Keeping Your Gage Blocks in Shape
In the dimensional metrology field, gage blocks are the universally used physical standards to which most linear dimensions are compared directly or indirectly. Prior to their existence, gage ‘sticks’ were used, each one being made to a specific dimension but the cost over time and their storage made them costly artifacts to use.
Carl Johansson of Sweden realized these problems and developed a series of sizes that could be combined to form a near endless number of dimensions. Once a dimension was no longer needed, the pieces used to create it could be used in other combinations or ‘build-ups’—a considerable cost saving.
Keeping the blocks in a build-up together was critical to ensure the accuracy of the required dimension as was making them to very precise dimensions. Johansson solved these problems by turning his wife’s sewing machine into a device that could be used to lap the precision faces of the blocks to precise dimensions while keeping them flat and parallel. This enabled those faces to be brought into intimate contact through the process we know as ‘wringing’ so they stuck together with little or no change in the overall dimension required.
Considering their importance, I’m surprised at how poorly gage blocks are treated since caring for them is relatively cheap and easy to do. I hope the following thoughts will help you in this regard.
Make sure that users of your blocks understand the importance of keeping them free of dirt and in the case of steel blocks, burrs.
Test user skills to ensure they can wring the blocks effectively. They should know when there is a burr present (or dirt) so they don’t ruin the blocks trying to force them together. A pair of old blocks that have good finish can be used for this training.
Encourage users to include wear blocks on their build-ups.
Keep the lid on the block case closed when you are not selecting or returning blocks to it. This will help keep dust from accumulating in the case. Vacuum the case from time to time.
Blocks used in the shop are subject to more dust, dirt, oils, etc., than those in a lab setting so cleaning them carefully before wringing is very important. Unfortunately, this is most often done by wiping with a shop rag. Many such rags are loaded with dirt and chips etc. and may do more harm than good when it comes to cleaning. Toilet paper is a low cost wiping tool and since it is white, dirt is usually easily seen. People tend to discard it when it is so cheap rather than shop rags that are often hoarded as they have a habit of disappearing when your back is turned. You can buy paper wipers made for this type of work that are free of lint, making them even better.
Blocks made from steel can be easily damaged from mishandling, leaving raised burrs that prevent wringing and/or change their functional length. An optical flat can be used in normal lighting to show the presence of burrs. These are removed by careful use of an Arkansas stone. The process is simple but only skilled people should do this as the risk of permanently damaging the blocks is extremely high. Some block makers offer a maintenance kit for this purpose which is well worth the small cost. Scratches on block faces cannot be removed without significant lapping of them that will alter them dimensionally among other problems. It’s cheaper to replace any blocks you’re concerned about.
Regular calibration of your blocks ensures they retain their high value as a reference standard. When selecting an outside lab to have this done, ensure they are accredited to ISO 17025.
Make sure their accreditation covers the actual material your blocks are made of since the different materials each have a different co-efficient of thermal expansion and deformation characteristics; all of which matter when millionths of an inch or parts of a micron are involved. Find out if the lab de-burrs and de-magnetizes each block in the set prior to calibrating them if they are steel.
We have calibrated a set of blocks that are over eighty years old that were made by Johansson that are still in excellent condition. If you look after your blocks as I’ve outlined, they could serve you for a similar period of time.