Minimum Required Gages For In House Verification of Production Tools

[Mikishots]

Our company has long used the tried, true, and detestable system of having the QC tools calibrated while calling the tools used by the operators be considered reference only. (Calipers, squares, protractors, and height gages for the most part.) I am looking to change this and begin doing some basic "calibration" in house. As we are too small to invest in a full metrology office, I am hoping to continue having the most critical tools used in final inspection calibrated by an outside service. But I would like to start cataloging and verifying the rest in house.

An early step in this will be to make a proposal to the owners to purchase gage blocks specifically for this purpose. But I am not sure how many we would need. How many sizes inside the range of a measuring tool do I need in order to verify a tool is functioning correctly?

If I am checking a 6 inch pair of calipers, I will need to check something like 1", 2", 3", 4", 5", and 6". Or some range similar showing that all along the measurable distance I am getting good results. But how many different checks are required? If I am checking a 12" or a 24" caliper, how many places along the range need checked? The longer rods and blocks are increasingly expensive, so I want to buy as few as possible in the higher range, rather than a 1500$-2000$ set to cover every inch from 12" to 24".

Say as an example you have 6" calipers that need to be calibrated. One must assume and expect that the device will be used all the way up to six inches, so...you have to verify that range. Use a set of blocks that will allow you to create those dimensions (wringing as needed). I'd select 1, 2, 3, 4, 5 and 6 inches. The accuracy depends on your requirements. Where I work, we use standards that are four times more accurate than the tightest tolerance needed on the floor. Some places need much more accuracy, others not nearly as much.

In agreement on your plan to move away from the current method. Because the Standard requires us to make sure we have a process to follow if a calibration fails, your current method leaves a lot of holes if a QC tool is found to be out. because some intervals cab be six months or a year, that's a LOT of measurements. A method we use - both production and QC tools are calibrated. Production tools can be shared, but QC tools are for the exclusive use of that particular QC person. and production is never allowed to use QC tools for production activities. If the production tool fails and the parts it was used on have already left the facility, we look at the QC tool that was used to verify those parts. If it passes, we're good. To have both fail would be a very rare occurrence and can be considered a non-issue.

 

[Steve Weber]

Say as an example you have 6" calipers that need to be calibrated. One must assume and expect that the device will be used all the way up to six inches, so...you have to verify that range. Use a set of blocks that will allow you to create those dimensions (wringing as needed). I'd select 1, 2, 3, 4, 5 and 6 inches. The accuracy depends on your requirements. Where I work, we use standards that are four times more accurate than the tightest tolerance needed on the floor. Some places need much more accuracy, others not nearly as much.

In agreement on your plan to move away from the current method. Because the Standard requires us to make sure we have a process to follow if a calibration fails, your current method leaves a lot of holes if a QC tool is found to be out. because some intervals cab be six months or a year, that's a LOT of measurements. A method we use - both production and QC tools are calibrated. Production tools can be shared, but QC tools are for the exclusive use of that particular QC person. and production is never allowed to use QC tools for production activities. If the production tool fails and the parts it was used on have already left the facility, we look at the QC tool that was used to verify those parts. If it passes, we're good. To have both fail would be a very rare occurrence and can be considered a non-issue.

We have a Ceramic Set of 9 gage Blocks we send out for Calibration to a supplier that is ISO17025 certified. We use those along with Calibrated Length Standards, Bore Rings, Pitch Mic Standards and Surface Finish Standards to calibrate both company and employee owned gages. We allow employees to use their own calibrated tools for in-process inspection, however; we use company owned gages to verify set up and final inspection. All equipment has unique numbers and are entered into our QT9 database. Each tool will have a calibration sticker affixed to it upon completion that specifies the tool number who preformed the calibration, the date the calibration was performed and the due date for the next calibration. We check a minimum of three spots along the range of every tool with the exception of tools exceeding 10" which is the length of our longest calibrated length standard. These has never been an incident where a customer part was returned due to tool being out of calibration. Management fully supports this method and perhaps it could work for you.