Because You Asked: Body Frets

I recently received a request for a "how to" blog post from Mr Bill Good, and I'm going to do it.
This time out, I'm going to describe my procedure for installing body frets.

It's one of the very last things I do when I'm making a lute, or a group of lutes. Everything else is pretty much done--all the woody bits have been glued together, a nice, shiny finish has been slapped on, I've fitted a set of tuning pegs (now there's a finicky procedure), made a nut, tied on a set of gut frets, and strung up the lute. I've tuned those strings and played the first, quivering, barely in-tune notes, and an indescribable thrill has begun to form in my being as I realize, again, that I've created a living, breathing thing. The lute is alive!

The Lute Maker, in the Throes of Creation

In such a state of creative fervour--and after such a long time putting the lute together, weeks and months of sometimes very difficult, demanding work--the impulse can be to... well... get into a bit of a hurry at this late stage, cut corners a little, just tape down some toothpicks, nobody will notice, nobody ever uses the body frets anyway....

No! Don't you even think about it! Calm down--you've still got work to do. An hour's work, if you do it right, and have a procedure to follow. Here's mine.

Finding the Octave

The first thing to do is to tune that lute. And I mean tune it, pretty much constantly, for a couple of days at least. Regardless of the type of strings you've used, they'll be continuously stretching and going out of tune for most of that time. I find it useful to actually play the lute while I'm tuning--in addition to waking the lute up and helping it to find its voice, my playing works and stretches the strings so that they will fall into tune that much faster. Besides, I'm a lute maker, it's my job, and I have built this thing to make music--at this point, what else would I rather be doing?

Once the strings are really well in tune, you need to find the octave, the point approximately halfway up the string where the 12th fret will be located. It's not exactly halfway as measured by a ruler--because of the physics of string stretchiness, the octave will always be somewhat less than half the distance between the nut and bridge. (Depending on a few different factors, the octave will be probably be approximately 1.5 to 2mm shorter than half the measured distance.)

How to find the octave? Allow me to demonstrate. This is a lute that I recently repaired--not one of mine, but one made in the early 1980s by a maker in British Columbia. Like many lutes of that vintage, the fingerboard had warped a bit over the years, and needed to be re-shaped in order for the lute to be playable. In order to do the re-shaping, I had to remove all the tied frets as well as the body frets that were on the lute. Now that the work on the fingerboard was finished, I needed to put a set of new body frets down. (By the way, the gut frets I've put on are placed provisionally, and will be adjusted when I find the octave and calculate all the fret positions.)

I have a small ebony body fret that I have shaped and keep just for the purpose of finding the octave (I'll talk about how to shape these body frets in a minute or two.) I put the fret down on the belly under the top string, and strike a note.

I move that fret up and down the string, making slight adjustments one way or the other, plucking notes until I've found the true octave. I use an electronic tuner to do this (because I am lazy), but if you're a purist you could probably do it by ear, comparing the fretted note with the natural harmonic produced at the 12th fret. Whichever way you do it, when you are convinced that you've got that fret and that octave in the right place, take a very sharp but not very hard pencil (HB works well), and make a crisp, light mark on the soundboard on each side of the fret. Move the fret away. Then, between those two marks, make another mark exactly in the middle. That is the precise position of the octave.

Next, get out your ruler, and underneath the top string, measure accurately the distance between the octave mark you've just made and the front edge of the nut. Now, double that number: this gives the lute's playing string length. Use it to calculate the position of all twelve frets, according to whatever system of temperament you are using (this lute, a 9 course, will be fretted in equal temperament.) Cut a piece of card stock about an inch wide and a bit longer than the length of your octave, and carefully mark out the fret positions on it. You'll use this card to lay out the positions of the body frets (there will be three on this lute), and to adjust the positions of the tied gut frets on the neck.

I also write down on this card the diameters of each of the tied frets, and when I'm done using it to lay out the body frets and adjust the tied frets, I put it in the lute case and keep it there. It's very handy to have when replacing worn out gut frets.

Making the frets

After erasing the marks I've made on the soundboard, I lay a strip of 1/2" masking tape down on the soundboard, so that the inside edge is more or less in line with the edge of the fingerboard (the strip of tape isn't quite parallel to the first course, but flares a little.) This tape will mark the location of one end of each of the body frets, so I lay my fret card down alongside it, and mark the locations of the body frets on the edge of the tape--like so:

I can then get out my ruler, and determine the length of each of the body frets.

