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Finalizing the Marimba Resonators

By Jim McCarthy | June 29, 2009

Ok – so I’m still a week or two behind with this marimba building blog – I’m actually almost finished building now, but I’ll make another couple of posts to take you all through the processes. This post follows the last by about a week or so.

Here’s what I managed to accomplish in that week:


So lets have a look at some pics and processes.
detail of extra timber pieces on small end of marimba frame
This photo to the right shows the extra timber pieces on the small end of the marimba frame. These have to be cut very carefully to match the gaps and angles required. Each of the lower pieces fits between the ends of the long horizontal timber struts that support the bars. The ends of these pieces are cut with angles to match the angles that the struts meet the frame ends at. They are also cut to be just a little loose. Actually I left exactly one millimeter between the ends of these little blocks and the struts. This is to allow room for the felt lining that will prevent movement noises at the junctions later on. Also whilst you don’t want these struts to be a sloppy fit, you don’t want them to be really tight either. You need to be able to easily fit the diagonal struts in particular as they are slotted in after the main center struts are bolted onto the end sections.
top end resonator banks almost finished
The smaller blocks which sit on top, are cut to width carefully so that they will fit exactly between the ends of the aluminium flat bars that hold the resonator banks together. I actually had to finish constructing the top end resonator banks before cutting these blocks so I knew what the width was going to be. I also made them a measured amount undersize to allow for a little padding between the aluminium and the wood. This will be done later with some thin rubber tube around the aluminium.
middle resonator banks almost finished
This photo here to the right, shows the middle banks of resonators. These ones are probably the easiest to construct. You might be able to see from the picture above that the top end banks actually get a little wider on the top few tubes. This makes these banks a bit of a pain to construct. The reason is that the tubes are extremely shallow – so much so that there is not enough tube past the end cap for the full width of the aluminium bar that rivets onto it. These last few tubes actually have a sleeve made for them from an extra end cap without the end. These are cut to length so that the tubes are the full diameter of the end cap for their entire length. This makes it easier to rivet them neatly into place, but is the reason these banks must be finished before those timber blocks can be fitted. The middle banks are easy because they use all the same diameter tube pretty much, and the tubes are all short enough to require no bends. The photo here shows them without the diagonals rivited on yet which will hold all the tubes straight. Also the “dummy” resonators for the “black note” bank are not yet installed.
fitting dummy resonators to the middle bank
The photo to the left here shows some of these dummy tubes being fitted to the middle “black note” bank. Note that in this photo the diagonal is in place holding the bottom of these tubes in position. These dummy resonators are not needed at all for the sound of the instrument – after all they are not below any marimba bars – they are purely for esthetics. They fill in the gaps between D# and F# and also between A# and C#. The dummy tubes help to hide the diagonal strut and anything else behind the tubes from the audience’s point of view which might look slightly messy. The tubes don’t need the end caps either of course, but they are fitted anyway for a consistency of look.
detail of bottom end resonator banks
This photo to the right shows the detail of the completed resonator banks for the bottom octave. There are a few tricky bends here. This design is created to allow the banks to fit completely under or inside the line of the timber frame diagonals. This means that the resonator banks can be adjusted a little higher and still not contact the diagonals. It also means that the resonator banks can be easily inserted between the main timber bar support struts after they are assembled onto the frame. You can see that there would have been room for the bottom tube to simply bend back up vertically inside the two diagonals, but this would have made inserting the assembly between the timber struts impossible. Notice how the lowest C# and D# have U bends which are angled away from each other so that the tube ends are inside the timber diagonal. This is possible because the C# is the lowest note of that bank, and there is a gap between the D# and the F# above it. This looks a little ugly from the front of the instrument, so there is a carefully constructed dummy tube with it’s back cut out to cover this, and the result is actually quite good looking. If these tubes were made from metal, the weight would be significantly out of balance because of all this tubing away from the main centerline of the resonator banks where they are hung from on the frame. The extra tubing would then need to be attached somehow to the framework to give them support. In this case however, the PVC tubing is quite light so the banks hang almost perfectly with no extra support at all. The tiny amount of imbalance is compensated for in the height adjustment mechanism which I devised for these banks.
height adjustment mechanism for bottom octave resonators
With the resonator banks constructed, the next important step was to create the height adjustment mechanism for the bottom octave resonator banks. You can see from the photo here basically how it works although this is the “unpolished” version. The little timber L construction holds the end of the resonator assembly by the aluminium strut with a single bolt which allows a little pivot as the height is adjusted. Two bolts go through the L assembly which have threaded inserts so as the bolts are done up, the ends force further past the bottom edge. The bolts have rubber tips which sit on horizontal parts of the frame end section. You might notice there is also a third bolt through the top of the L assembly. This one has the threaded insert threaded in a hole in the top of the frame, so the bolt holds the L assembly down. This not holds everything secure and prevents any twisting, but because it is on the side of the resonator bank opposite the extra weight, it holds the whole bank hanging straight.
lower detail of frame small end allowing enough width for castorslower detail of frame big end allowing enough width for castors
Now it was just the finishing stages to go, but before I could sand everything down ready for estapol I wanted to have all the timberwork finished which meant doing any extra work needed to allow wheels to be attached to the bottom of the frame end pieces. Without going to too much trouble. the best castors I could find for this instrument were ones mounted on a 60mmx60mm base plate which meant making sure there was a decent 60mm square surface on the bottom of each end piece to screw them to. For this I simply faced an extra plank on the opposite side of the verticals. You can see these in the photos here above and right.
leg piece after putty and sanding
The first stage of finishing was to fill all the screw head holes with sawdust putty. Even though I’d done most of these already in a previous stage, there was quite a few more – on the top of the small end section, the new bits on the bottom of each end section, and all the little blocks on the main horizontal struts. All of these got filled with putty, allowed to dry, and then sanded back flush with the timber faces with a coarse sanding disc. Then a few hours with the orbital sander and finer paper leaves all the surfaces smooth, rounded, and ready to paint with estapol. A little example of the results can be seen in this photo here.

Well that pretty much takes care of that week’s spare moments. The following week will see the final cosmetic stuff like sparypainting the resonators and staining/estapol coating the timber. Also applying the finishing touches like felt pieces and the string holders.

Of course as usual I have to mention that all of this will be discussed in WAAAY more detail in the building guide which will be written up soon and available from makeamarimba.com or the building guide link at the top of this page.
Jim McCarthy

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