Design for Manufacturing
Design for manufacturability is the general engineering practice of designing products in such a way that they are simpler to manufacture. Below are some design for manufacturability tips. When applied, these features will help reduce the cost and lead time for machined parts. In addition, it will help the manufacturer get started on making parts instead of requesting changes from the engineer.
Tolerances
Fill out the tolerances in the drawing title block and pay attention to how many decimal places are being called out in the dimension. If the drawing dimensions default to three decimal places (.XXX), then make sure that is the precision you actually want. A part can be more expensive than necessary if the dimensions are defaulted to the highest precision.
Figure 1. Typical drawing title block tolerance callout
Inside Corners
When CNC machining, parts with inside pockets should have round corners and not be square. An endmill will always leave rounded inside corners. A smaller endmill can be used but will still leave a round corner…just smaller. Square corners can be done but it will be a secondary machine or operation, which will add to the cost of the part. If a square part fits inside the pocket then use drill holes at the corners.
Figure 2. Left: Pocket with undesirable square corners. Middle: Pocket with desirable round corners. Right: Pocket with drilled holes at corners.
Machine shops usually stock endmills in fractional sizes (e.g. 1/16” diameter, 1/8” diameter, ¼” diameter, etc.). Therefore, don’t make inside corner radiuses the exact same size as endmill diameters. Instead of calling out an inside corner radius to be .250”, make it .270”. This way a 1/2” endmill (.250” radius) will have less surface area contact in the corner and leave a better surface finish.
Figure 3. Top: A .500” circle representing a ½” endmill cutting a .250” inside radius. Bottom: A .500” circle representing a ½” endmill cutting a .270” inside radius with less surface contact.
Deep Drilling/Tapping
Avoid using deep holes or deep tapped holes. Standard jobber drills are designed to go a certain depth. If that depth is exceeded then special drill bits are needed. The same goes with tapped holes. The deeper the tapped hole the harder and more expensive it is to achieve it. A good rule of thumb for depth is to tap the hole 1 to 3 times the diameter. Use a commercial fastening nut for reference. The thickness of a nut is roughly 1 to 2 times the bolt diameter. Anything more is unnecessary as the bolt will brake before the threads fail.
These are just a few tips to make parts more manufacturable. If you have any design or manufacturing questions then please reach out to us.