I want this 12th fret to cover the first three courses (the 11th fret will cover 4 courses, and the 10th fret will cover 5), so I measure the distance between the edge of my masking tape and the mid-point between the 3rd and 4th course. (You can see my mark on the masking tape on the left side of the ruler; you can also see the length of this fret will be 33mm.)

On this lute only the 11th and 12th fret will be on the belly, and the 10th fret will be on the fingerboard. In order to determine the length of the 10th fret, I need to measure from the fingerboard edge to the mid-point between the 5th and 6th course. You can see the fingerboard edge clearly here: the length of this fret will be 51mm.

Once I have the length of the frets figured out (and written down someplace), I can decide on the thickness of frets that I want to use.

This can be a bit of a tricky calculation, and it deserves some thought. The thickness of the body frets you use will depend partially on the thickness of the tied frets on the neck, and partially on how much and how quickly the belly slopes down and away from the strings, once you get past the body-neck joint. The slope of the belly at this point depends on the amount and severity of the "belly scoop" that the luthier has planed into the edge rib of the lute: some makers like a deep belly scoop which slopes quickly away from the fingerboard, while others prefer a more gentle, gradual slope (that's generally the way I like to do it, and that's the way the maker of this lute decided to work.)

Let me explain how I decided on body fret thicknesses for this lute. My general method of fretting a lute (which I normally use unless a client asks for a different setup) is to begin with a thick fret for the first fret--say, 1.05mm for the first--and then gradually tie smaller frets all the way up the neck. So, for a 9-fret neck like this one, this is how the tied frets go:

1st: 1.05mm
2nd:1.00
3rd: 0.95
4th: 0.90
5th: 0.85
6th: 0.80
7th: 0.75
8th: 0.75
9th: 0.75

The 10th fret is my first wooden fret and it will be on the neck, so  I will make it 0.75 thick as well, to give some uniformity of "feel" with the last three tied frets. Then, because the next two frets are on the belly--which is gradually sloping away from the string, and creating a bit more clearance--I will grade the 11th and 12th slightly thicker--0.85 for the 11th, 0.95 for the 12th.

Now let's make some frets.

I make fret blanks out of ebony and english boxwood, two very hard woods that will withstand years of playing, and generally choose the light or dark wood depending on the overall look of the lute. (On this lute, the original body frets that I removed were ebony, so that's what I put back on.) I make my body fret blanks 1.4mm wide, and I'm able to thickness them quite precisely to various thicknesses, as you can see in the photo above. This is the tool I use to size them up, a pull-through scraper:

I have described this tool and its use in detail in a previous blog post from a few years back. You can visit that blog post here (you'll need to scroll to near the end of the post.) It's well worth a look--I use this unit to make sets of rib spacers, ebony edge bindings, strips for striped veneers, and much more. It's an incredibly handy tool in the lute maker's workshop.

I take the blank of desired thickness, and cut a segment 3 or 4mm longer than the final finished length. Then I dress the edges of the fret, using this jig, held in a benchtop vice.

It's just a piece of particle board with some arborite on top--I think it came from a piece of old kitchen countertop. The arborite edge is nice and sturdy for holding the fret. I have a thin piece of some kind of plastic sheet stuck down on top with 2-sided tape, forming a ledge that's as wide as the fret, where I can hold the fret securely while working it.

With a #2 flat file, I break the top edges on both sides of the fret. I file them at about a 45° angle, then round over the two new edges I've just created. I'm careful not to go over onto the top of the fret--I don't want to lower it, I just want to make those edges comfortable to press a string down onto.

Then I relieve the sharp edges at the end of the fret. I put the end of the fret at the edge of my jig, and with my #2 file, give a quick downward stroke to put a 45° angle on it. Then I relieve the edges on that angle by trailing the end of the fret across the file. I then cut the fret to its final length with a sharp chisel, and dress that end of the fret in the same way. Here's how the fret ends up:

Now that I've got all the frets made, i can finish laying them out.

Laying out the frets

I need full access to the end of the neck and top of the belly to lay out the frets, so I unstring the lute. I just loosen off all the strings without removing them from their pegs, and tie them away from the top of the soundboard.

