help for vernier callipers and screw gauge??

[Green]

Hi,

I am currently having some problems in using a vernier callipers and screw gauge .I would appreciate a litle help in knowing how to calculate the measurement.

PLEASE AND THANK YOU

yours truly

[coquina]

Hi' date='

I am currently having some problems in using a vernier callipers and screw gauge .I would appreciate a litle help in knowing how to calculate the measurement.

PLEASE AND THANK YOU

yours truly[/quote']

 

... but were afraid to ask.

 

Is it a dial caliper or a straight vernier? Is it metric or English measurement?

Are the screws metric or english?

 

This page shows how to read an English measurement caliper:

http://hyperphysics.phy-astr.gsu.edu/hbase/class/phscilab/vernier.html

 

Same for metric:

http://www.physics.smu.edu/~scalise/apparatus/caliper/

 

Picture of different kinds of calipers - the digital one is self explanatory.

 

http://www.archaeologica.org/UsingCalipers.htm

 

Instruction for reading dial calipers - The scale is marked in inches and .1". One time around the dial is .1" You read the scale first, and then the dial. If the scale reads 2.3 and the dial reads "33" the measurement is 2.333"

 

About screw threads both standard English and Metric threads are 60 degree V threads.

 

In English, the first number is the major diameter (outside diameter measurement) of the thread and the second number is the number of threads per inch.

 

For example, a 1/4-20 screw has a major diameter of 1/4 or .25 and there are 20 threads per inch. That is known as a "coarse" thread and is abreviated "UNC". A 1/4-28 thread has 28 threads per inch and is a "fine" thread, abbreviated "UNF". The "pitch" of the thread is the length of one thread. To arrive at that number you divide 1" by the number of threads per inch. The pitch of a 1/4-20 thread is .05"; the pitch of a 1/4-28 thread is .03571.

 

Metric threads are designated a little differently, for example an M8 x 1 means that the major diameter is 8mm and the lenth of one thread is 1 mm. An M8 x 1.25 means the major diameter is 8mm and one thread is 1.25mm.

 

The measurements are "theoretical sharp V". The actual measurement of the OD of the thread will be a little under the actual measurement to allow the screw to fit properly.

 

The "minor diameter" of the thread is the diameter of the screw at the bottom of the thread. Unfortunately, I don't know how to put figures here, so I'll wing it. Your thread looks like this vvvvvvv except the the tops of the V is closed. As it is each triangle is a 60 deg. equilateral triangle. To find the minor diameter, you have to first bisect the angle to make a right triangle. The short side is 1/2 the pitch. The opposite angle is 30 degrees. You trig is out to get the dimension of the long side. Here's where is gets tricky - Because there are threads on both sides, you have to double the dimension of the long side and subtract it from the major diameter to find the minor diameter.

 

When machinists talk about cutting something in a lathe, you will here them use the term "on a side" - when they say this they are talking about the radius of what they are cutting.

 

Hope this helps.

[DQW]

About screw threads both standard English and Metric threads are 60 degree V threads.

I don't understand the relevance of this part of the post to the OP's query (which, I must say is too vague to answer specifically). The question was about micrometer (screw) gauges, not thread gauges or screw threads. Don't machinists refer to micrometers as screw gauges anymore ?

 

The "minor diameter" of the thread is the diameter of the screw at the bottom of the thread. Unfortunately, I don't know how to put figures here, so I'll wing it. Your thread looks like this vvvvvvv except the the tops of the V is closed. As it is each triangle is a 60 deg. equilateral triangle. To find the minor diameter, you have to first bisect the angle to make a right triangle. The short side is 1/2 the pitch. The opposite angle is 30 degrees. You trig is out to get the dimension of the long side. Here's where is gets tricky - Because there are threads on both sides, you have to double the dimension of the long side and subtract it from the major diameter to find the minor diameter.

Or you can halve it and subtract it from the radius. But either way, you will end up with a wrong answer - your number will be an underestimate because the threads are not sharp on the major dia (ie : the profiles are not complete equilateral triangles; the tops are chopped off), and without a knowledge of the exact profile (or the amount of flattening on the major dia) you can not calculate the minor diameter.

 

So instead, you look up a table, like this one.

 

To the OP : Coquina's second link tells you how to understand what a traditional vernier reads at a certain position.

 

A screw guage (or micrometer) is easier to understand than a vernier caliper. Both are explained here : http://www.phy.uct.ac.za/courses/c1lab/vernier1.html

[coquina]

I have never in my life heard a micrometer referred to as a "screw gage" and I grew up in a machine shop. Around here, we just call 'em "mics". (Pronouced "mikes".)

 

I thought s/he must be talking about a thread gage, but I couldn't figure out why s/he was having trouble reading one, since no reading is involved, it either fits or it doesn't.

 

Now that I have managed to get off on the wrong foot with you - welcome. In what part of the world do you call a mic a "screw gage"?

[DQW]

Hey, I apologize for being a grumpy ol fart.

 

The archaic usage of the term 'screw gauge' (for a 'mic') is a remnant from Imperial terminology that still exists today in the UK and some of its erstwhile colonies, including right here in the US. Try asking some of the older machinists in your shop - they will remember the name.

[coquina]

Hey' date=' I apologize for being a grumpy ol fart.

 

The archaic usage of the term 'screw gauge' (for a 'mic') is a remnant from Imperial terminology that still exists today in the UK and some of its erstwhile colonies, including right here in the US. Try asking some of the older machinists in your shop - they will remember the name.[/quote']

 

No problem, you were right about the threads. I will ask Marion tomorrow. He is 80 and still works 32 hours a week. He is an honest to God toolmaker, and will be extraordinarily hard to replace. He says he has no plans for retirement. I wonder when mics were invented? I have my Great-great grandfather's American Machinists bound volumes from the 1880's and don't remember seeing any pictures of them there.

[alibabba]

Or you can halve it and subtract it from the radius. But either way' date=' you will end up with a wrong answer - your number will be an underestimate [b']because the threads are not sharp on the major dia (ie : the profiles are not complete equilateral triangles; the tops are chopp/b]ed off), [and without a knowledge of the exact profile (or the amount of flattening on the major dia) you can not calculate the minor diameter.

 

 

If I remember correctly, the flat portion of an American standard screw thread is 1/8 of the pitch. That is to say that if you are cutting a 10 pitch thread, the flat on the top of the thread (and incidently also the bottom) will be 1/8 of 1/10 of an inch, or in other words, .0125 wide.

 

One can then calculate the precise hight of the thread by subtracting the two little bitty triangles first, then calculating the rest (or vice versa).

 

I never heard a micrometer referred to as a screw gauge, and I thought the the OP was refering to a thread pitch gauge. The only time I would use a micrometer to measure a thread is with the use of thread wires, and that is to determine the pitch diameter.

 

I am a Journeyman machinist with 40 years experience.

[DQW]

I never heard a micrometer referred to as a screw gauge' date=' and I thought the the OP was refering to a thread pitch gauge. The only time I would use a micrometer to measure a thread is with the use of thread wires, and that is to determine the pitch diameter.

 

I am a Journeyman machinist with 40 years experience.[/quote']The usage is mostly British, and may be dead in most of the US. Apparently, it's deader than I'd thought.

[alibabba]

The usage is mostly British, and may be dead in most of the US. Apparently, it's deader than I'd thought.

 

In considering the term, I can see where it would be an accurate definition of what a micrometer actually is. Although I might have opted for "screw caliper."

 

That is probably why the term was used. I just never encountered it is all, but then, there are many things in the world that I have not personally encountered.