Fits and Tolerances are Standards-Based
Did you ever wonder how Engineers decide tolerances when the way parts fit together is important?
The answer is something called “Fits and Tolerances”. There are actual standards that define exactly what tolerances need to be to achieve various kinds of fits such as Sliding Fit, Press Fit, Force Fit, Free Running Fit, and so on. The standards include ISO 286, ANSI B4.2, and DIN 7172.
The standards ensure nobody has to guess what is meant by a term like “Press Fit” so that parts are consistently made and their performance can be relied on.
Fits and Tolerances involve a fair bit of math, so the best way to deal with them is not as a Chart, but with a special Fits and Tolerances Calculator. G-Wizard includes a free ISO Tolerances Calculator that makes a convenient reference when you’re doing Fits and Tolerances work.
You access it from the “Geometry” tab. If you have G-Wizard Calculator handy, bring it up now so you can play with it as you’re reading. If not, take a minute to sign up for the free trial, we’ll wait for you.
The next thing is, “What exactly are Fits and Tolerances and why would I need a calculator for them?”
I thought you’d never ask!
Basically, whenever we’re designing a system with a shaft and a hole, Fits and Tolerances are a set of standards that define how large they should be relative to one another and what their tolerances should be for them to perform in the way you need them to. There are a series of different types of “Fit” you might want to consider. G-Wizard Calculator offers the following types of Fits:
– Loose fit: Used where the most clearance is needed on things like latches or parts that may corrode
– Free running: Shafts rotate easily without noticeable clearance or gap in the hole
– Easy running: The part slides easily
– Sliding fit: Precise clearances for guiding and centering
– Close clearance: Getting tighter
– Location clearance: Tight, but still without interference, used for precision location
– Slight interference: Assembly will take a rubber mallet
– Transition: Fit will depend on tolerances and could fall to either side
– Press fit: Cold pressing will be needed for assembly
– Medium fit: Cold pressing with large forces will be needed
– Force fit: Assembly will require different temperatures as well as pressing forces
As you can see, quite a range of applications are covered. One of the great things the Industrial Revolution and subsequent manufacturing sciences has brought us is various standards that define this sort of thing. A number of people had to do a great many experiments to arrive at what would work and what wouldn’t for these various types of fits. G-Wizard Calculator encapsulates the ISO 256, ANSI B4.2, and DIN 7172 standards, which are the most widely used.
G-Wizard’s Free ISO Tolerance Calculator
Let’s take a look at how it works:
G-Wizard’s Fits and Tolerances Calculator…
The easiest way to use the Fits and Tolerances Calculator is as follows:
1. Select a Hole Diameter (you can start from the shaft using the “Shaft-Basis” if you prefer, but it is customary and preferred to start from the hole and machine the shaft to fit.
2. Select a Fit Type. From that G-Wizard will select the “Preferred Fits”, which is a designation for the Hole and the Shaft using letters and numbers. In the screen shot, the hole is to be H11 and the shaft is c11. Holes are upper case, shafts are lower case.
3. The number reflects the International Tolerance Class, which has to do with how precisely made the feature is. You can find lots of discussions about what that all means. For now, just read it off as the Tolerance on each.
4. There’s a whole host of other useful numbers under there that describe the size and how precisely machined these two features will need to be. For example, the minimum and maximum dimension for each to fall within tolerance.
5. If you want to fine tune things, you can alter the specification for either the Hole or the Shaft. You might do this to tighten or loosen a fit, or to make things more compatible with a part you had already machined.
That’s about all there is to it. Having a Fits and Tolerances Calculator sure seems to make the principles involved easier to understand than trying to work through all the tables that are out there.
If you’d like to have a Fits and Tolerances Calculator, simply sign up for the 30-Day Free Trial of G-Wizard:
Now here’s what’s cool: even if the Free Trial expires and you don’t choose to purchase G-Wizard, most of the calculators including ISO Tolerances will go right on working!
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