I lay down the frets in their approximate positions, according to the marks I've put on my long piece of masking tape. I then stick down smaller pieces of tape at the other ends, and bring back my fret card to mark the fret positions on them. Then I move the frets away, and use my small ruler to draw a faint line on the wood of the belly from the mark on the treble side to the one on the bass side.

I then use a small, sharp, curved scraper to scrape away the lines I've just drawn. I'm not really scraping away very much wood--mostly just a thin layer of finish on the belly and neck so that the glue I use to stick the fret down will adhere to the wood below.

Gluing the frets

I use hide glue for this job, as I do for all the gluing jobs on a lute. You might be tempted to use yellow carpenter's glue for this, which would be fine, but I find that hide glue tacks faster and cleans up better--instead of waiting overnight for yellow glue to dry, you can use hide glue and be playing again in an hour.

I use a fairly thin glue for this job--you don't need a lot of glue to get these frets to stick. 

I wrap a little two-sided tape around the end of a small dowel, and just press that down onto top of the fret so I can lift it and hold it over the glue pot while I brush some glue on (the more dextrous among you could probably just as easily use tweezers for this job.) I just dip the brush in the glue, sweep upward onto the fret, then sweep back down to brush off excess. Then I stick the fret down quickly in position--locate one end of the fret close to its mark, then bring the other end down. It should be good--if not, adjust. Hold the fret down for about 30 seconds, until the glue begins to set. 

Then bring in the small metal ruler, and use it to press the fret down until the glue is well set (a minute or two of pressure should be good.)

Then move on to the next fret, and go through the same procedure, then the next. By the time you've got all three frets stuck down, the glue will have set well enough on the first one that you can go back to it and clean up the squeeze out. Just brush a little cold water on each side of the fret, give it a minute for the glue to soften, and clean up with a sharpened spruce stick. For a last measure, give each fret a quick wipe with a damp bit of paper towel, to remove any glue residue that might be lurking on the top of the fret.

And that, my friends, is that. Remove the masking tape bits, and start the process of tuning the strings. By the time the lute's back in tune, the body frets will be firmed up, and you're ready to play again.

So that's my procedure. It took a long time (and a lot of photos) to describe a fairly simple operation. Still, some might object that I go too far--that all this rounding and filing and fiddling is simply paying too much attention to a feature on the lute that isn't, after all, that important. To which I would reply: there is no detail on a lute that is too small to pay attention to. And in the case of these body frets, a lot rides on their position and shape. If they're not in the right place, the intonation will be wrong. If they're left square-edged, the note will sound thin, but more to the point (ahem), they will feel unfriendly to the fingertips. I've tried many lutes that had body frets with edges left square, and I've always had the uncomfortable feeling that I was stubbing my fingertips on them. Why would I want to leave something unfinished on the lute that I felt was impeding my playing?

When players and makers think of "playability", they're mainly thinking of big things like string spacings, action (string height above the fingerboard), fingerboard arching, and neck shape. But I would argue that apparently "small" things like body frets can make a significant contribution to playability too. There are a few other small details of finishing a lute that make similar contributions; perhaps I'll make them subjects of future posts.

Because You Asked: Making a Lute Mold (6)

Hi again.  Today's the last installment in the series of how to make a solid lute mold.  We've built a big block of wood, carved it down into a shape, drawn rib lines and carved rib facets--all we have to do now is refine these lines and surfaces until we're satisfied. 

Simple enough to say it, but in practice I've found the refining to be a very long process.  It demands a lot of concentration, decision making, and intentional looking--from different perspectives, in different kinds of light.   It also requires you to mentally step away from the fact that you're sculpting a shape, and think, instead, of how bent wood ribs will actually fit together on the mold.  The fact is, you can stop work on the mold anytime you like, and make your mold any shape you want, but it doesn't necessarily follow that your mold will be easy, or even possible, to build on.  With that in mind, what I want to do today is give some techniques and hints to help you get a good-looking and practical shape for your mold.

The first thing to remember throughout this process is not to try to do everything at once.  Instead, think of working through drafts, or versions, of the mold.  What we have right now is a first draft (and a rather lumpy one at that.)  We'll do some shaping, remove a modest amount of material, then have a careful look at the whole.  We'll make some decisions, then go on to a second draft, where we'll do the same thing.  All the time, we're thinking of moving toward a shape--not just landing there, but moving toward it in careful steps.

