Fits and Tolerances: How to Design Stuff that Fits Together

The vast majority of engineered products have
multiple components that need to fit together. Sometimes you need components to slip together
easily, while other times, you need components to press together and not come apart. There’s
a systematic way to design parts so they fit together exactly the way you want on the first
try, every time, and in this video, we’re going to show you how. The engineering term
for this consideration is “fits and tolerances”. Let’s define these terms. Most of the time,
especially in CAD, we just type single, exact numbers into the computer for our dimensions.
That’s called a “nominal” dimension. But when it comes time to actually make that
part, we won’t be able to manufacture the features to exactly those dimensions. Each
feature might be slightly larger or smaller than the number we originally typed into the
computer. So, we need to specify a “tolerance” which tells the person manufacturing the part
how much of a variation from our nominal dimension is acceptable. Different manufacturing processes
are capable of different tolerances. This chart shows a variety of different manufacturing
processes, as well as a general idea of the tolerance that can be expected. It’s important
to remember that tolerance capability corresponds to a large production run, where factors like
tool and machine wear, deflection, sharpening, and the natural variation of different processes
apply. A very common mistake that new engineers make is selecting tolerances that are far
tighter than necessary. As the tolerances become tighter, the cost to manufacture the
part grows exponentially. So how tight of a tolerance do you need? Tolerances typically
come into play when designing fits. In some cases, you might want components to slip easily
together, but not have a lot of perceptible play. In other cases, lots of relative movement
is okay. You might even need to have two components come together permanently and not come apart.
These different cases are called “fits”. Machinery’s Handbook is the go-to reference
for designing fits. Fits have a particular naming convention of a two-letter abbreviation
followed by a number. The letters denote the type of fit, and the number corresponds to
the tolerance class of the fit, with larger numbers representing looser tolerances. This
graph shows all the standard ASME fits. The solid bars represent the acceptable tolerance
of the shaft, while the hashed bars represent the acceptable tolerance of the hole. The
scale of this graph corresponds to a one-inch nominal shaft. This video deals primarily
with ASME fits in the inch system, but there is a similar convention for metric. The naming
convention is different, but the general concept is the same. For more information, you can
reference Machinery’s handbook, or ISO 286-2. Since the chart only applies for a one-inch
basic size, we need to use tables to design for other sizes. The process is easy, and
we’ll show you how with an example. The first fit we’ll show you is the loosest.
This is a LC11, and you can see there’s lots of clearance. To design this fit, first
we’ll look up the LC11 column. Then, we’ll find the row that corresponds to our basic
size. At the intersection of the size and fit, we have a tolerance range for both the
shaft and hole. Note that these numbers are in thousandths of an inch. You’ll use this
same procedure to design all the other fits that we’ll talk about. These tolerances
are extremely loose, so drilling the hole with a regular drill is sufficient. Hitting
the shaft tolerance on the lathe is also no problem, and other processes like forming
and extrusion will probably hit it as well. This fit is used where you don’t need accurate
alignment of the mating features. The clearance around a bolt is a good example of this, since
you typically don’t use a bolt for precision alignment. This is a slightly tighter locational
fit, LC9. It provides less clearance and has a tighter tolerance than the LC11. This is
probably about the tightest hole tolerance that can be drilled, but turning shouldn’t
be a problem for the shaft tolerance. You can see that the LC9 fit is a bit tighter
than the LC11, but both can still be easily achieved with standard equipment and tooling.
The next category of fits is called “running” fits. A running fit is typically used when
you need two parts to mate together freely, or that’s to say, without any force, but
you need some degree of precision in their alignment. We reamed this RC6 hole, but you
can probably achieve this tolerance on a CNC mill if you compensate for tool diameter in
the control. A drill is not likely to hit this tolerance every time. Depending on the
size, this is probably about the tightest shaft tolerance that you can expect to hit
consistently by turning. This is the fit to use when you need two parts to align to each
other, but some clearance is acceptable. This is the tightest fit that can be readily achieved
with normal manufacturing processes, and without undue effort. For the rest of the fits in
this video, we’re going to have to work a bit to hit the tolerances, and you’re
going to want to be absolutely sure you need this in your design, because you’ll be paying
for that care and attention. This is an RC3 fit, which is a tighter running fit. In general,
this fit is going to have almost no perceptible play. This hole will need to be reamed or
bored. You can also interpolate it on a CNC mill, but you will probably have to update
the wear offsets throughout the production run to compensate for tool wear, and you will
need to be sure that tool deflection isn’t causing the feature to taper with depth. A
dialed-in and rigid lathe setup might be able to hit this shaft tolerance consistently,
but as with the hole, you will need to perform regular inspections throughout and adjust
the wear offsets. A lot of parts, especially flexible ones or those made of tough materials,
will require grinding. Here is a comparison of the two running fits. The RC6 has some
perceptible play, while the RC3 has almost none. RC3 and tighter will precisely align
components but can still be assembled easily by hand. This next type of fit is called a
“transition fit” and it requires a slight amount of force to assemble and disassemble.
However, there is absolutely no play once the pieces are assembled. The tolerances required
for the LT3 fit are about the same as for the RC3, but the range of the shaft has been
shifted so that there is a slight amount of interference. If we add a bit more interference,
we create what’s called a “force fit” or “press fit.” These fits usually require
a press to assemble, and are intended to be a permeant connection. We hope you enjoyed
this video. If you have other topics in mechanical design engineering that you’d like to learn
about, leave a comment and let us know. We’re coming out with new videos soon, so please
be sure to subscribe. Thanks!

22 thoughts on “Fits and Tolerances: How to Design Stuff that Fits Together

  • Nice video guys ! post videos about all types of GD&T symbols & inspection , tolerance stackup analysis in the same way !! Thank you

  • Another great video. Does anyone know where one can get a set of parts to demonstrate fit classes? I always recommend new engineers keep a set of calipers and feeler gauges at their desk to help visualize specified tolerances. A set of class of fit demonstrators would be another fantastic learning tool to keep handy.

  • Really well done. I particularly love that this references accessible and common touch points in every shop, including the Machinery's Handbook. Everything you need to be a good machine designer and machinist can be found in a pretty small collection of references. Thanks for this video!

  • Ah this is awesome! So many people just say "Yeah these are the fits and tolerances" but you guys actually showed what those fits look like, which is so important for us to connect what we want with what we have to say!

  • I would like to know why Engineers will place tolerances on a print, (example) .500+.005-0.000 ,and/or
    .250+0.000-.005, as to opposed stating the mean tolerance along with the acceptable tolerances. It does make for a confusing adaptation of which dimension you are measuring and what the tolerances are.

  • This video is great. Its all fun on the computer like you say but I'd pay some money for a set of ANSI pin and hole fitments just to get a real world idea.

  • Can you please make vedio on difference between achieving surface finish by grinding and surface finish achieved by itself in the process of maintaining the tolerance.

  • Since I first started on Mechanical design this was one of the most challenging things to understand. I remember feeling incapable of imagining how these tolerances would behave beyond my CAD models. And that's why this video is so valuable. Thanks!

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