Technika pomiarowa
Technika pomiarowa
Basics of Measurement Gaging Tips
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Surface plates provide a broad, smooth, flat reference surface that can be extremely useful for inspecting incoming, in-process, or finished parts.
One of the biggest issues these days is customers trying to squeeze more performance out of their existing gaging.
If you're trying to measure microns, every micron counts.
Dimensional control is an important part of manufacturing, yet those trying to achieve precision measurements often forget the single largest source of measurement error: temperature.
Just like the people who use them, gages should have periodic physical examinations. Sometimes, gage calibration is needed to identify the seriousness of a known problem.
When we talk about differential gaging, we are usually referring to the process of using two sensing devices and combining the results into one measurement. The measured dimension is the change in the position of the two sensing components.
The five elements of a measuring system can be listed in an acronym. SWIPE, and rather than immediately blaming the instrument when there is a problem, a better approach is to examine all five elements.
Believe it or not, one of the most overlooked problems in qualifying gages is unaccounted for deflection of the fixture due to the force of the probe on the part. Who would have guessed? After all, fixtures are used to provide stability.
Once upon a time, an overly enthusiastic QC manager appealed to me, confused and dissatisfied. Here he was, spending good money to purchase very high quality masters, but his inspection process was no better than before. What was worse, his masters went out of calibration rapidly, pushing his costs even higher.
When we think about making a simple measurement, such as an outside diameter, many methods can be used. These would include calipers, micrometers, mechanical snap gages, air snaps, CMMs, and special fixture gages, just to mention a few.
"It takes a while to warm up in the morning, but after that, it runs great." I swear I've heard machinists say this of their gages, as if those instruments were like car engines with 50-weight motor oil and cold intake manifolds.
Familiarity may not always breed contempt, but in precision gaging, it can certainly lead to error. It happens when we do what we have done a thousand times before, but do it without thinking.
Selecting the best gage for the job requires a consideration of many variables, including the size of the part, the length or depth of the round feature and whether you want to gage in-process or post-process.
Quality Gaging tips has been being published for a long time and we have covered many tools and techniques for assuring accurate, repeatable measurement and gaging in machine shops. So I thought it would be appropriate to double back and talk about some oldies but goodies—things we should know all too well by now, but sometimes overlook or take for granted.
Specifications for metrology equipment are usually published in nice glossy brochures that have lots of great pictures highlighting all the features of the product. But specifications can be confusing and are often a hot topic of debate.
We have seen that there are other factors that influence gage performance, such as gage linearity, long term stability, and bias from the gage design. Combined, these measuring system-based factors that influence measurement results are called the 'Measuring Uncertainty.'
Most masters used for setting comparative gages look like everyday gage blocks, master rings or discs. However, ASME standards for these masters are very specific in defining their characteristics, including material, physical sizes and geometric conditions.
If you are planning on manufacturing to tolerances of .0001" or less, and have not done so before, your introduction to high order gaging may be an eye opener. Gaging at this level is a very different animal, especially when considering how to measure the new parts you will be producing.
Commonly Asked Questions - There are a number of choices: a dial bore gage, an inside micrometer, an air plug, a self-centralizing electronic plug like a Dimentron®, or any one of several other gages.
Plate gages are a mainstay in the bearing industry, or anywhere that fast, accurate readings of ODs or IDs are needed. You’ve seen them around: the bench mounted ID/OD comparative gage with the tilting stage plate to set and locate the part being gaged.
Whenever you use a hand tool or precision gage, you should be aware of typical pitfalls that prevent good gage performance.
There are a number of ways that the adjustable bore gage can be set to the nominal size. Some are good for a quick setup, some are more expensive but very precise, and some provide a good balance of reliability and versatility.
Single Minute Exchange of Die (SMED) is a lean manufacturing concept (or goal!) that was originated in the late 1950s and early 1960s to improve the efficiency of manufacturing by reducing or eliminating bottlenecks caused by process changeovers.
Zeroing sets a reference point from which all subsequent measurements are made. If a gage has been allowed to drift from zero, it will introduce error into the measurement process.
Gages and measuring instruments come in all different shapes and sizes. But whether it's a snap gage, a micrometer, or a bench stand, you will find a capital "C" in almost all of them.
Nothing is more basic than that big rock on the shop floor or in the center of the calibration lab. Flatness and repeatability are the two specifications that help define the accuracy of a surface plate.
Fixed body mechanical plug gages are one of the best ways to measure hole diameters. With this type of gage there is no rocking, as with an adjustable bore gage, and therefore it is fast and provides high performance for tight tolerance holes.
Say you've been making the same part for decades, running hundreds or thousands of parts a day, from the same machines, using the same process and the same gaging purchased when the process was started.
Circular run-out is a two-dimensional measurement using surfaces to control an axis. The tolerance is applied at any cross section. When it is used on a surface referenced to a datum axis, as with this part, it will control the total sum of all variations of circularity and coaxiality.
How many times have you heard an assembly operator complain that incoming parts are consistently out-of-spec? How many times have you heard the parts people trash assembly folks for not knowing how to use their measurement tools?
In the world of dimensional measurement, electronic gages make up a class of instruments that are capable of detecting extremely small dimensional variations on a surface element.
The call is a common one: tolerances are tight and better gages are needed to improve the measurement process. But what may seem like a simple thing can sometimes lead to unforeseen issues.
Do you remember learning the names of weird shapes in elementary school and then later in geometry? There were isosceles triangles, parallelograms, dodecahedrons. What good would come of all this bizarre knowledge in "real life?"
Like every other function in modern manufacturing operations, inspection is subject to management's efforts at cost control or cost containment. Its good business sense to try to maximize the value of every dollar spent, but it means that hard choices must be made when selecting gaging equipment.