Using thread micrometers
I have brought myself some thread micrometers so that each time I cut a thread it is firm and correct all the time, the problem I have is the information to get the correct measurement, is there any formulas or charts for this information. if I cut a eg:1 inch thread the measurement is always smaller, there has to be a correct size.
Hi Harold, the Machinery Handbook has all the information on threads. A thread micrometer measures the Pitch or Effective diameter.
Thread mics measure the pitch diameter. Machinery's Handbook has most of them listed in classes: 1A, 2A, & 3A. I generally go in the middle of 2A.
For example: 1-12 thread calls for a pitch reading of: 0.9441 Max, to 0.9382 Min. I try to shoot for the middle for general use. If you need a tighter or looser fit, then machine accordingly. Remember, a thread pitch mic has different anvils for the pitch ranges. They must be zeroed each time with the standard.
As everyone else has said get a copy of Machinery's Handbook it will have all the common thread specs listed. On the very rare occasion you will need to determine your own specs. I only run into having to do this when I am building or rebuilding suppressors. The ID of the tube never is a standard thread size.
Thanks for this, it has been a great help, we have just got to get thesse things right the first time.
[QUOTE=harold.;1784596]I have brought myself some thread micrometers so that each time I cut a thread it is firm and correct all the time, the problem I have is the information to get the correct measurement, is there any formulas or charts for this information. if I cut a eg:1 inch thread the measurement is always smaller, there has to be a correct size.
Well, Harold, I started machining in 1970 and have never used a "thread mic". I have nothing bad to say except for precision threads I simply do not trust indirect built in measurements. I would have a lot of them if all I did was thread and check. Most of the jobs I have done are ones of a kind with the outcome critical. I trust mathematics and the "3 wire method" as the outcome, while slower, is more on my head. The thread mics sort of do the math for you. With the "3 wire method", you can use any 3 wires of the same diameter and they seat on the V thread form in the right place for trust. If you are simply going for the precision every now and then, I would urge you to learn the math. Simple trig. The "3 wire method is more accurate in my opinion. I have never killed a precision critical part when it counted. Oh, yes, & in my day, I was a threading fool.
I find the in feed depth of the thread I am cutting (say 28 tpi = .023) and subtract that from the Major diameter of the part. This is the number I am looking for on my thread micrometer.
An example Ĺ-28tpi
.500 - .023= .477
.498 - .023= .475
I donít know if this is the right way but it seems to work for me.
When calculating the depth of in feed when threading: Divide 0.64952 by your TPI.
Example, for 28TPI threads:
Harold - another helpful tip. Look up Thread Tech at Thread Engineering Software, British, Buttress & Acme Thread Gages, Ring Gages - Thread Check, Inc. - it is a database that you can use to determine all the dimensions you would ever need for any thread you can dream up, or any existing one. They'll give you a 1-month free trial, I used my trial to make a book of all the common threads from 0-80 up to 1 1/12" and a lot of metric sizes too - very handy!!!
Thread mics are good, but like all measuring tools, only as good as the hands and mind operating them. That said, there are a couple of things that come to mind that are advantageaous about thread mics over say, a set of thread wires.
First, you eliminate the wires, which can be clumsy to use. I typically use grease to hold the top wires in place , then hold the bottom wire with one hand while using the other to adjust the mic. A bit more juggling than a novice would be comfortable with. In the hands of someone used to it, the wires can be very accurate. But trying to get astraddle of all three wires evenly, plus find the max diameter, and keep the mic pressure just right, then do the math. As I said, for the experienced hand at it, the wires work great. But for the novice, it's a learning experience.
Second thing I dislike about the wires is that they add one more interface for the introduction of a foreign particle. Not a likely scenario, but still possible. The tolerances that your shooting for preclude any grit or the tiniest chip, which could still get between the thread mic anvil and the part, but add the wires in there, and that foreign object has one more place to hide.
You still have to be sure you have the proper anvil if your using thread mics, or the correct wires if you using those. So that's a wash. Your thread mic should read direct in Pitch Diameter, while wires have to be run through a little math. Slight edge for the mic. Thread wire sets are vastly less expensive and more adaptive than thread mics. You can get a set of wires to measure nearly any thread you can come up with for less than a set on anvils for a thread mic, plus the wires don't require a special standard, so economy is on the wires side. The Machinist Handbook will be applicable to either method, so it's a wash. You need one anyway, trust me. Boring as watching paint dry as far as reading goes, but a treasure trove of information. Get one. The info you seek is under thread pitch diameters, and not only will the book have the PD that you need, most likely, including tolerances, it will have the formula for figuring out the pitch diameter for anything it doesn't list. Get you calculator out and have fun.
Originally Posted by harold.
charts and formulas work too but I use the tried and true most reliable method
1) tap holes with a tap
2) carefully set thread mic to tap and subtract 0.001 to 0.005" depending on fit desired
works for me every time
Last edited by DMF_TomB; 04-30-2012 at 03:51 PM.
I'd imagine you can find just about everything you need in this re threads and especially pitch diameter tolerances.
Originally Posted by harold.
Screw thread types
mm to inches divide by 25,4. You can just print out the pages you feel are relevant to you.
With thread mics you need to make sure you are using the correct anvils for your pitch. You are going to be measuring pitch diameter, you can get that from the Machinists Handbook, if you don't have one you should. Also remember that a thread mic isn't going to tell you that your thread form is correct.
Safest way to check threads is with a gage or mating part
One past job I worked we made a lot of parts that were threaded to fit the dreaded "timken nut"...and the old fart inspector had his own timken nut in his toolbox that was "the" gauge for the job. Well a cheap set of thread mics let us make parts that fit his hidden secret gauge every time.
I'll bite. What's a "timken nut"?
Originally Posted by willbird
If you have an inspector that acts like you write I'd wonder how much your competition is paying him to make life difficult for you.
Timken is a company. www.timken.com
Originally Posted by Gordon B. Clarke
That inspector is an idiot for relying on a purchased nut to test stuff with.