×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Position value
4

Position value

Position value

(OP)
Hello
I am not an expert in the GD&T. I want to know how do you get the values for the position tolerans? are the a standard value or you just choose by yourself?
I really appreciate if someone could explain it. I attach some picture with the red marked values. Those value I am intressted to know how to get?






Regards
NJ

RE: Position value

2
In general, the biggest factor to consider is the function of the part. But here are some specific approaches.

There are 4 methods to arriving at a tolerance number:
  • The "make-it-up" method, where the designer either copies a tolerance from a similar part, or looks up a value in some chart/table, or simply guesses
  • The "customer-based" method...also called the "top-down" method: if you know how the part fits into a larger assembly, then use the customer's specs to drill down to your tolerance amount
  • The "manufacturing-based" method, also called "bottom-up": your knowledge of the capability of the machine or manufacturing method drives the tolerance amount
  • The best one is the "effective tolerancing" method, which combines the previous three, juggling all factors until you find a sweet spot for your tolerance amount

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Position value

"Tolerance" is a verb - the amount of variation you will tolerate. As JP mentions, there are multiple motivations for which that tolerance for variation can be determined.

1) Peer pressure. That's normally the source of "make it up" and typical "manufacturing-based" This is done to get along with others, acceding to what they feel is an acceptable value.

2) Cost pressure. This is the one that is most often claimed under "manufacturing-based" to make the tolerance really large to allow for the worst manufacturing methods to be used. This works great, often forcing the design to have extra degrees of freedom that will then require extra work in the assembly stages to produce adequate fits and alignments, include hand re-work with files and grinders to custom fit the parts. Often these costs are not considered when only the piece-part costs are.

3) Engineering design. This takes into account the entire process in order to minimize design sensitivity to variations so that tolerances can be less sensitive to manufacturing methods. The entire design is evaluated for the cost of the complexity, the cost of limiting variation, the cost of assembly, the cost of maintenance, and the cost of maintaining strength and deflection limits that are part of the functioning of the entire design.

Peer pressure and cost pressure are the most common; anyone who says "Put some GD&T on the drawing" isn't doing any engineering.

RE: Position value

(OP)
Thanks JP and 3DDave for your valuable feedback.

Regards
NJ

RE: Position value

Position tolerance values come from a few possible places:
1) For clearance bolt holes, it's all about the difference in diameter of the fastener and hole. The general goal is to get the fastener to assemble without rubbing the inside of the holes or distorting either component. As a rough answer, I typically take that tolerance and put half of it on the hole of each part. There are more mathematically correct methods but for very small production quantities, the rough answer works.
2) For alignment features (bearing bores, dowel pins, etc), the tolerance comes from the stack-up of error for all of the features so that the final gearing or bearings are within allowable limits for misalignment.
3) If you know the manufacturing method for the feature, you may specify a tolerance that the method is known to produce. However, this ties your design to that manufacturing context and can lead to cost increases over time. This is the least effective solution. That said, it is very important to have an idea of the methods you want and the tolerances it can produce. Sometimes minor adjustments in the design can allow the next cheaper manufacturing method for the component features. Sometimes, spending the extra time to maximize the tolerances won't amount to anything. This is one of those valuable engineering experiences.

David

RE: Position value

First and foremost you should focus on this consideration: what tolerance value will result in a part that functions as it should?

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close