October 12, 2022
We have been devoting this year’s blogs to fundamentals. I frequently remind my clients that the gear geometry is the easy part of gearbox design. The devil is in the details and you need to make sure your project is not derailed by stupid errors that could have been prevented by a thorough checking before any material is ordered or chips are made. There is not a lot written about the checking process. The old hands that performed the function in the pre-CAD days were workers not writers. You will find some helpful guidance in Machinery’s Handbook but otherwise you are on your own. So, as a public service, the curmudgeon community offers the following tips:
- If at all possible do not check your own work. Familiarity breeds sloppiness and you will kick yourself later if you let mistakes slip by.
- Check on a hard copy not the screen. The human brain is incredibly efficient at spotting things that do not fit a pattern. It is much less efficient at picking up fine details at close range; call it the “forest for the trees” effect.
- Review everything, starting with the default notes and material description. Even the title block needs to be checked as a misspelled part name is likely to get you mocked.
- Do not assume anything. Some shops have well established manufacturing and drawing protocols that allow them to simplify their drawings. But what happens if the work is sub-contracted or subject to third party review? Have a reason for every note, section, and view. Keep reference information in the project file.
- Check the drawing from the viewpoint of the people who will be using it to make the part. Do not expect them to add up dimensions. Make sure tolerances match the stack-up calculations. Think about each step in the production process and provide the information needed even if you have to add another view, section, or note.
- Process capability involves tolerance, finish, and complex geometric relationships between features. If you need a hole pattern to be aligned to another feature you must say so; otherwise it may be considered just a “drafting convention” that can be ignored. The same with runout, squareness, parallelism, and other “geometric form control” information; do not expect it to happen unless you require it.
- Shop lighting is not the same as office lighting. Bigger fonts are appreciated.
- Understand “maximum material condition.” A machinist wants to get “in tolerance” as quickly as possible. To help them it was traditional to put the limit first reached on top or to the left in a dimension. Most CAD systems put the lower value first and this tiny little change can cause problems if the dimension is not read closely.
- Be careful about specifying both a result and a method. A good example of this is tap drill sizing; roll taps use a different tap drill than a cutting tap. Once the part is finished you cannot necessarily determine the method used so concentrate on what you can inspect.
- If you have to violate rule #1, do yourself a favor and allow enough time to do a thorough check. Develop the discipline to print out the drawing and physically check off each item as you review it. Wait a day if you can to see if that wonderful subconscious of yours rings an alarm or two while you try to sleep.