With design programs and CAD software now the default tools for drawing product concepts, there is the danger of believing that anything that can be designed can just as easily be made. However, when transferring a drawing from the perfect world of the computer to the practical world of the machine shop, certain essential considerations apply that should give the designer pause. Knowing how to design a product for manufacture requires the understanding of three key concepts: Accuracy, Precision and Tolerance.
Degree of conformity to a known standard or value.
The standard for physical dimensions is the meter, and its value is a universal one based on the distance that a beam of light travels in a certain fixed period of time. So, accuracy is the degree to which a measurement or movement in space conforms to this known standard or one of its derivatives (typically the millimeter or micron for metric measurements.)
Thus, when a machine tool is touted as being highly accurate, it means that when the programmer instructs the machine to cut a piece of metal 40mm long, then the machine actually does that. The more accurate it is, the closer it gets to exactly 40mm, with no error. There are many other factors involved in getting exactly the 40mm cut that you’re looking for, but just remember we start with a known value and accuracy describes how close the machine gets to that value.
Repeatability of a measurement without generating random errors.
Precision is the ability of our machine tools to repeat this 40mm cut, over and over again, with no introduced or random error. Precise machining, or precise workmanship, is a testament to controlling the process and not deviating – regardless of whatever the measurement or process may be. This is often confused with accuracy and indeed the two terms are casually used somewhat interchangeably however precision refers more to repeatability.
In our sense of precision, it also applies to the performance of a task, like taking a careful measurement or sharpening a cutter head or any other job in a machine shop whose performance ultimately effects the quality of the finished product. It is for this reason that we say our OMOM® tool making service is considered our most precise, since it involves a degree of careful handwork that is able to reduce any machine-induced errors.
Allowable or predictable deviation from a standard.
Every engineer knows that there is no such thing as guaranteed 100% accuracy. There is only an agreed upon standard. All machine tools will have a degree of free play or vibration in them – they must, or else they would be frozen, inert blocks. The tolerance therefore specifies the deviation away from a known value, expressed as a “+/-” number. So, if we have a machine tool that claims a tolerance of “+/- 1 mm”, this means the tool will potentially introduce a deviation of one millimeter with every cut or pass. Could be an extra millimeter, or too short by a millimeter or some fraction thereof.
This would be considered extremely poor performance, and no reputable machine tool is that sloppy. Taken together, this means that accurate tools come close to the standard, precise tools are reliably accurate over and over again, and tolerances are controlled to within a small, acceptable degree of variability.
How does all this affect Designing for Manufacturing?
As mentioned above, modern CAD programs are capable of generating 3D renders which are highly accurate, maybe too accurate. You will need to consider the following when designing your product:
- The accuracy and precision of the tools being used to make the product.
- In other words, what is the capability of the factory where your product is to be made? It’s good to know this in advance.
- The natural movement of the raw materials (metal and plastic of different types) in response to environmental conditions (temperature and humidity) and mechanical stress.
- Necessary allowance for movement.
- Draft angles are applied to parts which must be removed from a tooling mold, to make space for clearance without damaging the part.
- Other clearances must be allowed for pieces that have mechanical fittings: screws, axles, gears, etc.
- Kerf is the amount of material removed in the process of cutting. The computer model may not account for this lost material but the machinist must.
- More accurate is not necessarily better.
- Higher degrees of accuracy and precision require more effort, time and money for diminishing returns.
- Pursuing extreme accuracy in one feature of a design may compromise accuracy in another area.
- Many part features may need some free space to allow movement or room for thermal expansion/contraction between mating pieces of an assembly.
For more information about our capabilities and specifications contact us today.