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Modify Your Swingweight Scale to be Digital

Dave Tutelman -- May 28, 2016

After I posted instructions for making your own digital swingweight scale, I started getting questions about modifying a conventional swingweight scale to be digital. Yes, it can be done, and here is how.

In the past month, I've gotten two inquiries how to convert perfectly good conventional swingweight scales into digital instruments. So, Randy LaFoone and Russell Platt, here is how to do it.

Randy and Russell both have Golfsmith swingweight scales. Neither of them specified which model, so I'll assume it's the Auditor shown below (made by GolfMechanix and sold by Golfsmith). It's a very conventional design, and I'll use it as illustration of how to do the conversion to digital. If you understand the instructions for this scale, you should have little trouble modifying most other analog swingweight scales.

Let me suggest that, before you start, you look at my article on building a digital swingweight scale from scratch. You will need to know how to use and read your digital swingweight scale, and it would help to understand the theory behind it. You should also understand the construction of the arm, even though you are modifying an existing arm. By the way, I have not tried these modifications myself on any analog swingweight scale. What I say here is valid instrument engineering, but the tooling and construction required for your machine is up to you.

The digital scale pressure point

The first thing you need to do is to attach a point (screw head, knob, some protrusion) to press on the digital scale you will use as a readout.

Where the pressure point touches the digital gram scale must be between 4.96 and 5.00 inches from the beam's pivot, or fulcrum. This measurement is the horizontal measurement from the centerline of the pivot to the place where the pressure point touches the digital scale, with the beam level. That's a total range of only 1/25 of an inch, from 4.96 to 5.00. It is the most critical dimension you have in this project.

I don't know if the beam on the Auditor -- or on any other scale, for that matter -- can be drilled and threaded from the bottom, as the picture assumes. If not, you'll have to find another way to mount a pressure point. (Epoxy? A round-head pin in a tiny hole? Something else?) However you do it, the 5-inch measurement must be as described above.

With that measurement and a proper calibration, here is how to read digital swingweight. (It is copied from my article on the DIY digital swingweight scale.)

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.

Whatever you use for the pressure point, it will have some weight. The weight will affect the readings, both for the digital scale or any continued operation as an analog scale. How much? Probably not a lot, maybe not even enough to detect reliably. If you use a #8 machine screw, it weighs about 2 grams. A quick estimate suggests that will make everything read a fifth of a swingweight point higher than it should. So when you calibrate, set the sliding scale a fifth of a point light -- if you can set it with that resolution; frankly, I doubt you can. Consider:

I have seen very few analog swingweight scales with a precision of a fifth of a point. (Probably none. I've only seen one I can read to that resolution, and it is not repeatable to more than a half a point.)
My own digital swingweight scale, designed from scratch to be digital, is no better precision than a fifth of a point.
Private correspondence with Alex Dee of Fujikura (who is working on perhaps the ultimate digital swingweight scale) suggests that they view a fifth of a point as the limit of precision for swingweight in any practical shop instrument.

So the weight of the pressure point is probably not an issue. And it may become even less of an issue when you decide what you want to do for calibration. Next up!

Calibration

Calibration is simple; just move the sliding weight so it reads a swingweight of A-0 on the slide. If you can do that quickly and accurately, you are done with this step. The rest of this chapter is how to still achieve speed and accuracy if you can't (or don't want to) calibrate by hand every time you use the scale.

(Note: There is another way, probably superior, to find the proper calibrated position of the sliding weight. But it is worth reading and understanding this method first. Read the whole page before actually doing any modifications to your swingweight scale.)

The picture shows where the left edge of the sliding weight has to be in order to match the A-0 indicator on the beam. If the weight is in exactly that spot, then the scale is calibrated for digital use. So it would seem convenient to mount a stop there. Just slide the weight against the stop, and you know the digital scale is calibrated properly.

I have shown a machine screw threaded into the beam to act as the stop. Note that, unlike the pressure point screw, the line indicates the rightmost edge of the screw instead of the centerline. Remember this when you drill the threaded hole. Accuracy counts! You can see by the engraved scale on the beam how much of a swingweight error you will get from any error in placing the stop.

If you never intend to use the scale again for analog weight measurement, you can just epoxy a stop into place. Anything hard and light will do, as long as it is thick enough to prevent the weight from sliding past it. It needs to stop the weight sharply and precisely in the A-0 position.

In fact, if you never intend to use the scale non-digitally again, you could simply cement the sliding weight at the A-0 setting. I would not do that, simply because it is irreversible. But whatever your decision, don't act on it until you read the next section on removing the tray.

How much of an error does the weight of the stop introduce? Assuming it is roughly the same weight as the pressure point, it introduces roughly the same error. With this swingweight scale, where the stop is closer to the fulcrum than the pressure point, it introduces correspondingly less error for the same mass. Even better, that error is in the opposite direction from that of the pressure point; it reduces the error from the pressure point.

Removing the tray

The Auditor has a tray, so you can use it to weigh components, not just measure swingweight. Yours may also have a tray. And the tray may be in the way of the digital scale. Here is what you have to do if you want to remove the tray for digital operation.

