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Catnip's (John K) Bench


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Back to the workshop.  I have decided to use the flamed aspen.  Since the board was wide enough I was able to cut ribs from it and still leave enough width for a one piece back.  Three  430 cm rib lengths easily yields two 230 cm LB , two 190 cm UB and three 140 cm CB (an extra one).  There is a slight slant so I have numbered the ribs to match the blocks.

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After cutting the ribs as marked I use two  2mm riser boards that "pinned" to the corner blocks so that C bout ribs have a bit of overhang (2mm).  I used to use double sided tape but pinning the riser board is much more secure especially when I have to lift the mold to check the fit on the underside.  I like to use the parallelogram  blocks with a small Bessy clamp to tightly fit the C bout rib to the blocks.  Pressure adjustments can be made by sanding (tapering) the clamping edge.

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After C bout ribs have dried the rib extensions are trimmed. Then I use an oscillating sander to match the inside rib curvature to the corner blocks. 

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After the linings are in,  I drill a 2 mm location hole in the upper and lower blocks on the center line about 5 mm from the edge.  Then the back is clamped to mold and the location holes are extended about 2 mm into the back (there are half-holes since they do not extend all the way through) Next the outline is traced using the standard washer method and a fine 0.35 mm pen.

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Next I use the alignment holes to draw diagonal guidelines to establish the corner over hang. This is just personal taste. 

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Later I will use a corner template to establish more symmetry between the corners.  After cutting to the outline with a bandsaw (3/16" blade) and rough sanding to the line, I establish the approximate edge thickness in preparation for the purfling channel ledge as shown.  The ledge is wider in the Upper-Lower bouts (12 mm wide) and about 8 mm wide in the C bouts.

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Doing this first makes it easier to establish the final overhang when the back temporarily attached to the mold with double sided tape.

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  • 2 weeks later...

Now that the edge thickness is done and the corners have been refined I mark a guideline 4 mm from the edge to establish what the corner purfling will look like.  I use a handheld purfling jig with a 1.2 mm double fluted bit to cut the purfling channel. 

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The extensions to the corner are done by hand with an exacto knife afterwards.

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There are different methods to glue the purfling. I first compress the purfling to 1.0 mm then fit  and glue the C bouts. I clamp the corners then lift the purfling out of the groove so I can apply the hide glue with a syringe.

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After gluing the purfling I next finish the top arching by first doing the channel.  I like to see if I can do the arching by eye and then check it with my arching templates.  This flamed aspen has some deep curl and can easily chip out even with a sharp finger plane.IMG_3030a.thumb.jpg.608e3c567c88a36428be5b14cbde49b4.jpg

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How close are you getting just by eye?   What are the shaded pencil areas?  Have you tried a toothed blade plane to prevent tear out?  And finally... :D sorry for so many questions but what material is your template made of?  It looks like a thin piece of drywall.   

When using a template, I smear the edge with soft artist charcoal so it marks the high spots on the wood.   I also prefer the half templates.  

 

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I can get pretty close without templates (3 or 4 out 5 are OK) with some minor tweaking.  Usually it is either the upper or lower bout arching that is not low enough towards the channel as shown in the picture.  The pencil marks are equivalent to your charcoal marks.  I use cut lighting and pencil in the area the needs attention by rocking the template ever so slightly over the high spot.  I have toothed blades for my finger planes but have never used them.  I may follow up on your suggestion and give it a try.   Even a very sharp thick scraper (0.7 mm) will cause a bit of a tear out. I have some 40 - 50 grit sandpaper that I use for these specific trouble areas.  I guess 40 grit sandpaper is a bit similar to a toothed blade  My paper printout templates are  glued to 1/16 in or 3 mm plywood.

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To do the inside graduations I draw a rough map of general thicknesses. 

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Because my drill press has a bit of vertical give ( ~1 mm) the rough thicknessing will be about a mm more.  I simple depth stop drilling jig is used; just make sure that depth stop is tight.

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After drilling the holes I use a gouge to chip out the aspen followed by various small finger planes.

