Do-it-yourself Digital Swingweight Scale

Using the scale

Dave Tutelman -- May 9, 2015

Measuring swingweight



If you are familiar with conventional (non-digital, sliding weight) swingweight scales, this should be pretty familiar in operation. 

You place the club in it as you would any swingweight scale. The difference is that you read the swingweight on a digital display, instead of balancing the arm with a sliding weight.

For normal operation, the gram scale's meauring tray should be centered under the forward round-head machine screw. When the club is placed on the instrument, that screw applies a force to the gram scale.

Reading swingweight

The display is a digital gram scale. The scale in the picture reads to a tenth of a gram, but the last digit is wasted. You will use the reading to the nearest gram.

There should be no club in the swingweight scale when you press the "on" button of the gram scale. We want the zero reading to be a true zero. But if you happen to forget, and turn on the gram scale with weight on it, just remove the weight and press the button labeled "tare" or "zero"; every digital gram scale has such a button.

The display does not give a lorythmic swingweight like "D-1" directly, but reading it is very easy once you do it a few times. The output is in grams, and a swingweight point is exactly ten grams. Zero grams is a swingweight of A-0. So here is a table of the usual swingweights.

Swingweight Grams
A-0 000
B-0 100
C-0 200
D-0 300
E-0 400
F-0 500

Example: the reading in the picture, 352.0 grams, corresponds to a swingweight of D-5.2. That is:
  • The first digit, 3, means we are in the D range.
  • The rest of the digits, 52, means we are 5.2 points into the D range.

In my experience, this instrument is precise to about a fifth of a swingweight point, or 0.2 points. It may give a resolution higher than that, but the reading is not repeatable to better than 2 grams on the scale. Therefore, the extra resolution does not show up as precision. The precision cannot be improved by a more precise gram scale; the problem is not measurement of force, but the measurement of swingweight itself. First Walter, then I, discovered that on every variation of every model we made. Here are some of the factors that may limit precision, and there may be more:
  • The ball bearings that make up the fulcrum of the swingweight scale are very low friction. But they are not zero friction. The small residual friction may be providing small amounts of force that would otherwise be provided (and measured) by the gram scale. In fact, Walter and I tried various kinds of bearings -- two kinds of ball bearings and a polyethylene sleeve bearing -- with no better results.
  • The shaft is not perfectly rigid. It vibrates. There are small vibrations that have a tiny -- but measurable -- effect on the instantaneous force on the gram scale. (I would not have even thought of this one, but Alex Dee of Fujikura Shafts, who is himself working on an advanced digital swingweight scale, told me they encountered this problem. Their digital filters limit the effect of vibration, but it still prevents the precision from being much better than a quarter point of swingweight. That number is close to the 0.2 points I have observed.)

Placing the club on the scale

Place the club on the scale, then wait briefly for the scale to settle before you take the reading.

Don't drop or push the club onto the scale. Place it relatively gently. A thumping drop establishes a too-high initial force. The result can be a slower response of the gram scale, and perhaps even a too-high force once it settles. You don't have to be very delicate, but you must not be abrupt nor rough.

Measuring a completed club:
Place the butt of the grip against the butt stop, under the two pins. It is now positioned correctly, and hooked in so it can exert force on the arm of the instrument.
Measuring without a grip:
Much like the completed club. Place the butt of the shaft against the butt stop, under the two pins. It is now positioned correctly, and hooked in so it can exert force on the arm of the instrument.

The position of the pins has been chosen so that they support the club in a good, measurable position with or without the grip.
Virtual butt-trimming for swingweight:
Few people do this, because few swingweight scales are set up to facilitate it. I had modified my traditional scale to do it, and designed this one to make it easy.

The point of this is to measure what the swingweight would be if the club were shorter due to butt-trimming. You can position the club with a "virtual butt trim", and slide it around until it reads what you want. You now trim at the inner face of the butt stop.

To position the club, slide the gripless shaft through the large hole in the butt stop. You can now virtually trim quite a few inches from the club, just by sliding it further through the hole.

This does not give a completely accurate swingweight, because the shaft sticking out beyond the butt stop has a little weight. Every gram of shaft weight beyond the butt stop subtracts rough a third of a swingweight point. So you have to add that swingweight back to the reading to get the swingweight of the trimmed club. There's a table below in a math notes box, so you can see what the numbers look like.

But it's actually much easier than that. You really don't need to use any rules of thumb or do any math if you're using a cutoff saw to butt-trim the shaft. You just have to be willing to make two cuts. (Maybe three for steel shafts.) Here's how to do it:
  1. Slide the shaft in the hole until you measure the target (gripless) swingweight.
  2. Mark the shaft using the inner edge of the hole as a marking guide.
  3. Cut off the end of the shaft at the mark. (Be sure to allow for the thickness of the marker if it was not very sharp.) You will not have cut off too much, because the end hanging through the hole assures that the swingweight will be higher than measured once you have cut it. So it may still be a little long, but definitely not too short.
  4. Measure the cut club normally, with the shaft against the butt plate and under the pins. If it is within the instrument's tolerance (say about 0.2 swingweight points), you are done. Otherwise, the swingweight will be higher than the target swingweight. Go back to step #1 and repeat the steps.
With graphite shafts, it has not taken me more than two cuts -- two passes through the steps. Much easier than calculating using the table below. Even with steel shafts, it should not take more than three cuts, and very possibly two.

Shaft material Shaft
weight (grams)
Shaft length
(inches)
Grams
per
inch
Add this swingweight to get true effect of trimming.
Shaft extends...
1 inch 2 inches 3 inches 4 inches 5 inches 6 inches
Steel 120 40 3.00 0.9 1.8 2.8 3.8 5.0 6.1
Lightweight steel 100 40 2.50 0.7 1.5 2.3 3.2 4.1 5.1
Medium graphite 80 41 1.95 0.6 1.2 1.8 2.5 3.2 4.0
Lightweight graphite 60 46 1.30 0.4 0.8 1.2 1.7 2.2 2.7

If you're looking for a rough rule of thumb, add one swingweight point per inch for steel shafts, and half of that for lightweight graphite.

Calibration

Calibration is rather simple. With no shaft loaded in the instrument, move the gram scale so it is centered under the rear round-head machine screw, and adjust the counterweight until you get a reading of 500 grams on the gram scale.



The counterweight is a inch hex bolt, plus whatever nuts and washers you choose to add for weight. Do what you need to do to get the scale to read 500 grams. At that point, your swingweight scale is calibrated. Here are some hints about how certain components affect the reading:
  • A hex nut added to the middle of the inch bolt will increase the reading  by about 22 grams.
  • Screwing or unscrewing the hex nut the length of the inch bolt will change the reading by about 4 grams.
  • The hex bolt itself can be either 2 or 3 inches long. The difference will change the reading by about 12 grams.
  • Washers or lockwashers added to the inch bolt are lighter than a hex nut, and will have proportionally less effect.
  • A nylon-insert locking nut is slightly heavier than a hex nut, and can be counted on to stay securely where you put it. I have not found a locking nut necessary in my shop, but you might want one if you travel with your swingweight scale; otherwise, vibration in transit may throw off the calibration.
For reference purposes, being off by 1 gram on the calibration introduces an error of about 0.2 swingweight points, roughly the precision of the instrument itself.

Last modified - Sept 17, 2015