Benedetto Archtop Build

[Ron Sommers]

Hi all, this is my first attempt at constructing a guitar.

Thought I'd document the process.

Got the book a few years ago and am finally getting around to it.

I've made 5 violins to date, so it couldn't be much different than a cello in size, so why not give it a go?

Reading several blogs I learned one couldn't make it from the book alone, so I bought the plans.

The plans are in greater detail, but I think I could have made it from the book. I made violin arching templates from the Sacconi book with what the Benedetto book provides.

How I did it was to mark off the arching templates using a millimeter ruler by sliding the ruler until I hit 10 mm, or 11, 12, 13, etc. (I didn't want to deal with half 0.5 mm's). I then traced the shape to paper and finally to an aluminum sheet. Cut the shape out and I had a durable template.

For the archtop, I used the same method and transferred the arches to the body using a triangle and a straight edge. I then "connect the dots".

see 'transfer' jpg. below

I'll then 'transfer' the drawn lines to the wood, then drill holes etc.

I have the Benedetto plans and decided to do the same thing, however, there appears to be a conflict in the drawings!

If you'll notice the back Arching Layout jpg (from the bottom to top it starts at "A", "B" etc.)?
Looking at Arching "B" center it's 24 mm, but when you look at the side view to the left of the body it doesn't show 24 mm, but 22 mm! Find the '22' at the bottom, then scroll up to the center line. Same for the "D" arching. It says '24' but the side arch shows '22'.

see 'arching' jpg

Logically, this can not be. I think erring on the 24 mm side is the way to go here, i.e., start numbering from the 24 mm center and delineate from there to the side.

see 'conflict' jpg

I found these carved plates online and they tend to agree with what I'm thinking.

see 'carved' jpg

Anyone care to comment on my assumptions?

[Ron Sommers]

Still learning how to post jpgs.

Wouldn't edit to repost above, so here they are below!

[Jason Rodgers]

The Benedetto book was my gateway drug into instrument building. Still haven't built that archtop, but someday.

My suggestion with your 2 mm conundrum is to eyeball it and make it look right.

So, are the pictured plates some pre-profiled products you actually purchased?

[Chris Richards]

Hi

I don't know whether it will help, I was in much the same situation, I had made several solid body guitars but never an acoustic or archtop.

Hopefully it will help; have a look on my website (www.3rguitars.co.uk) under the "archtop build" menu where I've posted quite a detailed build story, the Benedetto book was superb but some of the tools and jigs would be quite expensive to buy/build just for a one off, you may get a few ideas from my page.

Chris

[Ron Sommers]

So, are the pictured plates some pre-profiled products you actually purchased?

No. I snagged them from google images, but credit can be applied to Scott here:

http://scotts-workshop.blogspot.com/201 ... lates.html

It's one way to hog out a lot of wood quickly using the Safe-T-planer.

[Ron Sommers]

Thanks for the link Chris, I'll check it out.

The Benedetto plans use 'inches' and I prefer 'metric'. It's very precise and easy to grok (in case anyone wonders why I switched).

Here are two examples of what I've gotten so far. The 1st uses a uniform spacing (10mm) along the arch templates and transferring an XY grid to the tracing paper over the plan.

The 2nd example is what I finally decided was the better route because it eliminates fractions. Instead of uniform horizontal spacing I used vertical spacing - as in moving the rule along the template and making a mark for each mm as it 'fit' the space.

You'll notice it comes out 'cleaner', or more even without fractions (19mm vs 19.5mm).

I didn't complete the right side because it should be a mirror image of the left side. I'll flip the tracing over to transfer the lines.

The 3rd pic I snagged from a guy in the east smithcreekmandolin who carves tops and backs to Benedetto specs with a CNC for a fee. It shows a good representation of the end shape.

[Ron Sommers]

Adding pictures is backwards to the way one thinks.

You have to add the last one first. To 'fix' the order, I'd have to delete all of them.

Here's the last one flipped to match the drawings.

