Modify Your Swingweight Scale to be
Tutelman -- May 28, 2016
After I posted instructions for
making your own digital swingweight
scale, I started getting questions about modifying a conventional
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
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.
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
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
With that measurement and a proper
calibration, here is how to read
digital swingweight. (It is copied from my article
on the DIY digital
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.
Example: the reading in the picture, 352.0
grams, corresponds to a swingweight of D-5.2
- The first digit, 3,
means we are in the D
- The rest of the digits, 52,
means we are 5.2
points into the D
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
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.
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!
- 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.
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
to) calibrate by hand every time you use the scale.
(Note: There is another
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
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
an ounce. Its removal makes a difference of about 200 gram-inches, or
four swingweight points. Way too much to ignore!
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
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
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
alternative calibration method
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
-- 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
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
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.
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
Glitches from the
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.
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.
pressure screw should apply force directly on its centerline. Randy and Russell both ignored a detail from the page on arm
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 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
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
Hope this serves you well if you modify your swingweight scale for
Randy and Russell are happy with their newly-digital swingweight scales.
Last modified - June 29, 2016