MECHANICAL ENGINEERING

THE lMEASURBWZNT QU23TION

ACKN0WLEM;MENT This article was authored by Donald E. Wendein of Monsanto Research Corporation. I t appeared in the June, 1964 issue of “Mechanicnl Engi- neering.”

“When gou can measure what you are speaking about and express it in num- bers. you know something about i t ; and when you cannot measure i t , when !iou cannot express it in numbers, your knowledge is of meagre and unsatis- fact- kind. It may be the beginning of knowledge, but you have scarcely in your thought advanced to the stage of a science.’’

--Lord Kelvin in 1848

WHY DO WE measure? The answer is that we have n o science if we have no numbers, and without numbers we have no idea of quality level. But for practical purposes we measure for one or more reasons:

1. To insure the part will function accordmg to the design intent.

2. To insure the interchangeability of fit at as- sembly.

3. To determine the characteristic of the produc- tion process so that it may be kept under con- trol.

4 . To control features used for locational purposes in later operations.

Some tolerances of a part are critical because they ,iffect the performance or are controlling factors in

the function of the device or product being manu- factured. In the case of a ball bearing it would be the tolerances that control the truth of rotation that are critical from a functional standpoint.

Other dimensions of a part are critical because they control the fit of mating parts at assembly. In the case of a ball bearing we expect the outside and inside & m e t e r s ground in Connecticut to fit a spindle and headstock made in Ohio; thus measure- ment of & m e t e r s must be made and tolerances applied.

Any inspection or measurement criterion that is set up must take into consideration all of these four factors. The inspection or quality-control personnel responsible for the design and procurement of any gages must coordinate with the project and process engineers responsible for that product.

Naval E n g i n * r % Journal. October. I964 697

THE MEASUREMENT QUESTION MKHANICAL ENGINEERING

The direct measurement method measures the part directly without the use of a calibration system or masters. Examples of this are micrometers, ver- nier calipers, optical flats, the precision level, and fringe counting or interferometric devices.

The functional or fixed-gage method is not a mea- surement method as no variable data are obtained. It IS a method used to sort parts into acceptable and questionable categories. It is the practice of most inspection departments not to reject a part by means of a fixed gage. Questionable parts are usually in- spected with a measurement-type gage to insure

W y r e 1. Tiny cylindrid prts with 0.040-dia. holes. with +0.001 tolerance. Shown are gages for inspection of this tolerance: CO no-go plugs. an air spindle with noncontacting orifices and pneumatic comparator. a mechanical-indicator bore gage, and a mechanical bore gage with an electronic transducer and amplfier.

that the part does not lie in the zone between the gage-maker's tolerance and the limit of the part. Measurements are also taken to determine the mag-

We use one of these methods to inspect a part: nitude of the out-of-tolerance dimension and condi- 1. Indirect comparison method. 2. Direct measure- tion so that it may be corrected. nient method. 3. Functional type or f i e d gages. In general, then, we have some latitude when we

The indirect comparison method makes u s of a consider "how to meawre," but an engineering calibration system or masters and a comparator evaluation should be made to determine which which can be of the mechanical, pneumatic, or el=- method is the most feasible from the application, the tronic design. Mechanical comparators are of the production. and the economic standpoint. contact design, and when using contact gagmg we Figure 1 shows a small cylindrical part having an must remember that the results obtained are a func- 0.040-in-dia hole with a ~ 0 . 0 0 1 tolerance that is tion of the gagmg load used. Pneumatic and elec- critical. Also shown in this figure are three differ- tronic comparators can use transducers of the ent methods used to inspect this tolerance. Go no-go contact or the noncontact design. Electronic com- plugs may be used; an air spindle with noncontact- parators have the advantage of being readily a d a g ing orifices connected to a column type or back- table to data-handling systems such as a typewriter pressure pneumatic comparator may be used; a printout, card punch, tape punch, an analog, or mechanical indicator type bore gage may be used; recording. Optical measuring systems are of the or a mechanical type bore gage with an electronic noncontact type using various principles of light.

H O W TO MEASURE

transducer and amplifier may be used

698 N a v a l Enqmew Journal, October, 1764

MECHANICAL ENGINEERING THE MEASUREMENT QUES"I'ON

If we were to use the go no-go plugs for this in- spection we are subject to conditions of fit between the hole and the measuring plug that may result in an erroneous measurement. This condition of fit may occur in the cross section of the hole, as shown in Figure 2. A condition of fit may also occur along the axis of hole that results in an erroneous measure- ment, as shown in Figure 3. When selecting this method we must keep in mind these errors due to conditions of fit and their effect on the quality of the product. It is general practice not to use go no-go gages when the tolerance of the part is 0.001 in. or less.

The part shown has an AQL of 0.065 and if we inspect by attributes our sample size is 150; if we inspect by variables our sample size is 40 in lot sizes of 3000 parts. This is a factor that should be taken into consideration when selecting a functional in- spection method.

A physical contact measurement of the 0.040-in- dia. may be taken by using a mechanical bore gage that actuates a dial indicator, a pneumatic gaging cartridge, o r an electronic gaging cartridge, Figure 4 .

