Closing the Corpus

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Time to close things up!

First things first:

The last time I posted, I had temporarily installed the neck, and (I thought) I had glued the neck heel root in place, as it was to be a permanent part of the corpus, glued to both the neck mortise and the back button.

So the bass looked like this:

Bass with temporary neck set.
Bass with temporary neck set.

 

Side view, showing neck heel break-away joint.
Side view, showing neck heel break-away joint.

 

The heel root looked like this:

Heel root positioned and glued to mortise, using hide glue.
Heel root positioned and glued to mortise, using hide glue, with paper break-away joint.

 

I was so confident that everything was right, that I even sawed off the excess heel root, visible in the above photo, and planed it flush with the back of the neck block, in preparation for installing the back plate.  But!  When I broke the paper “break-away joint” (see the above photo) the heel root gently let go of the mortise and was completely loose. (Rats!)

It turns out that hide glue doesn’t stick to carbon fiber plate! Ok… so I had to start over, and this time glue it home with epoxy. I put a plastic bag around the neck heel to protect it, coated the heel root with epoxy on the bottom and two sides, and slid it all back into place, this time bolting the neck in solidly, and clamping the heel root tightly against it. It all worked this time.

Beginning the purfling of the back plate

While the epoxy was curing, I decided it would be a good idea to at least install the “purfling weave” portion of the back plate purfling, before installing the plate. I figured it would be easier while the plate was still loose. The reason I wait until the back plate is installed to do the rest of the purfling, is that the corpus often changes shape a little when the mold is removed, so I can’t guarantee that the overhang will still be the same. If I have already installed the purfling, then I am stuck: but if I have not, then I can maneuver the overhang to being as close as possible to what I wanted, and install the plate, then plane away excess all around until the overhang looks right again, and finally put in the purfling so that it looks as though everything just worked out right, to begin with.

Cutting the slot for the purfling weave on the back plate for five-string double bass .
Cutting the slots for the lower purfling weave on the back plate.

 

This is a weave that I came up with for my very first five-string fiddle and which I have tried to incorporate on all my subsequent five-string instruments. It is just a modified “fleur-de-lis”…nothing really special, but I like it. I use the same design, upside down, in the upper end.

Picking the waste wood from the upper purfling weave.
Picking the waste wood from the upper purfling weave.

 

Upper weave slot nearly complete.
Upper weave slot nearly complete.

 

After cleaning the slots out, I used heat and water to bend the purfling strips to fit the curves of the weaves, and glued them in using hot hide glue.

Both purfling weaves completed on the five-string double bass back plate.
Both purfling weaves completed on the Oregon Big Leaf Maple back plate.

 

I planed the weaves flush with the plate after they were dry, using a gouge, a small finger-plane, and a scraper.  You can see the beginning of the rest of the purfling slots, how they will connect to the weaves.

Purfling weave planed flush.
Purfling weave planed flush.

 

Closing the corpus

I finished scraping the interior and then laid the corpus onto the back plate, positioning it carefully, adding spool clamps, and constantly checking the overhang all the way around. When everything was as close as I could get it, I removed a few clamps at the bottom block, used a thin palette knife to ladle in the hot hide-glue, and replaced the clamps, tightening them securely. I added more clamps over the glued area, then repeated the process for the next section on either side of the bottom block, and worked up around the sides that way: removing clamps, inserting glue, replacing the clamps and adding more…until it looked like this:

Closed Corpus for five-string double bass.
Closed Corpus.

 

I still had not put the magnets into the cover plate, because, when I added the reinforcements to the cover plate, it changed the curvature, and it no longer fits cleanly into the access port flange. (Rats, again!) So I kept wetting and clamping the cover plate in different configurations until I got it to a close fit, then I added the magnets.

magnets in cover plate
Here are the magnets in the cover plate.

 

Annnd… it turns out they are too weak. (Sigh…) I will have to order some bigger magnets after all.

At any rate, I was now ready to correct the overhang all around, and begin the final purfling.

Overhang corrected, beginning final purfling.
Overhang corrected, beginning final purfling.

 

Tracing in the slots for the final purfling.
Tracing in the slots for the final purfling.

 

The bottom end of the back plate has the same design.
The bottom end of the back plate has the same design.

 

Carving the Channel

After the purfling was completed, I still needed to trim back the purfling and carve the plate channel. This involved marking the edge crest all the way around, about 2 millimeters inside the outer perimeter, and carving the channel to barely touch that line. I used a sharp gouge, in the manner of a drawknife, to carve the channel, then used a riffler file to smooth the outer curve, where it meets the crest line.

Using a gouge as a drawknife, to carve the channel.
Using a gouge as a drawknife, to carve the channel.

 

(I actually made a very short video of how this works, but I was unable to successfully link it to this post. Sorry.)

After the channel was complete, it was time to begin final edgework:

Beginning final edgework on back plate of five-string double bass.
Beginning final edgework on back plate.

 

Beginning final edgework for the front plate of a 5-string double bass.
And the same for the front plate.

 

The goal is to make sure that the edge contour is correct all the way around, and that the plate channel fairs smoothly into the surface of the plate, without ridges or lumps. Getting the light at a low angle across the plate makes shadows which will show me where the lumps and ridges are so I can scrape them away.

Using shadows to complete edgework.
Using shadows to complete edgework.

 

It suddenly occurred to me that my bass-varnishing fixture requires that the end-pin hole be drilled, so I drilled the endpin hole but did not ream it to the taper it will eventually have. On smaller instruments I usually varnish before drilling the endpin hole, so that there is no likelihood of causing sags or runs where the varnish brush touches the hole. But on the bass, I have to have that hole as a place to attach the support for varnishing. (I can’t hold the bass in one hand, and the brush in the other, as I can with a violin!)

End pin hole drilled for a 5-string double bass.
End pin hole drilled.

 

And, that is pretty much where it sits, for the moment! The bass is ready for final varnish-prep, which will involve wetting down the whole surface to raise the grain, so I can sand it smooth again, then repeating until the grain no longer responds to moisture. Then I will rub in a compound to add color to the wood itself (not a stain, which might “reverse” the grain colors) and a mineral ground to close the pores in the wood. Finally a sealer locks in the mineral ground, and I will be ready for varnish.

So there is the bass corpus, ready for final varnish prep!

Five string double bass corpus ready for varnish prep.
Five string double bass corpus ready for varnish prep.

 

Thanks for looking.

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