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Help with Measurement Problems Cheat Sheet (DRAFT) by [deleted]

Help with Measurement Problems

This is a draft cheat sheet. It is a work in progress and is not finished yet.


In general terms, fixed limit gages are used to ensure parts made within prescribed limits will assemble. When it comes to actual size measur­ements, a lot of additional factors come into play one or more of which can indicate that what appears to be the best instrument for the job won’t work well at all. Here’s a list of some of the factors and/or questions you need the answers to in order to make a good choice.


The precision required of a measuring instrument to meet the tolerances a measur­ement is required to verify will be determined by the tolera­nces. The resolution of a particular instrument is not an indication of its precision no matter how many digits are in the display and is not a reliable indicator of its perfor­mance. Someone experi­enced in dimens­ional metrology should review the tolerances involved to determine what devices have a reasonable chance of doing the job—all other factors being equal.


The actual enviro­nment in which the measur­ements are to be made can be the most critical factor­—es­pec­ially the temper­ature. The ambient temper­ature is one thing but if the work is to be measured immedi­ately after machining, this must be allowed for. And if measur­ements are required during machining, other problems can arise regarding handling the measuring instrument in addition to temper­ature problems.


Orient­ation of the axis of the feature to be measured can mean an otherwise excellent choice for the applic­ation will not work precisely enough for the situation. Typical of this are long bore diameters that must be measured while their axis is horizo­ntal. Even short bore measur­ements can have problems if the instrument requires careful manipu­lation such as centra­lizing to produce reliable readings.


Despite the wonders of digital instru­ments, they require human (or robotic) handling to provide meaningful readings. Human skills are the most variable and the need for high skill levels increases as the tolerances decrease. Robotic handling also requires proper positi­oni­ng/­sensing abilities for the same gains and while offering high repeat­abi­lity, may not be as reliable as a similar device used by an experi­enced machinist or inspector.



Instru­ments and masters used with them may have a current calibr­ation report but it can be of little value if not done thorou­ghly. If the instrument was calibrated by a device with flat anvils but it is being used to measure a feature with cylind­rical surfaces or vice versa, measuring contact geometry can introduce errors. A review of how a device was calibrated can reduce the impact of these condit­ions.


Some instru­ments require more time than others to be used effect­ively and this can be a challenge when measur­ements are being made in a production situation. In some cases this is due to manual use of the device, in others, response time of the device may not be suitable even though it is part of an automated setup. A trade-off may be required to obtain the desired degree of precision for the applic­ation.


Hand tools such as microm­eters and calipers are regularly being promoted with claims they meet some standard or other that enables them to be used with machine coolant splashing all over them. Impressive as this may be, if you want precision, don’t use them under such circum­stances where you could be measuring over dirt or bits of metal, or grinding dust, etc. Cleanl­iness improves your precision every time whether it’s the instrument or the item being measured.


Calibr­ation labora­tories prepare uncert­ainty budgets in their work so readers of their reports can get a sense of how reliable their processes are. You should consider doing the same for instrument selection because a lot of guesswork and personal prefer­ences are taken out of the decision when it is based on a number of factors reduced to common denomi­nators. Some factors will be common to all devices (such as temper­ature) while others will vary. In the end, a lot of arguments can be avoided when the selection is based on budgets for the different instru­ments being consid­ered.