Quality Scan: Mastering Your Bore Measurements
By Chris Yost
F.V. Fowler Co. Inc.
Bore Gaging is a wide and varied field in precision measurement. Gages range from being general application to specialized and job specific types. Often the measurements taken with these instruments are carried out improperly due to poor technique and improper mastering.
Most bore gages have one thing in common. They need to be mastered or set to a known size diameter before they will be accurate and effective. Like other internal gages, the anvils of bore gages face away from each other; consequently it's not possible to bring the anvils together to touch and set zero. This introduces the need for master setting gages.
The characteristics of the measurement are critical in determining the way the bore gage should be mastered. A wise man once said: "It's always best to master a gage the same way you will be using it." This statement is very true. If you are taking a shallow bore measurement where only 0.100" (2.5 mm) of the anvils are being used, try to duplicate this with the mastering procedure. This way the different stresses that are active on the gage when it's being mastered will also be taken into account when measuring, giving a more accurate result.
Another important mastering technique to be aware of deals with the size of the master. It's always best to master a bore gage as close to the nominal measurement as possible. Doing so will ensure that the linear accuracy of the gage is kept to a minimum by utilizing less range. This will result in the accuracy of the measurement closely following the gage's repeatability, or the ability of the gage to duplicate an identical measurement.
Position of the bore gage in the master while setting is also important. Care should be taken to ensure the gage is calibrated in the master at the same location each time it's used. Rings are normally bell-mouthed, so the highest accuracy is in the middle section. In any case, it's best to test the bore gage for repeatability in the master before taking part measurements.
The three-point bore gage is excellent for determining tri-lobing as well as diameter. The instrument should be mastered--preferably with a ring gage, but alternatively with a calibrated master component. This is true because while measuring on a spherical surface a ring gage closely emulates the face of the object that will be measured. The three-point instrument is generally the easiest bore gage to master, removing as much of the operator input as possible, hence reducing repeatability issues.
A two-point cylinder bore gage is excellent for determining ovality of a bore in addition to the diameter. The cylinder bore gage must be swept forward and backward through the master to find its reversal point. When the indicator's reading moves from high to low to high, the diameter can be set into the gage's memory or tuned in with the bezel of the indicator for future measurements.
Since the contacts on a standard two-point bore gage are 180º apart, it's possible to use straight masters, including gage blocks with jaws held in a gage-block cage. This setup is often used when many diameters are being checked, and buying multiple ring gages is not economical. Straight mastering is a good way of setting only if attachments are available to keep all measuring faces parallel. The only time a straight master would actually be preferred over a ring would be if the operator were taking a straight measurement such as a groove or a slot. As mentioned before, it's always best to master a gage the same way you will use it. Outside micrometers, when locked, can be used as straight masters, but this is not recommended because there can be many sources of error associated with them. If such a master is used, verify the accuracy and make sure to check the calibration by taking a repeatability test.
Bore gages are available in many different styles and configurations. They can measure diameters, grooves, threads, tapers, slots and many other features. While the masters vary, the fundamentals discussed will help ensure that the measurements taken by the technician are correct.
This article was first published in the July 2004 edition of Manufacturing Engineering magazine.