Measuring the Perpendicularity of Small Metal Parts

March 27, 2019 Barbara Osborne

measuring perpendicularity

Where the End Cut Matters

Perpendicularity is one of the GD&T features sometimes indicated on an engineering drawing. These callouts on a drawing are used to communicate how a small metal part should be produced and to specify the acceptable GD&T tolerances for part features.

At Metal Cutting Corporation, where every day we produce thousands of metal rods, tubes, and wires cut to length, we typically talk about measuring perpendicularity in terms of the part itself in relationship to the end cut.

That is, our perpendicularity inspection looks at two surfaces: the side of the part compared with the end cut we put on the part, whether it is a rod, tube, or wire.

Other Ways to Look at Perpendicularity

This is somewhat different from GD&T perpendicularity, which typically refers to a tolerance that controls either:

  • The perpendicularity between two 90° surfaces or features, with two parallel planes acting as the tolerance zone
  • How perpendicular an axis needs to be to a particular reference point, represented by a cylinder around a theoretical, perfectly parallel axis

Measuring GD&T perpendicularity of a surface requires using a height gage, with either the gage or the part locked to a 90° datum, to determine how perpendicular the surface is. Alternatively, calling out the maximum material condition allows the use of a gage to measure axial perpendicularity for either a negative (hole) or positive (pin) feature.

Here at Metal Cutting, the perpendicularity of an axis might be called out on drawings that reference a feature such as a hole or pin. However, usually our customers are more concerned with the perpendicularity (or squareness) of the end on the parts we cut, rather than surface or axis perpendicularity.

Factors That Influence Perpendicularity

On engineering drawings, perpendicularity is represented by an upside-down “T,” and the tolerance can be called out in inches or in degrees.

For the parts we generally work with at Metal Cutting, the perpendicularity tolerance of the cut end depends on the diameter of the part; the larger the diameter, the greater the tolerance zone. The length of a part also affects the tolerance, with a longer part requiring a larger tolerance zone.

A square (that is to say, perpendicular) end cut on a rod, wire, or tube would form a perfect right (90º) angle. However, such a perfect angle will have its tolerances imposed by the method used. For example, the cutting wheel might travel a little or deflect a bit as it makes a downward pass.

While a “perfect” angle might not be possible every time, at Metal Cutting we come very close — typically achieving perpendicularity of 0.001” over a 0.125” diameter when cutting a part. In degrees, we can typically hold a 0.5º (half of a degree) angle of perpendicularity or better, depending on the diameter of the rod, wire, or tube.

The straightness of the material can also affect perpendicularity. If the material is not straight, it will not sit perpendicular to the wheel to begin with, which will become exaggerated by the movement of the cutting wheel.

Measuring Perpendicularity in Inches or Degrees

When measuring the perpendicularity of a rod, wire, or tube, we look at the triangle formed by the cut end in relationship to a hypothetical perfect right angle.

Typically, we put the part on a comparator, roll the part until the largest angle is visible, and then zero out the comparator to measure the leg of the triangle and determine if it is within the specified tolerance — for our customers, generally a tiny fraction of an inch.

Alternatively, we might turn the part and measure the angle itself in degrees, to see if it is within the allowable tolerance.

Depending on how a customer specifies their part, we may have to convert between inches and degrees when calculating perpendicularity tolerance. Here again, you can see the effect of diameter; for instance, 0.5º angle of perpendicularity over a 0.125” diameter is 0.001”, while 0.5º over a 0.500” diameter is 0.004”.

Manufacturing Methods Matter

When measuring the perpendicularity of the end cut on parts, the results will also vary depending on the manufacturing method used. For example, with lapped parts, Metal Cutting can hold 0.0002″ over 0.125” diameter, or 0.1º.

With the secondary operations that we perform, we might also need to take perpendicularity into consideration. For instance, suppose a customer asks us to polish or tumble a part to deburr it, but the part also has a stem on it with perpendicularity requirements.

In this case, we’d take great care to make sure the stem is not bent during the deburring process. (However, note that our customers are more likely to use a straightness callout for a part with a stem, rather than a perpendicularity callout.)

On the other hand, the perpendicularity of the end cut might not matter at all if a customer will be taking the parts we cut and doing something else with them, such as soldering a wire to the end, grinding a multi-angled face onto the end, or bending and trimming the end.

This, too, is something important for us to know. If the perpendicularity is not important, we will not add cost to your product by inspecting for it.

The Right Partner Helps to Ensure Quality

Whether your application requires perpendicular ends cuts or other important GD&T features, it is vital to choose the best production method and ensure that your specified tolerances are appropriate for your product’s functionality as well as your budget.

It helps to have a trusted partner such as Metal Cutting, where our goal is to work with you to keep production cost-effective while delivering precision parts that meet your quality requirements.

 
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