Tools

Off-The-Shelf Hacker: How to Use the Dial Caliper for Extra Precise Measurement

24 Aug 2019 6:00am, by

tape-scale-caliper

Off-the-Shelf hackers are certainly familiar with measuring parts using a common, everyday tape measure. If you’ve done any drafting or engineering design drawings, you also know how to use an Engineer’s Scale. These work great for measuring things down to a few tenths of an inch. What if you want to measure parts in the range of a couple of thousandths of an inch, say for fabbing up a few 3D printed parts?

No problemo, just get yourself a dial caliper, which can offer precision down to the micrometer.

Thumb’s Up for Measurements

A dial caliper is a sliding, F-shaped tool, typically made of hardened stainless steel that retails for around $20. It makes outside, inside and depth measurements. A small thumbwheel adjusts the jaws of the tool when making a measurement and a dial indicates the smallest increments. The 6-inch model from Northern Tool has served me well over the years. A 12-inch model is also available for about $45.

My 6-inch model is a good size for general fabrication duties. It came in a sturdy plastic case with protective, form-fitting foam inside. When not in use, the caliper should be stowed in the case so it doesn’t get dropped, banged-up or bent.

6-inch dial caliper

I’ve used the tool periodically for projects over the last decade or so. Recently I wanted to gin up some custom-sized gears to add new capabilities to Hedley, the robotic skull. The dial caliper is the perfect tool to take measurements of slotted motor shafts, sample gear thicknesses and hub diameters. Five or 10 thousandths of an inch accuracy is good enough for my resin 3D gear-printing needs. An average human hair is about 4 thousandths of an inch thick. Light barely passes through that gap in the caliper jaws at 4 thousandths.

Inside, Outside and Depth

Of course, we want repeatable accuracy, so the first order of business is to “zero out” the caliper, to adjust for any potential wear. I normally hold the tool in my right hand with my thumb pressing against the thumb-wheel on the bottom. Start by slightly loosening the locking screw on the top of the caliper and move the slide (jaws) all the way closed. Now, very slightly loosen the locking screw on the bottom of the dial. Rotate the dial face so the indicator needle lines up with zero and snug the bottom locking screw back down. Move the slide back and forth and the needle will rotate around the scale. It should return to zero when fully closed. You shouldn’t have to zero-out the dial again although you should check it from time to time.

Measuring an outside diameter

Taking an outside measurement is pretty natural. Roll the thumbwheel counterclockwise to spread the jaws so they fit over the object you want to measure. Try to hold the caliper perpendicular to the object and roll the thumbwheel clockwise to lightly close the jaws. It will take a little practice to know how to put just the right pressure on the thumbwheel while wiggling the caliper to find the minimum possible measurement. Watch the dial for the lowest number. Make sure you are also using the end or flats parts of the jaws and not the relieving area, up close to the slide.

Once you have the smallest reading, gently tighten the lock-screw on the top to anchor the slide at the current reading. Carefully slip the caliper off of the object and look at the measurement on the slide.

Beveled scale-edge and measurement markings

The beveled scale edge of the slide, to the left of the dial, shows the inch and thousandths of an inch measurement. For example, if the edge is between the first 3 and 4 on the slide, the inside measurement would be between 0.300 and 0.400 of an inch. If the pointer on the dial reads 75, you’d add that to the linear number, making the measurement 375 thousandths or 0.375 inches. Open the jaws up to the first large one with the pointer on zero and we’d have 1.000 inches. The whole inches are numbered zero through six on the slide, with the in-between numbers being the hundred-thousandths.

Inside measurements are handled similarly. The inside jaws are on the top and are significantly smaller than the outside jaws on the bottom. This time close the caliper and make sure the dial reads zero. Insert the inside jaws into the gap you’d like to measure. Now spin the thumb-wheel counterclockwise to expand the inside jaws. Again, wiggle the tool around a little bit looking for the largest reading on the dial. Snug up the top locking screw and withdraw the caliper jaws from the gap. Take the scale reading and add it to the dial reading for the final inside measurement.

Lastly, depth measurements are handy using the end of the slide. Say you want to measure a blind hole in a piece of metal. Simply balance the end of the outside part of the slide on the edge of the hole and move the thumbwheel counterclockwise to extend the inner part of the movable slide until it bottoms out in the hole. Snug down the top lock-screw and read the measurement as usual.

Measuring Up

That’s pretty much it for using a dial caliper. If you treat the tool gently and keep it in the case it will last a lifetime. It’s a very small investment for a lot of accurate measurement functionality.

Now comes the hard part. Transforming all those measurements into a CAD model for 3D printing or computer numeric control manufacturing. I’m using FreeCAD for my 3D modeling and gear design. It works well on the old ASUS duo-core Linux notebook and has a lot of useful engineering features. Reverse engineering and blank-sheet part design are an art and science of their own. We’ll look at those topics, down the Off-the-Shelf Hacker road.

Catch Dr. Torq’s Off-The-Shelf Hacker column, each Saturday, only on The New Stack! Contact him directly for consulting, speaking appearances and commissioned projects at doc@drtorq.com or 407-718-3274.

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