Both the noncontact air spindle and the mechani- cal contact gaging method can be designed to give a true diameter measurement. In the case of the mechanical contact gaging method, the part and the gage must be rotated slightly to pick up the low point or true diameter reading. In the case of larger diameters, air gaging has the flexibility of using either contact or noncontact spindles of the true diameter design or uf the average diameter design. In cases where data analysis or the recording of data is a significant part of the inspection problem, an electronic gaging system should be considered as it IS a natural to handle these problems.

WHEN AND WHERE TO MEASURE

In general, it is to our advantage to establish a measurement program as close to the process as pos- sible.

In the case of a development project, time scales are generally such that they allow little or no time for inspection. We should resist this tendency very strongly and advise our management on the many benefits of a good inspection program. If a develop rnent program is allowed to continue without the support of an adquate inspection program, decisions itre likely to be based on opinions and guesses rather than on facts. As a result, problems show u p at a later date when they are more costly and difficult to (Y)rrect.

It is during the development phase that the best measuring equipment and the most analytical in- \pection devices should be used. Much can be learned about the processes under consideration by [he use of this type equipment, especially with the .urface geometry recording equipment that exists :()day.

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Figures 2 and 3. If the hole is not round, or not uniform, the go no-go gage will give erroneous measurements.

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Figure 4. Use of a mechanical bore gage with an indicator.

An example of this is shown in Figure 5. A little cupshaped part is to be pressed with a ?0.001 radius tolerance. After several unsuccessful at- tempts to qualify a pilot lot, it was decided to make a surface profile study of the part. (See the left chart.) This chart shows the radius varying over a wide range from undersize on the right side to over- size on the left side. The internal dle cavity was then inspected on the surface profiling equipment using a replica material and exhibited a similar contour. These charts were discussed with the diemaker; the tool used on the electrical discharge machine was found to be the dlfficulty.

A new rotary table of higher accuracy was used in the generation of the radius on the new electrical discharge machining tool. The sphericity of this tool was inspected and found to be w i h n 0.0003. l h s new tool was then used to fabricate a new die cavity which produced parts shown on the chart a t the right.

In the case of receiving inspection we inspect the finished part after the processing has been completed in a vendor's plant often many miles away. When discrepancies are found in the receiving inspection department it is always too late, and it always costs both the supplier and the customer time and money.

Naval Engiwers Journal. October. 1964 6W

THE MEASUREMENT QUEsTION MECHANICAL ENGINEERING

Figure 5. Surface profile study of a small cup-shaped part with + 0.001 tolerance. The radius is seen to vary over a wide range.

It IS to our advantage, then, to work out an adequate quality control program at the source of the pro- duct ion,

I f your quality level is such that the statistical sampling plans being used do not break down and successfully accept a given lot of material, then the size of the receiving inspection department can be kept small. On the other hand, if sampling plans fre- quently break down and considerable screening is necessary to meet production schedules, then the receiving inspection department becomes a problem area due to the large work load.

W H A T WILL BE MEASURED IN FUTURE?

Let's start with the area of standards first. We have on the commercial market today micrometers with 2.00 in. of travel presumably accurate within 0.000010 in. and X-Y measuring machines with 18.00 in. of travel presumably accurate within 0.000035 in. To certify the calibration of equipment in this cate- gory we must develop our measurement capabilities within micrein. over an extended range, Production tolerances and the advent of numerically controlled machine tools are generating considerable pressure in this area.

Specifications on the performance and accuracy of numerically controlled equipment a re constantly becoming more stringent, and as a result the check- out of these machine tools is becoming more com- plica ted.

We must develop our ability to check such things as the truth of rotation of a machine-tool spindle, the growth of a machinetool spindle along i t s axis due to i t s own heat generation. We must develop our ability to measure such h n g s as flatness over a large area, straightness of a slide member Over a long distance squareness, and paral lehm of a slide and a spindle axis and system hysteresis. We must

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The tool used on the electrical discharge machine was found to be the source of the trouble A new die cavity produced parts charted above.

learn to check these machine-tool virtues statically as well as dynamically and under load.

There is much to be done in the measurement of surface finish and the establishment of surface-finish standards. Many drawings now call for both a maxi- mum and a minimum value for surface f i s h be- cause it has been found that too smooth a surface may be just as detrimental as too rough a finish. More emphasis is needed on the three-dimensional picture of surface finish rather than on the two-di- mentional aspect.

Every day we are faced with the inspection of close tolerance of a new material having a wide range of mechanical and physical properties. Some of these materials will vary dimensionally with changes in humidity, barometric pressure. as well as in temperature. In some instances we are faced with measuring parts the design of w h c h are such that they are not r isd in the free and unsupported state, and cannot be touched by a mechanical contact.

The use of electronic gaging systems with record- ing equipment or digital readout in the form of punch cards or punched tape is steadily increasing. Where a large number of inspection points is re- quired and a computer used to determine such things as range, standard deviation, and frequency distribution, the use of these systems should be con- sidered.

Most measuring machines in use today are of the X . X - Y , or X - Y - Z designs. These designs, of course, a re based on the grid system. We can arrive at the same point on a two-dimensional object by the u w of a polar coordinate measuring system, and we can arrive at a point on a threedimensional object by the use of a spherical coordinate measuring system.

In the space age, we are seeing more objects to inspect that are surfaces of revolution and more em- phasis on the use of polar and spherical measuring systems.

700 N a v a l Enpinears Journal, O c t o k r , 1964