Do-it-yourself Digital Swingweight Scale

Building the base

Dave Tutelman -- May 9, 2015

As noted earlier, the arm is where the critical tooling was done, critical to the measurement accuracy anyway. And we have finished that part. The base can be free-lanced, as long as everything fits and nothing binds nor interferes. Here are three base ideas. All three have been built (hey, we have photos) and result in instruments that work well. You can build any one or mix 'n' match.
  • The "standard base", a wooden base that is relatively easy to build. I have CAD drawings and tooling/construction details listed for this one.
  • A wooden alternative to the standard base. I built it because I wanted a smaller footprint allocated to swingweight in my shop, and a slightly higher-tech look. It's harder to build than the standard base; you really need a drill press. I won't present CAD drawings nor detailed instructions, but I'll say enough about it so a good craftsman with the right tools can make it.
  • An aluminum base. Walter prefers to work in aluminum, and this is the swingweight scale he has in his shop.
  • Yet another wooden alternative base, added after Carlos Mendiola built his own from the plans.

Standard base


Let's start with the CAD drawings. Again, we have a PDF (click the thumbnail) and a downloadable DXF.

I made my standard base out of poplar hardwood, in the readily available 1-by-6 size. (The actual dimensions of 1-by-6 lumber is 3/4" by 5.5"). I made a similar base of maple, and it tooled and worked just as well. I imagine oak would be an equally good choice. If you want to get exotic and use mahogany (which was a favorite of instrument makers 80 years ago), go for it!

I made a small (less than 1/8") chamfer around the top of the base. It was completely cosmetic, not at all necessary. If you are really squeezed for real estate, you might want to skip the chamfer. It does increase the footprint of the instrument; the flat top surface of the base must be big enough for the gram scale and preferably a rubber bumper under the butt stop.

In the CAD drawings, the pedestals that support the bearings are at the proper height for the gram scale that Walter and I used for our swingweight scales. (See the pedestals in the photo.) Because the force is transmitted from arm to scale by the round-head machine screws, there is a good half inch of latitude in the height of the scale tray. But it is possible that a different scale may require a taller pedestal.

The biggest effort in making the base is fabricating the pedestals -- the saddle and cap -- for the bearings. It requires several steps, and the steps and the necessary and logical order they are in are not always obvious. Let's go over how to do it, step by step.
 
Each pedestal is made from a block of hardwood. My standard base uses a block 2.5" by 2.5" by 0.75".

The first step is to drill a hole the size of the bearings' outer diameter. In the case of the ABEC bearings, a diameter of 7/8" is just about perfect. You can do it with a spade bit or Forstner bit. (The Forstner bit makes a cleaner, smoother hole, but it is expensive if this is all you ever use it for.) The most important thing is that the hole be drilled square to the block. A drill press is helpful here, but not essential. The squareness of this hole does not need to be perfect.
Next, we're going to drill pilot holes for the bolts that hold the cap to the saddle. Again, a drill press is helpful but not essential.

We will drill these holes as deep as we can. But we are limited by the length of the drill bit, and perhaps even more limited by the travel of the drill press if we used one. So we drill them as deep as we can. The bottom must be considerably below the  middle of the large bearing hole. If it is just slightly below, a drill press becomes more important, because we want the finish of the pilot holes to follow the same line, so we want some depth to provide a guide later.

How big a diameter should the pilot hole be? The most critical determinant is the bit you are going to use later, to counterbore a 3/4" diameter hole in the base, from the bottom. Whether a spade bit or a Forstner bit, it will have a pointed tip to center the hole. The pilot hole can be no larger than this tip is wide, so the tip does not wobble in the pilot hole that guides it.
  
Now we use a saw to slice the pedestal into two parts, the cap and the saddle. Most saw blades are about 1/8" thick, so the slice will be about 1/8" wide. (That is called the "kerf".) It is not wide enough, so you will probably have to make two cuts: one for the bottom edge and the other for the top edge. Why does it need to be so wide? Because a 1/4-20 hex nut has to fit flat in the space vacated by the wood, and it is 3/16" thick; that is the minimum amount the saw blade has to remove.

Before you go any further... Mark the two saddles and two caps, so you know which goes with which, and in which direction. The key here is that the pilot holes were drilled through and must line up. If you put a cap with the wrong saddle, or even turn the right cap end-for-end, it may not line up well enough. So mark them to be sure you know how they are supposed to fit.
 
Before finishing drilling the pedestals, we are going to glue the saddles to the main board of the base. We do this with the completed arm resting on the saddles, as shown in the photo. That way, we can be sure the saddles are placed so that:

» They support the arm exactly parallel to the edges of the base.

» They are centered on the base.

» They are separated enough for the bearings to be approximately centered on the saddles, yet the arm does not bind against either saddle.

