# The next instalment of “Teach the New Guy”: Thread Micrometer.



## Moderatemixed (Dec 2, 2019)

Hey everyone. I have a question regarding Thread Micrometers. I have in my tool chest a Starrett 210A Screw Thread Micrometer. I also have 3 Starrett 575 Screw Thread Micrometer Callipers for measuring pitch diameter (14-20, 22-30, 32-40 TPI) To my question..... Which do I use as a hobby guy when I want to see how close I am to being done threading. Essentially what measurement are each of these reading, or do I have this all wrong? When single point threading I turn the major diameter, then start threading..... which mic tells me when to stop?

Regards,

Derek. 











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## T Bredehoft (Dec 2, 2019)

No. 575 AP through FP are for differing pitches. If you are measuring 16 pitch you need 575 CP
, otherwise the W Anvil won't fit in the threads. Hope this helps


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## mikey (Dec 2, 2019)

Moderatemixed said:


> Hey everyone. I have a question regarding Thread Micrometers. I have in my tool chest a Starrett 210A Screw Thread Micrometer. I also have 3 Starrett 575 Screw Thread Micrometer Callipers for measuring pitch diameter (14-20, 22-30, 32-40 TPI) To my question..... Which do I use as a hobby guy when I want to see how close I am to being done threading. Essentially what measurement are each of these reading, or do I have this all wrong? When single point threading I turn the major diameter, then start threading..... which mic tells me when to stop?



You would use the 575's; choose which one based on the TPI you're cutting. 

The numbers on the barrel of the mic give you the pitch diameter of the thread you're cutting. It is a direct measurement of what the pitch diameter is of your thread. Every thread has a pitch diameter range you need to hit for a particular class of fit, and the more accurate your class of thread, the smaller the range of acceptable pitch diameters will be. You look that range up in a chart like the one I'm attaching below. Estimate your total depth of cut with the formula *0.750 / TPI*; this gives you only an approximate total depth of cut. Well before you feed in this far, start checking your pitch diameter with your mic and use small depths of cut as you get closer to your range. Stop cutting when your pitch diameter is within the range for your thread.


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## Moderatemixed (Dec 2, 2019)

I am trying to learn this by starting with a bolt from Home Depot, measuring Major, Minor diameters, Threads per inch and diameter then going into the Machinery’s Handbook to see what numbers are closest to what I have measured with each of the mic’s..... either I am completely helpless or bolts from Home Depot are absolute garbage.....   

Thoughts? 


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## Moderatemixed (Dec 2, 2019)

Thanks Mikey. I’ll give it a go here shortly. 

Derek


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## darkzero (Dec 2, 2019)

Moderatemixed said:


> I am trying to learn this by starting with a bolt from Home Depot, measuring Major, Minor diameters, Threads per inch and diameter then going into the Machinery’s Handbook to see what numbers are closest to what I have measured with each of the mic’s..... either I am completely helpless or bolts from Home Depot are absolute garbage.....
> 
> Thoughts?
> 
> ...



I understand your desire to learn the tool but measuring pitch dia from a questionable quality mass produced bolt is not going to tell you much. Pitch dia is not needed to identify a bolt size. Usually major diameter & thread pitch is all that's needed.

Pitch dia is for identifying class of fit for threads. Most hobbyist don't have a need to worry about pitch dia to hit a specific class of thread, single point till it fits. I rarely use my pitch mic set. I only use it if I need to make something for someone where I don't have the other mating part to test fit.

Don't be surprised if the bolt from the hardware store may not even fall within a class 1 thread. Bolts/screws made for sale at hardware store often made with rolled threads that have loose tolerances intentionally. Not saying they all are though.


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## Moderatemixed (Dec 2, 2019)

Ok..... thanks Mikey, I think I might be getting this.

Bolt details..... 

Diameter at Shank .3685
Diameter at Threads .3695
16 TPI advertised as 3/8”

My measured Pitch Diameter is .328 using the 14 to 20 pitch Starrett 575. So if my measurements are correct this bolt meets A2 specifications.

Your chart shows Major Diameter as .3737 to .359
Minor Diameter range as .3287 to .2992
Pitch Diameter at .3331

A quick addendum..... my Starrett 575 says 14 to 20P V and U.S. ST’D. Is that pitch correct for UNC and UNF Threads?

Thanks all!




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## Moderatemixed (Dec 2, 2019)

Darkzero..... you are correct, it didn’t meet spec. I guess the original direction I was going with this thread was to learn what each mic actually measures, (Starrett 575 and 210A respectively) and as a hobby guy what do I use (other than thread wires) to accurately measure the minor diameter.


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## darkzero (Dec 2, 2019)

Moderatemixed said:


> Darkzero..... you are correct, it didn’t meet spec. I guess the original direction I was going with this thread was to learn what each mic actually measures, (Starrett 575 and 210A respectively) and as a hobby guy what do I use (other than thread wires) to accurately measure the minor diameter.
> 
> 
> Sent from my iPad using Tapatalk



Sorry, I think I may have misunderstood part of your post too. Carry on, it's good practice, & fun to learn as well as compare threads from various things/sources.  