In general, it's best to be conservative at this stage.   It's much easier to remove material from the mold a little at a time than it is to add material if you've gone too far.

Refining the Rib Lines
You'll remember that before we put in the rib lines, while we were still working on the overall shape, I suggested that it was natural and desirable to leave the surface of the mold slightly 'humped up' between cross sections.  I also said that eventually we would be bringing down these areas in a very controlled way--and that's what we'll do now.

The best way to see these humps between the cross sections is by looking at each rib line separately, not from the front or the side, but directly across the rib line--from above, with a strong light (your bench lamp) above you, and all other lights shut off.  You can also lay a dark cloth on your bench below the mold, to provide a contrast against which the rib line will stand out.

Sighting across the rib line, you'll see a series of humps between the cross sections.  This is good.  The idea now is to lower those humps gradually and create a smooth, continuous line.  To do this, use a sanding block (with, say, 180 grit paper) to gently take them down.  Be careful not to sand down your cross sections--these should be preserved.  Sand off only material between the cross section lines.

It's best to have a plan of attack when you're doing this work, rather than just randomly sanding away at various rib lines.  Here's how I proceed:  I mentally divide the mold into quadrants--treble and bass sides in front of and behind the largest cross section--and I begin by working with one front quadrant, then the other, and then one back quadrant, then the other.  I might work from the outside rib in toward the middle, then from the middle toward the outside rib.  However, even though I'm working on one rib line at a time, and one quadrant at a time, I'm always keeping an eye on the whole, trying to see how the rib lines are shaping up all over.  

Here's a look at the mold after we've had a go at all the rib lines.

You can see that where the rib lines have been sanded and flattened, the pencil line has been obliterated.  However, the tattooed pin holes are still clearly visible.  With your flexible curve, re-establish the pencil rib line; then as before, use your finger plane and curved scraper to remove material between the rib lines, to establish a good flat surface between them.

That's round one.   How's it looking?  A bit less bumpy, a bit more streamlined?  Good--we're making progress.

Now take some time to really look at how the mold is shaping up.  Look not only at the rib lines you've been working with, but also the facets you've carved between them.  Look at each one separately: does the shape curve gracefully, curling and flattening in an organic way?   Sight across each facet, from one rib line to another: are there bumps on one side that don't match the other?  In other words, will you be able to bend a strip of thin wood to a shape that will fit that facet closely?

Turn on the overhead light, then turn it off again.  Use your bench light to highlight certain spots; then back it off to get a good sense of the whole.  Walk to the far side of the room, then come close again.  Turn the mold around, look at it from above, behind, in front, below.  Take the mold over to the window, and look at it in natural light, both direct and indirect.  Think.  Brood.  Sigh.  Furrow your brow.  Scratch your chin.  With a pencil, and mark some spots where you might want to flatten the rib lines further.  Then take a deep breath, take up your sanding block, and go on to draft number two.

A Couple of Tools
A couple of tools that will make your life easier at this stage are a contour gauge and a sliding bevel.

The contour gauge allows you to take an accurate impression of a curved shape.  It's useful in two ways: first, it allows you to compare contours from one side of the mold to the other; second, it can sometimes show a hump or a bump more clearly than just looking at the rib line itself.  This is especially true if you're trying to get a smooth transition over one of your cross section points, where the rib line tends to hump up on each side.

(By the way, comparing rib lines from one side of the mold to the other likely won't give you an exact match.  That's to be expected, and it's not a problem, as long as the shapes are comparable, and good on each side.)

The second handy tool is the sliding bevel.  This tool allows you to look at the angle formed by the outer rib and the bottom of the mold.  That angle will obviously change as you go from the widest point forward, or back; the sliding bevel will help you judge whether the rib angle is changing gradually and gracefully.  It will also allow you to compare the rib angle from side to side on the mold.

The sliding bevel is perhaps most useful around the bottom of the mold, as you develop the shape of the capping strip.  Again, the angle changes as you move around the mold, but the angles should be similar from one side of the mold to the other.  

