Official Luthiers Forum!

Alright gang...now that my 1st top is inlaid (thanks to ya'lls help & i'll post pics soon)...

I am building a Go-Deck and need to know what size forms to (build/have built/buy). What is the standard top and back radii for a Martin Dread??? I might build the forms, but, I think that I may be able to get my machinist friend to build them for 6 pack after hours (if I supply the wood). IF so any recommendations on the type of wood to use...oak/birch ply??? or MDF???

Thanks again for the help...oh yeah...and thanks for the HomeDepot plug...I think I'll look into getting some reflectors and harvest the rods for the go-bars...GREAT STUFF...

JP

[QUOTE=Dread1916] Alright gang...now that my 1st top is inlaid (thanks
to ya'lls help & i'll post pics soon)...

I am building a Go-Deck and need to know what size forms to (build/
have built/buy). What is the standard top and back radii for a Martin
Dread???   IF so any recommendations on the type of wood to use...oak/
birch ply??? or MDF???

Thanks again for the help...oh yeah...and thanks for the HomeDepot
plug...I think I'll look into getting some reflectors and harvest the rods for
the go-bars...GREAT STUFF...

JP[/QUOTE]

JP,

Not sure on exact radius for a Martin top but I use 20' and 25' radius
dishes for tops and 12' or 15' for backs on my steel strings and classicals.

As for wood for making dishes...I'd probably go for MDF.

Cheers Martin

Hi JP,

The most common size dishes I sell to steel strings makers (and use myself!) are 15 foot for the back dish and 25 foot for the top dish. Some like a 30 foot radius top also but it is not quite as common. My dishes are 24 inch diametre and that is pretty standard, some go 22 but I like the extra inch all around, never know when you might think about a BIG body guitar or acoustic bass. I use two layers of MDF, some just use one but I think the double ply just adds stability to the dishes and the extra weight makes them a joy to use. When you make them make sure that you get one piece sandpaper for them.

Good luck

Shane

_________________
Canada

Martin spec for tops
28 foot radius on all tops
BAcks
   Dreds and jumbo are 24
   000 and smaller are 20
these are the martin specs so take your pick.
john hall
blues creek guitars

JP is your machinist friend planning to cut these dishes on a vertical mill? If so that is very nice of him. One warning though! Bowling dishes is the dirtiest job in luthierie, the dust from bowling gets everywhere I also recommend that you use waterproof EXTIRA MDF.

I just made a double sided set from 2 pieces of 3/4 sandply from Home Depot glued up to 1.5" ....15' radius on one side and 28' on the other. There were some voids but Bondo patched them up real quick. Took about 20 minutes per side with a router after setting up a carriage jig. It was a bit messy but no more so than many other duties in a woodwork shop environment.

Did you coat them with shellac when done? Also I have always used MDF but tought about ply but have heard stories of the ply delaminating. It may not really be an issue but have not tried it with ply.

Another thing just crossed my mind! Having a dish with both profiles cut in it is convenient and should work just great as sanding dishes but, I would have concerns about using it for brace clamping. It would seem to me that not having the a flat continious surface backing up the bowl during clamping may lead to the bowl deflecting during brace clamping. You may have enough rigidity with the 2 -? ply’s laminated together, but before I used it to glue up braces, I would do a deflection test with a dial indicator, just to be sure. the greatest chance of deflection is going to be at the center because that is the thinest area but this could lead to braces not gulung up properly if there is deflection there.MichaelP38777.4639583333

Michael,
You are correct. We had tossed around the idea of doing this also for our dishes that we sell, but we found that there would be too much deflection. To correct this you could put another 3/4" mdf piece in the middle of the 2 dishes, but it would make the dishes so heavy, that we couldn't ship it! But if someone makes it themselves, it will probably work. If gluing the pieces together, never use a white glue, as the water content could cause warpage. Use a resin glue like gorilla glue, or some fiberglass, or epoxy glue.

Oh yea, you should make your dishes 24" round as this is the standard size for sandpaper discs. It will be a lot easier to apply the discs if the size is exact. Also, make your go-bar deck at least 24" square on top and bottom. Hope this helps, and good luck!
Tracytl50736238777.5169444444

There is another way to address deflection and that is to make a block that fits under your dish. You can make this block out of a scrap of 2x4 about 6 inches long. Sand the curve of the dish onto the bottom of the block until it fits into the dish curve, then use a straight edge and scribe a line across the block and re-saw to thickness. Place that block under your dish and you will be just fine! I have only made one double dish so far, for Luigi in Italy, and I didn't check for deflection but it sure seemed solid to me. I use 2 ply 3/4 inch MDF and I also do glue them togther with AR glue (yellow carpenters glue). There is not enough moisture in that glue to cause any problems at all in the 50 or so dishes I have made. Also, you are carving the dish AFTER the glue has dried so you should be correcting for any deflection taht is caused by any of the laminating process. Sorry Tracy, I am not meaning to be rude just showing that there are a bunch of different ways of doing things. Also, the next double dish I do I will check for defelction.

