inspection rules for measuring holes

Gary Whitmire

INSPECTION GUIDE Measuring a Hole

August 2003

Genium Publishing Corporation

Everyday we must measure holes and it may sound simple, but there are many rules associated with measuring holes, Measurements must be done in accordance with ASME inspection and engineering rules in order to guarantee interchangeability and part function. When comes to dealing with interchangeable parts, all inspection and engineering personnel must understand this very important subject. The hole must be measured:

1. for its smallest and largest size 2. for perfect form at Maximum Material Condition (MMC) size (RULE ONE) 3. for proper depth requirements 4. for out-of-round conditions 5. to establish the interchangeability virtual condition size 6. to establish the position tolerance zone size based on hole size 7. to establish if the hole is out-of-square to the datums, which will result in a tighter positional

tolerance zone 8. to establish if the center of the hole is located within the positional tolerance

The example drawing for this Inspection Guide is shown in Figure 1.

B

AC

1.000

3.00

.03±.01

2.50

2.38

.25

.25

4X R.19

1.0001.000

1.000

2X .511 -.516.005 M A B C

2X 1.50±.02

.38 X 45° CHAM(NON FUNCTIONAL)

Figure 1

Facts About the Two Holes 1. The limit of size is .511-.516 (See note 1) 2. Maximum Material Condition (MMC) size = .511 3. Least Material Condition (LMC) size = .516 4. Virtual condition size = .511 - .005 = .506 (See note 2) 5. Position tolerance zone at MMC = .005 6. Position tolerance at LMC = .010 (See note 3) 7. Critical depth of hole = 1.48 (See note 4) 8. The tolerance zones for the holes as a pattern are perfect to each other; perpendicular to datum

“A”; and located from datums “B & C”.

Gary Whitmire


August 2003


Note 1. - ASME B4.4M Inspection of Workpieces, Paragraph 1.4.5 Limits of Size of a Hole, states:

The diameter of the largest perfect imaginary cylinder, which can be inscribed within the hole so that it just contacts the high points of the surface, should be no smaller than the maximum material limit of size (smallest size). The maximum diameter at any position in the hole must not exceed the least material limit of size (largest size).

The above rule means that if the hole is everywhere at its maximum material limit (smallest size), that hole shall be perfectly round and straight, that is, a perfect cylinder. This is also RULE ONE in ASME Y14.5 Dimensioning and Tolerancing. See Figure 2.

Paragraph 1.4.2.1 of ASME B4.4M Principle of GO and NOGO Gaging states:

The Maximum Material Condition (MMC) (smallest size) is checked with a plug gage of a length equal to the length of the hole, or the length of engagement of the mating part, and a DIAMETER EXACTLY EQUAL TO THE MMC SIZE. The Least Material Condition (LMC) of the hole is checked with a gage designed to contact the hole at TWO DIAMETRICALLY OPPOSITE POINTS (this is not a NOGO diameter gage) separated by a distance exactly equal to the LMC. There are exceptions stated in the standard for large holes. The above may be simulated by using a Coordinate Measuring Machine (CMM).

Gary Whitmire


August 2003


.511

(MMC)

GAGE

PIN

RULE ONE

An individual feature of size, must

have perfect form at MMC.

Rule One does not apply at LMC.

.511MMC

.511MMC

.516LMC

.516LMC

.516LMC

.519

.516LMC

.511MMC

size of NOGO gage pin

NOGO gage pin does not check the LMC size.

LMC size must be checked with a two point

measuring device.

GOOD

NO GOOD

GOOD

If a NOGO gage pin was used to check the LMC

size of the hole, the part would have been

accepted because the gage pin would not go

into the hole.

Figure 2

Note 2. - Virtual condition for a hole is calculated by taking the smallest size MINUS the position

tolerance (.511-.005=.506). Virtual condition of an external feature, such as a pin, takes the largest size, PLUS the positional tolerance. Virtual condition (.506) is the critical size for interchangeability. That is, .506 must be interchangeable with the virtual condition of the mating part. As an example, if the mating part has two .495 -.500 diameter pins with a position tolerance of .005; this would make the virtual condition .500 + .005 = .505 giving a worst case clearance of .001 (.0005 per side between the hole and the pin). See Figure 3.