The Bottom of the Mold
The area back of the widest section is the most difficult part of the mold to shape.  All you have to guide you is the longitudinal section; every thing on either side of that is up for you to judge.  It helps if you have a good set of photographs of the original lute body you're working from, showing the bottom section from various angles, but you might have only a photo of the side view, and another of the whole back.  Or, you may not have any photos at all.  In any case, while it's true of the mold as a whole, it's especially true of this part of the mold: you are a sculptor, and you have to imagine this shape into being.

I can't say that much to guide you here, except to re-emphasize the principles I've given already--remove a little wood at a time, try to see the back area not in isolation but as part of the whole, and at the same time pay attention to individual facets, to judge whether you will actually be able to bend a rib into the shape you've made.  One other general principle is that of all the ribs, the center rib will likely be the one to have the sharpest bend at the bottom end.  All the other ribs will have a somewhat shallower bend, and they will likely become a little shallower the further they are away from the center.

The Capping Strip
Maybe the best way to help find a shape for the back end of the mold is by setting up the capping strip.  We've already put in a pin line to show its location (remember, we did this when we first set up the rib lines)--now, let's draw it in fully, and do some work to integrate it into the mold.

Once I've drawn in the location of the capping strip, I flatten the whole area, from the top of the capping strip to the very bottom edge of the mold.  Why do that?  Won't that spoil the nice rounded shape at the bottom of the mold?  Well, it will, but the fact is that once I actually get around to putting a back together on this mold, I'll need to flatten the ribs in that area anyway to create a gluing surface for the capping strip.  If I flatten the area on the mold first, then when I put the back together I can bend the tips of the ribs to follow that shape.  I'll be giving myself a head start in my flattening work, and I'll also save having to remove a lot of rib material to do it (which might otherwise thin out and possibly weaken the ribs in that spot.)

The more you refine the capping strip area, and the rib lines and facets leading up to it, the better idea you'll get of the shape of the whole bottom end.

Things are starting to look better here.  I can't remember how many drafts I went through on this particular mold to get it to this shape, but it was a few.  Each time I touched up the rib lines, and each time I flattened the facets between them, my work got more and more precise, and the mold got more and more refined.

Late-Stage Chores
As the overall shape evolves, and comes close to what you might hope is a finished shape, there are a few late items to pay attention to.

Keep refining the top block, but when the shape gets very close to finished, switch to working with a file (a #0 Grobet file works well), and file from the mold onto the top block.  Remove the block to have a look at how close you're getting to the marked cross section here--you should be very close indeed.

Another late-stage task is to scrape the facet of the center rib.  You'll remember I advised to leave that one alone for as long as possible, to preserve the longitudinal section you built into the mold.  At a certain point, though, hopefully when the mold is very close to its final shape, that longitudinal section needs to be scraped away, as the center rib takes its place as just another of the ribs on this lute mold.

While we're on the subject of scraping facets, another important late chore is to hollow-scrape the facets between all the rib lines.   Up until now we've been content to scrape them reasonably flat, for the purposes of carving a good overall shape.  However, we now have to take this scraping a step further, in anticipation of actually fitting a bent rib against the mold.

One characteristic of ribs bent with dry heat is that they tend to go somewhat concave across the width.  This makes for a really cool look on the finished instrument, but it can make for some difficulty in fitting and gluing ribs--unless you account for it on your mold.  That's what the hollow scraping is for.  The wider the rib you're working with, the more concave it's likely to go, but narrower rib facets--like the ones I'm working with here--need to be hollow scraped too.  Check with a straight edge all over the mold to make sure that they are.

Beyond the Theoretical
At this stage, your mold is very close to being finished--you've used the cross and long sections you built into the mold to carve yourself an elegant shape, with graceful, flowing lines.  But what if, after all this work, and all that careful attention you've paid, the lines aren't graceful, and the shape isn't elegant?  Well, the short answer is: you've got more work to do.

You may have gotten to a point where the practical shape of the thing you've made bumps up against the theoretical shape you're working with.  In other words, the cross sections you designed in two dimensions on your drawing just haven't translated well to three dimensions on the mold.  That's okay--it happens.  Usually, for me, the problem has been that I've ended up with a shape that's a bit too big and boxy on the front end of the mold (the area approximately behind the rose on the finished instrument.)  I'm not too upset if that happens, because it just means that I have a little more material to remove; it would be far worse if the area was designed too small, because once the material is gone, it's difficult to put back.

