Fixed Fastner

8/20/2019 Fixed Fastner

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For fxed astener applications where one part has

threaded/press-ft holes or studs and the other parthas clearance holes, assembly shit only needs to becalculated or the part with clearance holes. In fxed

astener cases assembly shit is added to thetolerance stackup once representing the amount theclearance holes may shit about the astener.

This ormula is sometimes expressed in terms o T

instead o t! " t# howe$er, T implies that thetolerances or the threaded and clearance holes arethe same. In most cases, it is desirable to assignmore tolerance to the threaded hole than to theclearance hole because the threaded hole is usuallymore di%cult to manuacture.

&nce the astener is determined, our pieces o

inormation are needed to complete the clearancehole dimension and tolerances.



! 'learance hole ()' diameter. Threaded hole location tolerance t!

*. 'learance hole location tolerance t+. 'learance hole ))' diameter

!. 'learance hole ()' diameter

The largest hole that will ft under the head o aastener is the sum o hal the diameter o theastener plus hal the diameter o the astener heador hal the distance across the ats o the head, asshown in Fig. -*'. The ()' clearance hole also canbe calculated by adding the diameter o the astener



and the diameter o the astener head and thendi$iding the sum by two.

ngineers may select any si0e clearance hole that isre1uired. The .**2 clearance hole diameter mightha$e been selected or our example, but instead, amore conser$ati$e hole si0e o .34 in diameter wasassigned

. Threaded hole location tolerance t!

The location tolerance or both the threaded holeand the clearance hole must come rom thedi5erence between the actual diameter o theclearance hole and the diameter o the astener, thetotal tolerance a$ailable.

6ince drilling and tapping a hole in$ol$e twooperations and cutting threads in a hole is moreproblematic than 7ust drilling the hole, it is commonpractice to assign a larger portion o the locationtolerance to the threaded hole. In this example, 849



o the tolerance is assigned to the threaded hole# theremaining tolerance applies to the clearance hole.

*. 'learance ole (ocation Tolerance t

There are those who like to assign a positiontolerance o .44: or .4!4 at ))' or the clearancehole location. owe$er, a tolerance at ))' isarbitrary because bonus tolerance is a$ailable. Ithere is any doubt about which location tolerance touse, speciy a 0ero positional tolerance at ))'. ;ero

positional tolerance at ))' will pro$ide all thetolerance a$ailable and gi$e the machinist the mostsi0e exibility in producing the clearance hole. ;eropositional tolerance at ))' has been assigned to theclearance hole in this example and will be used tocalculate the ))' hole diameter



+. 'learance ole ))' diameter

&nce the astener and the position tolerances or the

threaded and clearance holes ha$e been selected, itis a simple matter to calculate the clearance hole))' diameter. The positional tolerance or thethreaded hole t! is .4+ in diameter, and thepositional tolerance or the clearance hole t is .444.<hiche$er tolerance is selected, it is important touse this ormula to calculate the correct clearancehole ))' diameter. I the clearance hole ))'diameter is incorrect, either a possible no-ftcondition exists or tolerance is wasted.



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Fixed Fastner