View Full Version : Have a question

MotorHead19

12-18-2008, 02:12 PM

I recently took a nims certification test layout, and benchwork and something else I cant remember i passed but there was this one equation i coudlnt figure out i do believe the question was

with this equation solve for M with a 3/4-10UNC thread

M=D+3w-1.5155/N

I know M=Major Diamator and D=Diamater of the thread wich is .750

and i know N=number of threads but what does w? stand for my teacher wasnt even familier with this formula so if some one could help me with this and slove it that would be deeply appreciated

oldtiffie

12-18-2008, 07:09 PM

I recently took a nims certification test layout, and benchwork and something else I cant remember i passed but there was this one equation i coudlnt figure out i do believe the question was

with this equation solve for M with a 3/4-10UNC thread

M=D+3w-1.5155/N

I know M=Major Diamator and D=Diamater of the thread wich is .750

and i know N=number of threads but what does w? stand for my teacher wasnt even familier with this formula so if some one could help me with this and slove it that would be deeply appreciated

I don't know if this will help, but it seems to be to do with "thread accuracy":

http://www.google.com.au/search?hl=en&q=M%3DD%2B3w-1.5155%2FN&btnG=Google+Search&meta=

http://www.freepatentsonline.com/4590678.html

crews1

12-18-2008, 08:27 PM

M = Measurement over Wires

W = Wire Size

JBL37

12-18-2008, 08:35 PM

The "W" in your formula is for the best wire size to measure the thread size of 60* threads. Attached is a copy of this formula from a school book. Hope this helps. The only difference is the wire size label. The school book uses a "G" where your uses a "W". Jim

http://home.att.net/~itemforsale/threewire.jpg

Oldbrock

12-18-2008, 09:20 PM

Is your teacher a machinist? There are many things I don't know but this is pretty basic. Peter

GKman

12-18-2008, 09:24 PM

Is your teacher a machinist? There are many things I don't know but this is pretty basic. Peter

So, what is M?

Oldbrock

12-18-2008, 09:34 PM

Measurement over the wires. D is the thread diameter

miker

12-18-2008, 10:41 PM

What/where do the two numbers, represent/come from?

0.57735 and 1.5155 .

I hope all these 3 wires we have been reading about lately are disconnected from the mains before use. :)

rgds

Oldbrock

12-18-2008, 10:58 PM

Miker, those numbers are constants to save you doing the trig to figure them out. Peter

Astronowanabe

12-19-2008, 02:05 AM

What/where do the two numbers, represent/come from?

0.57735 and 1.5155 .

I hope all these 3 wires we have been reading about lately are disconnected from the mains before use. :)

rgds

.577350... is an approximation for the tangent of 30 degrees which is exactly one third of the square-root of three.

it shows up because 30 degrees is the complement of the 60 degrees the thread.

unless you are a squid it helps to have the current running through the wires creating electromagnets to hold then in place while you measure them

oldtiffie

12-19-2008, 02:20 AM

The previous answers are correct in general terms. But application for a specific case needs a "bit more leg-work".

The general answer is under the heading of "Measuring Screw Threads" in my Machinery's Hand-book" (27th. Edition). The specific pages are 1898, 1899 and 2000.

The general formula is at pages 1898 and 1899 and gives a very good "pointer" to the matters addressed in the OP.

The whole of the subject matter under the heading ranges from page 1893 to 1911, and "Thread Guages" runs from pages 1911 to 1918.

There is a lot of "heavy reading" in there.

I would caution against "jumping in too deep too soon" as there is a lot to absorb and to consider.

My guess is that it is incorporated in the software utility being discussed currently at:

http://bbs.homeshopmachinist.net/showthread.php?t=32090

LES A W HARRIS

12-20-2008, 08:58 PM

Thread measurement over three wires. (With NO compensation for Lead angle).

Long winded equations are often reduced to the simplest form for convenience,

Old fogies do not seem to pass on where the full equation started; so……

M = Measurement over three wires.

E = Pitch diameter of the thread.

G = Best wire size. (The diameter of wire that lies as close as possible to the)

( pitch diameter of the thread).

G = secant of half the thread angle (a) /2*threads per inch (n)

(secant is the reciprocal of the cosine). For 60° = 0.57735/n

or 0.57735 * pitch.

E.g. 13 threads per inch (TPI) = 0.076923” pitch (p).

M = E + ((( 1 + cosec a ) x G) – 0.5 x p x cotan a).

Cheers,

miker

12-23-2008, 04:08 PM

Brockley1, thanks, I couldn't think of the right word.

My question should have been " what do those Constants represent?"

Astronowanabe, thanks for the explanation that makes the formula make sense to me now. I find it easier when I know why a constant is being used.

Rgds