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Defective instrument usage in less precise works

Defective instrument usage in less precise works

Defective instrument usage in less precise works


I have a question regarding usage of defective instruments in production line. If an operator of assembly area is using lets say a defective Vernier caliper. The caliper has an error of 0,1mm so it can not be used by machining department, but the assembly department has acceptable error of 1mm. Theoretically if the assembly operator takes the defective caliper his work is not affected by it.

But is this practice allowable?

I am against it, since this caliper might result in an error, but my colleague says if the end result is not effected the caliper can be allowed in the job floor.

Is there any standard which governs whether this kind of instrument should be discarded or not?

RE: Defective instrument usage in less precise works

If the device (e.g., caliper) is being used to measure a CRITICAL dimension, it MUST be calibrated. If it is being used to "estimate" a non-critical dimension, then it is no different than using a body appendage. Just be sure that the device error falls well under the necessary estimate accuracy (typically no more than 1 to 2 % of total measurement). Personally, I believe in using calibrated tools to avoid issues later - but I also understand how hard it can be to police that capability.

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RE: Defective instrument usage in less precise works

It depends on the criticality of the problem. If you subtract the error in the tool from each end of the range then you will still (theoretically) only accept items that meet the original limits, though it may reject items that would otherwise be usable.

For example 2 +/- 1mm and a tool that is potentially off by 0.1mm - if the production range for acceptance is 1.1mm to 2.9mm then the tool may still accept 1mm and 3mm parts, but it won't accept 0.9mm or 3.1mm parts. However it may also reject 1.1mm parts and 2.9 mm parts.

There should also be an allowance for operator error and that will narrow the acceptable range even further depending on operator skill at using the tool.

If there is a percentage or scale-multiplication error, then the amount of adjustment will depend on the size of the feature. The reduction in acceptable range will be smaller for small features and larger for large features.

For some features the temperature of the tool and the part can also matter; assembly areas are rarely temperature controlled. If there is a place where the sun can shine on parts waiting for assembly that's even worse.

Many factories purchase fixed tooling to set and check items so assemblers don't need to take time and those mainly need to be checked for wear or operator modification. These are usually go-nogo gauges.

RE: Defective instrument usage in less precise works

Mark it as "For Reference Only" so it's not accidentally mixed with calibrated tools. Continue to use it where a reference dimension is being measured, and not a critical dimension.

Ensure that your procedures and QC/QA allows for reference tools in the assembly sequence and requires calibrated tools in machining. If your policies and procedures are unclear or do not allow for reference tools you could point that out and use it to justify your position.

RE: Defective instrument usage in less precise works

@ Nashanas
Your colleague is correct.


RE: Defective instrument usage in less precise works

With digital measurements you can get 'creative' values that can't actually be used by the tool. IE the caliper that measures out to .00005", the tape measure with 1/64ths marks that has a 1/4" wiggle end (yes I know the purpose of the wiggle end, do you think it repeats to 1/64ths?), or the golf laser that reads out in ft and inches at 250ft.

For QC checks you need to limit what the tool can actually be used on. The calibration requirements are then based on the needs of the tool. IE - You may designate that machining calipers are good to 0.1mm and assembly calipers are good to 1mm. Loosen up the calibration requirements for the assembly calipers to match the need.

From there it's simple - label the calipers 'Assembly Only' and write in your procedure that all calipers may be used for machining unless otherwise stated on the tool.

RE: Defective instrument usage in less precise works

The use of "defective" instruments is frowned upon, particularly if you are ISO-9000 compliant; if it's out of calibration, how can you even be sure that any given measurement is within even the loose specification? You'd have to constantly crosscheck your instrument, at which point, you might as well be using a properly calibrated and maintained instrument.

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RE: Defective instrument usage in less precise works

Send the caliper out for repair or buy a new one. A reject would probably cost many times the cost of a new one. Dial calipers run around $130, an off brand as low as $40. Using defective inspection equipment never works out!

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