»  The butt stop rests on the top surface of the base. It doesn't overhang or rest on the chamfer.

Once the glue cures, we can drill the remaining holes through both saddle and base so they are guaranteed to line up.
 
Finish drilling the pilot holes. Drill from the top of the saddle, using the blind holes already existing as a guide. Be sure you are continuing the holes in exactly the same direction they were originally. They can guide the drill reliably if they were deep enough before.

They should extend all the way through the bottom of the base.
Counterbore from the bottom of the base, using the pilot holes to guide the 3/4" bit. Keep checking to drill exactly 1/2" deep -- or use a depth gauge if you have one. (If you are using a drill press, definitely use a depth gauge.)
Finally, ream out the pilot holes to accept the 1/4-20 by 3" carriage bolts. The holes in the saddle must provide clearance. The holes in the cap should be a little bigger, to cover any imperfections in matching the direction and position of the holes.
Run 1/4-20 carriage bolts through the saddles from the bottom. When their square shanks are firmly against the clearance holes, spin on hex nuts and tighten them with a driver or wrench. This pulls the shank of the carriage bolt into the hole and locks it from turning.

A nice touch is a rubber foot attached to the base where the butt stop strikes it. You can see it in the photos from a previous page. Without it, you'll get tired of hearing the "Bang!" of metal hitting wood when you take the club out of the scale. The rubber makes it feel and sound very solid.

The base is now ready for finishing. I just gave it a couple polyurethane coats, but staining and even painting also works. Whatever you think looks good!

Once the finish cures, place the arm in the saddles. It is easier to calibrate the swingweight scale without the caps, because you can add and adjust weights to the counterweight bolt more easily by removing the arm and turning it over. (That is not necessarily the case with the alternative bases, but it works fine with the standard base.)

Now add the caps and secure with 1/4-20 locking nuts. You are ready to use your swingweight scale.

Alternative designs

Since I built my first digital swingweight scale, plenty of others built instruments from these plans. (In fact, even I built another one.) Most of the freelancing was done on the base. That's appropriate, beacause the base is not directly in the measurement chain. But there were even some functional changes worth talking about.

Wooden (Dave)

Much as I liked the look and operation of the prototype swingweight scale -- with the standard base -- I wanted a few differences. So I made a second prototype. What differences did I want?
  1. A narrower base. The spot in my shop allocated to storing my swingweight scale does not have room for the footprint of the standard base.
  2. A more "industrial" look. Extensive wood for the base gives the digital instrument an anachronistic antique appeal. I like it, but wanted something that looked just a little more like lab equipment.
I got the next narrower piece of hardwood, a 1-by-4 in maple (actually 3/4" by 3.5").  That would fit my space just fine. However, the gram scale hangs off the side of the base instead of resting comfortably on it. So I had to secure it to the base.

The way I got a more high-tech look was to use a plated U-bolt to hold the bearing in the saddle, instead of a wooden cap. This was more demanding to build, but I like the result. Here is another picture, before I go into detail on the construction challenges.



I secured the gram scale to the base by epoxying a threaded aluminum plate to the bottom of the scale. Then I bolted the plate to the base with a flat-head machine screw.

The gram scale I used has a battery compartment that opens via a door in the bottom. I removed the door, exposing a flat area directly under the center of the weighing tray. That was very convenient, allowing the following modification:
  • I cut a piece of 1/8" thick aluminum (we have some left over from the arm) that would fit the flat spot. I drilled and threaded a #8-32 hole in the middle of the aluminum plate.
  • I carefully drilled a hole in the flat plastic. The hole was
    • Centered, identically to the threaded hole in the aluminum plate.
    • Larger than a clearance hole for an 8-32 machine screw.
    • Only through the plastic, not into the working mechanism of the scale.
  • I epoxied the plate to the flat section of plastic, being careful not to let any epoxy get into either hole.
When the epoxy cured, the result was a strong threaded insert with enough flat space to provide a stable mount for the scale on the base, if held there by a machine screw. The threaded hole and the hole in the plastic combined to allow up to 3/16" of machine screw tip.



This picture of the bottom of the base shows a countersunk hole for an 8-32 flat-head machine screw to hold the scale. Two such holes are needed, one for the scale in the measurement position and the other (not in the photo) for calibration. Each hole is just deep enough to expose 1/8" of threaded tip above the surface of the base; more than 3/16" may interfere with the operation of the gram scale.

