Why Gages Can Grow In Dimension

//Why Gages Can Grow In Dimension
Why Gages Can Grow In Dimension 2017-09-07T13:37:43+00:00

Hardened steel does have the propensity to grow.

The growth is usually noticed in larger sizes because it is an inch per inch type growth. What happens is when the steel is hardened the molecules are trapped in stasis. Slowly the molecules relax and retained austenite is changed to martensite, as they relax they cause an expansion of the material. The gradual relaxation process takes about a year. The larger the size the more the change is noticed. The tighter the tolerance the more the movement is noticed.

To combat this phenomenon gage makers require that their hardened steel be triple cold stabilized. The process requires the gage to be cycled three times through a freeze/thaw process. This procedure will reduce the probability of gage growth. Note the use of the word ‘reduce’ instead of ‘eliminate’.  This a major reason to have gages calibrated on an annual basis even if they have been subjected to little or no use.

The probability of growth is most likely noticeable when:

  1. If the gage is over an inch in size. The growth is compounded and as size gets larger it will become more apparent. On smaller tools the growth will most likely be considered within the range of measurement uncertainty or be explained away as being from different technician, equipment or laboratory variation.
  2. Within the first year after it has been hardened. This is because most of the movement is aged out of the steel in the first year. Things like temperature variation and gage use will encourage faster aging. Being unused and in a box may encourage slower aging to a small degree. Any growth detected after the first year will most likely be considered within the range of measurement uncertainty or be explained away as being from different technician, equipment or laboratory variation.

Gauling can cause a gage to grow.
Gauling is when small amounts of the material being tested weld to the gage surface thus causing the gage to grow. There are some materials, like stainless steel and aluminum, that are more prone to gauling gages. Small amounts of material can be transferred from:

Abrasion
Several conditions may increase abrasion.

  • Product thread manufactured too close to the maximum material size. When this happens there is more gage to material contact as the gage is applied. To avoid this try to keep the product size near the mean.
  • No oil on the gage. Be sure that the gage is well lubricated. By lubricated I mean oiled. I know that the part is covered in cutting oil. While cutting oil is a cutting lubricant, it is not a gage lubricant. A gage needs real oil. The thickness of the oil will not effect the measurement ability of the gage. The oil will just move out of the way, yet it will protect the surface of the gage from gauling.
  • A fat thread will frequently cause the GO gage to fail the thread. A marginally fat thread will be again close to the maximum material condition and thus cause abrasion against the gage. A fat thread is caused by too fast of a feed at the start of the thread making process. The speed or feed causes either the work piece to move away from the cutting tool, or the cutting tool to flex away from the work piece, as the cut is begun. This condition needs to be guarded against to avoid gauling of the gage.
  • A fine pitch, larger diameter thread is a recipe for abrasion of the thread gage. The more threads per inch the more revolutions the gage makes to gain full engagement and thus a longer contact distance for friction along the helical path of the thread.

Chips
The machining process may leave material chips in the threaded hole. If these chips are not removed they may weld themselves to the gage surface or get caught in the root of the gage thread. The fix for this is simply to remove the chips. OK, it is not always simple to remove them but it must be done to extend the life of the gage.

Burrs
These are pieces of material still attached to the work piece but protruding into the machined space left after cutting the thread. Not removed they may weld themselves to the gage surface. If burrs are a problem a different style cutting tool, a second pass with the cutting tool, different speeds or feeds or just a wire brushing may solve the problem.