As Tracy says..Good Luck

Shane

_________________
Canada

I used 3 lb shellac to seal them. Since I'm using a vacuum system for for brace glue ups I don't think I need to worry about any distortions. I may be wrong but I hope not.

If you have a membrane on both the sides of your fixtrure drawing toward each other then there is no issue, but if you have only one membrane that draws down to a closed bottom or sealed fixture, then the loading is still coming from one direction and is likly to deflect the bowl. Please don't take this as a critisiim of what you have done. it is possible that you added enough section mass to overcome the deflection since you used two 3/4" ply sections laminated togather but a good way to check is to mic them plus the membrane thichness un-loaded then mic them and the membrane with the vacum on then you would know for sure.MichaelP38777.5744791667

Yeah, I should have said that I use a bag, and not a flip top system(yet). Thanks for the advice though.

Michael, I know you are probably trying to be polite, but Man! I don't know what you just said . Maybe you can help out on that!

Thanks

Shane

_________________
Canada

since I messed up my finger my typing is very suspect

Anyway Gasawdust built a single bowl profiled on both sides, and said that he used vacuum clamping. So the possibility of deflection should not be a problem.

I pointed out that if his vacuum fixture had only a top membrane that the bowl may still deflect just as it would with go-bars, because of the hollow on both sides of his bowl. and not having any support in the thinnest area of the bowl.

He told me that he uses a vacuum bag, so the force of the vacuum is applied from both directions, there by equalizing the forces exerted on the bowl, i.e. no deflection.


On the other hand if the force is exerted by a membrane from the top side only, then the possibility of deflection or deformation in the bowls shape, due to no support in the deepest areas of the bowl would still be possible.

Better

Yup, got it...not sure I concur, because the membrane has a LOT more give than the dish, but now at least I understand what you were saying....I can be a wee bit slow and somewhat reading impaired.

Thanks Michael!

Shane

_________________
Canada

You all are a wealth of knowledge...and I appreciate you guys for sharing this with a newbie.

Hopefully my machinist friend will help me with this venture (all depends on where the boss is and what kind of time he has)...Yeah a verticle CNC mill is the sweetest way to build one of these forms. Of course, with the shop vac hose mounted on the mill head to suck all the dust coming from the ball nose mill.

Since I am hoping to get bracing in about 2 wks, I think that I may end up buying 1 dish, if we can machine these in time. Let me know if you have any recommendations on suppliers.

Last thing...ya'll gave me a potentially good idea for a convex (domed) forms for shaping the back side of a bridge. Has any one just taken a top dish form and waxed it was car wax and filled in with fiberglass and resin??? This would make a quick dome form for just a few bucks. I'll probably try this once I get all braced up and I am ready for the dome form...if it works...I'll send pics and a how to.

Thanks again,
JP

Hey JP,

If you need a dish to get you going, both Tracy and I sell them. Tracy is listed above as Luthier Suppliers and of course mine is High Mountain (link below or above). For reverse form for shaping the back side of bridges just take a chunk of 2 x 4 and sand the curve on your dish and then put soe adhesive paper on that 2 x 4. Voila! You have yourself a form to sand the bottom of the bridge!

Good luck with it, and as Michael said, the dust WILL get every where. But MDF dust is heavy so it settles fast and does clean up well. Also, when your are done make sure you put a finish on your finished dish to seal it. Mine are covered all sides with two coats of Varathane Diamond Coat, a waterbased finish. I am very happy with it and have had good responces from those that buy the dishes.

Shane

_________________
Canada

[QUOTE=Dread1916]
Last thing...ya'll gave me a potentially good idea for a convex (domed) forms for shaping the back side of a bridge. Has any one just taken a top dish form and waxed it was car wax and filled in with fiberglass and resin??? This would make a quick dome form for just a few bucks. I'll probably try this once I get all braced up and I am ready for the dome form...if it works...I'll send pics and a how to.

Thanks again,
JP [/QUOTE]

JP, I believe that Tim McKnight did something like this but instead of fiberglass and resin he used that old standard Bondo. You might consider giving it a try. Also Tracy at Luthier Suppliers sells a small domed dish (10") for bridge sanding. Don A38778.6817939815

_________________
Don Atwood
Arlington, VA

[QUOTE=Dread1916]
Last thing...ya'll gave me a potentially good idea for a convex (domed) forms for shaping the back side of a bridge. Has any one just taken a top dish form and waxed it was car wax and filled in with fiberglass and resin??? This would make a quick dome form for just a few bucks. I'll probably try this once I get all braced up and I am ready for the dome form...if it works...I'll send pics and a how to.

Thanks again,
JP [/QUOTE]

Y'know what makes a great dome for bridges? About 4 minutes and a scrap piece of MDF. Seriously, you got the sandpaper on there already, this is absolutely trivial. Fibreglass, heck, even bondo sounds like entirely too much work.

Alright...I think that the epoxy resin idea may be overkill...MDF would probably be the best solution for the dome form...afterall...I have a ton of it laying around.

Thanks again for the help...I'll keep ya'll posted.
JP