In the acoustic guitar world it means Orchestra Model. Smaller then a dreadnaught, larger then a 000. I’m spewing that from memory, I could have the sizes wrong but that’s what OM is an abbreviation for.
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Building an OM Bass
- Thread starter Dave Higham
- Start date
In the acoustic guitar world it means Orchestra Model. Smaller then a dreadnaught, larger then a 000. I’m spewing that from memory, I could have the sizes wrong but that’s what OM is an abbreviation for.
I asked Google.
The Orchestra Model, or OM, is one of the most common steel string guitar sizes, and has the same body dimensions as a 000 (15” or so at the lower bout), but with a 14-fret neck.
The difference between an OM and the 14 fret 000 is scale length. The OM (Orchestra Model) has a longer 25.4” scale length, the same scale length as dreadnought guitars. The 14 fret 000 has the shorter 24.9” scale length. This subtle difference accounts for the unique qualities of each model.
The Orchestra Model, or OM, is one of the most common steel string guitar sizes, and has the same body dimensions as a 000 (15” or so at the lower bout), but with a 14-fret neck.
The difference between an OM and the 14 fret 000 is scale length. The OM (Orchestra Model) has a longer 25.4” scale length, the same scale length as dreadnought guitars. The 14 fret 000 has the shorter 24.9” scale length. This subtle difference accounts for the unique qualities of each model.
I'm 100% right,...60% of the time.Well, I was closer then usual!
Or, 60% right,...100% of the time.
I forget.
Anyway, close enough.
That's right. C.F.Martin modified their 000 12-fret model which looks rather round shouldered to a 14-fret to the body, giving it a more square shouldered look. Their idea was that it would replace banjos in jazz orchestras, thus Orchestra Model. Mine is the OM size but with the profile slightly modified to give what some call a 'Modern OM' shape.
The white line is the Martin OM, the green line is mine. It doesn't need much to radically alter the look.
The white line is the Martin OM, the green line is mine. It doesn't need much to radically alter the look.
As I said above, I want to make this bass as light as possible, so I’m now getting rid of as much of the centre block as I can, whilst (I hope) leaving it structurally strong enough.
That’s all the holes drilled in this side of it.
And cleaning them up with the router. The piece of plywood is screwed onto the router base to make it more stable.
That’s the front finished
Then the back had the same treatment. It finished up as a sort of ‘H’ girder section with a 10mm (3/8”) web at different heights. When the front and back are glued on it will effectively form a box girder.
That’s all the holes drilled in this side of it.
And cleaning them up with the router. The piece of plywood is screwed onto the router base to make it more stable.
That’s the front finished
Then the back had the same treatment. It finished up as a sort of ‘H’ girder section with a 10mm (3/8”) web at different heights. When the front and back are glued on it will effectively form a box girder.
As I suggested elsewhere, how about giving the bridge saddle minimal intonation adjustability? Something like two screws at the back of the bridge, between the B and E and between the D and G string ball ends respectively, pulling the saddle back and forth within a ...trench of limited width? Flats do last a long time, but you may be occasionally interested in exploring alternatives...
Acoustic guitars (mostly) have a slightly domed soundboard (around 25ft radius) and a more domed back (around 15ft rad.) and they are usually deeper at the butt end than at the neck. This is partly for structural strength and partly because a guitar with flat and parallel front and back just ‘don’t look right’. The domed front is hardly noticeable and not needed in this case so I’ll make it flat which will make some things slightly easier. But the back will be domed and the body will be deeper at the blunt end because it will look nicer. It’s what I did on the Electro Acoustic. This means that the back of the centre block has to be domed.
To do this I have to bandsaw some of the surplus off the back, and to do that I had to attach a piece of sacrificial MDF with double sided tape.
The 15ft radius spherical surface is then formed by sanding in a 15ft radius dish. I forgot to take a photo, but I first apply white chalk to the surface of the block to be sanded and then sand it in the dish until there’s no chalk left.
Et voilà!
To do this I have to bandsaw some of the surplus off the back, and to do that I had to attach a piece of sacrificial MDF with double sided tape.
