MACHINERY’S REFERENCE SERIES
EACH NUMBER IS A UNIT IN A SERIES ON ELECTRICAL AND
STEAM ENGINEERING DRAWING AND MACHINE
DESIGN AND SHOP PRACTICE
NUMBER 64
GAGE MAKING AND
LAPPING
CONTENTS
Principles of Gage Making, by F. E. SHAILOR
Lapping Flat Work and Gages, by F. E. SHAILOR
The Rotary Lap, by A. J. DELILLE
Miscellaneous Laps and Lapping Operations
Copyright, 1910, The Industrial Press, Publishers of MACHINERY,
140.148 Lafnyette Street, New York City
In the following pages are compiled a number of articles relating to gage making and lapping. These articles have been contributed from time to time MACHINERY by various writers. in some cases the opinions expressed differ slightly as to the best practice, each writer describing the methods with which he personally has had most experience or best success. The articles of each writer have been given in full, irrespective of the fact that, due to this, some statements are repeated.
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.