Accurately measuring Full Thread Length

#1
Hello All,

I am currently in a discussion with one of my supplier's in regards to measuring Full Thread Length on an external Thread. The Supplier is currently placing the Thread Gauge onto the external thread, and once it bottoms out, they are measuring from the bottom of the gauge to the top/bottom of the external thread.

I disagreed and stated that to measure the Length of the Full Thread, you have to begin at the "Peak" of the first full thread and measure to the "Peak" of the last full thead to get the accurate length.

My question is. Am I 100% accurate? Or is my supplier? I cannot find any tools/diagrams online to depict where to begin/end when measuring Thread Length. Any insight or help will be greatly appreciated.

Thank you.
 
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#3
What caused the question to come up? What is the exact specification?
Hi Jim,
There is a min thread length callout of 8.5mm on the drawing. Our Supplier is stating their results are 9.2~9.5

From my measurements I am seeing 8.2~8.5 So some are undersized, or just at min.

And of course, measurement method is playing a role in this scenario. I just want the measurements to be accurate and go from there.
 

Tidge

Quite Involved in Discussions
#5
My reference copy of ASME B1.3 (I'm not sure if you are referring to screw threads or not) reads:

5(b) The gaging system used to inspect/evaluate the screw thread of a threaded product shall be as specified in the product standard, procurement drawing, or purchase inquiry and order. In the absence of a specified gaging system on the purchase order, product drawing, or other applicable documentation, the supplier and customer must agree uopn a suitable gaging system. There is no implied default gaging system.

I found the following in my reference copy of the Machinery's Handbook (under structural bolts, I didn't dig any deeper... it would help if you described the application or some standard terminology... you could be thinking about pipe threads!)

Basic thread length, LT, is a reference dimension, intended for calculation purposes only, which
represents the distance from the extreme end of the bolt to the last complete (full form) thread.
 
Last edited:
#6
Currently we are measuring using an Optical Comparator, First Full Thread Peak to last Full Thread Peak.

Application is Automotive. External Mounting Thread. With a min Thread Length of 8.5mm.
 

normzone

Trusted Information Resource
#7
What really matters the most is what the end use of this piece is, and how it's mating components dimensions are called out and toleranced.

If everything will go together just fine regardless of small differences in measurement technique, then we're just splitting hairs for purposes of training and discussion.

If the assembly actually may or may not function due to potential interface issues, THEN this is not a drill, this is the real thing. Good engineers design their assemblies such that minor deviations from print won't affect the ability of the assembly to reliably go together and function reliably. But they don't always have that luxury when they work in confined spaces or with thin materials.

Do you have the luxury of knowing how this part gets used, and how much this matters? Or is it just a deliverable, and meeting print is all you have to work to?
 

Tidge

Quite Involved in Discussions
#8
I'm never going to argue against the concept that parts should have specifications that meet their intended use, but we shouldn't pretend that their aren't century-old standards. The customer and the supplier may have to co-educate each other and come to an agreement on many things; I highly recommend that they agree on an established standard rather than develop their own.

According to my copy of the Machinery's Handbook, ASME B1.7-1984 (R2001) includes the following definitions:

Length of Complete Thread: The axial length of a thread section having full form at both crest and root but also including a maximum of two pitches at the start of the thread which may have a chamfer or incomplete crests.

Length of Thread Engagement: The length of thread engagement of two mating threads which may have a chamfer or incomplete crests.


In the case described, it sounds like the area of disagreement may be with the start of the thread with incomplete crests.
 

Jim Wynne

Staff member
Admin
#9
I'm never going to argue against the concept that parts should have specifications that meet their intended use, but we shouldn't pretend that their aren't century-old standards. The customer and the supplier may have to co-educate each other and come to an agreement on many things; I highly recommend that they agree on an established standard rather than develop their own.

According to my copy of the Machinery's Handbook, ASME B1.7-1984 (R2001) includes the following definitions:

Length of Complete Thread: The axial length of a thread section having full form at both crest and root but also including a maximum of two pitches at the start of the thread which may have a chamfer or incomplete crests.

Length of Thread Engagement: The length of thread engagement of two mating threads which may have a chamfer or incomplete crests.

In the case described, it sounds like the area of disagreement may be with the start of the thread with incomplete crests.
Although it's unlikely that the quoted bits have changed, the current version of ASME B1.7 is 2006. If the ASME standard (or any other published standard) calls for more than is necessary, it's best that it gets settled between the two parties in the most convenient manner.
 

Ninja

Looking for Reality
Trusted Information Resource
#10
What really matters the most is what the end use of this piece is, and how it's mating components dimensions are called out and toleranced.
it's best that it gets settled between the two parties in the most convenient manner.
There's the point. Who is 100% right or wrong has little value. Clarifying what is needed, and whether the supplied parts meet that need needs to be the main and only point.

What method was intended to be used when the spec was written in the first place?
 
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