Sunday, July 18, 2010

Drilling & Finishing





For the four people actually following this blog, I apologize that I haven't been updating this blog. I have been working on this bandura, but my day job has been extremely busy as well so it's been difficult to find time to build and post. As you can see from the pic, she's nearly done.  But back to what I've been doing for the last 3 months:


Shemstok - drilling for tuning pegs:




To angle the pegs back which is typical of the Kyiv bandura, drilling for tuning pegs requires a compound angle - to handle this, I made an angled table that allows me to drill the holes on the drill press.  It's essential to use a drill press for this operation because if the holes are even a little bit too big, the peg won't hold.  

At this time, I also drilled the holes for the Підставки or "Pidstavky" which would be the equivalent to a "string nut" on a harp.  These holes are vertical so they don't require an angled table to drill the holes.


Gluing the Shemstok to the Frame:

Once all the hole drilling was complete it was time to glue the shemstok to the frame.  For this operation, I used West Systems Epoxy.  This is probably the most stressed joint in a bandura and it's got to hold forever.

In this process, I found that the sled I built for binding is very useful for holding the bandura off the work bench to fit the clamps underneath.



Unfortunately, even after all that messing around with the templates etc, I'm still not 100% satisfied with the tightness of the joint between the Shemstok and the Frame.  Normally, for this joint I'd pick a polyurethane glue but I chose epoxy because it has strength when filling gaps, unlike other glues. The down side is that it is heat sensitive and will release if it gets too hot.

As a little insurance, I ran some screws through the frame into the shemstok to make sure it stays where it's supposed to.  The trick is though that in the final instrument (the one with mechanisms. It will be impossible to do so because the mechanism lives where the screws are right now.  I'll have to be extra diligent when fitting for the next instrument to ensure that this joint is really tight and will hold.

Here's a final picture of the insides before the back is glued on.













One more step before the back goes on: the ball end of the string is held at the bottom of the instrument.  To give it a place to "hide", I routed a slot in the bottom of the instrument using the tool to the left.

The plastic pieces ride on the frame and the wooden block keeps the slot an even distance from the edge of the frame.


Here's the completed slot.  Once complete, there were two things I didn't like.

First, the slot is a weak point in the frame.  There is internal reinforcement but it doesn't extend much past the point of the slot.  For the next instrument, I might add a 1/4" or so to the internal brace to give it a little more backing. 

Second, I should have made three "stops" in the slot. i.e. instead of a continuous slot, there would be two places where the slot stops, and re-starts after about a 1/4".  That might not make much of a difference structurally but it would certainly help me sleep at night.

The next step was to drill holes from the string lift to the slot where I used the same tool I used on my last bandura to drill the string holes in the shemstok. 


And here's a picture of the back being glued on.  I used Titebond Original glue for this procedure.  

Unfortunately, I forgot to take pictures of the back bracing on this one.

Once the back was on I used a 1/2" round over bit in the router to ease the edge on the back.

In my prior instruments, I had an extra piece of frame I called the "poly frame" for Polyganal frame to add extra material to the back of the instrument to allow a 1" radius on the back for comfort.  That frame adds a lot of work and I think, not much benefit.

My client requested that type of construction for his premium instrument which I'll do but, I'll almost certainly not use it in the future.


The Bridge

The bridge was actually started a few months ago and you can see it in the pictures in the last post when I was laying out the strings. I'll cover the details now though.

For this instrument, I purchased a piece of Bolivian Rosewood for the bridges and various trim pieces.  The bridge is cut on the bandsaw to rough shape and then sanded to final shape on my oscillating spindle sander which can be seen in the background of a few of my pictures.

It's then on to the router table to cut two ledges, one on top and one on the bottom, using a rabbeting bit.  I make the bridge intentionally tall at this point because a lot of the middle will be removed on the domed sanding plate like when I was fitting the "string lift".  

With the ledges cut, I use this homemade tool to cut a slot for the wire which will serve as a saddle.  The slot is cut on the top and bottom.













Once the slot is cut, it's time to move on to drilling the holes for the strings.  The locations for the strings were laid out at the same time as the locations for the tuning pegs and Pidstavky.

Like most things on a bandura this step also involves complex angles etc.  In the picture to the left you can see that the hole through the bridge has an angle - this is to create down force on the saddle and to mute the "dead length" of the string.   The приструнки or "white strings" need a down angle and the півтони or "black strings" need an up angle. 
In addition to that, the the holes must be drilled parallel to the direction the strings will be running on the instrument (duh) which requires yet another fixture.