If you haven't figured it out yet, in the MH you'll see class 1, 2, & 3. 1 being the loosest & 3 being the tightest. A is for external thread & B is internal. Not often do make anything with a class 3 fit unless it's a one off but certainly not required & no reason at all except practicing.


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## mikey (Dec 2, 2019)

Moderatemixed said:


> Ok..... thanks Mikey, I think I might be getting this.
> 
> Bolt details.....
> 
> ...



Hardware store bolts are usually class 2A. Given that they are mass produced and that the coating on them varies in thickness, they will not be all that accurate when using a thread mic. Even the major diameter will vary greatly so as Will said, as long as it hits a general OD and TPI count, its usually okay for a hardware store bolt.

You will use your thread mics when you need to cut a thread to fit a part that is already made or when something calls for a specific class of fit. Say you're making a bolt to fit a manufactured part and the part is not available to you but you are told it is not a critical fit. In that case, you would cut a class 2A thread and bring it to class 2A specs using your mic. Or say you wish a precision thread for one of your projects. In that case, you would turn your OD to a specific range and cut your threads to a class 3A range and you would be good to go.

Cutting to specs is not usually necessary in a hobby shop. I do it because I can and I enjoy working like that but if I'm in a rush, I cut to fit like most everyone else does.


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## benmychree (Dec 2, 2019)

The #210 is only used for comparative measurements and is faster to use, the 575 series is used to measure pitch diameter, but is fairly fussy to use resulting in more time used up.  If I was making a thread gauge, I would be using the 575, if I was threading most other work, I'd be getting close to finish and be using a nut or gauge for the final fit unless specs were called out for a pitch diameter.


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## Moderatemixed (Dec 2, 2019)

All the input is greatly appreciated. Last question then. What is the difference between pitch diameter and minor diameter?


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## darkzero (Dec 2, 2019)

Moderatemixed said:


> What is the difference between pitch diameter and minor diameter?


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## mikey (Dec 2, 2019)

Look at this diagram:




On the right, see major, minor and pitch diameters? The pitch diameter is halfway between the major and minor diameters. You will not use the minor diameter when cutting threads except for cutting thread reliefs. Thread reliefs are the space we make for the cutter to run into and they are typically 0.003 - 0.005" deeper than the minor diameter and 2-3 thread widths wide when we have enough room.


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## mikey (Dec 2, 2019)

Okay, Will has a prettier picture!


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## darkzero (Dec 2, 2019)

Unless you need to make a thread to meet a spec, minor diameter is not so important but it is important.

Depending on your threading tool, if your using a tool that your ground on your own, the minor diameter will vary depening on the tip radius you grind on your tool if at all (sharp V). Same with partial profile inserts (cuts a range of pitches), they have a sharp point. Full profile inserts wil have the correct form to cut the root but full profile inserts can only cut one single pitch. You would need a different full profile insert for every thread pitch. So a thread cut with a full profile insert should have a stronger thread than a sharp V tool since there will be a radius at the root of the thread.

I say not important but important cause if you have too much of a radius ground on your tool, it will not cut enough clearance at the root of the thread for the crest of the mating thread to fit.


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## darkzero (Dec 2, 2019)

mikey said:


> Okay, Will has a prettier picture!



Actually I like yours better. The one I posted was just the very first one that popped up on google images.


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## Moderatemixed (Dec 2, 2019)

You guys are rockstars. I do understand what you have been trying to get across to me. What’s more is that I appreciate that I can ask a legitimate question, get some positive feedback and come away from the experience better because of it. Thanks guys!! Dinners over and I’m heading to the shop to put this into practice. Cheers. My respect and thanks to all who took the time to help me through this.


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## higgite (Dec 3, 2019)

Pitch diameter of a typical Vee thread is the diameter where thread thickness is equal to groove width, that is, where both thicknesses are equal to 1/2 of the thread pitch. It’s halfway between major and minor diameters only when the roots and crests are mirror images of each other, as in a 60 degree Vee thread where the crests and roots are actually Vee shaped.

Tom


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## DavidR8 (Dec 3, 2019)

mikey said:


> Okay, Will has a prettier picture!



That’s some nasty chatter in your picture! 



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## DavidR8 (Dec 3, 2019)

Moderatemixed said:


> You guys are rockstars. I do understand what you have been trying to get across to me. What’s more is that I appreciate that I can ask a legitimate question, get some positive feedback and come away from the experience better because of it. Thanks guys!! Dinners over and I’m heading to the shop to put this into practice. Cheers. My respect and thanks to all who took the time to help me through this.
> 
> 
> Sent from my iPad using Tapatalk



Yup these gentlemen totally amaze me in their willingness to share their knowledge and experience!


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