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Geometric tolerance and precision pin design Question
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Posted by: gs
Huckleberry
12/05/2003, 09:04:51

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I am designing a precision guide pin that mates with a bushing. The tolerances given are:
Bushing: .250/.251
Pin: .248/.249

The pin I am designing is 1.4" long. 1" of the pin has the precision tolerance described above. The last 0.4" is a #12-24 thread that will thread into a chassis.

I need to put a GDT on this part. The 3 that I am considering are straightness, concentricity, and positional tolerancing. My concerns are the thread and how accurately it can be toleranced and what the tolerance values should be. 

1. Are there additional tolerances I should consider?
2. What tolerance values would be adequate for this application?







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Re: Geometric tolerance and precision pin design
Re: Geometric tolerance and precision pin design -- gs Post Reply Top of thread Forum
Posted by: Soundcheck

12/05/2003, 11:44:07

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gs,

   Tolerancing the shaft for roundness and concetricity is fine, but when it comes to tolerancing the threads you can run into alot of problems if you are trying to use the threads as a means of aligning things. Threads should only be used as a means of attaching things to things. If you are using bushings and pins that have close tolerances and use a thread for alignment your tolerance will only be a good as the ones used to machine both the thread and the tapped hole it is mating with. On the average a thread fit of 75% -80% is standard, anything over that and you will have problems when you try to screw the shaft into the threaded hole. I t sound like you are needing a 98% fit for your thread and that is unrealistic. I would suggest using a class 2B fit for both the shaft threads and the tapped hole and somehow use the O.D. of the shaft as a means of aligning your system, i.e. a .250 reamed c-bore to ensure position. You should utilize the shaft tolerance for precision alignment. In short you shouldn't be overly concerned with thread tolerances just make them as close as you can and remember the limitations and cost of the manufacturing proccess.







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Re: Geometric tolerance and precision pin design
Re: Re: Geometric tolerance and precision pin design -- Soundcheck Post Reply Top of thread Forum
Posted by: RandyKimball
Barney
12/05/2003, 19:58:43

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If this is a learning process about tolerances, ... express the threads as a #12-24 3A J thread. "J" threads not only control the pitch diameter but also control the radius diameters of the roots and peaks. They are used often in aircraft where a thread must be able to both hold close locational position and also be able to be removed often. By controling the radii we are able to remove the biggest source of thread jamb problems, the break down of the feathered edges at the root and peaks. These edges tend to shed slivers off into the engaged threads. With this problem removed from the picture we can call for and use much closer thread tolerances. Also note that by controling a root radius we improve the strength of the thread shaft because of the radius having less tendency to crack than a sharp cut (ripped) trench all the way around the shaft several times. Such sharp rips beg to become stress cracks. To learn more about "J" threads seek books about aircraft construction and standards.

-randy-




The worst idea of you lifetime may be the catalyst to the grandest idea of the century, never let suggestings go unsaid or fall on "death ears".

Modified by RandyKimball at Fri, Dec 05, 2003, 20:07:41

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