Bracing question

[inthemountains]

Hi everyone,
I'm newly registered to the forum, though I've been reading it for a few months. I am currently building my second mandolin (I'll say more about No. 1 in another post sometime). This one is one of the Stew-Mac Army Navy style mando kits. I am building from a kit this time mainly because I can do it without a real shop. Basically I can do most of this on a tabletop. Plus I want a nice simple mando I can drag around without too much worry.

The instructions for the kit just completely skip over shaping the braces after they are attached to the soundboard and back. They show the square braces being clamped to induce an arch in the plates, then just like magic they show shaped braces.

Anyway, I'm going to be gluing on the braces as soon as my cam clamps get here in the mail, then I figure I'll first plane them to a parabolic shape w/ the violin plane, or possibly a chisel. After that I'll shape the taper to the ends of the braces. Would you do it in this order, or shape the taper first, and then shap to a parabolic profile.

Any tricks or suggestions on how to best complete this operation? How do you handle it? I've looked at my copies of Siminof's first book and Troughton's book, as well as on the web, but I haven't been able to find any really detailed illustrations of the process. I'm sure I can figure it out, but maybe someone has some good advice?

Thanks in advance, I expect I'll have more questions as I go.

--
inthemountains

[Chris Baird]

Get em glued in then taper the ends down and afterwards shape them into a rounded triangle cross section. I don't know the height dimentions that would be standard but I would guess .300"-.400"? Taper the ends down to nothing as this helps a lot in keeping the ends from coming unglued. Do some light flexing and tapping so that you may begin to build a good understanding and internal database for future instruments.

[K_D]

I built one of those campfire mando kits and it was a snap compared to a carved top.

One note of caution would be to make the main brace a little taller than shown on the plan. I took the top off and replaced the original main brace (cut to the stewmac plan spec) a couple of months ago because the top had started to sag significantly (>2mm). The plan calls for the brace to be 0.38" tall and my new brace is about 0.48" tall but a bit thinner (or tapered) toward the top edge. This will reduce the volume of the instrument a bit but seems to be holding up fine with light strings.
As far as shaping goes I used a small flat hobby plane to trim down the brace (and add a bit of taper from the base (glued to the top plate) to the top edge and sandpaper to round it off. You want to reproduce the basic shape shown on the plan. You could also use a file or even scraper blades for shaping. In both cases I scaloped the ends first then shaped the main body of the brace but I'm not sure if the order really matters.

One other thing they don't specify is what gauge strings to use and I probably over stressed the original brace with j74 (medium) strings. A friend who has a flatiron army-navy style (with the same bracing scheme) told me his instrument specified lighter gauge strings:
.010, .013, .024, .036 (like Martin M400 strings)

I would at least start out with light strings in any case.
Sorry for going OT a bit but I hope this helps,
Keith

[Luthier]

I use an induced arch in my kit as well but I use an X bracing pattern and not the transverse as the Campfire.
Perhaps this will help a little.

Don

[inthemountains]

Well, I feel a little dumb now, because after I posted I looked at Troughton's book again, and he clearly says to chisel the taper and then shape the cross section, just as Mandoplyr recomends. I guess I skimmed the chapter on building the back, instead of the top, where the info is.

Keith, I'd take your advice on using a taller main brace, but I don't have any suitable materials. I guess I'll call Stew-Mac or LMI and see if they'll sell a single piece of bracing.

Don, I came close to getting one of your kits, and I may still build one, probably the octave. How does the X bracing affect the tone? I know you use different woods than this kit. Walnut if I remember correctly? Could you post a link to the info about your kits, especially the octave mando?

[Luthier]

No problem. I can customize any kit now also by letting folks pick their wood as well as a sound hole or no sound hole options.

I am working on getting a better URL website and domain but here is the link for now:

http://mywebpage.netscape.com/kawalekd/homepage/kitpage.htm

I like the X bracing for a bit more stability and I think the general soundquality of the instrument is enhanced.
The design of my OM also requires the X bracing because the sides are more narrow at the neck/body joint.

Don

[inthemountains]

I built one of those campfire mando kits and it was a snap compared to a carved top.

One note of caution would be to make the main brace a little taller than shown on the plan. I took the top off and replaced the original main brace (cut to the stewmac plan spec) a couple of months ago because the top had started to sag significantly (>2mm). The plan calls for the brace to be 0.38" tall and my new brace is about 0.48" tall but a bit thinner (or tapered) toward the top edge. This will reduce the volume of the instrument a bit but seems to be holding up fine with light strings.

Keith
Keith (or anyone else who wants to comment),

I went and purchased some Sitka bracing material to cut the deeper brace. For about $10.00 w/ shipping I got enough raw stock to do this plus probably another whole Mando, so no problem there w/ cost.

So, I'll carve the new main brace to about .48" in height. Since I am making it taller, can I go a little thinner in profile? The original brace appears to be about .28" thick (I don't have a caliper). I was thinking of making the replacement about .22" thick perhaps. Will a narrower but taller brace still be enough to keep the top from sinking under medium strings? Or should I keep the full width as per the original measurements, make it taller, and accept a slight loss of volume from the extra mass?

