In my line of business as a build-to-print manufacturer (or Job Shop,) we see all kinds of drawings (or Prints) from Design and Mechanical Engineers. We’ve even bid from and made parts from napkin sketches having been sketched out by small business owners and inventors!
Throughout the years we’ve seen the best looking well thought out drawings which were vetted by another engineer or plan checker for accuracy. And, we’ve seen napkin sketches! Often, some of the most detailed drawings begin as a napkin sketch and graduate into an expensive CAD system.
Whatever the type of drawing, it just needs to have complete information, otherwise it becomes a liability for everyone involved in the project. Complete information includes very specific details about materials, finishes, dimensions and tolerances. For assembly drawings, the mating parts must be cross-checked for tolerance fits. Each sub-component drawing should have specific information too. If there are unusual manufacturing processes, often the drawing will have information about a preferred vendor to do the process which has already been contacted and qualified perhaps.
Your drawing doesn’t need to be produced in an expensive CAD system. The drawing just needs to have all the information so that the manufacturer can go from bid to build and delivery of a part that the customer intended to have made.
Sometimes even the best drawings are missing critical information that we overlook during the quoting stage, and it’s revealed to us during the manufacturing stage. At the point, we’ll contact the person that drew the drawing and they are usually surprised about the missing information. Usually, it’s just a quick phone call to our customer for clarification.
Another aspect of “clean” drawings is Revision Control which is managed by our customer. Uncontrolled Rev’s is a topic for a new book and not just a simple Blog entry.
On one occasion an engineer of a small company had us bid to a Rev B drawing, he then placed an order for the Rev B and we made and delivered parts to the Rev B drawing. Upon receiving inspection of our parts the owner (not the engineer) of the small company informed us that the parts weren’t correct and he proceeded to give us a Rev B drawing when he rejected the parts for re-work.
The problem here? The engineer rolled the drawing to a Rev B and so did the company owner so there were two Rev B drawings both having different information from two different people within the company.
We aren’t in the business of assuming what our customer needs made. Our customer is in the business of designing parts and we are in the business of making parts. If we made assumptions about every drawing across our desks, we would be perpetually assuming and never producing.
Unfortunately our customer didn’t take ownership of their mistake and we re-made the parts at our cost to keep the customer happy. After that job, the customer never ordered from us again which still baffles me to this day.
Bottom line, whether it’s a simple missed tolerance or improperly controlled Rev’s, dirty drawings are a liability. As a result of past experiences we over-scrutinize drawings, but we cannot make assumptions about a product that we didn’t design initially.
Had we been making the mating part there would have been a better chance of us catching a possible tolerance issue. But that wasn’t the case either, and it’s further proof of having mating parts produced by the same manufacturer even if they aren’t the lowest bid on one of the mating parts.
The extra price paid on that mating part should be considered as cheap insurance to improve the chances of the manufacturer to catch a drawing error. However, this kind of insurance isn’t guaranteed. It is always best to have an effective process in place about how to properly validate a drawing.
I’ve seen drawing issues in both small and large companies. Once your drawing validation process is in place, then enforce the rules! A liability is a liability regardless if it is a small liability for a big company or a big liability for a small company. The end result of a liability is generally lost time, money and strained business relationships.
Source: Jason M LaRock
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