Ten years ago this month, Quality Gaging Tips first appeared in Modern Machine Shop. Since that time, 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.
10:1 Rule. Whenever possible, the measuring instrument should resolve to approximately one-tenth of the tolerance being measured. So if the total tolerance spread is 0.01 mm (that is, ¦0.005 mm), the smallest increment displayed on the gage should be 0.001 mm. This amount of resolution allows you to make accurate judgments for borderline cases and makes it possible to observe trends within the tolerance band. Many people know of and can explain this rule, yet it can still sneak up on the best of us.
Simple SPC. It is amazing how many shops still don’t do SPC because they can’t afford digital gaging equipment or don’t have the right computers or software. One of the first SPC manuals was published by Federal Products in 1945, long before the digital revolution. It has gone through 14 printings. Back then, simple SPC empowered machine operators to keep their processes in control by showing them how to make simple charts and other visuals that gave them regular feedback.
SPC wasn’t conceived of as something you have to buy, but as something easy you can do. SPC can be that simple, or it can be very complex. By starting out on the simple side and taking just one step at a time, any shop can develop an SPC program to take it to the next level of manufacturing excellence. The trick is to get started. Later on, you can think about getting digital tools, more computers and software.
SWIPE. When results don’t live up to expectations, it’s easy to blame the gage. However, this outlook will rarely get you on the right track. A more helpful approach is to consider how good practices encompass a range of factors that can be summarized by a single acronym, SWIPE. The letters represent standard, workpiece, instrument, personnel and environment. Study how each of these might contribute to the effective use of gaging, and you will be on the path to setting up a solid gaging system.
How to Use a Rule. It is impossible to overemphasize how important it is to remember the basics. For example, one of the oldest and most pervasive of all measurement tools is the steel rule. But when was the last time you received any training in how to use it? The first grade?
What’s there to know, you ask? Plenty. You should know (1) rule styles: English or metric, rules with the zero point on the edge or inset a short way; (2) how to avoid parralax error; (3) how to use a stop, even if it’s your thumb, for better alignment; (4) how to start a measurement at a graduation instead of the edge; (5) how to measure similar parts from the same starting point and in the same direction for greater consistency; (6) the fine art of rotating the rule to get the longest dimension across a hole; (7) and when you need to use a more accurate measuring tool (See 10:1 rule above). If basic skills are so important for using something as mundane as a rule, imagine the influence they have on something more complex such as calipers.
The truth is that no matter how technologically advanced our manufacturing processes have become, we will never outgrow our need to revisit basic measurement. From the pyramids to the space station, measurement rules continue to be the foundation upon which anything of quality was ever built.
Editor PickNon-Contact Vision Probe for Multi-Sensor System
RVP, Renishaw's vision measurement probe, is designed for use with the Revo five-axis measurement system on coordinate measuring machines (CMMs).