraic equations.

This works. Been a long time since I was in school. Thanks for your help.

Paul Alciatore

03-05-2016, 03:38 PM

You gotta watch those parenthesis.

It was all my mistake in method. It's been a long time doing algebra, and working the equation left to right had skipped my mind. The formula works. The piece I'm working on now, a different diameter and TPI, but I want to establish the "control" since there will be multiple pieces made the same.

oldtiffie

03-06-2016, 02:23 AM

It was all my mistake in method. It's been a long time doing algebra, and working the equation left to right had skipped my mind. The formula works. The piece I'm working on now, a different diameter and TPI, but I want to establish the "control" since there will be multiple pieces made the same.

If it were me with that job (of multiple threaded parts),the last thing I'd be using is the "3-wires"thread measuring system.

It is possible and quite easy and practical to use the required OD of that and only one of those three wires - preferably located on the under side of the OD and measure over the single wire and the underside of the OD with a standard shop micrometer over the top of the single wire.

Limits and tolerances for many (all 60 degree Unified and 55 degree British BSW) "Vee" threads are all classes of these threads are in "Machinery's Hand Book" where they can be read off directly.

Personally, I use thread micrometers which make measuring thread pitch diameter directly.

https://www.google.com.au/search?q=google&rls=com.microsoft:en-AU:IE-SearchBox&ie=UTF-8&oe=UTF-8&sourceid=ie7&rlz=1I7IRFC_enAU360&gfe_rd=cr&ei=YdjbVqWGFKfM8gf77JagDA&gws_rd=ssl#q=screw+thread+micrometer+Wikipedia

https://www.google.com.au/search?q=google&rls=com.microsoft:en-AU:IE-SearchBox&ie=UTF-8&oe=UTF-8&sourceid=ie7&rlz=1I7IRFC_enAU360&gfe_rd=cr&ei=YdjbVqWGFKfM8gf77JagDA&gws_rd=ssl#q=screw+thread+micrometers

https://www.google.com.au/search?q=screw+thread+micrometers&rls=com.microsoft:en-AU:IE-SearchBox&rlz=1I7IRFC_enAU360&biw=1280&bih=538&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiev9KjwavLAhWIipQKHTVwCMkQ_AUIBigB

Three-wire measuring of the pitch cylinder diameter using the standard simple method cannot be used for say acme or buttress threads - it is limited to 55 and/or 60 degree "vee" threads with a screw pitch of 5 degrees.

But Google and Machinery's Hand Book are very good guides.

danlb

03-06-2016, 02:32 AM

Limits and tolerances for many (all 60 degree Unified and 55 degree British BSW) "Vee" threads are all classes of these threads are in "Machinery's Hand Book" where they can be read off directly.

The published tolerances are fine IF you have a part where the nominal diameter is one one the standard sizes and IF the thread was properly cut to the ISO profile. If you are doing a 16tpi thread on a 3.54 inch diameter part, the thread wires will ensure that all your parts are the same.

Dan

oldtiffie

03-06-2016, 02:51 AM

Sure - do the maths as per standard"3-wire"methodand adjust the figures for just one wire (not three) as its better and easier.

Here is my wire-measuring micrometer set:

http://i200.photobucket.com/albums/aa294/oldtiffie/measuring/Gear-measure6.jpg

Here is a selection of thread micrometers up to100mm/4" at my supplier in OZ:

https://www.machineryhouse.com.au/Screw-Thread-Outside-Micrometers

Illinoyance

03-06-2016, 02:59 AM

I have 1" and 2" capacity thread mikes. For larger threads I use standard mikes and wires.

I would think using one wire would work, but since the standard and referenced method of measuring is with 3 wires, that's what I've been doing. I use a dab of grease on each side to hold the wires in place while I measure with the Mike.