The 15ft radius spherical surface is then formed by sanding in a 15ft radius dish. I forgot to take a photo, but I first apply white chalk to the surface of the block to be sanded and then sand it in the dish until there’s no chalk left.
Et voilà!
I should explain that this thread isn’t happening in ‘real time’. I like to get quite a way ahead before starting a thread in case it all goes pear shaped. Normally the next thing would be to thickness and bend the sides (I should really call them ribs) but I had a problem. I have a heating blanket and controller which seemed to be playing at ‘silly beggars’. It would heat up rapidly to about half the programmed temperature and then take 20 minutes to get the rest of the way. Apparently the controller was faulty and the person who was repairing it for me had trouble finding another one.
The next step would normally be to thickness and bend the sides so, instead of that, I moved on to the back and front, starting with the back. I decided to use this rather nice set of Claro walnut. I glue the two halves together, face down, in the go-bar deck. Face down because the two halves aren’t always the same thickness and I want to sand as little as possible off what will be the outside surface. The more I sand off it, the less perfect the book-matched grain pattern.
Cutting out the profile of the back leaving about 3mm (1/8”) extra for trimming.
The next step would normally be to thickness and bend the sides so, instead of that, I moved on to the back and front, starting with the back. I decided to use this rather nice set of Claro walnut. I glue the two halves together, face down, in the go-bar deck. Face down because the two halves aren’t always the same thickness and I want to sand as little as possible off what will be the outside surface. The more I sand off it, the less perfect the book-matched grain pattern.
Cutting out the profile of the back leaving about 3mm (1/8”) extra for trimming.
14 years ago I built an acoustic bass guitar that I’ve hardly ever played, the reason being that it’s OK for noodling at home, but it’s so big and bulky that I never get it out of its case which is about the size of a coffin, and to play with other instruments it needs to be plugged in and when I do plug it in it’s the ultimate feed-back machine.
It had a D-TAR Wavelength under-saddle pickup and preamp which I’ve decided to take out and install in this new bass and to do that there’ll need to be an access panel to be able to get at the batteries. This is the template for the panel and the magnets which will hold it in place. It’s hardboard (masonite) with the edge soaked in CA to harden it up.
This is where the panel will be.
This is the template in place, fixed with double sided tape.
I drilled a small hole 0.5mm dia. (0.020”) next to the template, threaded a jeweller’s saw blade through the hole and cut out the shape keeping the blade rubbing against the template all the way round.
Here’s the panel cut out. I’m quite pleased with the result but I have to fill that hole. It’s only 20 thou diameter but sticks out like a sore thumb.
It had a D-TAR Wavelength under-saddle pickup and preamp which I’ve decided to take out and install in this new bass and to do that there’ll need to be an access panel to be able to get at the batteries. This is the template for the panel and the magnets which will hold it in place. It’s hardboard (masonite) with the edge soaked in CA to harden it up.
This is where the panel will be.
This is the template in place, fixed with double sided tape.
I drilled a small hole 0.5mm dia. (0.020”) next to the template, threaded a jeweller’s saw blade through the hole and cut out the shape keeping the blade rubbing against the template all the way round.
Here’s the panel cut out. I’m quite pleased with the result but I have to fill that hole. It’s only 20 thou diameter but sticks out like a sore thumb.
The panel needs a support on the inside, so I made one from 2 layers of spruce with the grain lines at right angles for strength and stiffness. It has 3 small neodymium magnets glued into recesses.
Here it’s being glued onto the back. It’s under those 3 blocks of metal. Before gluing, the surface to be glued was sanded in the radius dish. I work on the principle that if I want the back to keep its 15ft radius, everything that’s glued onto it should be sanded to that radius. Cross-grained reinforcement strips are also being glued onto the centre join. The 2 strips on either side are pushing the back down into the dish while the other bits are glued in place.
Here’s the result. The 4 little blocks together with the centre strips accurately locate the centre block .
A view from the outside.
