Do all your measuring instruments agree with one another?
One would think so, but what does your shop have in place to ensure micrometers, dial bore gauges, dial and vernier calipers, setting fixtures, size standards, ID and OD specialty bore gauges and other measuring devices all agree on size?
A typical small engine machine shop has no fewer but often more than twelve to fifteen measuring instruments for ID/OD bores and journals. A shop may even have setting fixtures and standards for these instruments in order to keep them accurate. Often missing is a method to ensure all these measuring devices agree with one another. Given increasingly tighter tolerances found in engines today, measuring instrument agreement is almost mandatory.
1. Find a competent employee (you?) that knows how measuring instruments work and assign him/her the task of Quality Calibration Specialist (QCS). You only want ONE INDIVIDUAL for this task and not more than one. More than one specialist creates a “he said, she said” issue if a problem with size agreement among instruments arises. You want just one person responsible.
2. Assign a clean, uncluttered place for all calibration to take place and make it lockable if possible. All standards and setting fixtures must be housed in this room/space. Exceptions would be fixtures that need to stay near machines.
3. Make it temperature controlled if possible. This is an important requirement because instruments and particularly calibration standards can be affected by temperature. So try and find a place that can meet this requirement.
4. Have a simple, one page inspection sheet for each instrument (See Figure 1 for an example). These sheets should be locked away to reduce chance of loss or tampering. They are an important record of your quality assurance so treat them with importance.
5. Establish a regular time interval to inspect each device. Inspections should be dated and signed off by the QCS. Time interval should be a matter of instrument use. Higher the use, more often the inspection interval.
6. Treat any unusual mishap with an instrument (dropping the tool is one big one) as a need to inspect and re-calibrate.
7. Adequate training on instrument calibration should take place before turning your new found QCS loose on your measuring instruments.
For our purposes, this is about all that is needed. Anything more is left to the larger shops or volume builders to perform and most of them already have QA in place.
A word or two about temperature; regardless how large or small your shop is, temp control is very important when precision measurements are involved. Keep them out of sunlight or extreme cold. It’s natural to carry a large mic by cradling the inside of the frame in hand but if this is done for any length of time, a measurable amount of heat from the hand will expand the head away from the anvil. Coefficient of thermal expansion for nodular or ductile cast iron is between 6.5 -10.5 millionths of an inch per degree F.
A mic warmed to 80 degrees by body temperature will measure a 6.0000″ diameter part at 68 degrees as 6.00047″. If 0.0005″ is all the tolerance you have for an engine component, well, you see the tolerance problem here. It could take up to an hour or more for the mic to reach thermal equilibrium so pick up and use a micrometer only when you need to measure something. The rest of the time, leave it in a safe place.
So you can’t keep your hands off the micrometer? What is the fix? When you use your micrometers, protect the mics and the work from heat transfer by using a shop towel as an insulator. Mechanical measurements worldwide are referenced to standard laboratory conditions at 68 degree F (20 degrees C). If the measuring instruments are steel and the work is steel, it’s ok if both are the same temperature. If one is bronze or aluminum … get out the thermometer and mic to determine the correction.
Lastly, be mindful of temperature differences in the shop. Measuring instruments, no matter how well calibrated, will not agree if the instrument and work are at different temperatures throughout the shop. Consider how many machining operations there are in an engine and the floor space it takes to house all your equipment and you begin to see how easy it is to create temperature variations within the shop environment. Throw in winter and summer temperature extremes and the problems of measurement agreement get even bigger.
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