If this is the case for you, and you've got more material to remove, then just do it the way you've been doing it so far--a little at a time, working both sides of the mold evenly, keeping an eye on the flow of the ribs, as well as the overall shape.  You'll get there eventually.  I've found that when I'm working the area behind the rose, it's very easy to go too far, and to weaken it (I mean visually, not structurally), so do be careful.

The Finished Mold
The mold is finished when you declare it to be--when you're either satisfied with the shape, or you're so sick of working with it that you can't bear to look at it anymore.  (They might be the same thing.)  Here is the final shape of my mold.

I think this shape looks reasonable--reasonable enough, at least, for me to build a body on it, which is after all the only real way to tell if the mold is good or not.  If it builds a good-looking back, then I'll be satisfied; if it doesn't, then I'll work on it some more before building on it again.  Most of the molds in my shop have been works-in-progress; some have gone through numerous versions before reaching their final shape.  I'm looking for a shape that's practical, but also beautiful, and sometimes those qualities can be elusive.

You can see in the photos above that I've done a couple of things to finish up: I've inked the rib lines, and numbered the ribs.  You can number them consecutively if you want--I prefer to number the ribs separately on the treble and bass sides from the center rib out (i.e., 0 in the center, then T1, T2, T3, on the treble side, B1, B2, B3, on the bass, etc.)  I find this system helps me keep things straight as I'm working outward from the center rib when building the back.

Here's another hint--cut some strips of mylar, stick them down with double-sided tape, and trace the pattern of the rib.  A set of these rib patterns is very handy to have when you're laying out shapes on your rib material. 

One last thing to do is give the mold some kind of finish: two or three coats of a spray-on lacquer sanding sealer work well.  (Remember to remove the top block before you do it!)

You are now well and truly done your lute mold, and you have my congratulations in advance.  Take yourself out for a refreshing beverage of your choice--it's on me.  Don't overdo it though--you've got a lute to start building in the morning.

* * *

Post-Script 

And there you are, for those who asked (and those who didn't): this is how I make a solid wood lute mold.  I told you it was complicated!  Perhaps it's unnecessarily complicated; no doubt there are many ways you can find to do this more simply, or more efficiently, or more precisely.  I encourage you to experiment, and find the method that works best for you.  If you have the time, I'd like to hear from you--let me know what your experience is like using this method, or if you've come up with ways to improve the process (I'm a fan of those who betterize.)

I hope this is a help to lute makers, especially those who are starting out.  When I was building my first lutes, there wasn't much information around on how to make this kind of mold.  I don't think a solid wood mold is necessary for all kinds of lutes, but for some of the more complex shapes of certain lute bodies,  I think a solid wood mold is indispensable.

Still, while it's a very important part of the process, making a mold is really only a small part of the total work of making a lute--and after all, the whole lute is the true test.  The mold shown in the pictures I've used for this series is for a 12 course lute that I made in 2012.  How did it turn out?  You can judge for yourself--here's the lute's happy owner, Evan Plommer, favouring us with a tune, his own lovely setting of Walsingham.

Until we meet again...





 

Because You Asked: Making a Lute Mold (5)

Last time we met, we had turned the very rough shape of the glued-up mold into something vaguely resembling a lute. Today we'll continue refining the overall shape, and then at a certain point, we'll mark out the rib lines and begin to think of it not just as a shape but as a mold--a sculpted object upon which separate pieces of wood will be glued together to make a lute back.

Refining the Shape
Two tools will make our work a lot easier.  First, a small curved finger plane is very useful for carving the top block; for most of this work, the plane cuts best working uphill, that is, from the tip of the block back toward the mold. 

The second tool is a Veritas low-angle spokeshave, available in Canada from Lee Valley Tools.  This  tool can be set closely to take very fine cuts, and it works well across the grain as well as on end grain--which we have to deal with down at the bottom end of the mold.

I've evolved a way of holding the tool so that the left hand is a guide, while the right hand supplies power.  I hold it lightly but firmly, and take fine, quick cuts.  Most of the time I'm working out and downward, from the middle of the back out toward the edges, cutting directly across the grain of the wood.  I try not to dwell on one area too long, but keep moving and refining the entire shape of the mold.

