June 2009 - Bilateral vs. Unilateral Tolerances
Pro Tips - June 2009
Bilateral vs. Unilateral Tolerances
Last month's newsletter discussed Surface Profile tolerances. This month we are going to stay on the topic of dimensional tolerance but focus on unilateral vs. bilateral tolerances.
Fig. A: Part with both unilateral and bilateral tolerances.
This is a pretty common example. In this case, the easiest way to program and manufacture this part is with a profile cut from the top of the part which establishes both the 4.000" and 2.500" dimensions at the same time with the same cutter. The CNC program by default will drive the tool at the nominal size of the CAD model. It will create a rectangle that is 4.000" x 2.500". But the sides with a 2.500" dimension will be practically out of tolerance as programmed since we cannot be on the plus side of nominal at all. And with the typical small amount of cutter deflection outward, the 2.500" dimension will likely be out of tolerance. This makes the programming of the part take much longer as the programmer will have to compensate for this with manual adjustments to the program, adjustments to CAM parameters or a separate program with tool paths for each side. The set-up of the part on the CNC machine may also take longer if the machinist needs to apply cutter compensation in order to bring the part closer to the middle of the tolerance zone. In an extreme example, if you had a tolerance of + 0.005"/- 0.000" on the 4.000" dimension, it would be virtually impossible for either dimension to be in spec without major tweaking to the program. This brings up the other point about unilateral tolerances. Most CNC shops want to run the dimensions in the safest range to minimize the chance of scrap. So in the case of the part in Fig. A, they would likely try to run the 2.500" dimension at 2.4975" which is exactly in the middle of the + 0.000"/ - 0.005" range. This somewhat defeats the purpose of specifying a unilateral tolerance in the first place. It would be easier for everyone involved if the engineer put a symmetric bilateral tolerance at 2.500" or 2.497" because more than likely it is what will be delivered anyway.
Fig. B: Unilateral tolerances on holes are not a problem.
Armed with a bit of knowledge about which tolerance scheme is easier to machine the engineer can achieve their desired design objectives without adding cost to the product... an idea that we can all unilaterally agree is a good idea.
Part of the Month
Every month we feature a really cool part that we have made recently. June's Part of the Month is multi-axis mill-turn part used in an aerospace application. Just over 1" long, this part made from 6061-T6 has many off-axis mill features which required the use of our 6-axis Mori Seiki NL2500 SY. This highly capable mill-turn CNC machine was able to machine this part complete in one operation. After machining, the part was nickel plated.
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Inside the US, call 866-4PRO CNC. Outside the US, call 360-714-9000.