When we here at Redwood Plastics and Rubber get our estimators to work on a project we are required to fill out an internal form that, among other properties, specifies surface finish of the part and tolerances. Assumptions in these matters can occasionally result in customer disappointment but equally important the customer may be paying for tight tolerances or a surface finish that isn’t required for proper performance of the part. To reiterate: requiring tighter tolerances and a smoother surface finish than is required for a part means you are wasting money!
One of the biggest barriers to figuring out what tolerances and surface finish are required is that technical drawings rarely provide accurate information. Most commonly, drawings are supplied with metal tolerances – these are tolerances that are difficult or impossible to hold with plastics. Certain plastics such as acetal can be held to fairly tight tolerances but other commonly specified plastics such as UHMW-PE cannot. Questioning about how critical tolerances are at the very least can delay a quotation until we figure out what is required and doable.
A similar issue is that drawings often come with parts that have 90 degree square corners. This is also very difficult to supply and definitely adds cost. Our standard edge radius would be .125″ and knowing how critical the edge is or not will potentially save you, the customer, money and time.
Surface finish is a little bit of a tougher issue to tackle. Many customers do not know that a variety of surface finishes are available. Our standard is a “125 end mill” but do you know what that actually looks like? The tough part here is the customer may be expecting a “smooth” (very smooth!) part but if that isn’t specified we’d default to the end mill, which has small, but noticeable, grooving. Likewise, a rougher surface finish like a “500 shape turn” will potentially save the customer money.
At the end of the day we rely on you, the customer, to give us as precise information as is possible. It helps us not only supply a quicker quote but also a more competitive quote and that helps you as much as it helps us!