Measuring up: When you need to be accurate.

Measuring up: When you need to be accurate.

January 28, 2022 0 By Ray Bohacz

In the early days of machinery, many concepts, skills, and theories that did not apply in everyday life were essential to industry.

It was one thing to take a young man from a rural lifestyle and put him on an assembly line, but often there were more skills required than a good work ethic.

When you design and build machinery, especially engines, the need to measure is never too far away. The ability to accurately identify length, width, depth, thread, or any other size is imperative.

Proficiency in measuring is an essential element in almost any aspect of manufacturing. For example, there may be an instance when there is a need to calculate a linear measurement for a nonlinear item such as a brake or fuel line. Other situations may need a precise dimension determination, such as the clearance in a bearing or something not as critical as the size of the heater core outlet to specify the correct hose.

In some applications, a go-no-go gauge can be employed. It is a tool that is made to a defined dimension, and if the part fits, it goes, and if it does not, it needs to be either reworked or discarded. But this simple apparatus had limited use, especially as machinery became more sophisticated in the 1930s and tolerances were made tighter.

The following are some of the standard measuring tools found in a factory or engineering center:

  • Accurate tape measure
  • Vernier or dial caliper
  • Machinist’s steel ruler/straight edge
  • Feeler gauges
  • Micrometer
  • Dial indicator with magnetic stand
  • Snap or telescoping gauges
  • Depth micrometer
  • Dial bore gauge

There are many other measuring tools such as radius gauges, small-hole ball gauges, calipers, and dividers, along with engine-specific tools and those used by a tool and die maker.

Unlike years back, today, digital measuring tools offer the ability to read the dimension the old-fashioned and proper way (on the scale of the instrument) but allow a novice to get started quickly in the measuring field.

What a digital tool does not supply as readily as the scaled design is the feel for the instrument and component. In addition, the tool itself is often clumsy, especially when dealing with a micrometer. Though some may argue this point, most in the industry believe a better learning experience is gleaned from a traditional scaled tool than a modern electronically enhanced version.

The critical element of accurate measurement lies in the sight and touch of the operator. The sense of touch is essential when using any measurement device. This varies with each individual but needs to be cultivated.

A skilled machinist will have a highly developed sense of feel that can detect the slightest change in the tool’s contact with the measured piece.

In the human hand, touch is the most prominent in the fingertips. Thus, contact-measuring instruments should be appropriately balanced in hand and held lightly and delicately in such a manner as to bring the fingers into play handling the tool. If the operator is clumsy or grabs the tool with authority, the feel is greatly diminished.

There are times when accurate measurements are unnecessary, and only estimation is needed. For example, when doing a cursory inspection of a part such as a brake drum or non-critical bolt. A good worker will measure and remember the length of their one finger. Also, a dollar bill is around six inches long.

As the author, I know that just past my first knuckle on my index finger is one inch, and my Red Wing work boots (size 8) are just shy of 12 inches.

Even if you do not mentally assign a length, dimension estimation can still be employed.

Let’s say that you are comparing the backspacing of two different rims.

Stick your hand in one and, with the other, identify the length on your arm. Then compare that dimension to the other rim. It will not be a precise measurement but will allow you to quickly determine if the part will fit in instances when no measurement tools are available.

Measuring tools

Micrometers can be considered either inside, outside, or depth. The most common micrometer is the outside version.

Outside micrometers are usually produced with a measuring range of one inch. Thus, it will be cataloged as  0 to 1, 1 to 2, 4 to 5, etc.

Depending on what you believe you will be measuring will determine the size needed. Regardless of its range, all micrometers are read and used the same way.

If there were a need to measure pistons in most American V-8 engines, then a 4-to-5-inch micrometer would be required.

A standard small block Chevy 350 and a 302 Ford both use a 4.00- inch bore piston with the common overbore being 0.030 inch. But that instrument would not be helpful to check a piston on a Buick V-6 with a 3.800 -inch bore.

For this reason, many tool rooms have a myriad of micrometer sizes. A 0 to 1-inch micrometer would be a better choice when measuring much smaller parts such as the valve stem.

Many teachers of measuring suggest getting a 0 to 1-inch micrometer as the first instrument since it will be easier to measure things such as a ballpoint pen, pencil, steel tube, etc. and learn the proper feel instead of working with a clumsy part such as a piston and a rather large micrometer.

A standard micrometer is scaled in thousandths of an inch. For example, one-thousandth of an inch is the approximate thickness of the cellophane in a pack of cigarettes and is read as 0.001 inch.

To read the micrometer, multiply the number of vertical divisions visible on the sleeve by 25. Then add the number of divisions on the bevel of the thimble, counting from zero to the mark that coincides with the long horizontal line on the sleeve.

In some instances, there may be a need for a more accurate measurement that is read down to 0.0001-inch (1/10,000). To accomplish this, the micrometer must have a vernier scale. Except for one, the vernier scale lines are mismatched with the thimble scale. The number that matches is the ten-thousandths value.

When gripping the frame of an outside micrometer, the thumb and forefinger need to be free to operate the thimble. Therefore, the very light pressure of the thumb and forefinger is required when contacting the item to be measured. Some better micrometers have an internal ratchet stop to identify when the proper contact pressure has been reached.

Some helpful tips for the beginners

  • Wipe the micrometer clean of dust and oil after every use
  • Do not open or close by holding onto the thimble and spinning the frame around the spindle axis.
  • Never drop a micrometer. This can spring the frame and cause misalignment between the anvil and the spindle faces.
  • Ensure the spindle face does not touch the anvil when storing since temperature changes can spring it.
  • Do not touch the anvil and spindle faces with your fingers. Moisture and oils from your skin can promote corrosion.
  • Before using a micrometer, clean the measuring surface by sliding a piece of paper held between the anvil and spindle faces.

Slide calipers can be considered either vernier or dial designs. They are handy tools since they can measure round and flat surfaces while acting as a depth gauge but with less accuracy than a dedicated depth micrometer.

A vernier or dial caliper is easy to use to obtain a quick measurement, and usually provides a much more extensive range such as 0 to 6 inches or even beyond. In addition, the tool is excellent for measuring bolt hole depth using the depth rod at the end. Place the bar of the caliper on the outside of the surface and send the depth rod down into the hole. When it stops, read the scale for the maximum bolt length.

Another helpful tool is the dial indicator with a magnetic stand. This allows for accurate and fast movement measurement such as the backlash in a differential gear set, the free play in a crankshaft or camshaft, and the lift of a valve. The dial indicator and magnetic stand can also determine run out on a surface like a flywheel or a brake rotor.

The straight edge and feeler gauge is essential to determine any warpage on a flange, such as where the intake manifold mates to the cylinder head. It can also check the straightness of a cylinder head or block deck.

To check for warpage on any part, place the straight edge on the surface, and then using the thinnest feeler gauge you have, try to slide it between the two. Keep increasing the feeler gauge thickness until it does not fit anymore.

Once the maximum deviation is recorded, use either your caliper or micrometer to measure the gasket thickness. This will determine if the part seals.

For example, if the flange has a 0.20-inch variation, but the gasket is 0.45 inch thick, it will seal with no problem. The gasket is more than twice as thick as the most significant amount of warpage.

The best way to measure items such as fuel or brake lines that have kinks and bends in them is to follow the contour with either a soft welding rod, thin electrical wire, or string. Then, keep feeding the measuring material along the line, and when done, use a tape measure to determine the entire length.

Measuring is not only enjoyable, but it brings you into harmony with the mechanical device since you are now intimate with its dimensions.

And a machine that you are one with… always runs the best.