Did you change the width of the brace, or just carve it to a narrower curved profile?

There is a good chance this instrument will go to someone else, so I want it to be structurally sound. I'd really prefer not to have to redo the top a year or 2 later.

Thanks

[inthemountains]

^ bump

Anyone???

[K_D]

Sorry about the delay but I've been away at music camp all week.

I haven't had a chance to try medium strings on my ver 2 bracing yet but given that the one main brace supports the whole top I would only try mediums for one day if I did at all... Given that I'm now a bit paranoid about the subject I would not use medium strings for any length of time for this bracing scheme (might also CYA if it goes to someone else).
One thing I noticed on the rev 1 brace with med strings was that each time I checked the tuning it would be pretty flat (half step or so) and continued to do so for a week or so. By the time the tuning situation stabilized the top was noticably deflecting inwards and the damage was done. I wish I would have been paying attention to this potential issue at the time. Almost all tops sink some under string pressure but should settle out within a couple of days (and hold tuning).

With regard to brace thickness I would make the brace the same thickness as the original but then taper it toward the top edge which would be about half the thickness of the base. The base needs to be be thick (wide) enough to make a good solid glue joint to the top. One very important thing about the brace is to be sure the wood grain runs vertically from the base to top and a tighter grain will be stiffer than a loose grain. On mine I used a piece from a stewmac tonebar set for carved type mando kits.

For strength I would use the same basic brace shape as the plan shows with the end-scallops proportionally the same with similar end heights as shown.

BTW I would guess that the x-brace scheme used in Don's kits is stronger which is one reason why my next flat top will be x-braced.

Keith

[Luthier]

If you have ever noticed, a flat bed tractor trailer without any load on it is not flat at all. #They build them with an upward arch to compensate for the weight that will be added. #The top of the trailer is in compression and the bottom is in tension. #Not that different from your induced arch bracing. #I have just found that the X induced arch bracing for the flat tops has worked well structurally and for sound quality.

Don

[Dolamon]

As Don said above the built in tension in the top will help normalize and stabilze the instrument when it's strung up. The other issue is to go to Black Diamond mandolin strings. These are recreations (but much better made) of the old silver wound strings which were common about a hundred years ago. Their tension is very low but the playability is VERY good and the sound - amazing.

I've got them on a couple Larson's, a Vega Bowl back and even tried them on a marvelous KM110 S with a standard length scale. (The other three are short scale instruments @320 to 340mm.) These will require an adjustment in playing approach but they are loud and precise and express about 1/3 less top tension. D'Addario and GHS just don't make a string light enough for the "softer" top flat instruments.

(Oh yes - Just Strings stocks these - I don't know of many other stores which have them. FQMS perhaps?)

[K_D]

Thanks for the black diamond string info Dolamon! I'll definitely give them a try.

Hey Don,
Can your kits handle medium strings (like j74s) or do you recommend something lighter?
Keith

[toddjoles]

K_D, in regards to the STEW-MAC kit, did you induce the 8-10 degree arch in ALL the braces with the sanding jig as per instructions? I use D'Adarrio mediums (J-74) and after 4 months have had no sinkage at all. It sounds much better than my pacrim with nice highs and adequate bass. My only complaint with the kit has been neck angle, it's awful. Even after setup it plays with a "manly action". After I finish my F from the tree, I'm going to pull the neck on the Campfire, rework the joint and shim the top under the finger board to solve my problem. http://www.mandolincafe.net/iB_html/non-cgi/emoticons/mandosmiley.gif

[K_D]

Yes it did (rev 1) and now (rev 2) does have the induced arching called for in the stewmac instructions. Maybe the original piece of bracing spruce was lighter or more flexible than normal but the end result has made me very wary of overloading the top... I'm probably overly cautious now..
Keith

[inthemountains]

Thanks for the replys. I'm going to carve the new brace tomorow I hope. I guess I'll keep it the same width, and just add the extra height. I'll play this one with light strings at first, maybe the Black Diamonds, and if I ever try mediums I'll be sure to keep a close eye on the top. After that I get to notch the linings and glue on the top w/ the new spool clamps I just made.

I think I like getting new tools almost as much as building. Is that a bad thing? I bet I'm not alone.

Another question: How important is it to notch the linings so the braces go all the way to the rims? Is it ever OK on a mando or guitar to just have the braces reach the inner edge of the linings instead? I assume this is to more tightly bond the top and rims, but is it for tone production or structural strength (or both)?

[Luthier]

[QUOTE]Can your kits handle medium strings (like j74s) or do you recommend something lighter?

Yes. the medium gauge should not pse any problem. #Sometimes the string slots on the nut may need to be adjusted however to compensate.

[QUOTE]Another question: How important is it to notch the linings so the braces go all the way to the rims?

I would notch the linings to accept the braces.

Here is a picture of the jig I use to carve my braces