By Ted Visaya 6/6/2011
When you start to create fabrication drawings draw them as if you were the fabricator creating the part. Think like the machinist machining the part, or the sheet-metalist cutting and bending the part, or the injection mold operator pouring the plastic into the mold you designed. A detailed part drawing is needed to define a part with dimensions, tolerances, materials, and finishes. The drawing will be used to generate vendor quotes for fabrication. You as the designer may interface with the vendor directly while creating the drawing to adjust dimensions, loosen tolerances, and change materials to reduce costs and meet budget.
The machine drawings I've created in the past varied from heavily detailed piece parts with GD&T, Geometric Dimensioning & Tolerancing, to simple parts without the need for tolerancing at all.
I've learned throughout the years to interface with the machinist vendors directly to first see how they are equipped and the extent of their CNC capability. In the Silicon Valley there are the small mom & pop machine shops that specialize in quick turn around time for R&D parts and can do it cheaper then the bigger large quantity producing shops. I use the larger shops for mass production after the part has been tried and tested to work during R&D.
I create machine drawings as if I'm fabricating the part myself and dimension the part as if I'm going thru multiple machine passes on each side. Multiple auxiliary and section views may be added to thoroughly define the part and machining process.
All CAD designers must have formal training in Geometric Dimensioning & Tolerancing. GD&T is an engineering language understood between engineers, designers, and fabrication vendors when communicating throughout the design process.
When adding GD&T I put myself in the place of the machinist reading the drawing and inputting CNC numbers into his machine. A designer should be able to inspect the part by reading the GD&T tolerances on the drawing while using a pair of calipers. The designer may have to use more sophisticated measuring devices in the QA department to inspect the part.
I have found that most critical tolerances are for tight mating surfaces and hole patterns. Commonly used materials for machining are steel, stainless steel, aluminum, and polymers like delrin. When there are mating parts of dissimilar metals the designer should take into consideration different thermal expansion rates when tolerancing hole patterns.
A note on the drawing should be referenced for GD&T and look similar to this: "DIMENSIONS AND TOLERANCES ARE IN ACCORDANCE TO ASME Y.14.5" or something to that effect. ASME stands for the American Society of Mechanical Engineers. (back to top)
I draw sheetmetal parts in a finished folded form with minimal to no GD&T Symbols unless the mating hole pattern locations are critical to another part. In most cases loosly fit hole dimensions are suffice.
The three most common stainless steel grades available in sheet metal are 304, 316, and 410. The four most common aluminum grades available as sheet metal are 1100-H14, 3003-H14, 5052-H32, and 6061-T6. Although 6061-T6 is not as formable as the others, it is stronger and heat treatable. Not good for tight radius bends and crack lines may occur. I only use 6061-T6 for sheetmetal when absolutely required for strength other then that I will reserve 6061-T6 for machined peices.
Metal thicknesses above 6 guage (.1943" / 4.94mm) are no longer considered sheet metal.
I use a combination of conventional dimensioning
for feature size and ordinate dimensioning to locate multiple hole patterns. Most likely a hole schedule will be added to identify holes and a list of self-clinching fasteners as show in the drawing on the right. (back to top)
The main design feature for injection molded parts are the draft angles. I've been involved with redesigning many injection molded parts. My tasks involved incorporating a new design feature into an existing part for upgrades. The only feature I feel a designer has to be aware of when modifying these designs is the draft angle. There other inportant factors to consider but since the parts I've worked on had already been engineered the only thing to be aware of and stay consistant with is the draft angles and wall thicknesses. The drawing to the left was a project I was assigned to enlarge the inside cavity at the lower half of the housing. It was easy just widening and extending dimensions in the geometry but also maintaining the draft angles and wall thickness. notice that there are not any GD&T. This is a finished part drawing and the GD&T will apply to the 2 molding die cast that forms the part.
(back to top)