[Patrick Hanna]

Hello, Ron,
Right at the outset, I'm going to say that others might disagree with the advice I'm about to give you. Okay?

Now that is understood, so I'll just say that I don't think you need to get all hung up in discrepancies on your paper. In my view, what you need to do is clamp your back or top plate to your carving cradle and start planing away. Go slowly. Take your time. Work evenly on either side, and from the neck end to the tail end. Use the templates provided in the Benedetto book (or the take offs on his plans) as a general guide to make sure you are carving with symmetry. Mark your edge thickness all around the perimeter and DO NOT violate that thickness while you are planing. When you think you are close, rotate the cradle through 360 degrees several times under a strong raking light. Study the highlights and shadows and make sure they appear symmetrical to your eye. Then finish up the outside with a well tuned curved scraper. Use the scraper for your recurves. Check continually. It's like sculpture. Lots of nuances in the curvature will give you a beautiful looking and a beautiful sounding guitar. There is no absolute here. You just need to proceed slowly with patience and with an overall eye toward symmetry. Benedetto himself says that he never uses templates anymore. He just does it all by eye. Of course, he's built a thousand more guitars than you or I ever will. But his point, and my point, are both well intentioned. I just described to you how I made mine. I referenced his book continually, but I left myself total freedom to build it the way I wanted it. Mine turned out fine for my purposes.

[Jason Rodgers]

Good advice, Patrick. I played with those topographic sketches for a long time, but I can't imagine using them (at least not with my construction style now).

After reading a bit more outside of Benedetto, mostly in American Lutherie, it seems like there are a few basic parameters that folks consider when designing an instrument arching profile.
1. Arch height. Generally speaking, a higher arch can give a more strident sound, closer to the midrangy, early jazz comping machines. A lower arch brings out more low end for that smooth, marshmallow tone. Bridge height is part of this consideration, too.
2. Recurve width and depth. This obviously has an effect on the shape of the belly, but aside from the thinning of the recurve, I'm not sure how this contributes to the resultant tone. Some are very slight, with the arch coming nearly to the edge, and then there's the dramatic "German carve."
3. Arch profile. Folks get downright nutty about the shape (x and y directions) of the arch. Spline, cylindrical, curtate cycloid, Golden proportion... potato potahto.

Listen to Patrick. Get a few under your belt, then decide if you want to take the blue pill or the red pill.

[Patrick Hanna]

Hey, Ron,
One other detail I forgot to mention: The carving templates in the Benedetto book include the recurve. That recurve will make your templates sit high on your plates near the edge and make it difficult to judge the fairness of your curve as you work down to the "ledge" on the edge. Things will go easier for you if you eliminate that recurve from your templates. Make your templates to the general arch of your choosing, coming down from the high point and resting on the ledge right at the edge of your plates. You will plane or, better still, you will scrape the recurve in after you've established basic curve. I hope that made sense. I'm attaching a quick, rough schematic drawing to illustrate what I mean.
Patrick

[Dave Stewart]

That's the way I've preferred to do it, as Patrick says. Reduce the plate edge initially to 1/4", rough carve the arching (ie remove the bulk of the material) then reduce the edge to 3/16" and, using templates with the recurve removed, finalize the arch along each station, starting along the centerline. When all templates "bottom", fair the areas in between 'till you're happy. Then using the thickness gauge, carve the recurve close to final & blend with the arching.
(I like to use full width templates BTW.... much easier to tell if you're doing things symmetrically)

[Ron Sommers]

Good advice from everyone.

It's not much different than carving a violin. Just bigger!

The 'difference' is the cutaway. If I was making a regular guitar (no cutaway) it would be nearly the same process.

Violins have a sound post and one bass bar vs guitar with no sound post and two "bass" bars.

Going over the drawings as I have gives me a mental picture of what the final shape will be.

My plan is to use the drawings to layout the drill pattern on the wood.

After that, it's hog out what doesn't look like a guitar!