Gary Whitmire


August 2003


.506 virtual condition (hole)

.505 virtual condition (pin).511 diameter hole center of hole

center of pin1.000

1.000

.0005 clearancebetween hole and pin

.005 tolerance zonefor the hole and pin

.500 diameter pinNO SCALE

Figure 3

Note 3. - Any time there is a M in the feature control frame, the positional tolerance can become larger

as any of the two holes depart from MMC (.511). Virtual condition, which is the critical size for interchangeability, will always remain the same size regardless of the hole size, as shown below:

Hole Size Tol. Zone Size Virtual Condition Size .511 MMC - .005 = .506 .512 - .006 = .506 .513 - .007 = .506 .514 - .008 = .506 .515 - .009 = .506 .516 LMC - .010 = .506

Note 4. – The critical depth of the hole is 1.48 because whatever gets inserted into the hole must be equal to or shorter than 1.48. When inspection measures the size of the hole with the largest gage pin that will go into the hole, they should not measure deeper than 1.48 as shown in Figure 4. Note that the ASME standard states that ALL DEPTHS ARE UNDERSTOOD TO BE FROM THE OUTER SURFACE OF THE PART. On the drawing, (Figure 1) a depth dimension of 1.48-1.52 was given. In this case it would not be wise to use a depth symbol because the top surface is not clearly defined.

1.48

Gage pin to establish size of holewithin .511 to .516

1.52NO SCALE

If measured deeper, the gagepin could be smaller than .511

Figure 4

Gary Whitmire


August 2003


The next step is to determine the SIZE of the hole and the POSITIONAL tolerance zone. The drawing states .005, but the .005 only applies to the MMC size (.511). To check the actual machined size, insert the largest gage pin size into the hole, 1.48 deep. This is illustrated in the upper part of Figure 5. At this time we only know the actual size of the hole. To determine the correct POSITIONAL tolerance, we must simulate the assembly of the mating part. To do this we determine the size of the best cylinder perpendicular to datum “A” (1.48 deep), called Actual Mating Size, as shown in the lower part of Figure 5. The Actual Mating Size is .512, giving a POSITIONAL tolerance of .006.

2X .511-.516 .005 M A B C

HOLE PERMITTEDSIZE TOLERANCE.511 .005.512 .006.513 .007.514 .008.515 .009.516 .010

.515 DIAMETERGAGE PIN

1.48

1.48

.512 DIAMETERGAGE PIN

The .515 diameter gage pin size tell us theactual hole size with a potential tolerancezone size of .009. However, the interchangeability size will be the largest gage pin size perpendicular to datum "A" as shown below.

.512 Working Diameter-.506 Virtual Condition .006 Position Tolerance, not .009 because the hole is tilting .003.

SQUARE TODATUM -A-

A

Figure 5 The next step is to determine if the center of the hole falls within a .006 diameter tolerance zone. Inspection has a number of options:

1. Use a functional gage (See Figure 6) 2. Use a milling machine (See figure 7) 3. Use a Coordinate Measuring Machine (CMM) (See Figure 8) 4. Manual open inspection

Gary Whitmire


August 2003


In Figure 6 (functional gage) the part is flipped upside down and inserted over the .506 functional gage pins with three points of contact on datum “A”; two points of contact on datum “B” and one point of contact on datum “C”. Functional gages do not check hole sizes; the hole size is checked separately.

.506 dia. x 1.48 longVirtual condition

gage pinsFUNCTIONALGAGE

Datum Reference Frame is three mutually perpendicular planes

Figure 6

Figure 7 (milling machine) illustrates the setup on a milling machine for simulating the functional gage shown in Figure 6. The .506 diameter virtual condition pin size is moved in the X and Y direction the amount of the BASIC dimensions stated on the drawing, and then lowered into each hole. Note that a larger pin may be used to check CPK requirements for statistical process control.

.506 dia. x 1.48 longVirtual condition

gage pins

Figure 7

Gary Whitmire


August 2003


In Figure 8 (CMM) the datums are established by probing the simulated datums, not the surface of the part. The reason for this is the actual surface of the part may not be flat or not square from one surface to another. By definition, the simulated datums are 10% more accurate than the part. This is called the 10% rule.

Gary Whitmire


August 2003


C

B

A

A

B

C

Touching of the CMM probe

Computer generated

least square plane

Computer generated

least square plane

Touching of the CMM probe

This is the real datum “B”

This is the real datum “C”

C

1.000 1.000

1.000 1.000

1.000

B

EXAGGERATEDDRAWING

Figure 8

inspection rules for measuring holes

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