Maple plate carving

[Steve Mazy]

Would you experienced archtop makers share your methods for the initial “hogging off” the majority of the glued up plates in preparation for getting down to the fine carving of the final shape. Gouges, power tools etc. thanks for your replies.

[Barry Daniels]

What kind of maple? Any figure? How much do you need to remove? What tools you got?

[Alan Carruth]

I've been using a Wagner Safety Planer in the drill press for the outside of the plates. I use the lengthwise and crosswise arch templates (or make up your own) to develop a set of contour maps at 2 mm intervals. These are marked out on the wood, starting from the top, and the DPP used to hog off material a little above and outside of the actual desired contours. This takes off about 90% of the waste, and I use planes, gouges, and scrapers, to smooth things off from there.

Once the outside arch is done I set up a raised stop on the drill press table, and set the depth stop so that a sharp bit stops at whatever distance above that. This is used to dimple the inside of the plate. Then gouges and planes are used remove the bulk of the wood down to the dimples. I tend to do this is stages, taking the plate down to, say, 10 mm first, then going for something a bit more than the thickest part all over, and then working out to the edges. Naturally you will want to make the initial grads thicker than you think they'll end up, and sneak up on the final graduations using whatever method of testing you've decided on. I use Chladni patterns, but I'd follow the same sort of process if I was using tap tones or displacement, or mystically channeling the spirit of John D'Angelico or Orville Gibson.

One caveat with dimpling from the inside: get a drill press that has a good column stop. The ones that have the setting over on the handle side in a rotating sleeve with a setscrew tend to fail badly. That is, when you lose the adjustment (which happens when you repeatedly bottom the thing out) it tends to drill deeper, and you can end up drilling right through the plate. Hard to fix.

The good sort of column stop has a threaded rod attached to the column above the chuck that runs through a block on the side of the upper housing. Nuts on the thread rod allow you to set both 'up' and 'down' stops, and usually the down stop uses two nuts; one as a jam nut to secure the other in place. If this sort of stop fails because the nuts get loose they tend to drift downward, which stops the quill before it reaches the setting you want: the plate ends up too thick rather than too thin which is a drag but not a problem.

[Steve Mazy]

Alan, Thanks so much for the response. I must say that I used the Benedetto arch templates to derive the outside arches---the longitudinal center seam arch and latitudinal arches at the waist and lower bout tangent points and drilled holes and went from there. I was aware of the "topo" method you described, but didn't feel I could suss out the contours from the information I had. I do have a safety planer and would surely wish to employ that method, as it seems the most reasonably efficient way of having a visual map of the shape before you. Is it simply a mathematical exercise to create the 2mm contour map? After gluing up the wedges would you first cut the outside profile of the guitar and then start with a 1/4" plateau of the width of the recurve an go from there? Makes perfect sense to me but having never done that it is somewhat intimidating. But then...so is carving into a nice piece of maple for the first time. I know, there's always practice but...when I approach the top I need to have a better idea of where I'm going.
Thanks, Steve

[Steve Mazy]

Barry, Thanks for your questions. I thought I had replied to your post last night. I was using my phone because there isn't internet at my shop, where I've been staying most nights. As you probably noticed from my response to Alan, I've already carved the back plate but in a much cruder and haphazard way. It came out relatively well, no disasters like drilling through the plate, but it was time consuming and nerve wracking to go at it like that. To answer your questions: It's highly figured curly maple, wedges !-1/4 " thick at the center, I've got most every tool I could need. Planes, gouges, sanders, chain saw wheel on a 4-1/2" grinder etc.. I was basically asking after the fact for some other methods used by readers.
Thanks, Steve

[Alan Carruth]

Making the contour maps is mostly a drafting problem. You start with the outline, and lay out the lengthwise and crosswise arches on it. Start from the low point of the arch as being at some reasonable level above the bottom of the plate. Measure up from the base line of each arch to where the height is 2 mm higher than the low point, and mark that point along the arch base line. When you have all the points you connect the dots with curves. Naturally the more cross arch lines you have the easier it is to come up with smooth and 'correct' contours. You do this for every level.

A friend on mine, Mike Mahar, has written up a nice Android app he calls 'Lutheir Lab' that you can download for free. It has a number of useful tools, including one that calculates cross arches and contour lines using the 'curtate cycloid' method. This gives the whole contour including the recurve all the way out to the low point. You can save the graphic as a file and have your local print shop print it out full size. I have not used this particular feature of the app; since I already have my arch patterns I have not needed it. Mike (and his wife; both software developers) have done such a good job on everything I have tried that I have no worries, though.

I make the low point of the arch just inside of the inner edge of the liners. I have seen guitars that were made with the low point further in, and the tops tend to collapse over time. Maybe you lose some bass response this way, but I doubt it, and, at any rate, a very good guitar you can play beats a great one you can't play.

[Steve Mazy]

Alan,
Thanks so much for all your help and advice.
Steve Mazy

[Alan Carruth]

I have a few photos of that stage on the most recent archtop: I hope these are not too big.

