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CHAPTER I

PRINCIPLES OF GAGE MAKING*

Possibly there is no branch of tool-making that demands more skill and accuracy than does gage making. Little has been published, however, regarding this exacting line of work; one reason for this is that while the gage maker might describe very thoroughly, in detail, the manner in which to make this or that gage, this detail description may not apply to the methods employed in another shop. The object of this chapter is to touch briefly upon the general principles of the different methods generally in use.

Material for Gages

It is becoming the general practice to make gages from machine steel and case-harden them. Machine steel hardened to a depth of 0.003 to 0.005 inch would seem to answer fully as well as tool steel for gages, because a wear of a small fraction of a thousandth of an inch would in any case render the gage useless as a standard. To obtain the very best results from plug or ring gages, however, they should preferably be made of tool steel. This is principally because the gage may spring slightly during the hardening process, and if the gage were made of machine steel and hardened only to a depth of 0.003 inch or thereabouts, the case-hardening could easily be lapped away, leaving soft spots in the gage. This not only shortens the life of the gage, but the soft spot would be charged with emery when lapping the gage to size, so that the finished product would be partly gage and partly lap. On the other hand machine steel is superior to tool steel for snap gages and profile and receiving gages, owing to the fact that these gages are not appreciably distorted during the hardening process.

Fig. 1. Profile Gage

Making Plug Gages

When making plug gages, the best results are obtained by using stock considerably larger than the finished gage size. For instance, if the plug gage is to be 1 inch in diameter it should be made from a bar of steel 1 1/8 inch in diameter or larger. In this way the scale and outer stock that has been decarbonized to a certain degree is entirely removed. The same precaution is applicable to reamers, mandrels, dies and numerous other tools that require hardening. If a plug gage, 1 inch in diameter, were turned from a bar of steel only slightly larger than 1 inch, it would be found after hardening that spots would appear on the surface which would seem to bulge. These spots are hardened, but the surrounding stock is apparently soft. However, if this gage were ground down to 15/16 inch diameter, it would be found to be hardened over its entire surface.

Fig. 2

Fig. 3

The methods of making gages vary greatly in different shops, according to the accuracy required. Some manufacturers do not require gages of greater accuracy than those turned and filed nearly to size, after which they are hardened, and polished to size with emery cloth. In other cases it is necessary to grind the gages to size after hardening. Then, again, in another manufacturing plant the requirements may be more exacting and the gages are ground and lapped. Going still further we find manufacturers who are still more exacting, and demand that gages should be hardened, rough ground, aged, finish ground, lapped and the minute ridges caused by circular lapping entirely removed by lapping the gage lengthwise to size. About 0.0001 inch is removed by this operation. The lapping operation is dealt with in detail in Chapter III.

Profiling Gages

When making a profiling gage of the type shown in Fig. 1, it is a good plan to first make a sheet steel templet to accurately fit the model. A planer tool is then fitted to the templet, and the impression is planed through three gages at the same setting, the three gages afterward being used as master, inspector, and working gages, respectively. Should the profile be of such a size as to render it impracticable to plane the entire surface at once, a series of formed tools are made, together with a male templet, and the impression is planed reasonably close to this templet and then finished by hand. As it is absolutely necessary that the profile be the same over the entire surface, the knife-edge square shown in Fig. 3 will be found exceptionally well adapted for this work. The gages, after hardening, are lapped by hand by means of a flattened copper rod and flour emery, so that they will fit the model perfectly. The appearance of this copper rod is shown in Fig. 2. Should the gage open up a trifle during the hardening process, a common vise will prove an admirable tool for correcting this, as the interior of the machine steel gage is soft.

Fig. 4. Common Snap Gage with Clearance for Lap

Fig. 5. Cast-iron Lap Adapted to Lapping between Jaws

Snap Gages

The common snap gage, Fig. 4, is carefully machined to within 0.002 inch of the finished size, care being taken to make the faces smooth. The holes C are made to allow clearance for the lap, Fig. 5, which is a cast-iron disk. The gage is case-hardened and gripped in the vise of a milling machine, a hand-operated machine being preferably used for this operation. The lap is placed on the arbor, smeared with emery paste, and set in motion. By moving the table back and forth, the gage can be lapped until the model can just be started to enter, after which the gage should be finished by hand. If the gage is made to dimensions on a drawing instead of to a model, it is advisable to make a temporary end-measuring gage of drill rod and fit the snap gage to this temporary gage.

Fig. 6. Easily Duplicated Snap Gage

Fig. 7. Receiving Gage

Fig. 6 shows a very simple style of snap gage. This type is easily duplicated. In this gage spacers E are used which are made to the required size for the piece to be gaged. The plates F are parallel pieces of hardened steel which have been ground and lapped. When the gage becomes worn, all that is necessary in order to duplicate the original size is to remove the plates, lap the surfaces true, and lap off the required amount on the spacer. A limit gage can be made by adding another plate and using spacers of proper length as shown at G.

Method of Making a Receiving Gage

The receiving gage, Fig. 7, is a very difficult gage to make, and on account of its cost, it is rarely used except where it is absolutely necessary to do so. The gage is made to fit accurately the entire profile of the piece to be gaged, and is made of a series of small pieces fitted together, the object being to overcome as far as possible the distortion of the steel when passing through the hardening process. The base H is of machine steel, case-hardened, and its upper surface lapped perfectly level. The pieces J are ground and lapped on the bottom, and the formed edges are lapped by hand to fit the model. To obtain the best results, fit the pieces J to the model while the pieces are soft, and fasten them to the base H by screws. The dowel holes are now drilled and tapped with a fine pitch tap, say 5/16 inch diameter, 32 threads per inch. After the pieces J and the base are hardened, soft steel screws are turned securely into these holes and dressed off flush with the top and bottom of the pieces J and the base. After the pieces are lapped to fit the model, they are tightened in their places, and the soft screw bushing drilled and reamed through for the dowels. It is impracticable to attempt to lap dowel holes true, especially when they are tapered and do not line up. This soft screw bushing will be found useful on many other tools where dowel holes are apt to change during hardening.

Fig. 8. Universal Snap Gage for Large Work

Making a Universal Snap Gage

Fig. 8 shows a universal snap gage that is designed especially for large work. All that is necessary to make one gage cover a wide field is to set the gage to the required diameter (from standard length rods, so that the pointer stands at zero), then as the gage hangs on the piece to be gaged, it is swung so that anvil L passes over the highest point, and the pointer will record 200 times greater than the actual error. Any one who has used a large micrometer for measuring such work as is found in arsenals knows the difficulties under which one is obliged to obtain measurements. One man will hold the micrometer on the breech of a large gun and another man will do the measuring. With this form of gage one man can measure very handily, as all that is necessary to do is to note the number of graduations traversed by the pointer when the gage passes over the work.

* MACHINERY, June, 1905.