The picture also shows counterbored holes for the nuts that hold the U-bolt in place, which brings us to the hardest part of construction. Using a U-bolt instead of a wooden cap resulted in quite a few things that made precision tooling more important -- things that really require a drill press rather than a hand drill.
  1. The U-bolt requires holes that are absolutely parallel. I would not think of building this base without a drill press, if for no reason other than this.
  2. I shopped for a U-bolt, and the best I could do for the right width was a bit too short for comfort. That required lowering the arm; I cut almost 1/8" off the height of the saddle.
  3. In order for the lower arm to work, I had to shave off 1/16" from the bottom of the butt stop. Otherwise it would interfere with the base when the arm was level. I also needed to make a lower rubber bumper for the the butt stop. So instead of a commercial rubber foot, I used a small rubber grommet and counterbored the base so just enough of the grommet was above the surface.
  4. Even with these changes, the counterbore from the bottom had to be deeper than before. On the standard model, the bottom of the counterbore was 1/4" from the top surface of the base (that is, a 1/2" counterbore in 3/4" thick wood). For this model, I had to get the counterbore within 1/16" of the top surface. A good depth gauge -- the one on my drill press -- was essential here.
  5. The U-bolt is a lot thinner than the wooden cap was. That means that the bearings have to be really centered on the saddle blocks, or the U-bolt will not hold the bearings in. The result is that the saddle blocks have to be closer to the arm. They come so close, I have to use a business card as a shim when tightening down the U-bolts. Otherwise, there would be rubbing friction between the arm and the pedestal.
This combination of factors makes the narrow, industrial-look base harder to build than the standard base.


Aluminum (Walter)

I like working with wood, and I like the look of a wooden base and polished metal for an instrument. Walter prefers working with aluminum, and likes his instruments to be painted metal. So he built his base of aluminum, and painted it black.

Here are a few pictures of what he did. They are thumbnails; click on them for a closer view.

This top view of the swingweight scale shows a few clever features that I left out of my models.
  • A pair of screws to act as a positioning guide for the scale. Just set it down against the screws, and you know the round-head machine screw is centered on the weighing tray.
  • A level for the base. I trust my workbench surfaces to be level, but you can never be too careful. Note that I did use a level on the arm (not the base) for setting the depth of the round-head screws.

Other than that, it also incorporates Walter's signature shiny black finish.
 
Here is a closeup of how the arm is mounted to the base. Walter uses a Pex Talon pipe clamp to hold the bearing, and screws it to an aluminum platform just below the arm. The platform is mounted to the base with bolts threaded into the base, and metal standoff spacers to control the height of the platform, and thus the height of the arm.

A word of warning: Walter did not use skateboard bearings. He had a pair of bearing assemblies on hand. The race on his bearing has a smaller outer diameter than the ABEC skateboard bearing. That made it easy to find a Pex clamp that fits. A Pex clamp for a 7/8" OD pipe is not easily found; I'm not sure it even exists. But there are certainly other models of pipe clamp that could be used.

Wooden (Carlos)



Carlos Mendiola emailed me, saying that he had built a swingweight scale of his own from the plans. If you look at the photo he sent along, you'll see he freelanced the base. It's a nice piece of woodwork, and yet another way to mount the bearings. He left the support blocks as plain rectangular solids and held the bearings with a ½ inch conduit clamp. Of course, this clamp only works for a smaller bearing than the skate wheel bearing. Carlos also added a bubble level, a nice touch that Walter initiated; see Walter's aluminum base below.

It works, too. Carlos' email said, "I took a couple of my clubs in to get measured, and the scale I built is spot on."

Wooden (Darrell)



Darrell Dorsey is something of a woodworker. He likes the ability of plywood to lay flat; wooden boards can warp, and you don't want warp in this instrument. But plywood edges don't look very finished. So he built his base out of cabinet-grade birch plywood and made edges of solid poplar. Looks really nice, doesn't it?

You will notice a different model scale from what most of us used, though the specs are the same.

Metric (Maxime)



Maxime Frysou lives in France; that means his local sources of parts and materials are metric. So his metal for the arm, as well as the fasteners, are metric sizes and not quite the same as the plans call for. The biggest difference here is that the arm is considerably wider and thicker than the original design. (It is 50mm wide, which is almost 2" as opposed to 1.5". And it is 15mm thick, almost .6" instead of .5".) The result is that his scale has much more weight behind the pivot, and by quite a bit. So much, in fact, that Maxime needed to put the counterweight at the front of the arm rather than the rear. You can see that in his pictures.

Another novel difference is that he has not put a cap on the bearing pedestal. It greatly eases the effort of assembly and disassembly for calibration or maintenance. Also, it makes construction much easier. It does not affect the operation of the scale, though you have to be a little more careful when transporting it, or even moving it around in the shop.



So there you have it! Wooden bases. Aluminum bases. Wide bases. Narrow bases. Build yours like one of these, or freelance your own design. It isn't critical to measuring swingweight, it's just what works in your shop and catches your eye. You can even change some things about the beam -- but that is critical to the measurement, so you'd better know what you are doing.


Last modified - Feb 3, 2017