Categories

 

G-Wizard Calculator:
Fast, Easy, Reliable Feeds and Speeds

Subscribe

Join 50,000+ CNC'ers!  Get our latest blog posts delivered straight to your email inbox once a week for free. Plus, we’ll give you access to some great CNC reference materials including:

  • Our Big List of over 200 CNC Tips and Techniques
  • Our Free GCode Programming Basics Course
  • And more!

 

GCode is Complicated
G-Wizard Makes it Easy

The High Cost of Tight Tolerances

Apr 29, 2017   //   by Bob Warfield   //   Blog, Manufacturing, Techniques  //  6 Comments

We’ve all heard the stories about part drawings coming through that specify ridiculously tight tolerances.  We know that holding tight tolerances is harder, and we know it is more expensive to make a part the tighter the tolerances are.  But how much does it cost to have tighter tolerances really?

I came across this great chart from the University of Illinois that quantifies the relative costs of various tolerances:

Cost of tight tolerances

Starting with rough machining (a tolerance of 0.030″), we can see it costs roughly twice as much to make the part with an accuracy of 0.005″ and four times as much to take it to 0.001″.  That mythical 0.0001″ tolerance, and I say mythical because it is often overused as a bragging point, will cost 24 times as much to do!

Postscript:

As a commenter pointed out, the data above came from a book published in 1951.  Has CNC radically changed these figures?  A little, but not incredibly.  Here are some University of Ohio figures that reflect the CNC age:

ToleranceCostsThe thing to note is that while the absolute differences in costs have changed somewhat, the shape of the curve is still logarithmic.  Costs still go up exponentially as tolerances are made tighter, they just ramp up more gently.  Going from 0.001″ to 0.00025″ costs about 4 times, which is pretty similar to the older figures.  If CNC has is helping, it is largely at tolerances greater than 0.001″.

Tools to Relieve Tight Tolerances: DFM and GD&T

Before committing a part to manufacturing, be sure to do a careful inventory of the tolerances and make each one is as relaxed as it can possibly be.  We’ve written a guide that offers tips on how to reduce the cost of manufacturing at the design stage, a field known as “Design For Manufacturing” or DFM.

Consider also how you’re specifying tolerances.  The old-fashioned way is to slap +/- tolerances on every dimension (or perhaps specify a default for dimensions that have no tolerancing).  But Tolerances are nuanced.  The more modern approach is called Geometric Dimensioning and Tolerancing or GD&T for short.

Consider just the lowly tolerancing task of specifying the position of a bore center.  This can be pretty important for some applications and hence can attract some tight tolerances.  The bore is round, but if you think about simple +/- tolerances along say X and Y dimensions, they specify a square region.  So, we’re putting a round peg in a square hole?

We can visualize it like this:

Imagine the red is the allowable area for the bore center, so we try to capture it with +/- dimensions and get the yellow square…

Imagine the red is the allowable area for the bore center, so we try to capture it with +/- dimensions and get the yellow square.  But the rub is that it really is okay if the bore center falls outside the yellow square as long as it doesn’t fall out side the red area.  In fact, we’re having to use overly tight tolerances because the shape of our tolerance zone doesn’t match our needs.  The drives up the cost of the part needlessly.

GD&T recognizes this problem and introduces a type of tolerance called True Position.  You guessed it–True Position allows a circular tolerance zone and is ideal for our problem.  It actually allows looser tolerances and can therefore lead to a part being cheaper to manufacture.

If you’d like to learn more about Geometric Dimensioning and Tolerancing (GD&T for short), check out our free mini-course on the topic.

 

Like what you read on CNCCookbook?

Join 50,000+ CNC'ers!  Get our latest blog posts delivered straight to your email inbox once a week for free. Plus, we’ll give you access to some great CNC reference materials including:

  • Our Big List of over 200 CNC Tips and Techniques
  • Our Free GCode Programming Basics Course
  • And more!

Just enter your name and email address below:

Full Name
Email *
100% Privacy: We will never Spam you!

The High Cost of Tight Tolerances
4 (80%) 3 votes

6 Comments

  • I wanted to share this at my office but needed to research it a little more before doing so. I found that while the image shown came from a University of Illinois PowerPoint, the information in that image came from a book published in 1951 (http://openlibrary.org/books/OL13731016M/Machinability_and_machining_of_metals)

    Surely the differences in cost between tolerances shown here are no longer relevant with CNC machines produced 70+ years after this book was written.

  • […] An important goal of GD&T is to ensure that a part defined using GD&T and manufactured within the proscribed limits will fit and function with the largest possible tolerances. Proper use of GD&T can add quality and reduce cost at the same time sincehaving tolerances that are too tight can drive up costs rapidly. […]

  • I have been telling folks for more than 20 years now that tight tolerances almost always cost more than fancy geometry.

    I once worked for a bunch of Physicists who regularly called out two tenths true position for holes in FRP, not tenths of an inch, the other one. These PhDs refused to consider that an M.E. and former metal butcher might possibly know something that they didn’t. I have found in my little career that excessively tight tolerances are the result of pride or a lack of experience or both.

  • The customer just has to be reminded that to call for tight tolernces on anything but a one off piece may get extremely expensive. In a one off you might kill a couple of pieces before getting all the stars to align. Some of them you may have killed early in the process and you didn’t have to finish the piece. In a large serial production were inspection happens further down the line, you might find an out of tolerance piece and twenty more after it 🙁 Particulary if it only takes one bad dimension to kill the part.

    The customer may need to be reminded that he is not just paying for the parts he get, but also for the parts that had to be scraped because of him demanding un realistic tight tolerences.

  • Thanks for this very useful post.

    As a University lecturer teaching Design to ME students, it is always a struggle to get students to use meaningful tolerances. They tend to not specify any limits and finishes, or cover the drawings in ridiculous finishes and Geometrically tolerance everything in sight.

    I have been directing my current class to your GDT posts and have highlighted this post as well.

    Thanks!

  • oooh …..I also did a research study on tight tolerances on the machine operator’s mental health. surely there is a linear proportionality. tight tolerances are costly not only on production but a machinist’s health as well.

Leave a comment

 

Do you want to be a better CNC'er?

Get Better Tool Life, Surface Finish, and Material Removal Rates.

 

Start Now, It's Free!

Home

Software

  GW Calculator

  GW Editor

  Gearotic

  Conversational

  Deals and Steals

CNC Blog

  Software

  Techniques

  Beginner

  Cool

  Projects

 

Cookbooks

     Feeds and Speeds

     G-Code Tutorial

     CNC Machining & Manufacturing

     DIY CNC Cookbook

     CNC Dictionary

CNC Projects

Resources

     Machinist's Search

     Videos

     Online Groups

     Individuals

     Reference Data

     Books

     CNC Dictionary

     Suppliers

     Tool Brands

Workshop

     Hall of Fame

     Organization: Soon!

 

About

     Customers

     Partners

     Our History

     Privacy Policy

 
All material © 2016, CNCCookbook, Inc.