Geometric Dimensioning and Tolerancing for Mechanical Design Part 8 ppt

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Position, Location 127

Ø .250-.290

Figure 8-2 Floating fastener with a zero

positional tolerance at MMC.

available and give the machinist the maximum size flexibility in producing the

clearance hole. The calculations could not be easier. The MMC hole size when

toleranced with a zero positional tolerance is the same as the diameter of the

fastener.

H = .250 + .000 = .250

What is the actual location tolerance in Fig. 8-2? The location tolerance for

a given hole size at MMC is the same no matter what tolerance is specified in

the feature control frame. If the clearance hole is actually produced at Ø .285,

the total location tolerance is:

Geometric tolerance + bonus = total positional tolerance

.020 + (.285 − .270) = .035

or

.000 + (.285 − .250) = .035

If the machinist happens to produce the hole at Ø .265 and zero positional

tolerance is specified, the hole size is acceptable, but the hole must be within a

location tolerance of Ø .015. No matter what tolerance is selected, it is important

to use the formula to determine the correct MMC hole diameter. If the MMC

clearance hole diameter is incorrect, either a possible no fit condition exists or

tolerance is wasted.

The next step is to determine the LMC clearance hole size, the largest possible

clearance hole. The LMC hole size is, essentially, arbitrary. Of course, the clear￾ance hole must be large enough for the fastener plus the stated tolerance, and

it cannot be so large that the head of the fastener pulls through the clearance

hole.

Some engineers suggest that the clearance hole should not be larger than

the largest hole that will fit under the head of the fastener. If a slotted clear￾ance hole, Fig. 8-3A, will fit and function, then surely the .337 diameter hole

in Fig. 8-3B will also fit and function. How is the clearance hole diameter in

Fig. 8-3B determined? The largest hole that will fit under the head of a fastener

is the sum of half of the diameter of the fastener and half of the diameter of the

fastener head, or the distance across the flats of the head, as shown in Fig. 8-3C.

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Position, Location

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128 Chapter Eight

.125

.425

.337

.212

(a) (b) (c)

.250-20 UNC-2A

Figure 8-3 Clearance hole size at LMC.

The LMC clearance hole can also be calculated by adding the diameters of the

fastener and the fastener head and then dividing the sum by two.

H @ LMC = (F + F head )/2

= (.250 + .425)/2

= .337

This method of selecting the LMC clearance hole size is a rule of thumb that

will allow you to compute the largest hole that will fit under the head of the

fastener. Engineers may select any size clearance hole that is required, but with

the use of the above formula, they can make an informed decision and do not

have to blindly depend on an arbitrary clearance hole tolerance chart.

Fixed Fasteners

The fixed fastener is fixed by one or more of the members being fastened. The

fasteners in Fig. 8-4 are both fixed; the fastener heads are fixed in their coun￾tersunk holes. The fastener, Fig. 8-4B is also fixed in the threaded hole at the

(a) (b)

Figure 8-4 A fixed fastener and a double-fixed fastener.

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Position, Location