MikeC's bench

[ernym]

I remember you asked me about glue ghosts in the first Sultana build. My reply was...

"I don't worry about glue ghosts because it will all be carved away when finishing the arching and edgework. The glued edges help to keep the edges from tearing out when finishing....it all works out in the finished end. Besides this I'll size with a very thin coat of gelatin/alum before coloring the wood."

[MikeC]

yeah, I can see advantages in doing it that way.  No worry about glue ghosts, no worry about cutting the groove too deep. 

 As I was cutting the groove in this back plate I kept thinking how deep can I go.  Too shallow and the purfling is not stable in the groove, too deep and cut the edge off the plate

 So for the top plate I'm going to make a flat edge so that I can inlay the purfling full depth,  which isn't all that much, only about 2mm.   

I would really like to make a copy of the purfling cutter in the Strad museum, just for fun.   

I started on this one the same way as the last build, using the double blade to mark the channel and then cutting it with an xacto.   But then I tried using the marker with only one blade, turned the other balde around as a depth stop and like that much better.  

[ernym]

Even if you don't cut too deep the wood can tear out when using the pick so you can end up with uneven channel depth. 

[Sean Couch]

I think your purfling looks very nice. I’d be happy with it.

When I did my purfling, I used Rodger’s method, but I did not size the channel, just the purfling. I did not do the fluting first, I left the ledge flat and cut the purfling channel to a depth of 2mm.

My ledge thickness was 4.3mm. I haven’t seen any signs of my purfling coming loose, but sizing the channel sounds like a good idea. I fluted the purfling channel down to 3.1 ish mm, so theoretically, I have about 0.8mm of purfling still in the channel.

[MikeC]

10 hours ago, Sean Couch said:

I think your purfling looks very nice. I’d be happy with it.

When I did my purfling, I used Rodger’s method, but I did not size the channel, just the purfling. I did not do the fluting first, I left the ledge flat and cut the purfling channel to a depth of 2mm.

My ledge thickness was 4.3mm. I haven’t seen any signs of my purfling coming loose, but sizing the channel sounds like a good idea. I fluted the purfling channel down to 3.1 ish mm, so theoretically, I have about 0.8mm of purfling still in the channel.

I'm going to try that on the top plate.  use a flat ledge and 2mm channel. 

My fluting is more shallow than that,  I'll measure it later and post some numbers.  I fluted to the edge so when I round it over it will be a little lower than it is now.  

I'm not sure about glue sizing the channel or the purfling or both.  

If you size the purfling I assume you would do that after it's bent to shape, but then you have to cut the ends to join them at the corners so you would have to add a little glue there right? 

[ernym]

On 8/13/2024 at 1:13 AM, MikeC said:

I'm going to try that on the top plate.  use a flat ledge and 2mm channel. 

My fluting is more shallow than that,  I'll measure it later and post some numbers.  I fluted to the edge so when I round it over it will be a little lower than it is now.  

I'm not sure about glue sizing the channel or the purfling or both.  

If you size the purfling I assume you would do that after it's bent to shape, but then you have to cut the ends to join them at the corners so you would have to add a little glue there right? 

 How I understand, Roger does not apply glue to the channel or purfling but uses the glue already on the purfling from the making.

On page 75 of "Making a Double Bass" he explains...

"Having thus cut the channel, I prepared the mitres and inserted the purfling into the channel dry, making sure that the strips fit right to the bottom. With the purfling thus inserted, using a brush, I simply (but carefully) run boiling water over the channel in order to reactivate the glue on the outer surface of the purfling sandwich."

I apply glue to the channel and edges and let dry. I like having a glue size covering the entire channel. I clean the channel of any remaining residual glue before it dries.

Then like Roger I insert the purfling dry. Once everything fits tight I brush hot water over the purfling to reactivate the glue. I don't end up with any glue ghosts because I purfle before arching and edgework are finished. Once finished all glue has been carved away leaving fresh wood. Plus I always size the spruce top (especially the edges) to avoid uneven absorbtion and blotching. Everyone has their own working method and this is mine but you may find a result that works better for you and that's what counts.

[MikeC]

You do awesome work E.  

Well,  I have started carving the inside of the back plate.  I mention carving it down to about 4.5mm overall.  That's to start but then I'll thin out the upper and lower bouts obviously, just leaving the thick area in the middle.

I didn't have a lot of time to work on it that night so the rest will be this weekend hopefully. 

 

[MikeC]

More carving out the back plate.   I really need a better plate holder.  I have trouble with this one the plate is not held steady enough.   I'm thinking of re-engineering it before I start the top plate.  

 

[MikeC]

Roughly 4.5 to 5.5 mm  overall.   After this I'll start thinning it down in the upper and lower bouts leave a central thickness.

 

[MikeC]

Before continuing with final thicknessing of the back plate... I have been re-reading the Librum Segreti de Buttegha  translation by A. Dipper.  Also comparing that to the violin pattern at the NMM.  Which I can't seem to find there anymore, online I mean.   Do they still have it?  Why is it not on their site?  

There was an interesting discussion of that pattern some time ago. 

Since I build as a hobby and have no customers, I can do something a little out of the ordinary when it comes to thicknessing ideas.  

 

 

 

[MikeC]

After going over the above mentioned documents, I started marking out some lines.