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Then it is just matter of using scrappers to thin the back working the graduation towards the edge.  I like to leave a good 10 mm edge before I finalize the weight with the m5 tap tone. 

At this stage it is about 124 gm with m5 of 370 hz.  Still too stiff.   Because Aspen is less dense than maple I plan to leave it a bit thicker and follow a Cannone graduation pattern rather than a Strad graduation.

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The back graduation is more or less finished at 110 gm with a M5 of 340 hz using  approximate Cannone (heavy) graduations.  This still leaves me with the option to do some more refinement (if necessary) after setting up the violin in the white before varnishing. 

Here is how I do a contour map using Magic-Probe thickness sensor and software. I am using an older model Magic-Probe but it works well.  It does take some time to setup the Master files but the  5 second auto detect is a nice feature that allows for easy data input.  Just move the probe to the new location and once it has stopped at the new location there is a 5 second delay before the measurement is taken.   I use tracing paper with 120 fixed points which is temporarily taped to the back.

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Here is the Magic- Probe contour map.   I like to use the colors that would be seen if I were using a light box.  That is, thin spots would be yellow and thicker graduations would be more orange to brown.   This "cannone-like" graduation is more oval than the "strad" bull's eye circles.  The center is around 6 mm

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Here is my Octave program (Matlab equivalent) that works from the same 120 point data file.

% data file aspen.txt
% 1243 202 3.15
% 1244 261 3.05
% 1242 321 3.25
% 1249 381 3.58
% 1244 442 3.30
% ...

xyz=dlmread('aspen.txt'); % read delimited data from file
x=xyz(:,1);  % create x vector from all 1st elements
y=xyz(:,2);  % create y vector from all 2nd elements
z=xyz(:,3);  % create z vector from all 3rd elements

colormap(flipud(autumn)); %reverse the autumn color scheme
[X,Y] = meshgrid(min(x):max(x),min(y):max(y));
Z=griddata(x,y,z,X,Y); % format the data for contour plot
contourf(X,Y,Z,25);    % plot the contour map filled
colorbar();            % show the colorbar range
hold on;               % keep same graph
plot (x,y,"k*");       % plot all points as black dots

filename = 'aspen.png';
print ( '-dpng', filename );  %save the plot as image file

=================================================

 

And here is the Octave output.  I am not sure why there are islands around the upper and lower blocks, but the output agrees with the Magic Probe software

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This is my first aspen violin.  I was a bit hesitant after researching this topic on MN because of the negative comments but this piece of wood was very compelling.  This flamed aspen is a bit tricky to plane because it can easily chip out. I have various small finger planes with a different "set". So I choose the one that works the best ie.  small cut and extra sharp.  I saw a technique for sharpening finger planes without removing the blade. I use a various fine grit diamond plates  1000x to 6000x

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I used 30% hydrogen peroxide (very dangerous) to "bleach" out the dark stripes.  I will wait until it is ready to varnish before deciding if it needs another application.

 

 

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  • 2 weeks later...

Back is now glued using the "dry clamp" method.  The corners are clamped and adjusted first, then the upper, lower blocks followed by a few clamps in the bouts.  This allows some adjustments to made to even out the overhang.  Then the remaining clamps are applied (~30 clamps and 2 F clamps).  Starting at the top block 3 clamps are removed and the hot hide glue is applied with thin spatula while another spatula is used as a wedge.  Two clamps are replaced and two more clamps are removed until the whole violin back is glued.   Clean up is down with a brush and hot water as the gluing progresses.  Here is an illustration from an earlier build.

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Aspen back is glued.  After removing the clamps I remove any residual hide glue both on the inside and outside with hot water and a scraper.

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Now I use alignment pins (sharpened nails) to transfer the alignment holes to the spruce top.  The spruce top density is around 0.39 - 0.40 which is heavier than what I normally use.

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It is easier to initiate the bottom alignment hole on the center line with a scribe first, then set the alignment pin into that hole.   The top hole is done by is done by eye.

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The alignment pins along with two clamps will hold the top securely so that the outline can be traced using the washer-pen method.

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Thanks for sharing.