The piece cut out of the back was thinned down a bit and then had two cross grained pieces of veneer glued to the back. Again I did this in the radius dish. I made a clamping caul by sanding a piece of 18mm MDF in the dish and clamping the panel and the veneers between the dish and the caul. Although it’s relatively small, it took on the 15 ft dome perfectly. Three shallow holes were drilled and 3 magnets glued in the holes. The whole thing was calculated so that the magnets in the panel don’t quite touch those in the support, so the panel is always being pulled down onto the support.
Here it’s being glued onto the back. It’s under those 3 blocks of metal. Before gluing, the surface to be glued was sanded in the radius dish. I work on the principle that if I want the back to keep its 15ft radius, everything that’s glued onto it should be sanded to that radius. Cross-grained reinforcement strips are also being glued onto the centre join. The 2 strips on either side are pushing the back down into the dish while the other bits are glued in place.
Here’s the result. The 4 little blocks together with the centre strips accurately locate the centre block .
A view from the outside.
The piece cut out of the back was thinned down a bit and then had two cross grained pieces of veneer glued to the back. Again I did this in the radius dish. I made a clamping caul by sanding a piece of 18mm MDF in the dish and clamping the panel and the veneers between the dish and the caul. Although it’s relatively small, it took on the 15 ft dome perfectly. Three shallow holes were drilled and 3 magnets glued in the holes. The whole thing was calculated so that the magnets in the panel don’t quite touch those in the support, so the panel is always being pulled down onto the support.
On to the front, which was joined, cut out and sanded to thickness in the same way as the back, so I didn’t take any photos of that. It will have a ‘soundhole’ and a rosette as an acoustic guitar would. I found an off-cut of quilted maple about 7mm (9/32”) thick and managed to saw it through the thickness and glue the 2 pieces together.
Sanded it down to 1.5mm thick (1/16”) and rubbed a bit of alcohol on to see what it would look like.
I made this circle cutting jig recently. Up to now I’ve used a Dremel in a Stewmac jig but this works better. Apart from being held by the centre pin, the maple is also held down by small pieces of double sided tape in the area of the circle.
To route the recess in the soundboard I made trial cuts on a piece of ply for the inside and outside diameter. Actually, I doesn’t matter if the recess is slightly too big, as the channel for the purfling will be cut later.
Sanded it down to 1.5mm thick (1/16”) and rubbed a bit of alcohol on to see what it would look like.
I made this circle cutting jig recently. Up to now I’ve used a Dremel in a Stewmac jig but this works better. Apart from being held by the centre pin, the maple is also held down by small pieces of double sided tape in the area of the circle.
To route the recess in the soundboard I made trial cuts on a piece of ply for the inside and outside diameter. Actually, I doesn’t matter if the recess is slightly too big, as the channel for the purfling will be cut later.
Gluing the maple circle in.
Scraping it flush.
I find it easier to glue in the purfling strips if they are bent first. It doesn’t have to be an accurate circle or diameter. That’s a piece of steel tube I found in the scrap bin at work. In the bottom it has a wooden disc holding a lamp socket and a 200w halogen lamp. In the top is the bottom of a beer can. That’s just to avoid being blinded by the lamp.
Two pieces of B/W purfling and a piece of pear veneer just fit nicely into the channel made by a 1/16” dia. downcut router cutter. I set the cutter so it just takes a smidjin off the inside and outside diameters of the maple. I give the channels a couple of coats of thin shellac (to prevent the CA from seeping into the end grain of the spruce and discolouring it) and, when that’s dry, fit the purfling strips and flood them with thin CA. Then the whole thing is scraped flush with the spruce.
I gave the rosette a coat of Z-Poxy Finishing Resin to make the grain pop and this is what it looks like.
I added the four little blocks and the cross-grained centre strips which accurately position the centre block, and a few more cross grained strips just to strengthen the front, although they probably aren’t really necessary.
Scraping it flush.
I find it easier to glue in the purfling strips if they are bent first. It doesn’t have to be an accurate circle or diameter. That’s a piece of steel tube I found in the scrap bin at work. In the bottom it has a wooden disc holding a lamp socket and a 200w halogen lamp. In the top is the bottom of a beer can. That’s just to avoid being blinded by the lamp.
