Choosing a Shaft Measurement System

Considerations for Choosing a Shaft Measurement System
George Schuetz, Mahr Federal Inc.

  The vast majority of machining that goes on in the world is related to either making shafts, holes, or the pieces that hold them together. And just as there have been great leaps in the productivity of making shafts over the years, there has also been a dramatic increase in the ways that shafts can be measured. (For discussion purposes, a shaft as defined here is a round part where the length is greater than the diameter(s) and that can run in size from having to be picked up with tweezers to ones bigger than my house.)

  Here are some the very many ways that a shaft might be inspected after manufacture in today's world. Many of these solutions are also fairly new to dimensional metrology:

• Surface plate layout with gage blocks and comparators
• Micrometers/indicating micrometers
• Vernier/dial/digital calipers
• Mechanical indicating snap gages
• Air snap/ring gages
• Measuring microscopes
• Dedicated fixture gages
• Height gages
• Automatic shaft gages
• CMMs, measuring arms
• Precision length machines
• Laser micrometers

  There are probably a few more but the point here is that between these options there exists a price differential from about $99 to $150,000. There are also differences in gage accuracy, operator influence, throughput, data output, and so on. It's confusing, to say the least.

  So how do you choose which is the best one for your company's situation? A good approach is to first define the functional requirements of the inspection task, and let that steer you toward the hardware that is capable of performing the tasks as identified. In order to do this, you might consider the following factors:

1. What is the nature of the features on the shaft to be inspected? Are they only ODs, or are there radii or lengths that also need to be inspected?
2. Are there multiple ODs that need to be inspected in the same operation, or are they measured in sequence as manufactured?
3. What level of accuracy are you looking for?
4. How much are you willing to pay for super accuracy? Before setting up a gaging operation for extremely close tolerance, verify that a high level of accuracy is really necessary.
5. Will your gaging process be subject to a GR&R study, and if so, how will it be structured? If passing GR&R is one of your requirements, you should be prepared to discuss the details with your gage supplier.
6. How important is gaging throughput? If a fixed gage will save a thousand hours of labor over the course of a production run, it may pay for itself.
7. How long is this job going to last? If the particular job has a short life, high-throughput measurement may be too costly.
8. How about flexibility? Sometimes it's appropriate to buy a gage based on overall shop requirements instead of one that measures a specific dimension with optimal efficiency. Will fast changeover be needed or are production runs long?
9. What do you intend to do with the readings once you get them?
10. How important is ease of use? Especially for shop floor gaging, you want to reduce the need for operator skill and the possibility of operator influence.
11. Is your ideal gage one that can be maintained or is it a throwaway? Gages that can be reset to a master to compensate for wear are generally more economical, but may require frequent mastering to ensure accuracy.
12. Is the part dirty or clean at the stage of processing when you want to measure it? That may affect labor requirements, maintenance, and the level of achievable accuracy, or it might steer you toward air gaging, which tends to be self-cleaning.
13. Will the gaging environment be subject to vibration, dust, changes in temperature, etc.?
14. Would it be better to bring the gage to the part, or vice versa?
15. What happens to the part after it's measured? Are bad parts discarded or reworked? Is there a sorting requirement? This question may alert you to the potential of automated parts handling for improving efficiency.
16. Is the part compressible? Is it easily scratched? Many standard gages can be modified to avoid such influences.
17. Does the machine tool impose certain geometric and surface finish irregularities that require measurement? If so, what is the nature of these deformations? Are there lobing conditions that may affect the gage type?
18. What kind and grade of master do you need? Masters are graded as Z, Y, X, XX, and XXX, with Z being the least accurate and the least expensive and XXX being the most. The class you buy is determined, again, by the ten-to-one rule; but this time in comparison to the gage, not your part.
19. What about master materials? This will depend on your gaging environment. Steel is least expensive and is preferred where there is temperature cycling, because it expands and contracts in proportion to most parts. Chrome plating protects against corrosion. Carbide masters are the most expensive, but are highly resistant to abrasion and corrosive chemicals. Unlike steel, however, they exhibit only a third of the thermal expansion and contraction.
20. What's your budget? If you absolutely cannot come up with the funds for the gaging solution of your dreams, you'll have to go back over the questions to see where you can compromise.
21. Is there a need to measure the part in the machine by the operator?
22. Is the gaging to be part of an automated process? Will the gaging be robot loaded?
23. Is speed critical? How many parts need to be measured per hour/day/week?
24. What's the surface finish of the part? How compatible is it with the part tolerance, gaging, etc.?
25. What's the size of the part? Will it require special handling?
26. Is it important to measure relative to the axis base on centers or diameters?

  All of these factors may be important when making the decision of what gaging solution to choose and instituting the inspection program. Define as many as you can to help narrow the field, but remember that help is readily available from most manufacturers of gaging equipment: you just have to ask.

  The vast majority of machining that goes on in the world is related to either making shafts, holes, or the pieces that hold them together. And just as there have been great leaps in the productivity of making shafts over the years, there has also been a dramatic increase in the ways that shafts can be measured. Here a handheld digital micrometer measures a gear shaft.


  Newer high-end systems, like this MarShaft™ Scope Plus, utilize matrix cameras for optical shaft measurement of turned parts right in the production area.