[Patrick Hanna]

Ron,
Yes, the cutaway and its horn get tricky, because you have two options: First, you can carve symmetrically, which will leave a much higher binding ledge in your cutaway. I have seen Gibsons done this way. Or, you can carve down to the perimeter ledge all around, as Benedetto does, and as I did. That will give you a uniform binding channel, but an a-symmetrical shape on the top and back right in your cutaway. There is no right or wrong. It is all up to your personal preference.
Patrick

[Ron Sommers]

Ok, a little update.

Been working on a few things. Transferred the tracing paper to 1/8" thick 'chip board' I purchased at the local art supply store. It's fairly thick and 'card-like', but when I drilled holes it shredded like the paper it's made of. Put on my thinking hat and decided that spraying one side with several coats of white enamel and the business side with several coats of clear enamel it would stiffen up. It did.

Thought about using metal, which is more permanent, but cutting it to shape was problematic and it has sharp edges.

Observations on carving the back.

The back is fairly complete. I want to lightly sand the gluing edge one more time. The edges need truing and I need to thickness route them to 5mm.

It's really hard maple! Violins seem easy by comparison. Maybe it's the size of the piece??

The pattern I drew from transferring Benedetto's plans 'works', but the end result isn't exactly the shape of the template especially around the waist portion. I noticed the waist template was a little more "fuller" and rounded. I relied on the technique I use to layout violins. Probably should have depended on the template?

As I would a violin, I scraped and cajoled the wood off the outside using my chisel and mallet.

For the inside, I made use of the ArborTech Turbo Plane. If you really want to speed up the process, get one of these. At $150 they are pricy, but so is the Saf-T-planer these days, which is limited to thicknessing from what I could tell? I don't think the latter would hog out the inside like the ArborTech. You'll also need a side angle grinder to attach it to. It will create mounds of finely chipped wood in short order!

It cuts through fast. I think I spent 35 minutes carving the inside down?

It was a short learning curve to keep from going too deep. I kept going until I had about 1mm of drill hole remaining. Then I used my toothed finger planes to get it down to about 5.5 mm. Tap tone was "F". My end goal was 4.75mm (3/16"). I didn't want to get over zealous being my first attempt.

In the process of drilling the holes to depth on the inside, the 'stop' left dings in the topside! I had to scrape them clean before proceeding to thickness the inside. This alone will change the profile a little. I reshaped the 'stop' and put a thicker piece of cork on it to eliminate the possibility.

I drilled the next level of (final) holes using this setup and it left no dings topside. Finger planed the excess and scraped etc.

I measured the thickness and it varies slightly from 4.85mm to 4.4mm, but overall it's fairly close considering the crude setup I'm using. A duplicator would really be handy to own!

The pictures that follow are of the enameled chipboard with lines and holes. Inside and out of the carved back. One of moi using the early morning sun to find the dings!

I haven't weighed the piece yet, but the tap tone is a definite "C". I suspect it will drop to a Bb, or "A" by the time the edges are trued and the recurve is installed.

I know Benedetto says the actual tap tone doesn't matter, but I figure taking notes like this is a good idea. It is also relied upon in the violin making world.

[Jim Hepler]

"In the process of drilling the holes to depth on the inside, the 'stop' left dings in the topside! I had to scrape them clean before proceeding to thickness the inside. This alone will change the profile a little. I reshaped the 'stop' and put a thicker piece of cork on it to eliminate the possibility. "

I have had that issue as well. Rather than scrape down to the dings, I was able to steam them out quite easily. Mine were just dents - that is no broken fibres. I used the steam wand on my coffee machine, and they were all gone in a few minutes. I then moistened the entire surface and let it dry to make sure that nothing reappeared. This was on hard maple, and I had no probelms with it after that. Since then, I've put a piece of rubber gasket material over my stop to prevent the dents. Seems to work.

As I understand it the problem with scraping down to the dents is that the compressed wood may resweel at some point leaving lumps. I haven't experienced that myself, but it sounds reasonable since the dents aren't actually the result of any material having been removed.

