Design Realization/Detailed Design - Medical Device Design: Innovation from Concept to Market - page 179

Let us now assume that sub-assembly X1100 comprises four parts. These individual parts
would then be numbered X1100.1, X1100.2, X1100.3, and last of all X1100.4. If the main
body also had a single component (such as a covering sheet) this would then be X1000.1.
Simple isn't it? All part numbering systems need to be simple, and work for the designer and
the end-user, not just for the warehouse staff.
The benefit of this system is that it is easy for all healthcare staff to know which bits go
together. Imagine if they were numbered X1234256, X156719, X17823076, and X145629a:
Would you be able to tell that they were from the same assembly? It also helps your packing
staff to understand what goes with what, and hence minimizes packing errors.
This numbering system may not be to all tastes. Also some hospital purchasing systems
cannot cope with the usual - ;. / symbols used as divisors, so make sure that no number is
identical even with the symbols removed. Also watch out for mixed text and numbers - the
number 1 can so easily be confused with the letter l, etc.
You need to enforce a logical part numbering system from the outset - do not leave it for your
stores to sort out at a later date. Renumbering parts at a later date is really difficult, and really
time-consuming.
8.6.7 Tolerances
Once again, most CAD packages allow you to build tolerances into your components.
However, when you are producing your models at the start take care as you will set a fixed
value. Hence if you have a shaft that is 10 mm diameter +0.1 mm and −0.0 mm, what do you
use for the initial solid? Do you start with 9.9 mm (bottom), 9.95 mm (middle), or 10 mm
(top)? Some would argue that if you want the tolerances to work, always draw to middle
values. Others say stick to the original. Whatever you decide, stick to one method. I have to
admit that for some critical tolerances I use the “draw to middle” concept as it seems to make
the shop floor happy as they like to work to the “main dimension.”
Once again, and this is where CAD really helps, all forms of tolerancing and respective
options should be built-in.
8.6.8 Sign Off
Do not forget that the drawings of your components and assemblies are controlled documents.
They all need revision numbers (stated earlier), but they will all need to be signed off as valid.
This need not be done on paper; nowadays CAD packages allow for electronic sign off. I am
old fashioned - I like to have a paper copy in a file, but for larger companies this would be a
mountain of paper (imagine the number of drawings for a Boeing 747) and hence electronic
data sign off is acceptable.