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As a buyer looking for the best possible price from your suppliers, it cannot be emphasized enough that the best opportunity for reducing the cost of manufacturing begins in the design stage.  There are many things that can be done in the design of a component part that will have a large impact on the price.

There are extensive resources on the internet for DFM guidance.  One such resource is: DRM Associates.

With the constant pressure to reduce costs, it is imperative that engineers and designers follow best practices for DFM… Below is our condensed list of recommendations for reducing your purchase price by Designing for Manufacturing

Although the material that a component part is made from will be dictated by several factors during the engineering and design process, it’s still worth noting that the material type is a significant factor in the cost of a machined component. For example, the raw material costs of an aluminum part vs. a stainless steel part will be very different. Additionally, the machinability of the two materials is significantly different. CNC machines can remove material from aluminum at a much higher rate than stainless steel for example, and therefore the manufacturing costs will be significantly different. Even within the same class of material, aluminum for example, there can be significant variations in raw material costs as well as machinability between different alloys.

Even though the material for a component often cannot be changed, it’s worth understanding why similar parts from different materials may have vastly different prices.

Without a doubt, one of the largest influences on cost is that of tolerances. The simple fact is that the higher (tighter) the tolerance, the higher the cost of manufacturing is and the lower the tolerance the lower the cost of manufacturing.

The following chart from the University of Ohio illustrates this:

Tolerance vs mfg costs

As can be seen from this chart, a tolerance of +/- .00025 will cost about four times as much to produce as a tolerance of +/-.001.

An excerpt from Wikipedia on GD&T states: “Proper application of GD&T will ensure that the part defined on the drawing has the desired form, fit (within limits) and function with the largest possible tolerances. GD&T can add quality and reduce cost at the same time thru producability.”

Sometimes it appears to us that certain aspects of GD&T specified on a machined part are excessively tight. In those cases all we can do is ask if they can be loosened up.  If not, it’s worth understanding why the part may have a very high price tag.

Another significant manufacturing cost riser is that of corner radii. When designing interior features, ask any shop and they will tell you; the larger the radius the better. Here’s why… The most economical way of producing an interior corner radius, or fillet, is usually with an endmill on a CNC milling machine. As such, it’s important to realize that the stiffness of a cutting tool changes by the third power of length and the fourth power of diameter. Using a cutting tool twice as long makes it 1/8 as rigid. Using a cutting tool twice the diameter makes it 16x more rigid.

A good guideline to follow is to keep an internal corner radius at least 1/6th (.166) the depth of the pocket, or thickness of the part. For example, if a pocket in the part is .75 deep, the corner fillet should be at least .125. (Preferably .130 as noted in the next tip)

Another tip… whenever possible dimension the internal corner radius slightly larger than a nominal fraction. That allows the shop to contour the corner with a nominally sized cutter rather than plunge into the corner. Doing this will reduce stress on the cutter and avoid chatter in the corner.

There are several things that can be done to reduce the cost of producing threads. They are:

  • Minimize the length of threads in holes
  • Avoid blind holes whenever possible
  • If through holes are not possible, allow as much hole depth beyond the thread length as possible
  • Don’t over-specify thread percentage.  A 75% thread has 95% of the strength of a 100% thread, but requires much less torque to produce –  greatly reducing the chance of breaking the tap.
  • Avoid tight tolerences on thread depth in a hole

A significant advantage of designing parts with 3-D CAD systems is that a 3-D Solid Model file can be provided to the supplier.  Provided the model is complete and accurate, it can save the supplier a fair amount of time because they can program their CNC machines directly from the Solid Model.   As a buyer you will will realize the savings by receiving a lower price than would be quoted if an accurate solid model cannot be provided.

As much as possible consideration should be given to the raw form factor that the parts will be produced from. For example, in most alloys extruded stock is much more economical than calling for the parts to be made from plate stock.

Also the availability of the raw material can become a big issue for lead-time as well as cost factor when non-standard material forms are specified.

You might wonder, what does quantity have to do with Design For Manufacturing? Although it may not have a lot to do with design, it does have a lot to do with costs… Each custom manufactured part that you purchase goes through many, many steps before being ready to ship. Just a few of the steps include; a manufacturing review, work-holding design and manufacture, programming the CNC machines, setting up the machines, verifying the CNC programs, producing a first article part, first article inspection, finishing, assembly, plating and/or painting, final inspection, etc, etc…

Some of these costs are non-recurring and some are recurring for every batch that we run. All of these costs need to be accounted for one way or another. Sometimes we can bill for non-recurring costs separately, sometimes the customer wants everything amortized into the piece price. Either way, it’s easy to see that the quantity ordered at one time has a large impact on the actual piece price.