Typical Tolerances

For engineers and purchasers interested in a switch from traditional machined parts to powder metal (PM) parts for their next project, there’s often one hangup. Can you achieving the tight tolerances you believe are critical to your product?

Design Considerations for Powder Metallurgy: Dimensional Tolerances

To some degree, it’s possible to control tolerances with powder metal through careful engineering and an experienced manufacturing partner who’s mastered the process. Key considerations when designing for tolerances include:

  • Component size
  • Component complexity (shape, etc.)
  • Material choice
  • Density
  • Sintering considerations

Most small parts producers use multiple quality control techniques during production to achieve the desired end product.

Although powder metal components can be held to closer limits than you might think, tolerances should still be as liberal as possible. In most cases, however, the benefits of powder metallurgy you’ll see below are a worthy trade-off for the inability to produce super-tight tolerances

A Chart of Powder Metallurgy Tolerances

The following tolerances are based on material composition and sintering parameters.

General Tolerances
Diameters (& Height*) 7 mm 13 mm 25 mm 40 mm 50 mm 65 mm 75 mm
Diameters (Tooling Features) 0.04 mm 0.05 mm 0.080 mm 0.10 mm 0.10 mm 0.13mm 0.13mm
Heights*/Lengths in Pressing direction 0.15 mm 0.15 mm 0.20 mm 0.20 mm 0.20 mm 0.25 mm 0.25 mm
Flatness 0.025 mm 0.040 mm 0.050 mm 0.065 mm 0.075 mm 0.09 mm 0.10 mm
Parallelism (top to bottom surfaces) 0.040 mm 0.05 mm 0.07 mm 0.08 mm 0.10 mm 0.12 mm 0.15 mm
Concentricity ( Bore to OD) 0.05 mm 0.07 mm 0.08 mm 0.10 mm 0.10 mm 0.13 mm 0.15 mm
Surface Finish Diameters In Sintered Condition Top/Bottom Faces in Sintered Condition
1.6 Ra µm 3.2 Ra µm

Please note that, for example, to have a capability of 1.33 Cpk, multiply existing tolerance by 1.6 Tighter tolerances &/or Higher Cpk requirements will require secondary operations which will add to the total cost.

How to Weigh Tolerance Vs. Other Factors

1. Do you really need your design Tolerances?

Consider whether tolerances need to be as tight as you think. There’s a vast difference between a design that looks great on paper and something that can actually be manufactured cost-efficiently.

Asking your powder metal parts manufacturer for highly precise tolerances when they’re not necessarily required can result in higher cost.

Does your product really need the tight tolerances of a machined part? Or can those tolerances be relaxed so you can go with the powder metal process?

2. Are Your Tolerances Hurting Your Profit Margins?

In most cases, tighter tolerances result in higher prices due to the need for more secondary operations, as well as more tooling wear and tear.

As you add tighter tolerances,  we may have to add more secondary operations, which of course adds more costs. The point is that replacing traditionally machined parts with powder metal parts requires consideration of the tolerance-vs.-cost balance. Can your design “tolerate” specs that are just good enough, or do you insist on true precision

3. Are the Benefits of Powder Metallurgy Worth It?

Powder metal materials can be highly cost effective with benefits such as:-

  • Unique design shape options  including flat sides, key-ways, splines, multiple levels, or square holes
  • Excellent surface finish
  • High accuracy in large-production runs
  • Minimal required secondary machining
  • Greater material control through the sintering process
  • Corrosion-resistant options