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How to use Digital Calipers for Woodworking

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  • How to use Digital Calipers for Woodworking

    Digital calipers like these are available for 10-30 dollars. The size quoted as in "6-inch digital calipers" refers to the maximum measurement. The resolution and accuracy of these are typically .001 or one-thousandth of an inch. Some have an additional digit that shows 0 or 5 for half a thousandth.
    I recommend the six-inch Harbor freight ones, they work very well and are of surprisingly high quality. The stainless body ones are better than the composite plastic body ones although the plastic ones are cheaper. HF has the Stainless ones on sale for $10-20 frequently with "regular" prices at $30. Links at the end of the article.


    Familiarize yourself with the parts of a caliper:


    First we will measure the outside of an item (could be a piece of wood measuring the thickness or width - or the length if less than 6"). Make sure the lock screw is loosened to allow the readout to slide. Slide the readout and jaws all the way closed. Make sure the readout reads 0.000, if not you may need to calibrate the zero by pressing the zero button. Also make sure the readout is in inches (or metric if you prefer) by pressing the "in/mm" button. Some calipers also display fractional inches i.e. 1/8 or 7/64 and the button will say in/mm/frac or something similar.

    Now that the calipers are in the right units and properly zeroed, open the external jaws a bit wider than your workpiece. Holding the calipers in your right hand so that the thumbwheel is under your thumb makes it easy to slide the readout. The scale on the beam gives you a rough idea of the jaw opening - Look at the numbers just to the left of the readout housing on the beam.

    Now close the jaws on your workpiece gently but firmly. When closed there should be no gap between the jaws and the sides of the workpiece on a width measurement, if measuring a round or diagonal, then the flats of the jaws should be against the edges. Read the readout display for the measurement. If the piece is round, you may want to rotate it within the jaws to see if there is a low or high spot, while keeping the jaws pressed gently against the sides.

    Here's how to make an external measure:


    Calipers are also very useful to measure dado widths, slots and hole diameters. Use the internal jaws for this as follows:


    A really useful thing calipers can do it measure slot depths and hole depths. The depth probe is used for this. With the jaws closed, Place the end of the calipers flush to the surface of the workpiece with the probe over the slot or hole. Use the thumbwheel and lower the probe until it hits bottom, then read the display:


    Another use is for steps and dropoffs like rabbets - you can use the offset between the top of the jaws to measure in this way (this is also great to measure the thickness of an item sitting on a flat surface):


    A final use is scribing work lines. Who needs marking gauges? The jaws are sharp, hardened stainless steel and I mark wood as well as aluminum and mild steel without concern. Just set the jaws to the desired width, lock the jaws, then draw the calipers with one jaw on the outside edge just below the surface and press the other jaw into the workpiece as you drag it along - I had to enhance the pic a little so you can see the line but you get the idea:



    A few final comments:
    you can lock the readout to hold the readout and jaws at a certain opening, just tighten the lockscrew - this is useful for go-no-go testing if items are smaller than a certain size you lock in, if you can slide the jaws over it, it smaller than the readout value.

    Difference measurements: you can measure the difference between two items. Close the jaws around your reference item. Press zero. The readout will now read zero at the size of the reference. Now measure another part. If the number is .010" then it is .010 bigger than the reference. If it reads -.020 then its .020 smaller than the reference. This is also useful for measuring inferred values, such as the thickness below a stopped hole in a piece of wood. Measure the thickness of the wood, press zero, the use the depth probe to measure the hole depth - the readout will show the thickness of the bottom under the hole.

    The narrow part of the jaws (at the tips) are good for measuring the inside diameter of small holes or the narrow parts of a necked down item. Otherwise use the thicker part of the jaws if you can get the workpiece between them.

    Because the jaws align to both ends of an object being measured, its far superior to using a ruler because of the problems getting it precisely aligned to both ends. I have checked the HF calipers I own (yes several of them) and I find that they agree perfectly with the $150 Mitutoyos I use at work.

    Cautions:
    Be careful, the jaws tips are really, really sharp-you can hurt yourself. Since they are hardened stainless you can actually use them to scribe items for cutting or drilling.
    Make sure you have the jaws parallel, and the probe perpendicular or you will get measurement errors - this is a very precise instrument.
    DOn't overmeasure, the calipers show 1/1000th inch but in woodworking getting within .01" or 1/100 is usually quite good enough for assembly. Wood will shrink and grow a few thousands with temperature and humidity.

    here's links to HF calipers (normal prices, not sale prices)
    Fractional: http://www.harborfreight.com/6-digit...ngs-68304.html

    inch and mm:http://www.harborfreight.com/6-inch-...per-47257.html

    • Carlos
      #36
      Carlos commented
      Editing a comment
      Since they only cost 35 cents each, I just keep a pile in the garage.

    • tfischer
      #37
      tfischer commented
      Editing a comment
      I have several digital measuring tools: a couple calipers, a thickness gauge on my planer, a couple angle gauges, etc. All of them have this problem: they use batteries whether you are using the tool or not. It's VERY frustrating to me.

      And it seems very wasteful to keep throwing them out. I'm not sure I even get "a few months" out of mine before the screen gets very dim and/or starts blinking on/off (which seems to be some sort of low battery indicator)

    • LCHIEN
      #38
      LCHIEN commented
      Editing a comment
      I buy LR44s by the 20 pack. Still even at 20 cents or so it seems wasteful to replace the everytime I get out a tool if I haven't been very active for a few months.
    Posting comments is disabled.

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