Saturday, May 8, 2010

Long and Tedious

Really, I got a lot done today honey!!

Today was another productive day in banduristan.  I managed to shape and glue on my "lift" (which I'll get to in a moment), make the bridges and lay out all the drilling to be done - and there's a Ton of it to do.

The String Lift - In Kyiv banduras, there's quite a bit of downforce on the bridge from the strings, particularly the "diatonic row" or приструнки which start low, go up to the top of the bridge and then down do the level of the deck.  Something I do in my Харків instruments is create a "lift" at the bottom of the instrument to take some of the load off of the bridge. Many would argue that this downforce affects the timbre of the instrument and I'm sure it does but, as with any changes, this won't necessarily have a negative effect on the timbre.

Typically, instruments that use a tailpiece type of design, the downforce from the strings keep the bridge in place.  I glue my bridge down so the need for that massive downforce is not necessary and allows me to keep my top and bracing lighter.

One thing about having a domed top is that everything must be arched to match the dome of the top.  Using a spherical dome makes matching that arch much simpler.  In the picture to the right, you can see my 25' domed workboard on my motorized sander.  The "lift" is glued to a piece of larger wood using hot melt adhesive to give a little something more to hold on to.  The lift is 3/8" tall.

I purchased a matched set of a 25' dished workboard and a 25' domed workboard at the same time.  If you search the internet you can find many techniques on how to make your own but in my mind, the $80/board I spent was well worth it when you consider how much time and effort is spent making your own.

Here you can see how I keep track of how much sanding I've done.  I want to lose as little height from the lift as possible.  I draw on the bottom with a white pencil.

Here you can see I've almost made it to the edge

Once sanded to match the domed top, I glued the piece on and used my flush trim bit  to get it right.  There were some mistakes made but I have no intention on going into them now.  Needless to say, good thing I'm building a prototype first!

The reason I needed to get that done was because I need to lay out the шемсток (shemstok) for drilling.  It needs to be drilled prior to being glued to the rim because the holes for the tuning pegs are angled.  If I'm doing a layout, I may as well do everything at once.   To that end, I made the bridges as well.

Below is the setup I use for layout - which I've covered in previous posts.   The rub with this method is it takes a very long time but, it's ultimately flexible.  While it's not suited for serial production, it's very suited toward one offs.  Seeing as I'll only be building one more of this style of instrument, I see little point in making dedicated jigs for this.

Not the greatest shot, but here you can see the шемсток with all the layout marks for drilling.  Next step is to build a fixture to hold it in the proper angle for drilling the tuning peg holes

And finally, here's a shot of the perfectly fitted bridge with the spherical dome.  I found a local source of Bolivian Rosewood so I'm using it for the trim and bridge pieces.
Starting to look like a bandura - I managed to have several good evenings of work and have gotten to the point below (note to Yuri P. - those are not the bridges I'll be using and are there just for the photo!). Soundboard is glued on, shemstok is fitted and the instrument is bound and purfled. Fitting the shemstok to the curve on the top of the bandura was a nightmare and I do not look forward to doing it again but, here's the steps necessary.

Template Routing - This is a trick I remembered from my non-luthier woodworking days. Basically, using a router with offset collars, you can make two objects which fit together. I made a template from the shemstok (upper string holding thingy) that I'll be using on the next instrument (purchased from the Lviv Factory). I then vary carefully made a second template of the curve I'm fitting using a 1/2" flush trim bit that ends up 1/2" under sized. With that template carefully aligned and using a 1.5" diameter collar (providing a 3/4" offset to make up for the 1/2" offset plus 1/4" radius of the bit) with a 1/2" diameter bit, you can cut a perfect mating surface.

In the shot below, you can see the template on the bandura. What's not in this shot is an alignment template screwed on top used to position the 1/2" under template. The alignment template is nothing more than a "practice run" to make sure everything's o.k. I left it screwed on so that I could properly align the real template. The goal in alignment is to remove as little material as possible and still get a good fit.

It should be mentioned that the template is very hard to position on the top, even with the alignment template because the top is not flat remember. I had to make several tapered shims to get the template to sit 90 degrees to the edge and still be solid.

Here is a picture of the router with the 1.5" offset collar and a router bit.
And finally below is a picture of the material removed. It may seem like nothing but it's essential to have a perfect fit not only for the glue to hold, but so that it looks good too. Because of the shape, I ended up hand fitting the very end of the shemstok because to do otherwise would have required removing a ton of material from the rim.

And here's a shot with the shemstok clamped into position - perfect fit!

Actually there is a small gap near the neck and a small gap on the back side in one spot. Neither should be structurally significant and I should be able to hide them. The next step is binding and purfling.

Binding Channels - Bindings serve two purposes - they hide the end grain of the top and also provide sort of a bumper to protect against mild hits. I think that mostly it's an esthetic thing. I cut binding and purfling channels using a bearing piloted rabbet bit. I use a home made fixture to hold the router.

The binding channel must be cut parallel to the sides but the top is domed. Simply placing the router on the top to cut the channel would result in a binding channel that was wider at the bottom than at the top to you need some method of holding the router parallel to the sides. The fixture I use is a copy of a common design that's been floating around. I think it cost about $10 for the drawer slides and probably around $10 for the plywood I used.

Here is a shot of the router indexing along the side of instrument. I've found that single good passes make for the most uniform channels, but not necessarily the cleanest. I had a lot of fuzz to clean out by hand.

A shot of the purfling and binding channels.

And at long last, a shot with the bindings and purfling in place. Because this instrument is a prototype, I didn't do anything fancy. The binding is white ABS plastic left over from my guitar kit and the purfling is a simple BWB (Luthier code for Black White Black). I tape them into position using Stew Mac's Binding Tape . Once in position, I flood with CA glue . It should be noted that some types of CA glue can stain the spruce used for the top when absorbed by the end grain. To avoid this, I seal the binding channels with a coat of shellac. My current bottle of CA does indeed stain and the shellac has prevented staining.

Scraping the sides - Purflings are installed a little bit higher than the top and then scraped flush. Bindings are usually installed perfectly flush or a tad inset from the sides which are then scraped to be flush with the binding. We scrape the sides instead of the binding because we want a uniform thickness for the binding.

In scraping the sides I noticed what might be a big shortcoming of the compwood I'm using for these builds: there was serious cupping in the sides, perhaps 1/16". The only way to get rid of that on this build is to sort of taper the sides so that they get thinner toward the back of the instrument. I worked on it with card scrapers for about an hour and then needed a break. I checked the other frames and luckily, the frame for the "real instrument" is crowned which should be easier to deal with.

Today should be another productive day and I hope to post pics soon.