These next two photos show the kind of progress that you can make with these tools.  The first shows an overall smoothing both on the mold and on the top block; the second photo shows how I've used the spokeshave directly over the cross section to carve things down in a controlled way.

We've almost taken down the overall shape to the cross section lines, but not quite.  We need to go a little farther, and get a little closer.  Something like this:

That's close.  Notice, though, that you can still see the cross section, and you can still see the rib location points that I marked on the beveled edges of the cross section blocks before I glued up the mold.  Okay: I think we're almost ready to start laying out rib lines, and carving rib facets into our mold.

Once we start doing that, however, we will have to say goodbye to our cross section lines, because they are going to be (mostly) carved away.  We'll need to mark our rib location points so we can find them again as we work.  Here's how.

Take a sharp pin, and prick each rib point deeply.

Then take a felt-tipped pen, and press it into each pin hole.  By doing this, you're actually tattooing the rib points, ensuring that you can find them easily throughout the rest of the carving process.

Laying out the Rib Lines
To lay out the rib lines on your mold, you need to connect the dots--all of the rib points on each of the cross sections in your mold--from the very tip of your top block all the way back to the bottom end of the mold. 

The ribs are laid out on the face of the top block using a template derived from your cross section drawing. Notice how I've used a chisel to chamfer down to the line here, just as I did on the cross section blocks of the mold.

On the bottom end of the mold, you can use your cross-section drawing to lay out the orientation of the ribs as they disappear under the capping strip (in fact, you can lay out the location of the capping strip at the same time as you mark out the ribs.)

Use a pin prick through the drawing to show the location of each rib line below the capping strip.

Now, to connect the dots and make an accurate rib line, you'll want to make yourself this very handy tool.  Take a thin, flexible strip of plexiglass, between about 15-20 mm wide, and about 70 or 80mm long.  Scribe a straight line down the center of it, and every 10mm or so, drill a small hole for your marking pin to fit through.  Take this marking strap to your mold, and set the scribed line over the dots of one of the rib lines from the top to the bottom (you can fix the strap in place with pins).

 

When everything's aligned correctly, use your marking pin to prick through the holes into the mold.  Then remove the marking strap, and tattoo the pin holes with a felt-tip pen. 

All that's left to do now is to connect these tattooed dots, and I do this with a sharp, hard pencil and a flexible curve laid on its side.  (You'll need to cut away the inking 'lip' on the side of the flexible curve for it to lie close to the mold so you can mark an accurate line.)

Here are the rib lines, neatly laid out. 

I like using the marking strap and the flexible curve, rather than just the flexible curve, because the marking strap ensures that I'm creating a single, continuous line.  Because it's too short to reach over the entire length of the mold, the flexible curve by itself can only give segments of a line, with no real guarantee that the segments will align well.

Creating the Rib Facets

Now that the lines are laid out, the mold's facets can be carved.  This operation is pretty straightforward: all we're doing is removing material between the rib lines.  As long as you can plane, and scrape, between the lines, you'll be fine.

For removing the bulk of the material, you can use your small finger plane.

Just a word of caution though--you should leave the center rib, with our longitudinal section, unplaned for now.  We'll need the long section for reference for a little while longer.

Once you've removed most of the material between the rib lines, you can start working with a curved scraper to really flatten the facet.  You should scrape pretty much right out to the rib lines, and the facet should be quite flat between the rib lines.

Down at the top block we can really start to do some very accurate work as we shape up the rib lines and facets.  You might remove the top block and chamfer down to the cross section line on the front of the mold:

Then replace the top block, and chamfer the back edge of it to match.

As for the bottom end of the mold, you should stop flattening the facets of the ribs as you approach the top edge of what will be the capping strip.  Instead, use your finger plane, scrapers, files and sanding blocks to flatten the whole capping strip area.

I think this is a good place to stop for today.  We've taken the rough shape and refined it; we've laid our our rib lines and capping strip, and the project's actually started to look a lot like a lute mold.  Next time out, I'll discuss some ways we can refine the shape further--and further, and further still--until we reach a point where we can do nothing more, and we declare our mold finished.

The part of the process that we've just begun, of carving and refining, takes a long time, I've found, and a lot of concentration and stamina.  Have a good sleep and eat a good breakfast, and I'll meet you back in the shop in the morning.