The center of the three compass circles on the violin pattern from NMM as well as on the ones in the Stradivari museum attributed to the workshop of Ceruti,  exactly matches the location of the axis of focus described in Librum Segretti.  

Also when drawing the equilateral triangle at one third the length of the plate, the lower points fall exactly on the outermost of the three compass circles.  

The centers of oscillation described in Librum are bounded by the outer compass circle and the apex of the triangle in the upper bout and the arc of the triangle in the lower bout.  

Librum mentions not only the axis of focus but also it's compliment, which I find is exactly at the point where the outermost compass circle crosses the centerline towards the upper bout. So I used that point to draw out three more compass circles.   

Does any of this matter?  Probably not but it's fun to contemplate.   All I really need to do is determine where I want the thickest part to be and how much to thin it out. 

 

Oh one other coincidence I forgot to mention.  During the last build, looking at the CT scan of the Titian I noticed that the thickest area of the back plate on the long axis is kind of flat, which kind of makes sense. If you have a concave surface and then have a thick area in the middle tapering out to the thin area in the upper and lower bouts, the resulting contour would be kind of flattish.  Unless you make it really thick then you have a bulge.   Well anyway at that time I made a short flat template to mark out that area.   Now I notice that template matches the distance between the two small circles that represent the centers of oscillation mentioned in Librum Segretti.   That was kind of interesting and unexpected.  

 

[MikeC]

I continued thinning the back plate yesterday. I'm going to use the software that I got from Hans Pluhar to make a map and see what it looks like. I have too many data points, it gets tedious.

After looking at the later Strads, the Cremonese, Messiah and Tuscan -Medici, there is a clear pattern, so I went with something like that as a concept.

The thickest area tapering off about at the upper corners and extending somewhat past the lower corners.

Here's my drawing. Others have said that the thickness in the Sacconi book is not bad, use that and you'll have good results. But I like to explore and come up with new ideas or new old ideas 

I used the three compass circles from the NMM pattern but positioned higher with the Librum Segretti centers of ocilation marking the upper and lower boundary.

There is something about geometry  that appeals to my very analytical brain, it gives a sense of structure and reasoning, rather than just winging it.

4.5mm in the center, broad area of 2.5mm in the upper and lower bouts.

 

[ernym]

Better quit drawing circles and get er' done. ;^)   Can you post the Hans Pluhar info please...

[MikeC]

Lol,  I got er done!   Spent some time thinning it down today.   

Here's a link to his website.  I don't know if he still sells the mapping software but you could email him and ask.  

https://www.pluhar.com/

It's been a while since I was in contact with him.  

 

[MikeC]

I just weighed the plate, it's 116.3 grams, that may be a little heavy,  I may be able to thin it out some more but it's close to where it should be with the graduations.  

I could still thin it some more near the edges and maybe a little more in the lower bout.   Here it is with my drug dealer gram scale.   

 

 

And here's the end block area.  No steep drop off but a little thicker than it needs to be,  I should have had a little more recurve / fluting near the ends but I'm still learning to do a proper arching.  

 

 

 

[ernym]

Sorry if you mentioned it but what is the density?

[MikeC]

5 hours ago, ernym said:

Sorry if you mentioned it but what is the density?

I'm not sure what the density is.  What's the best way to measure it?  

[ernym]

I usually measure it behorehand but If you have scraps you could square up a piece then weigh and measure it.

I like using Salve Hakedal's sg calculator for measuring density.

  https://www.fiolinmaker.no/tips-triks.php?uside=eigenvekt.no&flagg=no

[Sean Couch]

Looking good Mike.

I calculated the density of my spruce the way Ernym described. It’s another chance to use your drug dealer gram scale, that’s what I used to weigh mine too. Lol

[MikeC]

for the maple I have some leftover scraps.  I could cut off a centimeter cube and weigh that.   I'll have to cut it very carefully though to make sure it's a cubic centimeter.   

The spruce is still uncut so I can measure the whole billet.  That should be more accurate.  

[MikeC]

I just measured the spruce and used the calculator on makingtheviolin.com 

it comes out to 0.38    There is a 0.40 marked on the billet so that should be about right.  

[MikeC]

Specific Gravity of the maple is 0.64

[MikeC]

Well it's been a few days since I posted. I used the software that I got back some time ago to visualize the thickness. I had kind of planned on something like the Strad maps that I posted earlier but then decided to adjust the curvature in some areas and it seemed too thick near the C bout edges so I thinned it some there. I'm using a homemade dial caliper and it's hard to get a good reading but this should be close. The thick area at the top is the neck block glue area. At the bottom that part got cut off of the scan so it isn't mapped.

[ernym]

Interesting, what's the software name?

[MikeC]

57 minutes ago, ernym said:

Interesting, what's the software name?

It's the software I got from Hans Pluhar,  I don't think it has a name,  something he had a programmer make for him.   

I don't know if it's still available.   

it gets tedious using that many data points.  I used a paper grid to mark points on the plate.  I think it's about a half inch or centimeter grid.    If I do this again, I might try using a larger grid with half as many points.   

For measuring, I attached a millimeter dial caliper to a wood frame.  It works well but is sensitive to how it's held against the wood surface so wiggle it one way or another and the reading can change several tenths of a mm.