Do-it-yourself Digital Swingweight Scale

The working parts

Building the arm

Let's start this part with the CAD drawings of the arm. Click on the thumbnail for a PDF of the drawing. If you have a CAD program and want to download a working CAD file, here is what my TurboCAD was able to save as a  DXF file. The PDF includes a 3D rendering of the assembled arm, a dimensioned orthographic drawing of  each piece, and a page of notes. There are three aluminum parts you will have to machine; the rest of the arm is assembled using standard hardware. The arm itself is made from a 19.5" length of 1.5" by 1/2" aluminum bar. The butt stop is made from a 3.75" length of 1.5" by 1/8" aluminum bar. The front support is made from a 2.25" length of 1.5" by 1/8" aluminum bar. The aluminum I used was extruded aluminum rectangular bar stock, alloy 6061-T6511, from Online Metals. Walter gets his aluminum from Metal Supermarkets. Both stores will cut it to length for you. But you can do a better job (and save their cutting charge) with a circular saw and a good carbide blade with lots of teeth. The carbide blade should be rated for non-ferrous metals, and most are. A power miter saw or radial arm saw will give better control than a hand-held carpenter's saw. Be sure to take all safety precautions, especially safety goggles, when you cut aluminum. It cuts a lot like wood, so visualize metal chips flying all over like sawdust.

You will need a drill press and careful measurements for three of the holes: the one for the bearing bolts and the two for the #8-32 round-head machine screws. These holes must be precisely positioned and exactly square. This figure shows the holes needing precision, highlighted in yellow. If the position or squareness of these three holes is off, there will be swingweight measurement errors. In particular, the positioning of the 8-32 threaded holes must be correct within 1/32 of an inch. If you are off by a bit but can measure the actual dimension you drilled, you may be able to offset some of the damage through calibration. Here are the considerations: The 5.00" dimension will be OK if it is slightly less than 5.00". If it is within the range of 4.96-5.00 inches, the errors can be kept to no more than the difference of opinion between designers of swingweight scales. (Some use 50 inch-grams for a swingweight point, and some use 1.75 inch-ounces. Details in the section on theory.) If you are using a ruler that does not give decimal, 9.1" is between 9-1/16" and 9-1/8", just slightly more than midway between. If you have set it to exactly 9-3/32", you are less than a hundredth of an inch off. If either dimension is off, you should fix the calibration. Instead of calibrating to exactly 500 grams, select your calbration number from the table below. Measurement length (nominal 5") Calibration length (nominal 9.1") 9 9.03 9.06 9.09 9.12 9.15 4.94 507.8 506.1 504.4 502.8 501.1 499.5 4.97 506.8 505.1 503.4 501.8 500.1 498.5 5 505.8 504.1 502.4 500.8 499.1 497.5 5.03 504.8 503.1 501.4 499.8 498.1 496.5 The other holes in the arm are not as critical. I used a drill press for them on one of my models and a hand drill for the other; the difference is not noticeable. Once the main aluminum pieces have been machined, the arm can be assembled.

The front support has a shallow rounded groove to rest the shaft of the club being measured. I cut it first with a hacksaw to set the centerline and depth, then widened it with a round file, and finally widened it further with a half-round file. It is important to get it smooth, since any sharp edges or jaggies may scratch the finish on a graphite shaft. It is attached to the arm with a pair of #10-32 machine screws. The ones shown had built-in lock-washers, but that is not at all necessary.

The butt stop is similarly fastened to the arm with 10-32 machine screws. The large .65" hole used for "virtual trimming" must be big enough to easily pass the butt of a shaft. Except for that criterion, the exact size is not very important. Since I don't have drill bits for metal of that size (not many do), I drilled a 1/2" hole and enlarged it with a tapered reamer. You can see from the reflections that the hole is tapered. It is important to run the reamer from the back, not the front. You want the shaft to be supported on the inner edge of the hole. The pins for holding the grip in place are #8 machine screws that are 1/8" longer than a pair of cylindrical spacers (the brown things in the photo). I used knurled-head screws with heads about the same diameter as the OD of the spacers. The two prototypes I built used different combinations: 1/2" screw with 3/8" spacer. 5/8" screw with 1/2" spacer.  Both combinations worked fine. At the bottom of the butt stop is the hex bolt for the counterweight, which we discuss next.

Here's a better look at the counterweight bolt. The arm is upside-down, so you can easily see what is on the bottom. The counterweight is tightly secured to the butt stop with a hex nut. The other hex nut and the lockwasher are just ballast; they are chosen for their weight and placed during calibration. The bolt may be 2½ or 3 inches long, depending on the weight you need. The round head machine screw is one of two, one in the hole at 5.00 inches and this one in the hole at 9.10 inches. It exerts a force on the gram scale, which creates the reading for either calibration (this screw) or measurement (the other screw). It has a nut which is tightened to lock it at the proper height. That proper height is determined by what is necessary to keep the arm level while the round head rests on the tray of the gram scale. Note that the slot of the round-head screw is parallel to the long axis of the arm. That is not an accident; it helps accuracy. Why be finicky about the position of the hole if this contacts the gram scale with an uncertainty of the width of the slot? So be sure to orient both round-head screws with the slot lengthwise on the arm.

The pivot, or "fulcrum", of the arm is implemented by a pair of skate wheel bearings. If you look up ABEC bearings on the Internet, you will find them for sale at a wide variety of prices. Don't be scared by the high prices for some of them. You will also find sets of 8 perfectly respectable bearings for less than $10, quite OK for our purposes. (You'll only need two bearings, not all eight. But it's amazing how many golf instrumentation projects can use bearings like this. My NeuFinder 4 shaft deflection instrument uses 14 of them all by itself.) The bearings fit very nicely on a 5/16-18 bolt. Secure each one on its own bolt with a hex nut, then screw the assembly into the threaded hole in the arm. Here are a few things you can free-lance about this part of the instrument: If you have suitable bearings that take a different diameter bolt, then drill and thread the hole for those bearings. The hole diameter and thread are not critical dimensions, so get them right for the bearings you use. While the hole must be drilled through, you don't have to thread it all the way through. Just thread deeply enough from each side for the bolts you use to secure the bearings. But you must drill the hole all the way through. That assures that the bearings are on the same axis, because a single hole has a single centerline. If you drill separately from each side and the holes do not line up perfectly on the same axis, there may be a bit of binding force in the bearings. This force will be hard to detect, but it will behave like bearing friction and will reduce the precision of the instrument. Now the arm is complete.

The gram scale

The other working part for the swingweight scale is a gram scale. Here are the specifications it must meet: A range of at least 600 grams, and preferably somewhat more. An accuracy of at least one gram. Anything beyond that is optional. A higher measurement range allows measurement above a G-0 swingweight -- in case you ever happen to run across such a club. Better accuracy is not usable; a gram is a tenth of a swingweight point, and the whole instrument will not do better than two tenths of a point. But I should mention that I could not find a digital gram scale for less money than I spent on this one, and it measures to a tenth of a gram. The major additional, optional specifications are price and size/shape. I found plenty of scales on eBay that would work, priced at less than $12. in fact, Walter and I both use a scale that is listed "Buy-it-now" at six dollars and change. There have been several vendors offering this same scale for the past six months (as of early 2015). If you pick a scale of a different size or shape, you may need to reflect that in the dimensions of the base assembly. Feel free to do so; the base is not a critical part of the measurement chain.