Do we need to calibrate gauges on process equipment?

Al Rosen

Staff member
Super Moderator
#11
mjoakin said:
Well, the temperature and the preassure (analog) gauges show only the parameter which the machine was set, I can not remove them from the machine. If I'm making tests to verify the adhesion was succesful, and all of them are Ok, so I don't need to calibrate those gauges, is that correct? I don't use any special tool to pull them, as soon as the piece comes out the machine, the glue is really hot and the parts can be easily separated from each other (the foam can't be used again). So, with this test I can validate the process/product and I don't really need to calibrate the gauges, you know, may be I'm using the word "gauges" incorreclty (sorry for my english) they are indicators that show the current setting of the machine.
IMO calibrate them since you are using them to CONTROL the process. You will need a set of calibrated spares to use when they are being calibrated if you don't want to shut down.
 
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Jim Wynne

Staff member
Admin
#12
mjoakin said:
Well, the temperature and the preassure (analog) gauges show only the parameter which the machine was set, I can not remove them from the machine. If I'm making tests to verify the adhesion was succesful, and all of them are Ok, so I don't need to calibrate those gauges, is that correct? I don't use any special tool to pull them, as soon as the piece comes out the machine, the glue is really hot and the parts can be easily separated from each other (the foam can't be used again). So, with this test I can validate the process/product and I don't really need to calibrate the gauges, you know, may be I'm using the word "gauges" incorreclty (sorry for my english) they are indicators that show the current setting of the machine.
If you're laminating foam to leather or vinyl for an automotive customer, somewhere there are specifications for adhesion, and those specifications almost certainly include environmental cycling requirements. If you're pulling the on the product while it's still hot, you're not proving anything except that the glue and the foam and the leather are hot. What are the specifications? How can you be sure you're meeting them if you're not testing? Do you have machine setting parameters in your control plan?
 

mjoakin

Involved In Discussions
#13
We are Tier 3, I've been asking for those specifications to my customer (which is not GM, directly) and they say that there are no specifications, and they gave me the parameters I said before (Temp 140C, Preassure 80 lbs, Time 40 secs), they are laminating their own product and they are not required to enviroment control or have their gauges calibrated (and they are TS 16949 certified, :applause: !), so, I don't know what to think or what to do :confused:
 

Mike S.

Happy to be Alive
Trusted Information Resource
#14
Marc said:
Part of the point here is it is process equipment rather than for an inspection or test. Just because it is not used for measurement and test does not mean it isn't important. The more critical the process parameter(s), the more important calibration of the associated gage is.

But - I've seen lots of process equipment which had process parameters which weren't 'critical' and one or moe gages were essentially references.

So - Evaluate how critical the process parameter is and go from there.

Essentially I agree with Hershal and the others, but I do advise making an evaluation rather than blindly calibrating every gage on every machine in the house.
FWIW I'm inclined to side with Marc here and not just blindly say that unless the measurement is meaningless you should always calibrate.

Is every electrician who doesn't calibrate his multimeter careless or irresponsible?

Must every ammeter on every welder be calibrated, even if the weld itself is inspected?

Must every pressure gage on every press be calibrated even if the size of the pressed part is the critical parameter and that is being checked after pressing? I'd say no -- but having the gage on the press might speed-up setup to start.
 
D

Dave Dunn

#15
Calibration of the indicators you mention could be determined by evaluating such factors as scrap costs, time lost, etc. should the process go beyond a window of making good product. The window could be determined as mentioned earlier by doing a design of experiments to test the limits of what the process can take before producing bad product.

For example in our company, we have a part which has a silicone gasket cured by a high temperature oven in which the parts travel for about 15 minutes. The temps and speed of the conveyor in the oven are not extremely sensitive, but we do know that if the parts are cooked too hot or travel too slowly, the silicone gets burned to an unacceptable state. Similarily if the heaters aren't up to the right range of temps or the conveyor is too fast, the silicone won't cure and will be runny.
At any given time, the oven will have around 500 parts in the cooking process. That's a lot of scrap if we don't monitor the process and instead rely just on output testing.

Additionally, if your process proves robust enough, you could potentially eliminate the need for output testing by proving the capability of the process and process controls.
 

mjoakin

Involved In Discussions
#16
We never get scrap from the adhesion process, I mean, if the final product looks "loose" because the glue was not melted correctly, then we just "iron" that part for 20 more secs, and the part is done, but that doesn't happen that frecuently.
 
G

gavinqce

#17
when is a process parameter critical

How do I simply define when a process parameter is a critical parameter. After all, if it is not critical why was the parameter set? Is there a simple definition in terms of final function, as opposed to interim manufacturing stages?
 
D

Dave Dunn

#18
gavinqce said:
How do I simply define when a process parameter is a critical parameter. After all, if it is not critical why was the parameter set? Is there a simple definition in terms of final function, as opposed to interim manufacturing stages?
One of the big questions is "how is this characteristic measured for part acceptance". If there is a measurement made downstream of the process parameter in question, then that measurement is where the decision to accept or reject the product is made and the equipment used to make that measurement must be calibrated. Equipment prior to that stage are not required to be calibrated, however, doing so where it makes sense to do so can help save on scrap.

For example, if your process proves out that you can run with a variance of 10 or 20 degrees plus or minus from your nominal and not have scrap, it might not be value added to calibrate the equipment that controls the temp. If, however, you find that you get defective product if you can't control the temp closer than a degree or two, it could prove valuable to make sure that the equipment is accurate.
 
D

Dave.C

#19
Dave has it absolutely spot on - control your process and do not rely on output testing.

Ensure those gauges, indicators, whatever you wish to call them, that you use to set the process are actually giving what you are asking for - is a setting temperature of 140 degrees the actual temperature achieved? Are you getting 180 or 110 instead?

Also, this could change over time so setting you machine to 140 when you started the job might have given you (pulls a figure out of the air) 145 degrees. Six months later the same setting of 140 could be giving you 120 degrees. How would you know that if the gauges weren't calibrated?

Do some simple tests to prove the correlation between the temperature, pressure, and adhesion strength characteristics. Use design of experiments or simple scatter charts to prove correlation of the effect of changes in those parameters on adhesion strength. If the results prove there is no correlation between them (unlikely) then this will justify you not needing to control them or calibrate your equipment.

If you can prove the correlation, then control the process parameters with SPC, you can eliminate costly destructive testing.

We never get scrap from the adhesion process, I mean, if the final product looks "loose" because the glue was not melted correctly, then we just "iron" that part for 20 more secs, and the part is done, but that doesn't happen that frecuently.
Have you measured the extra cost of that rework? The extra 20 or so seconds on cycle time could have a significant impact on the product margin.

How effective is the visual check on detecting poor adhesion? Have you performed an FMEA on this process? Are the RPN's high?

What would happen if GM found seat covers coming loose in the field - can your company stand the cost of a recall at the sort of numbers of vehicles that could be affected by a simple failure mode that could be avoided?
 
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