3 wire thread measuements.

[Kevinstj]

I was looking for a simple formula or if someone could tell me what size wires to use to measure an external 7 3/8" - 10 TPI. The pitch diameter is 7.308 - 7.301. I usually use thread mics ,but I don't go that big on the mics. I never have used 3 wires before. I looked in the Machinist Handbook and that was just a waste of time. Any help would be greatly appreciated.

 

[mrainey]

Max wire diameter = 0.10104
Min wire diameter = 0.05052
"Best" wire diameter = 0.05773

Specify what wire diameters you have available and the constant value can be calculated. The constant is then subtracted from the measurement over wires to determine the pitch diameter.


Software For Metalworking

http://closetolerancesoftware.com

 

[ornerynorsk]

Your wire set should have a chart for the thread pitch you are measuring. I'll try to remember to post the constants later today after I've been out in the shop.

 

[mfgenggear]

meas over (3) .05774 wires
7.3946
7.3876

 

[mfgenggear]

measurement =P.D.+(3W-0.86602 P)
W=wire
P=pitch
P.D.=pitch diam of thd

MfgEngGear

 

[Cockroach]

Sounds like you have a UN thread geometry, matching 7.307/7.296 inch pitch diameter. In this case, MfhEnggear is correct.

But the in the larger scheme of things, you should specify the thread geometry, this would affect your three wire measurement over wires. For example, both Acme and Stub Acme thread forms have 0.052 "best wire", but the Acme has a pitch diameter of 7.342/7.368 inches as opposed to the Stub Acme which would be 7.362/7.388.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[ABritTurner]

For American 60 degree included angle form best wire is 0.577p (where p = pitch) limiting sizes for best wire calculations is .535p to .62p. Best wire sizes should be used when there is doubt over the correctness of the included thread angle form i.e.as in the case of a hand ground tool.

maximum wire size is 1.01p
minimum wire size is .505p
These are for the effective diameter measurements. These size may be used when one has ground forms as in indexable carbide tools.

Depth of thread is .6495p
Width over the wires (3 wire measurement) is given by W

W = D +3d - 1.5155p

Where D = maximum major diameter
and d = roller diameter
and p = pitch of thread

This information is taken from Workshop Technology Part Three W. A. J. Chapman printed by Edward Arnold (Publishers) Ltd Page 410

I hope this helps. You can buy propriety rollers from Buck and Hickman or turn your own.

Hope this helps