The first step, of course, is to remove the tray. But the tray had some weight, so its absence will affect the swingweight readings. Unlike the pressure point (where the error was only a fifth of a point), removal of the tray will introduce an error of one or several swingweight points. For instance, let me guess that the Auditor's tray weighs an ounce. Its removal makes a difference of about 200 gram-inches, or four swingweight points. Way too much to ignore!

We can account for loss of the tray's weight by changing the calibration point. This picture shows a green line to the left of the original red line. It has to be to the left, because the sliding weight has to make up for the loss of the tray's weight left of the fulcrum. So the question is, how much to the left should it be?

You can figure it out by using the scale (analog, as intended, with the sliding weight) to measure the swingweight of a club. Pretty much any club will do. Measure both with and without the tray in place. Measure the difference in the sliding weight's position with and without the tray. That is exactly the difference between the red and green lines in the picture.

Having determined the position of the green line, install the stop to calibrate the scale without the tray.

Here's another alternative, in case you were planning to hand-set the sliding scale at A-0 to calibrate it every time you use it. Take the red-to-green distance you just determined, and make a mark on the sliding weight that distance to the right of the mark you use to line up with the graduations on the beam. Then to calibrate with the tray missing, line up your new mark at A-0 graduation.

An alternative calibration method

Here is alternative way to calibrate your modified swingweight scale. It is superior to what came before. But it was worth reading and understanding the previous sections before starting the modifications, because you will better appreciate how the measurement works. Now here is the procedure for the calibration approach I prefer.

Before you do anything to your swingweight scale -- anything at all -- take a club and set it aside as a calibration club. It can be any club, as long as you trust you can get a stable, precise measurement of its swingweight on your original, analog swingweight scale.
Measure the swingweight of the club on the original scale. Write it down and don't lose it. This will be a key number, the calibration number, for the scale and that club forever after.
Make whatever modifications to the scale you intend to make. Add the pressure point. Remove the tray. Make any other changes, except... Do not add the calibration stop, because you don't yet know where to put it.
Set up the scale as a digital swingweight scale, with the pressure point resting on the digital gram scale. Place the calibration club in the scale and note the reading of the digital scale. Move the sliding weight unitl the digital reading corresponds to the calibration number you measured earlier.
Mark the position of the sliding weight, and set up any stop so it positions the sliding weight exactly where you marked it. Now your scale is calibrated.

Glitches from the field

Both Randy and Russell modified their swingweight scales for digital readout. Here are a few things we have learned the hard way, based on glitches they encountered along the way.

The fulcrum bearings have to be pretty good. Randy and Russell have the same Golfsmith swingweight scale, an older model than the one in the pictures above. It has a bearing that is a metal (steel? probably) rod seated in a semicircular recess in the pedestals. (See Randy's photo at right.) Both experienced inconsistent readings, and traced it to the bearing. Russell happened to have ABEC bearings around, and replaced the sloppy semi-sleeve bearing with ball bearings. That solved the problem. Randy solved the problem another way; he used steel wool and cleaned up the steel pin and the recess for smoother operation, along with a wipe of Tri-Flow lubricant.

How did Golfsmith ever sell a scale with this problem? Remember that the digital scale is easy to read to a tenth of a swingweight point, while the analog scale as sold never claimed to have a precision tighter than one point. (Note: while you can read the gram scale that closely, the swingweight instrument is really only precise to two tenths of a point. Still, that's much more precise than Randy and Russell's bearings before they fixed them.) The bearing introduced enough friction for the force on the screw head to be off a good ten grams. That's a full swingweight point, easily noticeable with a digital readout, but hard to see looking at tick-marks on the analog scale.

The pressure screw should apply force directly on its centerline. Randy and Russell both ignored a detail from the page on arm construction from the original DIY article. The screw that exerts a force on the scale should be a round-head screw, with the slot aligned parallel to the arm. That sounds anal-retentive, but there is a reason for it. You have gone to the trouble of placing the screw's centerline at exactly 5.00 inches from the fulcrum, and oriented it exactly perpendicular to the arm. The reason for the precision is to assure that the contact point where the force is exerted is exactly 5.00 horizontal inches from the fulcrum.

The picture at the right shows Randy's pressure point. The screw has a Phillips head, with a recess in the center. So the force is never exerted at the centerline of the screw. The red lines emerge from the "high points" of the head, which is where the force will be applied. There is a "dead zone" of almost a tenth of an inch, where the force is never applied -- the center of which is where it should always be applied.

Russell used the tip of the screw instead of the head. But most machine screws have cupped tips, so the problem is nearly identical. Russell fixed the problem by grinding the tip of the screw to a point, so the force is always on the centerline. Randy used the same fix, reversing the screw head-for-tip and filing the tip to a point.

This will not result in a huge error, but it is an error that is so easily avoidable. The error is something like one or two tenths of a swingweight point for every ten points on the scale, so it is within the precision limit of the instrument for 99% of the measurements you will ever make. But it is so easy to get it right, it's a shame not to.

The arm must be level. Another point in the original DIY article that you might miss: the pressure point screw's height should be adjusted so that the arm is level when the screw rests on the digital gram scale. Randy and Russell both applied bubble levels to the base of their instruments. It is at least as important to check that the arm is level when taking a measurement.

Conclusion

Hope this serves you well if you modify your swingweight scale for digital readout.

Randy and Russell are happy with their newly-digital swingweight scales.

Last modified - June 29, 2016