Here is the layout for the pattern I use to transfer the contour lines to the plates. Since I generally make the back arch lower than the top I only make a 'top' pattern. The manila pattern on the top is one I've been using; the lower one for the new instrument has a different lengthwise arch.

Here I'm milling of the first step after taking the blank to the thickness for the high point.

I've trimmed the plate to the exact outline of the ribs, and routed a channel down to the low point, which, as I say, is just inside the liners.

And here are the plates with the contours finished. I drilled the holes for the 'eyes' of the F-holes before taking out the inside to thickness to help avoid chipout.

[Barry Daniels]

Nice looking plates, Alan. Can you tell me why you leave the lip around the perimeter?

[Alan Carruth]

The lip is there to support the router when I make the rabbet for the purfling and binding. It's easy enough to carve it down to fair into the recurve once the binding is on, and I don't think it adds so much stiffness around the edge that it messes up the plate tuning noticeably.

[Steve Mazy]

Alan,
Great pics. I’ve done some drawing and from looking at your sketch and pattern I think I’m on the right track.
A couple questions:
Where is the bridge in relation to the peak of the dome. Is it a consideration when drawing the contours?
My arch is 25 mm high which means the top plateau is somewhat smaller than yours, hence the previous question.
At the bouts do you have particular arches established?
I’ve been using the Benedetto book as a guide to the arching patterns although I’ve modified the cutaway slightly, and am getting a little bogged down from lack of experience. Obviously the ramps at the waists and cutaway are a little steeper, but at what point do I extend that influence to the adjacent contours?
I’m feeling like I need to just jump in and do it and hope for some empirical knowledge along the way. Thank you immensely for your guidance. Steve

[Barry Daniels]

It is always good to have a nice archtop that you can study to see how it all flows together.

[Steve Mazy]

Well put Barry. I must have spent an hour or more just looking at Alan’s pattern sketch trying to absorb the flow of the contours. It’s really an amazing piece of work. Invaluable to see the physical representation of his methods at our disposal.
Many thanks to Alan for his generous contributions.
Alan,
I think I have answered most of my own questions in my last post simply by viewing your pics. Thank you and beautiful work!

[Alan Carruth]

I make the high point of the arch at the bridge location on the top. As I may have mentioned I only make a pattern for the top, since, in the way I do it, the back is lower.

I'm using the 'curtate cycloid' method of laying out the cross arches. Once you have determined the lengthwise arch along the center line everything else is defined down to the low point. It produces a nice arch shape, which has worked well for me so far. There are several ways to do the layout for this. I'm told there is an on line calculator that will produce the cross arch shapes given the width and the height at a given point. The 'Luthier Lab' Android app that was developed by my friend Mike Mahar has a module that will also generate the cycloid cross arches and save the file in a format that can be printed out by most print shops. I tend to do it the old fashioned way.

Background:

People have been discussing the cross arch layout methods that Strad et al used for a long time, and lots of ideas have been put forth. Some time back a maker name Quentin Playfair realized that the sexy math at that time was in cycloid curves, which were used in the reigning Ptolemaic astronomy models. It had all been figured out about a generation before Nicolo Amati; time enough to filter down to the masses, so to speak.

If you mark a point on the edge of a wheel and roll it along a surface the point traces a cycloid curve, with humps in between the points where the edge of the wheel touches the surface. If you use a point that is off center within the wheel the line undulates up and down. If you make a wheel that will roll exactly once across the width of the instrument at a particular point, and then have the hole off center by half the arch height you can generate a curve that starts out at the low point at the edge, and sweeps upward to an inflection point where it switches to a down facing curve in the center, and then sweeps up again to the other edge. The only tricky part is making the wheel so that it rolls just once across the width. If you remember that pi~22/7 it becomes the sort of drafting problem that some of the old boys were really good at.

Several years ago one of the members of the on line violin makers group said that many makers used bad arch shapes. I contacted him off the list to see if he could tell me what a good shape was, but he said that the only way he knew to do that was to have me spend some months at his shop. A couple of years later, when the curtate cycloids came out he tried the system, and posted that it was even better than the templates he had made from Strad instruments. After all, the old instruments have all suffered distortion over the years. Nobody is perfect; there will always be errors in execution of the model, but this system gives a simple shop method for determining the cross arch.

As to the lengthwise arch: so far I don't know of any 'magic' system for that. I've always used a bent stick. Depending on how you do it you can end up with all sorts of shapes. On the one in the picture I was going for a 'acoustic copy' of a Lloyd Loar L-5, and the lengthwise arch I wanted was a lot different from what I've usually done, since it rises faster art the ends and is flatter in the center. One of my students who builds archtops had a lengthwise template that he took from an old Gibson. I took the liberty of making some corrections for what I thought the changes would be from aging, and developed the cross arches from there. It looks about right. Sadly, we were unable to get the one I made back together with the original to do a direct comparison, but the customer is happy with the sound, so what the heck.

[Dick Hutchings]

I spent some time doing the math and cutting a circle from cardboard and making a hole about 3/8" from the center. Low and behold, I've drawn a nice arch. Thank you.

In the past I've used templates or just eyeballed my mandolin tops. I'll give this a try next time and see how it lines up with my templates made from the Siminoff book.