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Understanding CNC Machining Tolerances to Maximize quality, reduce cost, and navigate design challenges

Eli Whitney, the inventor of the cotton gin, is credited with the concept of interchangeable parts. During a presentation to the United States Congress in 1801, he illustrated how all of the components needed for an assembly should be produced according to a set of exacting standards— in other words, to specific dimensions and tolerances—thus ensuring that the firing pin or gun barrel from one musket will fit into any other musket. His idea helped pave the way for the Second Industrial Revolution and what became known as the American System of Manufacturing, which has the standard method of part design and print for almost two centuries.

Although the concepts of component interchangeability and dimensional tolerancing have since become an accepted part of manufacturing, unfortunately, the lack of understanding and proper use of dimensional tolerancing can cause many problems. For instance, an overly stringent tolerance might require that parts go to a secondary operation and/or extra “finishing” passes, unnecessarily increasing costs and lead-time. Tolerances that are “too loose” or that aren’t in line with those of mating parts can make assembly difficult if not impossible, leading to required rework, or in the worst case, making the finished product unusable.

To help avoid these unpleasant situations, this design tip includes some guidelines on how to properly apply part tolerances, along with a few definitions of the more commonly used callouts.

The World of Engineering Thermoplastic and Thermoset Materials – How They Differ From Metals

First, it is important to understand that, unlike metals and ceramics, with engineering thermoplastics the property-determining particles are not atoms, atomic cores and ions, but organic macromolecules, and this differs greatly as compared to the lattice structure of metals.

These macromolecules can also differ within a plastic in terms of their size and chemical structure, meaning that these factors exert a far wider influence on the properties of the material as compared to metals. Most plastics are termed “semi-crystalline”, meaning they have both crystalline and amorphous structures within the material. Such a complex structure enhances some properties (such as impact resistance), but always results in compromises in material stability as compared to metals.

As a result of these differences, plastics offer lower dimensional stability in comparison to more historically specified

• Non-metallics have higher coefficient of thermal expansion, lower rigidity and greater elasticity
• The moisture absorbing properties of plastics, which can result in phenomena such as swelling of the material and the respective dimensions, also have a determining role to play (particularly in the case of polyamides [nylons]).

Combined, these attributes add to the difficulty of adhering to very tightly specified tolerances during machining, in shipment and in storage. Therefore, proper storage of engineering thermoplastic components over a long period of time (especially in summer months) is required to maintain the dimensions achieved during machining. High heat (over 80F), especially combined with high humidity, is to be avoided.

To a lesser extent, this is also true of thermoset materials – the various “phenolic” formulations. The fabric or fiberglass matrix makes these more stable than thermoplastics, but still less than metals.

The recommended guideline to use when determining machining tolerances is a minimum of 0.2% of the nominal value (Tighter tolerances are achievable when using very stable and fiber-reinforced composite materials).

Standardized Tolerances for CNC Machining

At WS HAMPSHIRE, our standard machining tolerance is +/- 0.005 in. (0.13mm) on standard L/W/T dimensions. Hole locations and other critical dimensions can be held more closely. This means any part feature’s location, width, length, thickness, or diameter will not deviate by more than this amount from nominal. For example, the 1 in. (25.4mm)-wide bracket you’re planning to order will measure between 0.995 and 1.005 in. (25.273 and 25.527mm) across, while the 0.25 in. (6.35mm) hole on one leg of that bracket will come in at 0.245 to 0.255 in. (6.223 to 6.477mm) diameter.

When specifying feature locations, be sure to reference the datums, or “start measuring from here”, points. This is usually from one or more edges, making clear where the centering point of a given feature needs to be located.

Something that usually helps in those discussions is sending us an assembly drawing, and/or drawing of mating parts. This allows cross-reference and can prevent “tolerance creep”, which is where individual tolerances all tend to one side which can hinder part alignment, especially at attachment points.

Tolerancing Guidelines for CNC Machining

Also, be aware that these are bilateral tolerances. If expressed in unilateral terms, the standard tolerance would read +0.000/- 0.010 in. (or +0.010/- 0.000 in.) while a limit-based tolerance in our bracket example would be 1.005 / 0.995 in.

All are acceptable, as are metric values, provided that you spell them out on the design. And to avoid confusion, please stick with one system and use “three place” dimensions and tolerances, avoiding the extra zero in 1.0000 or 0.2500 in. unless there’s an overriding reason to do so, which may require further discussion.

Surface Roughness Considerations for Machining Tolerances

There’s more to part tolerancing than length, width, hole size, etc. There’s also surface roughness, which in the standard offering is equal to 63 µ in. for flat and perpendicular surfaces, and for curved surfaces, 125 µ in. or better.

This is an adequate finish for most uses, but for cosmetic surfaces on certain parts, we’re generally able to improve appearance through adjusting the feeds and speeds of the equipment. For wear surfaces, the material will smooth out during operation. If aesthetics are important, that needs to be specified on the print and understood (samples always help!)

Geometric Dimensioning and Tolerancing

Here’s another consideration. As mentioned earlier, we can accept GD&T tolerancing. This provides a deeper level of quality control that includes relationships between various part features as well as form and fit qualifiers. Below are a few of the more common ones:

• True position: In the bracket example cited earlier, we called out the hole location by specifying X and Y distances and their allowable deviation from a pair of perpendicular part edges.
• Flatness: Milled surfaces are generally quite flat, but due to internal material stress or clamping forces during the machining process, some warpage can occur once the part has been removed from the machine, especially on thin-walled plastic parts. A reasonable GD&T flatness tolerance controls this by defining two parallel planes within which a milled surface must lie.
• Cylindricity: For the same reasons that most milled surfaces are quite flat, most holes are quite round, as are turned surfaces. However, using a +/- 0.005 in. (0.127mm) tolerance, the 0.25 in. (6.35mm) hole in the bracket example could potentially be oblong, measuring 0.245 in. (6.223mm) one way and 0.255 in. (6.477mm) the other. Using cylindricity—defined as two concentric cylinders inside of which the machined hole must lie—manufacturers eliminate this unlikely situation.
  • (NOTE – due to composites higher coefficient of linear thermal expansion, sometimes a slight “slot” is preferred as it allows part movement without buckling)
• Concentricity: The rings on a bullseye are concentric, just as the wheels on your car are concentric to the axle. If a drilled or reamed hole must run perfectly true to a coaxial counterbore or circular boss, a concentricity callout is the best way to assure this.
• Perpendicularity: As its name implies, perpendicularity determines the maximum deviation of a horizontal machined surface to a nearby vertical surface.

There are additional considerations to GD&T, including parallelism, straightness, profile, and angularity, all of which should be indicated on the print. Again – composites are less rigid than metals, and slight irregularities will conform to the mating surfaces, so avoid using “metal-think” when specifying these additional features.

Summary

• Remember that composites are less structurally stable than metals, which requires composite-specific tolerancing but also allows for greater conformability with mating parts
• Don’t over-specify tolerances that aren’t actually required, it adds cost rather than functionality
• Fine-tuning tolerance dimensions in your designs for CNC machined parts can help maximize those parts’ quality and reduce cost

We at WS HAMPSHIRE are happy to discuss appropriate part dimensioning, as well as material alternatives and other design considerations with your design team – with over 125 years of non-metallic manufacturing experience, we can help! Give us a call!