The primary cause of cost overruns with machined parts is poor specification. A part can be under-specified, as often happens when a badly worn out part is presented and the machine shop is asked to “make one just the same, but without the broken bits”. At the other end of the scale is the beautifully drawn part with all the standards and all the tolerances and all ground and polished surface finishes.
There is a sweet spot somewhere in between. This is where the requirements of the part are clearly communicated to the machine shop and each requirement is necessary and sufficient for the function of that part. To achieve this the draftsman needs to have a good understand of how the part functions and how it interacts with other parts. The machine shop is often missing the big picture of how one part interacts with the whole assembly. As a result the machine shop can only manufacture the features specified on the drawing.
Machining costs are driven by two main factors:
Cost of the material the part is manufactured from; and
How long it takes to produce the features specified in the drawing
The choice of exotic materials usually comes with a higher up front cost for material supply. Depending on the material, there is less competition for the supply due to generally lower demand. A price premium on the material results. Exotic materials often require the use of specialised tooling and sometimes specialised machines to manufacture the part further adding to the cost of the part.
The choice of material is almost always well understood by the designer and chosen to be fit for purpose. Although the material affects the final price of a part it is rarely the cause of cost blow outs.
The aspects of part specification which drive how long it takes to produce the part include:
In the machine shop we say that the customer pays for the finish cut and we make our money roughing out. When the surface finish is changed the time needed for roughing out remains constant. Each improvement in the surface finish causes a doubling of time required for the finish cut. With this in mind each machined feature should be specified with the roughest acceptable surface finish allowed by function and form. This includes the use of mill finish on stock materials and flame cut finish on profiled materials.
Every dimension needs to have a tolerance. Features which do not interact with other parts can usually be given ±10% tolerance unless noted otherwise. Any feature which goes inside another part or which has another part go inside it needs to have a specified tolerance. Selecting tolerances from the ISO System of Limits and Fits is recommended.
The smaller the tolerance is, the more care is required to successfully machine on size. The smaller the tolerance is the more tool wear affects size. Changing worn tools and checking size to compensate for wear slows down the manufacturing process and cause the part to be more expensive.
Overall size of a part impacts the time required to handle the material. Handling involves receiving the material, deliver the material to the machine, loading the material into the machine, manipulating the material for each setup, deburring and surface treatment, and packaging the final part for delivery. Part with stock material weighing up to 5kg can be handled manually. Above this weight manual handling equipment is required to maintain safety and to prevent fatigue. The bigger the part becomes the slower it is to handle.
The complexity of the part will dictate which manufacturing processes can be used to produce each feature. Flat sided shapes or cylindrical shapes are the simplest to produce. Overhangs are always difficult and 3D surfaces with enclosed cavities are limited to additive manufacturing techniques. It is important to know the capabilities of machine tools and design your part for the simplest of them. Generally, less simple features are faster to manufacture.
Some questions which may help:
Is a square hole necessary, or will a round hole work just as well
Is a radius needed on an outside corner or is a chamfer Ok?
Does the 3D surface perform a function or can a flat surfaces be used
Can all the features be machined with only one setup?
The final driver of how long it takes to produce a part is how many do you want to make. When the quantity is one, or a few, the investment in redesigning a part is often not recovered by savings in manufacture. When the quantity increases above one thousand there are real savings to be uncovered.
For instance, if you had a part which cost $100 to machine and you invested $1,000 to redesign the part you may be able to reduce the manufacturing time by 5%. This would reduce the cost of making 1,000 parts by $5,000. The net saving in this example is $4,000 or a 400% return on investment.
When the quantities are in the millions, seconds count. Decisions made in part specification directly affect the time to manufacture the part. These decisions directly affect the cost for the machine shop and for the customer. One final aspect of part specification needs to be mentioned. That is the clarity of the drawings and how easily they can be interpreted. This directly affects the quality and consistency of manufactured parts. Clear drawings lead to fewer mistakes. Modern CAD tools make it easy to produce auxiliary views, section view, detailed views, and hidden details. These should be used where they add clarity. The drawing should be laid out on the sheet so that it reads easily. Graphic design skills can make the difference between a good drawing and a great drawing.
The role of the machine shop is to make real what has been specified in the drawing. Sometimes the drawing asks for more then we expected and sometimes this can lead to bill shock. Understanding what it takes to make a part is essential to be able to specify in a cost effect way.
A good machine shop will help you to improve your drawings and designs to reduce your cost of manufacture.
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