Posted June 22, 2021

Tolerances: Guide For Turning Standards

Tolerance guides for turning are essential for CNC lathes like this one

There are many manufacturing methods available for component fabrication, but to get a part built quickly and at the highest levels of quality, CNC machining is often the best choice. Of the different types of equipment the machine shop has available for building your part, they typically choose the CNC lathe to turn cylindrical-shaped components. These machines produce high-quality parts quickly, but their productivity is dependent on the specifications of the design. This article will examine the turning process on a CNC lathe, providing a tolerances guide for turning.

Using CNC Turning for Manufacturing Parts

Machine shops fabricate components using two primary methods; milling on CNC mills and turning on CNC lathes in a subtractive manufacturing process. CNC mills create various shapes and contours in stationary parts secured to the mill. The cutting tools maneuver in three axes and rotate at high speeds to mill out unwanted material from the workpiece. For more complex parts, machine shops use CNC mills that move in five axes.

CNC mills are versatile, but CNC lathes offer a more cost-effective manufacturing method for cylindrically shaped parts. With this machine, cutting tools remain stationary while the workpiece rotates under them, shaving the material off quickly and precisely. Additionally, some CNC lathes use live tooling, which augments the lathe with milling capabilities. During turning, the workpiece stops rotating, and the milling head maneuvers to drill or cut features. This capability allows multiple machining operations without moving the workpiece over to a standard CNC mill.

All CNC machining equipment can manufacture parts with very tight tolerances. However, tolerances vary depending on the part’s chosen material, which we will look at next.

Choosing Materials for CNC Turning

Many materials are good candidates for CNC turning, including:

  • Stainless steel
  • Steel
  • Aluminum
  • Copper
  • Brass
  • Bronze
  • Plastics

Softer materials such as aluminum, copper, and plastics are simpler to machine, while harder materials require slower RPMs and feed rates, which slows down the productivity of the equipment. For example, as a general rule, stainless steel takes about four times as long to machine as steel, and steel takes about four times as long as aluminum. Harder materials can also be more expensive than softer materials.

Not only does the type of material change the investment of time and money for CNC turning, but it also affects the tolerances. The softer the material, the more challenging it is to hold tighter tolerances. Thus, machine shops often post slightly looser tolerances for plastic materials than metal. CNC turning maintains tighter tolerances with softer materials when required, but this comes with a higher price tag. To hold the tighter tolerances, machine shops must slow down the processing, adding time and cost to the project. Machine shops will sometimes recommend different materials or altering a quoted part’s tolerances to avoid these extra steps and costs.

Specifying the Tolerances and a Guide for Turning Standards

For a precise fit, designers should set tolerances for parts that will be interfacing with other parts. For instance, a turned shaft must be machined precisely to fit the designed bearing. If the bearing has an inner diameter that will be no smaller than 2.000 inches, then the shaft should have a tolerance of 2.000 +.000/-.005 inches. This tolerance will ensure that the shaft is fabricated no larger than the smallest allowable diameter of the bearing.

Good practices include calling out specific tolerances and fits on the 2D drawing for the machine shop with the CAD model. For example, if a drawing specifies a ¼ inch hole. Without any further instruction, this hole could be for a bolt, pin, or it may require tapping. However, if the intended purpose of the hole involves an easily removed pin, then the hole should be dimensioned with a tolerance of +.005/-.000 and called out as “slip fit to standard ¼ dowel pin.” Although the dimensions and tolerances provide the necessary information, the additional callouts will help keep the machinist fully informed. The following section highlights other steps to help communicate turning standards to the machine shop.

Working Together with Your Machine Shop

Designing a part for efficient machining will not only take less time and money to manufacture, but it will also be a better part. Here are a few ideas to keep in mind that can help you to design for efficient manufacturing:

  • Use a standard tolerance: Specifying a standard tolerance on the 2D drawing will save a lot of time calling out tolerances on every dimension.
  • Avoid unnecessarily tight tolerances: Tighter tolerances increase the time for manufacturing, and that spent in inspection and quality control. It can also require specialized tools and additional fixturing during manufacturing, increasing scrap material. In short, tighter tolerances can end up costing a lot and should be avoided whenever possible.
  • Only specify unique tolerances when necessary: As with overly tight tolerances, multiple unique tolerances will also have high costs. So make sure only to specify unique tolerances when required.
  • Use geometric dimensioning and tolerancing: GD&T will specify more detail and clarity on a drawing than standard coordinate dimensioning and tolerancing.

At Plethora, we have years of experience in building components for many different industries and applications. We understand the challenges that engineers face in designing their parts and can help simplify a design by reviewing the design needs. We are an ISO 9001 certified shop, and manufacturing parts to the highest level of quality is our primary goal. Our online DFM and quoting systems are ready to receive data so we can begin working with you on your next project. To get started, upload your design files to Quote My Part or call us at 415-726-2256.

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The Plethora Team

The Plethora team is your go-to CNC manufacturer for hardware done right the first time. We have the tools and experience needed to create high quality custom parts quickly and with precision, whether you need a prototype or production run.

Topics: materials, Manufacturing, finishing, CNC machining, Prototyping, quality design, Tolerances