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.)

 

[supergee]

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.)

Thanks for the comment. I do teach them that there are generally 4 general risks when selecting position tolerance :
1- Will it run out of material?
2- If not, will it collide or be misalign?
3- If not, can the manufacturing process anticipated allow this position? (though we don't design FOR manufacturing, it needs to be taken into account)
4- if not, is it ugly?

As for thue position, I am sorry. I am from the French part of Canada, hence I teach in French. English is my 3rd language so I tend to make a few mistakes like "true position"

 

[supergee]

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
View attachment 8970
View attachment 8971
View attachment 8972
+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.

Thanks... I am ambiguous about the use of AI... As an educator I "should" be agaisnt it because we don't know the sources, the answers are sometime wrong, and.... I am trying to create NI (Natural Intelligence) in those students... I don't want to jump out of an airplane if I learn it was designed by one of my former students...

On the other hand... well... not usign AI IRL is a really dumb move. It's the way of the future.

I will take your advice though. Next session I will do an excersise where they'll research modern machinery and processes (sheet metal, CNC milling, lathe etc.) and make a list of acheivable tolerances. Thanks.

 

[3DDave]

It is a frustration that the training typically does not include a serious segment of analyzing tolerance schemes in the context of the related assembly.

I think there should be:

a brief introduction to the underlying reason
an overview of the symbology
a demonstration of how the symbols interact, such as in FCFs, to create tolerance zones
- at this point the students should be able to read a drawing and create diagrams showing the expected tolerance zones
an intense period of reading drawings and combining the expected tolerance zones to find the overall result in the assembly
after that, an examination of variations to datum and characteristic symbol selections
then examining the range of process variations, the cost of deviating from those typical variations, some amount of statistics with regards to uniform vs. normal vs. skewed distributions
then comes tolerance allocations to meet assembly performance requirements
at the end is when students are given tasks about assigning tolerance values, but also selecting datum features and creating FCFs that are useful; they can apply their analysis skills to showing how the overall allocation can affect the assembly.
---
AI, right now, is dangerous garbage. If it wasn't the companies that make it would simply use it as-is in a directly productive manner. Instead it is packaged as if it is useful without a clear use case for the capabilities it currently has. It's like selling knives made of lead. Toxic, dull, computationally expensive.

 

[supergee]

It is a frustration that the training typically does not include a serious segment of analyzing tolerance schemes in the context of the related assembly.

I think there should be:

a brief introduction to the underlying reason
an overview of the symbology
a demonstration of how the symbols interact, such as in FCFs, to create tolerance zones
- at this point the students should be able to read a drawing and create diagrams showing the expected tolerance zones
an intense period of reading drawings and combining the expected tolerance zones to find the overall result in the assembly
after that, an examination of variations to datum and characteristic symbol selections
then examining the range of process variations, the cost of deviating from those typical variations, some amount of statistics with regards to uniform vs. normal vs. skewed distributions
then comes tolerance allocations to meet assembly performance requirements
at the end is when students are given tasks about assigning tolerance values, but also selecting datum features and creating FCFs that are useful; they can apply their analysis skills to showing how the overall allocation can affect the assembly.
---
AI, right now, is dangerous garbage. If it wasn't the companies that make it would simply use it as-is in a directly productive manner. Instead it is packaged as if it is useful without a clear use case for the capabilities it currently has. It's like selling knives made of lead. Toxic, dull, computationally expensive.

I COMPLETELY agree with you! I have relagated the teaching of symbology to youtube video so I can concentrate my class on THINKING. My student told be yesterday that GD&T is fun ?!?! because they need to think a lot. they need to fill a form for each fearures to explain why they selected the Datum, Why they selected the size tolerance and why they selected the FCF. We have debates on what datum to chose and why. Then I've included metrology in my class so they know what the tolerance means for real.

I am trying to give them all the basic I never had myself and that I had to learn the hard way. I base my class on the training I used to give to a major aerospace company. It's an intense 60 hours over 15 weeks.

Honestly I am confident in there knowledge after my class. They already make better drawing than some "experienced" designers I've worked with.

 

[mfgenggear]

Thanks... I am ambiguous about the use of AI... As an educator I "should" be agaisnt it because we don't know the sources, the answers are sometime wrong, and.... I am trying to create NI (Natural Intelligence) in those students... I don't want to jump out of an airplane if I learn it was designed by one of my former students...

On the other hand... well... not usign AI IRL is a really dumb move. It's the way of the future.

I will take your advice though. Next session I will do an excersise where they'll research modern machinery and processes (sheet metal, CNC milling, lathe etc.) and make a list of acheivable tolerances. Thanks.

I agree with Dave's and your thoughts on AI.
But not all books all equally. I could make a spread sheet from 45 years of experience as a guide. How ever it would be obsolete in short
Time. When I started. I used manual machines.
Manual engine lathe, manual turret lathe,
Hand operated OD and ID grinders, manual mills.
Lol a lot has changed.
S o back to figure out tolerance produced on machines. There are standard tolerance, precision and super precision. Some of these machines are run on a cold room. So are certain
Shears and punches with close tolerance.
I had projects of gears design by hand engineering drawing drawn 50-60 years ago.
That are challenge to manufacture today.
With very tight tolerance .
The machinist, and sheet metal guys were very
Skilled at their profession.
I think the best bet is contact different manufactures. For different machines.
The AI I posted is very close and realistic.
Buy use other resource with factual data.
The issue is reliability and precision can also rely
On the operators skill, experience and knowledge. I would always seek their consul
On very difficult project. And still do.