Gage Contacts: Get the Point?

In spite of their apparent simplicity, gage contacts represent a source of potential measurement errors.

In spite of their apparent simplicity, gage contacts represent a source of potential measurement errors. When the simple act of touching a part can change its dimension, it’s important to understand the ramifications of a contact selection and application.
 
The first consideration must be whether you actually touch the part. Air gaging, as a non-contact method, has many advantages but is not always appropriate. Air gaging tends to average out surface finish variations on a part, providing a reading that lies between the microinch height peaks and valleys. In some instances this may be desirable, but if the critical dimension lies at the maximum height on the surface, then contact gaging might be more appropriate.
 
Contact size and shape are critical. Contacts with small radii may nestle between high spots of surface and form irregularities, or might sit on top of them, depending on exactly whether he gage contacts the workpiece. If the critical dimension is the low spot, it may be necessary to explore the part with the gage. Larger radii or flat contacts will bridge across high spots. The choice of radius depends at least partly on whether you want to “ignore” surface and geometry irregularities on the high or low size.
 
Contact size and shape also influence measurements because all materials compress to some extent as a function of pressure. When measuring obviously compressible materials such as plastics or textiles, gaging practice is commonly guided by industry standards. For example, ASTM D-461, “Standard Methods of Testing Felt,” specifies the size of the bottom anvil (minimum 2 in2), the size and shape of the upper contact (1 ±0.0001 in2; that is 1.129" diameter, with edge radius of 0.016 ±0.001 in2), the force of the contact (10 ±0.5 oz.), and the amount of time allowed for material compression prior to taking the measurement (minimum ten seconds). Similarly detailed standards exist for measuring the thickness of wire insulation, rubber sheet stock, and dozens of other materials. Not all the contacts defined in the standards are flat, parallel surface: other shapes such as knife-edges, buttons, cylinders, or spheres may be specified.
 
Even materials that are not thought of as compressible do conpress somewhat under normal gaging pressures. Because of the higher and higher levels of accuracy required in metalworking industries, it is often essential to compensate for this.
 
Under a typical gaging force of 6.4 ounces, a diamond-tipped contact point with a radius of 0.125 inch will penetrate a steel workpiece or gage block by ten microinches. The same contact will compress tungsten carbide by 6.6 microinches, and fused quartz by 20 mincroinches. If microinches count in your application, it is important that workpiece and master be of the same material. Alternately, one can refer to a compensation table to make the necessary adjustment to the gage reading. Compression can be minimized by using a contact with larger surface area.
 
Contact material also makes a difference. For the sake of durability, one normally selects a contact point that is harder than the workpiece. Typical choices include (in increasing order of hardness): hardened steel, tungsten carbide, and jeweled tips—ruby, sapphire or diamond. Tungsten carbide is a good choice for measuring steel parts unless millions of cycles are anticipated, in which case diamond might be chosen for longer life. One should avoid using tungsten carbide contacts on aluminum parts, however. Aluminum tends to “stick” to carbide, and it can build up so quickly as to throw off measurements between typical mastering intervals. Hardened steel and diamond are better choices for measuring aluminum.
 
Differently shaped marts may produce different readings, even though they are dimensionally identical. This is especially true when the contact points are worn. It is often possible to obtain accurate gage readings with worn contacts if one masters carefully and frequently. This includes using a master that is the same shape as the workpice. Periodically confirm that the gage contacts are parallel by sliding a precision steel ball or wire on the anvil from 12 o’clock to 6 o’clock, and from 3 o’clock to 9 o’clock, and measuring for repeatability m Measure again with the ball in the middle of the anvil to check for wear there.
 
Make sure the contact is screwed firmly into its socket so there is no play. On rare occasions, a jeweled insert may some slightly loose in its steel holder. A simple repeatability check will detect this. Unfortunately, there’s no good fix for it.
 

Not all gages use perpendicular motion. If yours has angular motion, be aware that changing the length of the lever contact will change the reading. On the mechanical test indicators, you may be able to install a new dial face with the proper magnification, or you can apply a simple mathematical compensation to every measurement. If you’re using a lever-type electronic gage head, you might be able to program the compensation into the amplifier.