Test & Inspection: Get a Grip on Shaft Measurement
Round or cylindrical parts come in variety of sizes and shapes, from a wide range of industries including aerospace, automotive, defense, power sports, heavy equipment and medical just to name a few. They are critical to motion or function in every case and their quality can dramatically affect the efficiency of complex assemblies. That is why it is important to make the best choice of available metrology technologies.
Shaft-type parts can require inspection of an incredibly diverse set of characteristics and generally will be manufactured utilizing widely different techniques. Each one of these characteristics is further classified according to one of several existing measurement fields including dimensional, surface, form and geometry, hardness and nondestructive testing.
Select the solutionSelecting the best measuring solution for your part can be a very confusing and frustrating proposition. In trying to make the best selection you will want to ask yourself some of the following questions:
What type of characteristics will require inspection, are they dimensional, form, surface finish, or hardness?
Where will the inspections take place, on the shop floor or in a metrology lab?
Can the measurements be performed in-process or post process, either in-line or off-line?
What are my productions needs (frequency of inspection)?
What kinds of operator skills are necessary to perform the inspection?
How flexible does the inspection equipment need to be?
Each of these questions, when answered, will begin to limit the available options when attempting to select a measurement solution. However, in evaluating specific application requirements it will often be necessary to accept tradeoffs as different solutions may offer advantages in one area, while offering disadvantages in another. It therefore becomes very important to prioritize your requirements as you will quite often be unable to choose a single solution which meets all your needs.
As an example, dimensional measurements represent the single largest characteristic measurement category in the industry. Using various types of acquisition technology, operators have quite a few options to choose from when selecting a measurement solution to best fit their evaluation needs. For example, here are three dimensional measuring techniques each of which has its own pros and cons when inspecting different manufacturing characteristics.
Pneumatic. Dimensional pneumatic gaging has been around longer than most of us have been in manu-facturing. It is accurate, repeatable (1/10th micron), robust and cost effective. Pressure requirements for pneumatic systems is usually seen as a disadvantage, mostly due to the costs involved, but modern systems have advanced to the point that very little air is necessary for accurate measurements.
Typically used on either low- or high-volume tight tolerance parts, pneumatic gaging can deliver fast, accurate results time and again with very little to no maintenance.
Measurement ranges vary based on low or high pressure systems, but top of the line systems can deliver a measuring range of ±0.080 millimeter, delivering repeatable accuracy as tolerances as low as ±0.0001 millimeter. Part sizes vary, but can be as small as 1 milllimeter outside diameter all the way up to the largest manufactured shafts in any given industry.
Typical applications for pneumatic gaging of round parts would mainly consist of post-process applications, both in-line and off-line systems.
Contact. Contact gaging comes in many varieties and has taken tremen-dous strides over the last decade in terms of accuracy and reliability. Pencil probes (or LVDT probes) are the mainstay of this type of measurement, along with various versions of contact tips-such as ruby and car-bide.
Typically contact gaging has been much more susceptible to environmental conditions than pneumatic systems, particularly very dirty environments such as on inline measurement systems.
Newer technology, though, has led to more robust devices that are much more capable than their predecessors, capable of delivering high accuracy and reliable repeatability even under rather adverse conditions. In addition, altering contact designs permits flexible shaft measurements machines to be produced that can perform form, surface finish and other high-end characteristic evaluations.
Typical applications for contact gaging of round parts would be post-process (in-line, off-line and point-of-use systems) and in-process (size control for journals, lobes, etc).
Optical. The latest technology to gain acceptance for dimensional inspections are optical systems. The increasing capability of optical measurement systems during the past few years has led to an explosion in the popularity of this type of technology.
Better lighting systems, high-resolution sensors and better software have resulted in a marked increase in the overall capability of these systems to perform industry-leading evaluations on a diverse group of parts.
The single biggest advantage to this type of technology is its flexibility in evaluating almost any part type that will fit within the confines of system itself, as well as its ease of use when setting up these new part types. Simply load the part into the gage, perform a part scan with the system optics, and then select locations on the scanned image for the evaluations you want to perform.
After all the evaluations have been assigned to the part, select “start” and the entire part can be inspected in a matter of a few seconds. Applications for optical gaging consist exclusively of post-process systems, both in-line and off-line.
CharacteristicsTwo other very important factors to consider when discussing dimensional metrology are frequency of inspection and characteristic limits.
Frequency of inspection is critical in determining where in the process (location) the evaluation will occur and what type of technology will be used (pneumatic, contact or optical).
If inspecting 100% of part production and you produce hundreds of parts an hour, then an in-line system might be the only way to go. If only inspecting every 100th part, then an offline system might be the best bet.
Frequency also determines the best technology since having to inspect a ±0.010 millimeter tolerance every 5 seconds would best be handled by pneumatic technology; the same tolerance being inspecting every 20 seconds might best be handled using contact technology.
Characteristic limits-for example, tolerances-probably play the single largest role in determining the time, place, frequency and technology used to evaluate any given part. If the characteristic limit is ±0.010 millimeter, then a pneumatic system might be advisable. If the limit involved is ±0.080 millimeter, then a contact system might be all that is needed. In the end, tolerance limits play a significant role in selecting the proper technology in any given inspection.
OverviewThese represent just some of the options for dimensional measuring technologies applicable to shaft-type parts; and, as a result, do not begin to address the application of each technology to your specific needs. To obtain a best-practice solution, you will need to consult with your preferred gage system supplier, remembering to bring with you at a minimum the answers to these questions along with an open mind.
In the end, selecting the right time, place, frequency and technology will help you control your part quality efficiently, effectively, and most important, economically. Q
Tech TipsRound or cylindrical parts are critical to motion or function in every case and their quality can dramatically affect the efficiency of complex assemblies.
Tolerance limits play a significant role in selecting the proper technology in any given inspection.
In evaluating specific application requirements it will often be necessary to accept tradeoffs as different solutions may offer advantages in one area, while offering disadvantages in another.