Your guitar looks really nice by the way.

[Ron Sommers]

No pictures, but the top is carved.

The top is a smidge thin in the center. It's supposed to be 1/4", or 6.35mm and it's closer to 6-6.1mm or 0.236". On the other hand, the 1/16" section might be a bit thicker?

I went with the 3 way carving 1/4", 3/16", and 1/16" and blended the remains.
That's why the middle is a bit thin. I used a sanding block to achieve the blend.
I should have started blending one operation sooner when the center was 6.6mm.

I can see where having a router duplicator, or a CNC to do the carving would speed things up a bit.

Tap tones.

I said earlier the back was an "F". I'm now thinking it's "Bb"?
Of course it depends upon where one holds the node.

For the "Bb" I got today I held 2" at the waist and a little bit forward toward the neck.
Tapping in the center gave me the "Bb".

The top is a clear "E" using the same technique.

I still need to trim the edges of both back and top to 5mm.
I figured I'd tackle them at the same time, as the router is set up on a table with wheels and I have to roll it out to use it.

I'll retap after I trim. It should change the tone a little. Especially after the 'F' holes.

On to the recurve.

Benedetto says to carve it after the guitar is assembled.

Violins are carved and assembled with no 'recurve' after the fact.
It's carved as part of the plate before hand.

To my way of thinking, how does one know how thin the recurve is if it can't be measured?


Anyone have advice here?

Pros? Cons? to carving it before assembly? After?

If after, what "tone" does one listen for while carving?


Next up, thicknessing the sides and bending.

[Dave Stewart]

Ron, you really need some form of Hacklinger gauge to measure thickness if doing much carving after assembly.
One method I favour is carving the bulk of the recurve before, by marking a line (bottom of recurve trough, 1" in from edge or whatever) around the perimeter of the plate. Then stopdrill from the top along that line to .130 - .140" depth. and carve & smooth the recurve. This will get you most of the way there re fairing of all the outside curves before ass'y and leave a bit of thickness (quite a bit for the back) to refine the recurve after ass'y, depending on the feedback from the plate. (The drill will compress the wood under it, so you'll find you have to wet the trough /raise the grain half dozen times to finally get rid of the bumps).

BTW, don't use cork on the DP pin... it compress & will yield inaccuracy
Also, I hope you meant 1/8" rather than 1/16" thickness above.

[Ron Sommers]

Yes, 1/8"! Thanks for the correction. Was thinking 3/16" and one less... Tired after 4 hours of carving.

As for the Hacklinger gauge... Isn't that the magnetic one that costs a few hundred?
I wonder how the old timers did it before that was invented?

I'm going to do as you suggest and carve the bulk before.

Cellos are the same size (in the tonal family) and there's no 'recurve' after carving the plates.
Same with violins. It's 'figured' into the carving.

Still wondering what I'm 'listening' for when tapping the plate...

[Dave Stewart]

Also posted here, though I don't think they chose to keep it!
http://www.luthiersforum.com/forum/view ... hacklinger
(You'll definitely hear a change as the recurve thins ...hard to explain. Tap at bridge position and feel vibrations right down into the recurve with pads of fingers....... should feel vibrations everywhere.)

[Ron Sommers]

Thanks for the link. Just joined. Waiting for approval...

Routed the back and top edge thickness.

Thought I covered all the bases, but one slight error on the upper bout edge on the top.

It's supposed to be 3/8" thick but the top flexes when pressed down and the edge rises which caused the corner to be 2/8" thick at the curve! Grrrr...

This will effect the binding I'm sure.

Toss it in the bin and start anew, or make it work?

Fortunately, the spruce is a lot easier to carve, but I'd hate to redo it.

Suggestions?

Next time I'll cut the edge thickness 'before' carving! I thought I was being conservative with the edge. I didn't want to ruin it with a plane.

The back, being much stiffer, is perfect.