General Information about Comtorgages

Comtorgages are hand-held, variable readout, comparative measurement gages, each capable of measuring a single dimension. They can provide an economical and rugged method of checking most dimensions on a part.

Each Comtorgage consists of an expansion plug that expands to contact and measure the part, an amplifier or coupler to display the measurement value, and a ring gage for setting the gage to the MEAN dimension. The expansion plug and ring gage are built specifically to a size with a small measurement range, but the amplifier or coupler can be used with other expansion plugs and rings that are made for equivalent part tolerances.

The most common Comtorgages are bore gages. Comtorgage bore gages must be retracted to enter a bore, and they are self-centering and self-aligning as the gage is expanded into the bore. The expansion plug expands using a spring-loaded outward force, and a locking taper "locks in" the measurement reading. The spring-loaded contact forces are uniform from part-to-part, which assures repeatable measurements, even with operators of different skill levels.

A Comtorgage bore gage can measure geometric conditions such as taper, ovality, hourglass, or barrel shape conditions in your parts. However, the gage must be retracted and repositioned in the bore to do this because of the locking taper in the measurement transfer system. Simply rotating or sliding the gage without retracting the contacts will cause unnecessary wear to the contacts. However, a Comtorgage does not require the gage to be rocked over-center like many dial bore gages.

The Comtorgage Bore Gage Principle of Operation:

The following drawing of a Comtorgage bore gage has an exaggerated amount of clearance to the part to explain the principle of operation.

  • The expansion plug is made from a single piece of steel which is saw-cut to a diameter, creating a flexure hinge. Expansion plugs larger than .8700" (22.098mm) also use the flexure hinge, but they have separate gage "shoes" or "anvils" attached to the hinge.
  • Then, the gage head is ground round and straight at a diameter which is .001" below the minimum amplifier size reading. For the CM2 amplifier shown in the drawing which has a total range of +/- .006", the gage head would be ground to .007" below the MEAN part size. This assures the retracted gage will always be able to measure the nominal size bore within the tolerance of the amplifier's dial range.
  • In the side view, the flexure hinge is the pivot point where the split plug expands, and the gage contacts the part at the very tip. As the gage tip moves outward to contact the part, a small clearance is created at the back of the gage head caused by the hinging action. This clearance is small enough to position the gage contacts perpendicular to the bore centerline, and measure the part without any measurable cosine error. The clearance also allows the gage to check taper, hourglass, or barrel shaped bores.
  • In the end view, as the undersize split plug expands, the contact points are perpendicular to split line, and there is an increasing amount of clearance between the gage head and bore as you get closer to the split. This clearance is small enough to position the gage contacts very close to the true bore diameter. With the "settling" technique described below, the gage head will be positioned in the center of the bore.
  • Thus, the Comtorgage is a two contact gage that measures the true diameter of a bore.
  • There is another big advantage of using a Comtorgage. The locking taper "locks in" the reading, so the gage operator doesn't need to watch for the smallest reading, like with many bore gages. The gage operator can insert the gage into the bore, release the retraction button, and "settle" the gage without looking at the amplifier dial. After he has done this, he can then read the dial to get the measurement. The locking taper even prevents the operator from changing the reading by "wrenching" on the gage. The gage mechanically "locks in" the true diameter and displays the reading until the retraction button is retracted.

Using A Comtorgage Bore Gage Properly:

  • Use the identical method for mastering the gage and using the gage to check parts.
  • The tip of the expansion plug must always remain within the master or part bore. This is because the contacts are at the tip. If the contacts are allowed to go past the master or bore diameter, the gage will indicate the diameter as being larger than it actually is. To prevent this, various styles of depth stops are available.
  • Realease the retraction button smoothly. This doesn't necessarily mean slowly...just don't release the retraction button without any resistance. If you let it "snap", the gage reading will have a slight error due to overtravel of the locking taper.
  • At this point, the diameter reading will be somewhere within one graduation of the actual part diameter. Because the expansion plug is slightly smaller than the part, it is possible for the contacts to be slightly off-center in the bore (sideways), measuring a chordal dimension instead of the true diameter. This positioning variation can produce a slight undersize reading of up to one graduation.
  • If you "settle" the gage, you can remove this slight error. "Settling" the gage involves gently rocking the gage from side to side a couple of times. This uses the back of the gage head for leverage to move the contacts to the largest measurement...the true diameter. The locking taper "locks in" the true diameter, and doesn't allow the contacts to go past the diameter to a smaller chordal dimension.
  • Be sure to rock the gage in the proper direction. If the gage is assembled correctly, the split line at the tip and along the sides of the of the expansion plug head will be 90 degrees to the long dimension of the amplifier or coupler case. This puts the contact points in line with the retraction button. For "settling", the gage should be rocked by pivoting on the contacts, which means a slight back-and-forth rocking motion in the direction of the narrow dimension of the amplifier or coupler. There is very little movement involved...it might be better described as a slight pressure in the two different directions.
  • The "settling" is actually a slight relative motion between the gage and master or part. With small masters or parts, this is usually easier by holding the gage and moving the part or master. For larger masters or parts, it is usually easier to move the gage back-and-forth.
  • "Settling" only takes a fraction of a second, and it soon becomes a natural part of the measurement process that you don't even think about.
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