Here you can see that fixture.  Each style of bandura requires its own fixture to make sure the strings are "straight up and down"  For my Kharkiv instruments, I built the break angle in  to bridge holder but since I need to make opposite angles for the different strings.  I did not build the angle into the fixtrue but instead decided to upgrade the angle table I built for the shemstok operation to be adjustable.






Here's a picture of the revised adjustable angle table.  I found the hardware at Rockler Woodworking and although it's not the greatest, it's good enough for this job.













And here's a shot of me drilling a string hole in the bridge.  You can see I'm wearing loups so that I can get the hole lined up perfectly.

Because the drill bit is so small, it can tend to wander (flex) when drilling.  I used an awl to put a divot at the start point of each string and then carefully align the bridge under the drill bit to minimize the wandering effect.

The bass bridge construction is similar but easier because it's not curved like the treble bridge.

With all of this completed, it's time for finishing.






Finishing!

With all of the construction complete, it's time for finishing.  I used basically the same finishing schedule as I did with the guitar and bandura # 4 so I'll keep this part kind of brief.  Because this was the first time I've built this style of instrument, it came out a little sloppier than normal so step one was to fill in any little gaps with wood filler of some sort.  To help hide the mistakes, I decided to do sun burst finish on this one just like the last instrument.

The back is made from the same piece of fiddleback maple as the last instrument and while the last instrument looked good, the figure of the maple wasn't "popped" to my satisfaction.  To help bring it out, I used an old trick where you stain the figured wood with a dark stain then sand it back to bare wood.  The figured grain absorbs the stain more than the non figured and it really pops the figure well.
The whole instrument is now sanded to 220g using 100g in some areas first that need a lot of sanding.  I used Koa for the rosette and that needed some pore filling as well.  I didn't mention it but I filled the pores with epoxy at the same time that I was gluing on the shemstok as there was plenty left over.  It was at this time I learned that the "compwood" I used for the frame had cupped a bit which required a lot of extra sanding. Once sanded, the entire instrument gets a wash coat of super blond shellac.

For the sunburst, the body first gets a coat of amber. I mix the amber dye in with the finish (Target USL, just like last time) and here's a shot of the instrument after the amber coats.  I think you can really see how the dark staining & sand back brings out the figure of the maple.  It really makes me want to build an instrument that's "clean" enough to finish without a sunburst.





Next, I used an Iwata Eclipse airbrush to add the black parts of the burst.  For this, I used an opaque black pigment mixed with the USL.


And then we finish the burst with transparent red. The bursting took about two days to allow the various coats to dry.  To help minimize mistakes, I use pretty mild color mix that takes a long time to build a dark color.  That way you can't make drastic mistakes in the finishing.
Once the burst was complete, it was about another 18 coats of clear.  Now the instrument needs at least two weeks for the finish to cure. While the finish was curing I made the Підставки  or "String Nuts".

I purchased a few rods of 3/16" aluminum for the string nuts then sanded them with 220g so they looked nice and polished them up a bit with some nevrdull. The Pidstavky need to be very accurately cut to length so I made a jig to cut them on the table saw.  The picture on the left shows how it works.

And here's another shot.  The toggle clamp holds the pidstavka securely so it doesn't go shooting straight in my eye.  The rod to the right is the length stop to ensure that they're all the same length.  To get the correct length, I strung up two strings a while back and determined that the short ones needed to be .4" shorter than the long ones to get the string crossover in the correct location.  For this first round, I made the long ones 1.1" and the short ones .7".  If that's too long, I can always cut them all down to around 1".


So yesterday I spent about 5 hours sanding the finish starting with a light sanding with 400g paper on a large felt block followed by a sanding with 600g and then 800g.  Some areas like the scroll won't be sanded too much because it's really easy to sand through the finish and when you do sand through an area that's dark with a light wood underneath, it's quite obvious.

I bought a new coarse buffing compound for my buffer so I'm hoping I can stop at 800g and will be running a test later today.  If not, it's just means that I've got to go to 1000g with an Abralon pad on my random orbit sander.

Once the finish is fully buffed, the bridges need to be glued on and the instrument strung up - hopefully that will be done by the end of the week!