Functional Gages give different results - Thread Plug Gage

Wayne

Gage Crib Worldwide
#11
Re: Functional Gages give different results

… Even the amount of torque the calibration tech uses versus the inspectors can create a variation in passing or failing a part. It is a very dicey measurement technique, especially for tight tolerances. For that, you may have to section the part and measure the threads - or use a tri-roll thread gage - to get variable data.
I've seen these situations arise as completely theoretical exercises, and threads (male or female) that would work perfectly well in the application get rejected because of questionable gaging.
... One would need to look at this aspect and assess the "Risk" involved in accepting defective threads vs rejecting acceptable threads.
Threads are very forgiving for the most part.

The design of threads allows for assembly regardless of many expected errors in the manufacturing process.

The design of the thread gages allow for torque variation between operators and calibration technicians.

IMHO: If a minor deviation on a product thread causes a great “Risk” of failure, then the product design is faulty; pushed too close to the operating limits of the thread.

Threads are not intended to be inspected by micorscope.
 
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Wayne

Gage Crib Worldwide
#12
Re: Functional Gages give different results

… The Johnson gage company founded thread gaging….
Please, lets not rewrite history.

Grandpa Johnson was a very capable engineer.

While working for Pratt & Whitney during WWI he designed the Adjustable Thread Ring Gage.

He also, as an employee of Pratt & Whitney, designed the original Tri-Roll Gage System; which is now being sold by Southern Gage.

After he left Pratt & Whitney he established Johnson Gage Inc and made some design changes to his original tri-roll design; thus producing the Johnson Gage product line.
 

Wayne

Gage Crib Worldwide
#13
Re: Functional Gages give different results

… I have not heard of a tri-roll thread gage. Can you give me more info on that? How does it provide variable data?
Casey;

I know that Johnson Gage was mentioned, but please be aware that there are six independent gage makers each manufacturing their own version of the variable thread gage measurement system.

Here is a link to a good tri-roll primer. The Thread Gaging Reference Guide was written by Glenn Allen while working for Starrett.

IDK if the hard copy version is still available from Starrett since at this date Starrett has discontinued selling tri-roll gages. This linked webpage is appearentaly intended as a sales promotional page, but it gives a nice history of the discontinued Starrett tri-roll systems, and some nice interchange data between Starrett and Johnson Gage thread gage products.

When considering a tri-roll gage, be causious of the segment style. It has the same wear problems as the GO gage talked about earlier.

I hope my barage of posts has in some way helped you with your screw thread gage dilemma. :agree1:
 
T

TOOLMAKER

#14
The problem is that there are many "hard" gages being used and that the variation between their respective geometries is most likely causing interferrence issues between measurements.

The best suggestion by far has been to use variable thread gaging. This will give a dimensional measurement for Pitch Diameter and Functional Size. These are your primary concerns.

System 21 Hard Gages are not going to prove anything but "fit".

System 22 Variable Gages will prove conformance to requirements.

My suggestion, if economically possible, is to have your incoming inspection outfitted with System 22 Variable Gages.
 

Brizilla

Quite Involved in Discussions
#15
Re: Functional Gages give different results

Casey;

I know that Johnson Gage was mentioned, but please be aware that there are six independent gage makers each manufacturing their own version of the variable thread gage measurement system.

Here is a link to a good tri-roll primer. The Thread Gaging Reference Guide was written by Glenn Allen while working for Starrett.

IDK if the hard copy version is still available from Starrett since at this date Starrett has discontinued selling tri-roll gages. This linked webpage is appearentaly intended as a sales promotional page, but it gives a nice history of the discontinued Starrett tri-roll systems, and some nice interchange data between Starrett and Johnson Gage thread gage products.

When considering a tri-roll gage, be causious of the segment style. It has the same wear problems as the GO gage talked about earlier.

I hope my barage of posts has in some way helped you with your screw thread gage dilemma. :agree1:
Since it wasn't mentioned. Tri-Rolls are excellent gages for checking OD threads. It consists of two static thread rolls for whatever size thread you are using and one movable arm thread roll. You pull the arm back and put the piece between the three. It gives you an actual measurement for your pitch diameter and can detect egg shaped or lobed parts which is difficult for fixed rings to do.

Johnson gages have both ID & OD gaging. Their are portable and table units. They are good high end gaging. They're great for setup as you can tell exactly where within the tolerance you are.
 
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