Two pieces of B/W purfling and a piece of pear veneer just fit nicely into the channel made by a 1/16” dia. downcut router cutter. I set the cutter so it just takes a smidjin off the inside and outside diameters of the maple. I give the channels a couple of coats of thin shellac (to prevent the CA from seeping into the end grain of the spruce and discolouring it) and, when that’s dry, fit the purfling strips and flood them with thin CA. Then the whole thing is scraped flush with the spruce.
I gave the rosette a coat of Z-Poxy Finishing Resin to make the grain pop and this is what it looks like.
I added the four little blocks and the cross-grained centre strips which accurately position the centre block, and a few more cross grained strips just to strengthen the front, although they probably aren’t really necessary.
This build is top tier lutherie, but that bending iron beer can lid just takes it to a whole new level.
I finally got the heating blanket controller repaired, so now to make the sides. Because the front will be flat, the front edges of the sides will be straight, but the back edges won’t because of the radius and the difference in depth of the body. I prefer to shape the sides before bending them, leaving just a little extra width for trimming.
This is the mould I’ve used to make 2 OM-size acoustic guitars. It’s sitting on blocks on the 15 ft radius dish and the blocks are adjusted so that at the widest part of the body shape it’s the same distance from the surface of the dish, but at the tail end it’s10mm higher than at the neck end. It has a piece of card lightly stuck to it with spray adhesive, parallel to the top surface of the mould.
That little triangular piece of plywood has a hole in it with a piece of pencil lead stuck in the hole
It’s used to mark the profile of the sides onto the card.
This is the result. I don’t think it’s quite what you’d expect the shape of the sides to be and you can see how much wood you’d have to cut off if you left the sides parallel and shaped them after bending.
The sides were cut out and thickness sanded to 2mm (0.080”)
They were then bent in this contraption.
A sandwich is made of:
Spring steel slatAluminium foil
Damp kraft paper
Sides
More damp kraft paper
More aluminium foil
Spring steel slat
Heating blanket
Spring steel slat
The waist clamp is raised, the sandwich is inserted and the heat is turned on. At about 200°F the waist clamp is screwed down, the upper and lower bouts are pulled down and the temperature raised to about 300°F and maintained for 15 minutes. Then the whole thing is allowed to cool completely and reheated to set the bends. I don’t take photos of that.
The contraption seems to be working as it should again as the sides bent OK. They were put into the mould and marked and then cut to length, but I forgot to take any photos.
So they then had to be glued to the centre block. First the tail end (left hand in the photo). The sides have been pulled apart at the RH end so the clamping caul is in contact with the centre block. I used Epoxy for this as I was gluing on to end grain.
Then the other end, although I turned it round so it’s the LH end again.
This is the mould I’ve used to make 2 OM-size acoustic guitars. It’s sitting on blocks on the 15 ft radius dish and the blocks are adjusted so that at the widest part of the body shape it’s the same distance from the surface of the dish, but at the tail end it’s10mm higher than at the neck end. It has a piece of card lightly stuck to it with spray adhesive, parallel to the top surface of the mould.
It’s used to mark the profile of the sides onto the card.
This is the result. I don’t think it’s quite what you’d expect the shape of the sides to be and you can see how much wood you’d have to cut off if you left the sides parallel and shaped them after bending.
The sides were cut out and thickness sanded to 2mm (0.080”)
They were then bent in this contraption.
A sandwich is made of:
Spring steel slatAluminium foil
Damp kraft paper
Sides
More damp kraft paper
More aluminium foil
Spring steel slat
Heating blanket
Spring steel slat
The waist clamp is raised, the sandwich is inserted and the heat is turned on. At about 200°F the waist clamp is screwed down, the upper and lower bouts are pulled down and the temperature raised to about 300°F and maintained for 15 minutes. Then the whole thing is allowed to cool completely and reheated to set the bends. I don’t take photos of that.
The contraption seems to be working as it should again as the sides bent OK. They were put into the mould and marked and then cut to length, but I forgot to take any photos.
So they then had to be glued to the centre block. First the tail end (left hand in the photo). The sides have been pulled apart at the RH end so the clamping caul is in contact with the centre block. I used Epoxy for this as I was gluing on to end grain.
Then the other end, although I turned it round so it’s the LH end again.
