Determining SPC tolerance

[Jim H]

Can anyone direct me to where I can find information regarding the calculation of an SPC dimension tolerance and which factors should be taken into consideration? Or better yet offer your own opinion.

 

[Jim Wynne]

Please describe what you mean by "SPC dimension tolerance."

 

[optomist1]

yep, sounds like youre refering to an engineering drawing specification or dimensional tolerance rather than SPC UCL/LCL control limits?

 

[Bev D]

Are you referring to "Statistical Tolerancing"

 

[Jim H]

I am not referring to control limits, but the actual tolerance applied to the SPC dimension. Example: 112.56±0.1. Is there a specific formula that would be applied to arrive at the ±0.1 tolerance?

 

[Bev D]

Sorry for being so dense but I don’t know what a “SPC dimension” is?
In general there are several ways to set tolerances on any characteristic. Some are good some are ‘meh’ and some are just dumb.

 

[Mike S.]

I'm not sure what you are asking but I'll take a stab at it and ask, is this the kinda thing you are referring to?:

ARP9013/3 recommended practice specifies a product acceptance system using process
control methods. Its purpose is to assure conformance for specified characteristics. Use of
process control techniques for product acceptance requires maintaining process stability
and capability. A stable and capable process is the best assurance of conforming hardware
for the customer. There can be lower inspection costs associated with a process control
acceptance approach.

 

[Jim H]

Sorry for being so dense but I don’t know what a “SPC dimension” is?
In general there are several ways to set tolerances on any characteristic. Some are good some are ‘meh’ and some are just dumb.

An SPC dimension is a specific dimension designated to be monitored for process control. Typically I use the DIN 16742 standard to establish the tolerance based on the size of the dimension. I am attempting to find more information on the process for establishing the tolerance size other than relying on the DIN standard. I hope this is not too ambiguous but I do not know how else to explain what I am trying to achieve.

 

[Jim Wynne]

An SPC dimension is a specific dimension designated to be monitored for process control. Typically I use the DIN 16742 standard to establish the tolerance based on the size of the dimension. I am attempting to find more information on the process for establishing the tolerance size other than relying on the DIN standard. I hope this is not too ambiguous but I do not know how else to explain what I am trying to achieve.

The tolerance should be based on what the thing is expected to do, and how that might fail if the size is wrong. There are risks involved, such as making the tolerance too loose or tight, but there are no tables that will help you to determine that. How do you know the dimension must be monitored for process control if you don't know the tolerance?

 

[Jim H]

The tolerance should be based on what the thing is expected to do, and how that might fail if the size is wrong. There are risks involved, such as making the tolerance too loose or tight, but there are no tables that will help you to determine that. How do you know the dimension must be monitored for process control if you don't know the tolerance?

I am the dimensional engineer and I determine which dimensions are to be used as SPC. They do not necessarily need to be functional dimensions, as I am choosing them for process control only. Functional dimensions are typically toleranced much too tightly to ever achieve a 1.67 Cpk.