Is there a preferred true position standard?

[supergee]

So one of my students gave a .005 inch true position tolerance on a hole used to attach a P-clip that will hold a loose electrical wire. I asked why and he said because the position is not important. I had to tell him that .005 is a rather tight tolerance concidering the use of positioning a P-clip in the middle of a part. The student said : How am I suppose to know if a location tolerance is tight or not ?

Since location precision is dependent of the machining process, I used to contact my suppliers to know if my location made sense with the machining they used and have some personnal experience to guide me but... experience is what students don't have.

There is a preferred FIT list that can be easily found I Machineries Handbook, internet, etc but is there such a list that exist for true position?

Just to be clear, I am not asking how to calculate position or do stack up analysis: this I know and teach that already. Once the stack up analysis is done, or in cases where it's not required, I am trying to know if the location tolerance is hard or easy to make when using different manufacturing methods. Is there such a guide?

 

[jassco]

Yes. There is an IT grade for different manufacturing processes. If you use a different process, you may want to know what economical precision is. But you should focus on what you need according to your design intent.

 

[mfgenggear]

So one of my students gave a .005 inch true position tolerance on a hole used to attach a P-clip that will hold a loose electrical wire. I asked why and he said because the position is not important. I had to tell him that .005 is a rather tight tolerance concidering the use of positioning a P-clip in the middle of a part. The student said : How am I suppose to know if a location tolerance is tight or not ?

Since location precision is dependent of the machining process, I used to contact my suppliers to know if my location made sense with the machining they used and have some personnal experience to guide me but... experience is what students don't have.

There is a preferred FIT list that can be easily found I Machineries Handbook, internet, etc but is there such a list that exist for true position?

Just to be clear, I am not asking how to calculate position or do stack up analysis: this I know and teach that already. Once the stack up analysis is done, or in cases where it's not required, I am trying to know if the location tolerance is hard or easy to make when using different manufacturing methods. Is there such a guide?

not a simple answer. I completed college course in steel manufacturing and plastic manufacturing as a starting point.
as technology improves so does the precision of cnc machines. I would suggest
to focus on the design fit form and function.
then a review of the design if the tolerances are practical.
I would suggest to research the actual precision of newer cnc machines.
here is an AI quote
Overview



+11

Machine precision tolerances are expressed as a range of acceptable variation from a nominal measurement. Standard tolerances for CNC machining are typically around ±0.005 inches (0.127 mm), while tight tolerances can reach ±0.001 inches (0.0254 mm) or even better. These tolerances are crucial for ensuring parts fit together, function correctly, and meet performance standards.

Here's a more detailed breakdown:

Standard Tolerances:

• ±0.005 inches (0.127 mm): This is a common standard for many CNC machining applications.
  
  
• ±0.002 inches (0.0508 mm): Some CNC mills and lathes can achieve tolerances in this range.
  
  
• ±0.0254 mm: A typical average tolerance for CNC machines.
  

Tighter Tolerances:

• ±0.001 inches (0.0254 mm):
  This represents a very tight tolerance, roughly the diameter of human hair.
  
  
• Sub-1 micron (0.00004 inches) and below:
  While sub-1 micron work is challenging, some high-precision machine shops can achieve tolerances in this range.
  

Other Considerations:

• Unilateral vs. Bilateral:
  Tolerances can be bilateral (allowing variation in both positive and negative directions) or unilateral (allowing variation in only one direction).
  
  
• Decimal Places:
  The more decimal places specified, the tighter the tolerance. For example, ".00x" (e.g., ±0.006) is tighter than ".0x" (e.g., ±0.02).
  
  
• Engineering Tolerances:
  Precision machining shops often use engineering tolerances to define measurements, assuming a general tolerance grade unless specified by the client.
  
  
• ISO Standards:
  Standards like ISO 2768 and ISO 286 define different tolerance classes, such as fine, medium, and coarse, which help manufacturers communicate tolerance requirements.

 

[Garland23]

There is no way to say if a tolerance is tight or not, without understand the function and the mfg process juxtaposed to each other. (What is the purpose of the clip?)
Why is .005 inch a tight tolerance? What if we were making something at a microscopic level? Then it would be a huge tolerance.

Also, since you're an educator in this topic, you might already know that it's not really a "true position" tolerance, but rather a "position tolerance." (Sorry... just had to get that in there.)