When Zero is More
By James Keith
We have been led to believe that more is better -- more speed, more
filled seats per flight, more miles per gallon, more inspection tolerance, and
so on.
With tolerances, one might think that a large value in a feature control
frame is better than a small value. When
using Geometric Dimensioning & Tolerancing and
talking about position tolerances, the greatest allowable variation actually
comes from the application of a “zero” tolerance on a Maximum Material Condition (MMC) basis.
Okay, let’s put
this in simple terms: Tolerance is
air! We can see this demonstrated by the
formula shown in the ASME Y14.5 (GD&T) Standards, where Tolerance = Hole
– Fastener, or T=H-F.
Example: assume
a hole diameter of .260 - .270, and a fastener diameter of .250. Both H and F will be the MMC value of the
respective features. So, H (.260) – F
(.250) = T (.010). The problem with this
method is that holes less than .260 (down to .250) are not allowed regardless
of their location.
Now this is
where zero is more! We are going to
modify the formula slightly to read: T(0) = H-F. Now we have a value of zero that the MMC hole
minus the MMC fastener must satisfy. H(.250) – F(.250)
=T(0).
In other words,
we have modified the MMC hole size to equal the MMC
fastener diameter. The new hole size is .250 -.270 diameter and the location tolerance
is zero at MMC. Virtual condition
remains unchanged. See the example
below.
2X Ø.260-.270 2X Ø.250-.270
Virtual Condition = .260 -.010 =.250 Virtual Condition = .250 -.000 =.250
As you can see,
the change to zero tolerance is not difficult.
This little change allows acceptance of the full range of parts that
will assemble and meet the hole size and location
tolerance.
Virtual
condition remains unchanged for both methods!
Caution: do not apply Regardless of Feature Size with Zero
tolerance. Remember, all the tolerance
comes from the bonus and with no bonus you get no tolerance at all.
Manufacturing
may not warm up to this idea right away, but when they see that they get more tolerance with this method, they
should come around and may even ask for it.
In the example
below we see that, for the non-zero toleranced part
(left side of graphic), the range of acceptance falls off at the MMC hole size, which is far short of the virtual condition. Any hole size less
than .260, regardless of its location, should be rejected.
The range for
the zero tolerance part (right) is equal to the virtual condition and will
allow acceptance of all parts that meet the hole size and location requirements
and will assemble to be accepted.
|
|
Hole Ø |
Total Tol
Ø |
V Cond |
|
|
Hole Ø |
Total Tol
Ø |
V Cond |
LMC |
.270 |
.020 |
.250 |
|
LMC |
.270 |
.020 |
.250 |
|
|
.269 |
.019 |
.250 |
|
|
.269 |
.019 |
.250 |
|
|
.268 |
.018 |
.250 |
|
|
.268 |
.018 |
.250 |
|
|
.267 |
.017 |
.250 |
|
|
.267 |
.017 |
.250 |
|
|
.266 |
.016 |
.250 |
|
|
.266 |
.016 |
.250 |
|
Accept |
.265 |
.015 |
.250 |
|
|
.265 |
.015 |
.250 |
|
|
.264 |
.014 |
.250 |
|
|
.264 |
.014 |
.250 |
|
|
.263 |
.013 |
.250 |
|
|
.263 |
.013 |
.250 |
|
|
.262 |
.012 |
.250 |
|
|
.262 |
.012 |
.250 |
|
|
.261 |
.011 |
.250 |
|
Accept |
.261 |
.011 |
.250 |
MMC |
.260 |
.010 |
.250 |
|
|
.260 |
.010 |
.250 |
|
Reject |
.259 |
|
|
|
|
.259 |
.009 |
.250 |
|
Reject |
.258 |
|
|
|
|
.258 |
.008 |
.250 |
|
Reject |
.257 |
|
|
|
|
.257 |
.007 |
.250 |
|
Reject |
.256 |
|
|
|
|
.256 |
.006 |
.250 |
|
Reject |
.255 |
|
|
|
|
.255 |
.005 |
.250 |
|
Reject |
.254 |
|
|
|
|
.254 |
.004 |
.250 |
|
Reject |
.253 |
|
|
|
|
.253 |
.003 |
.250 |
|
Reject |
.252 |
|
|
|
|
.252 |
.002 |
.250 |
|
Reject |
.251 |
|
|
|
|
.251 |
.001 |
.250 |
|
Reject |
.250 |
|
|
|
MMC |
.250 |
.000 |
.250 |
When using functional
gaging (ASME Y14.43) that is designed for
verification based on Virtual Condition (absolute policy), the use of Zero
Tolerance on the work piece is the preferred combination.
If a virtual
condition functional gage is used with the toleranced
method, the gage will accept parts with holes smaller than the specified MMC,
provided they are located within the acceptable location. These are parts that would have been
rejected, based on size check, per the drawing.
There are
circumstances where this method is not the best choice, but for most
conventional assemblies designed for assemblability,
this method will work well.
If you want to
do more with less, try doing the most with nothing (zero that is).
Mr. James Keith is a member of the ASME Y14.5
Standards Committee, and a consultant and instructor with Crucial Knowledge (www.crucial-knowledge.info), based in Wichita, Kansas.
He can be reached at 301-412-5966.