I was a bit nervous the first time I used the dry clamp method to reglue a plate after repairs, but it has become my preferred method because it takes all the pressure off of getting the plate aligned with a bunch of glue cooling on the ribs.

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Even after finishing the overhang with the back temporarily glued (double sided tape) to the garland on the mold I find a bit of movement (error) after the garland is removed from the mold. The dry clamp method allows me to micro adjust the overhang back to where it was.

So I now temporarily attach the top so that I can repeat this process of finishing the overhang with my over hang tools.

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Here is a sample of my over hang tools.   These can be made using sandpaper and large flat headed 10-32 screws.  I now used metal sandpaper (dura-grit).  The C bouts are done with a sanding dowel with the middle part wrapped with tape to a thickness of  ~3.0 mm (personal taste).  You can see a picture of it in an earlier post on Nov 20

 

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I also made a pencil gauge to highlight areas that are over 3 mm.  

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Instead of using a digital caliper a simple tire thread gauge can be used... I use the mm scale not the imperial scale.

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After removing the top, I use a compass to draw a 4mm wide guideline for my handheld purfling router.  With practice it is possible to get very close to the corner point to minimize the amount of final corner cutting with the knife.

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There a few different ways to insert the purfling.  Glue the C bouts first, then cut the 45 degree bee sting and insert the bout purfling.  Or, fit all the purfling in one go.  When it comes to applying the glue I clamp the corners, lift the purfling a bit and apply thin hide glue with a hypodermic needle. 

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This method works for me because I compress the purfling from around 1.35 mm to 1.00 mm using compression rollers.  It is a copy of Peter Westerlund's jig which comprises of one horizontal roller and one slightly slanted roller.  This allows for a range of compression thicknesses for cellos, violas and violins by using a wooden spacer.  My spacer is set for 1 mm. IMG_3064.thumb.JPG.da914cc7e28f8ba5c34163ade64dc7a5.JPG

 

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After the purfling fluting and outside arching is almost complete I draw the f-holes to see if any central arching corrections need to be done.  If not then it on to the inside graduations using the same method that I used for the back (step drill to approx 4 mm all round).  The sequence I use is: gouges followed by finger planes followed by scrappers.   This is higher density spruce than I normally use and I have estimated that it will finish around 71 gm with no ff holes with reasonable flexibility.

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Reading through the last page of your build here, and I saw bleaching. I didn't read about bleaching, did I? The page before has it, and the result is amazing! I never would have thought to do that. It is like night and day.

I like build "blogs," especially when well done.

Happy New Year.

Ken

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I only use hydrogen peroxide 30% and sunlight and repeat the process as needed.  I have read about the two step solution process which is to use a) Sodium Hydroxide followed by b) Hydrogen peroxide and then a neutralizer such as vinegar.   I don't know the long term effects of NaOH on spruce.

I had to do a replace a broken peg.  After researching MN the following method works quite well.

Drill a small pilot hole and then use a spring loaded center punch with adjustable tension to gently knock out the stuck peg.  The first impulse moves the peg about 0.25 mm outwards.  Of course support the pegbox while doing this "chiropractic" treatment.

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Graduation of the inside is done in stages.  I generally try to get an overall thickness of 3.5 mm around the f-holes - soundpost area and edges.  The other inside areas are reduced to about 3.0 mm.  At this stage m5 tap tone and weight is recorded and an estimate of the final weight can be roughly determined.   Here it is at 74 gm with m5 of 340 hz.  My goal is to get it under 70 gm with the appropriate flexibility and stiffness.  Using my graduation caliper I measure a spot that is 3.0 mm then I set my pencil gauge to just clear that spot.  This allows me to refine the central area.  I also have a 2.6 mm spot so that I can refine the upper and lower bout areas

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The final plate is now at 71.5 gm with m5 of 325.  It is time to move on to laying out the f-holes.  I use an acetate printout  which allows me to draw one f-hole and mark alignment dots to help orient the second f- hole when the template is flipped.  I will revisit the inside graduations after the f-holes are cut to see if I can reduce the weight by a gram or two before putting the bassbar in.

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