Violin Forum/Message Board

[jethro]

I am working on some highly figured maple back wood- trying to get the
center joint planned perfectly. I have glued it twice or three times, each
time to discover the spring gap wasn't quite enough and one or both ends
had a slightly visible gap. (heated faces with rubbed joint)

It seems like there are several variables at play which would determine how much gap ( water content of wood, moisture content of wood, air
temp and humidity ect...... )

I have been planning it so I have about 1 to 1+1/2 inch on each end
that touches flat and the part in between has a tapering gap- with most
of the slope near the ends. using fairly thick mixed glue.

Is there a way to determine how wide is enough ( or too much) on the
central gap ??????? (so it's just levle 3 minuites later after all the swelling
is over )
This seems to be one of the last skelletons in my skill closet !
Any thoughts will be kindly appreciated .......

Tim

[Jack H.]

Make it dead flat. Forget this spring joint.
Once you can make it perfectly flat consistently, then do what you will with it.
there is no need to glue a center joint 3 times.
Use clamps.

[Michael Darnton]

It sounds from your description of "spring" that you have made it too complicated, and in doing so have done yourself in. The most important thing for you to do at this point is to

STOP THINKING!!!!

If you're going to spring, it should be a consistent curve, not something that changes at the ends. But basically Jack is right: just do it straight, and use clamps. It's not exotic, but it gets the job done.

[jethro]

sorry, i didn't intend to portray my spring as having "hips" at the ends.
It is just basicly made with the 18 inch lie-nielson jointer plane- so it really
isn't possible to make anything but a pretty smooth curve. Sometimes I
put a little flat at the ends instead of a point ( where the plates touch at the ends)

I tried again tonite with a little bigger spring. Will find out tomorrow.

I am not opposed to clamping but I have found that a simple application
of a single bar clamp in the center tightened lightly usualy results in the
plates being torqued open very slightly causing the joint to open up on
one side or the other. Is there something like a pannel clamp that can be
used to keep the thrust from the clamp straight through the middle??????

I have been specificly avoiding a straight joint (OK on spruce...) because
I have had 3 back plateds un-zipper starting at the ends several months to 2 years later..... I think because of not enough spring at the ends when
the joint was formed.

But Yes you are right..... I do tend to overthink many problems and make
life harder than it has to be. Most of the time there is no one arround to ask or watch!

What do you think of using a pannel clamp like woodcraft sells ?
( I can get the joints flat fairly easily )

Thanks for the pep talk yall !

Tim B.

[Michael Darnton]

When I clamp, which I usually don't these days, I use a pipe clamp, and put the wood in roof-peak down. I slide the wings of the wood down a bit in the clamp faces so that the side you can see is open just a hair. Then when you tighten the clamp, first the bottom (the side you can't see) closes, and then the top side closes, last at the center where the gap from springing is.

As you tighten the clamp, keep the glue squeeze-out wiped off on the top side, tightening and clamping, and when that last quarter-turn of the clamp doesn't bring out any more squeeze-out, stop turning, right then--don't turn the clamp a bit tighter, or things will start getting worse.

It doesn't take much clamp pressure.

[Andres Sender]

I've found that it's very easy to plane off too much at the ends with for instance a metal jack plane unless you put a lot of effort into good technique. A surprising amount of effort, I think some would find it.

I've also found (thank you Michael) that a good straightedge makes a huge difference in really knowing how the joint is going.

[CT Dolan]

When I was learning how to manipulate a file (from an old English gunmaker), one thing that really struck me was the importance of proper stance, which meant working at the proper height over a sturdy bench and vise (basically elbow height, which allows the body to use the proper motion). With this foremost in mind, you could file surfaces so true you could ring them (get them to stick tight) like a precision-ground set of gauge blocks, however without taking such considerations into account you'd be filing until the metal was worn to a nub, and never get the result you were after. The thing I learned most of all is that, given the opportunity, the human mind and body are capable of amazing results.

[Jack H.]

If the problem is too much space in the spring gap then WHY are you making it with more spring??? I really don't get this at all!
The spring should be MAYBE 1/20th of a mm nothing more. Get a quality straight edge and check your work.
<Shaking head>


J

[Michael Darnton]

The rule is that you've got to hold the joint up in front of you, close it with your hands easily, and then check for light leaks while you're holding it: there shouldn't be any. You're not trying to make a window. If you can slide a small slice of paper through the unclamped gap, that's too big.

[jethro]

Jack: perhaps you misread the first paragraph of my question where I
mentioned that the spring wasn't quite wide enough so that AFTER the
glue set about 30 secconds (wood absorbed moisture and swelled) there
came slight cracks at each end. The trick seems to be getting the geometry just right when its dry so that after the surfaces absorb the moisture the whole width of the joint is paralell and perfectly sealed up.
1/20 mm is a little less than a sheet of thin typing paper. Do you measure
your spring with a .001 inch feeler gague (?) !
It seems to me the gap is so small I can only rely on the floodlight and
good plane technique (probably where I'm going bad) and Michaels hint
of using a light squeeze to flex the gap closed to get an idea if its too big
or too narrow.

I tried it one more time and I may have a acceptable joint this time.
not perfect but maybe acceptable. I got the plane cutting very thin
shavings (1/2 tissue paper) and planed opposite directions.
then I reduced the cut depth to half (nearly none) again both directions
untill no more cutting happened. That resulted in a deasant nicely tapered
spting with no lumps and I could close it with a couple of pounds of force
against the light. joint rubbed only- no clamp.

Wow Michael, the clamp thing sounds REALLY scarey watching the thing
torque when tightening. (hoping it will go back the other way ....)

I will let this last joint set for a while and see what it does .......

Thanks for the help... sometimes yall keep me going when I want to quit !

Tim B. !

[jethro]

C T Dolan:
I havn't tried that with metal but I know exactly what you mean.
I grew up as a machinist and then worked for a few years as a gunsmith.
The exact thing you are talking about technique is why I built my wood
working table out of 2 x 6 boards. It weighs about 600 pounds and is 4 foot by 7 foot. I wanted something so heavy that it wouldn't move a wiggle when I was gougeing or planning. That part worked great. I stacked 2 x 4's on edge to make the top work surface (like bowling alley
floor) Everyone laughed at me for making it so heavy ........ but the wood was free so why not !

I just discovered doing this last wood with lots of dark stripes and grain
swirls in the faces where I was planning that the plane blade would encounter very large changes in resistance down the length of the joint.
Much more than a plain grade of wood. I noticed that to get a STEADY
constant travel speed (with no sticks or stops) I had to put the rear handle
of the plane near my sturnum and move my whole body along. That way
when it hit a hard spot I could slowly push through it without the blade breaking contact with the surface when the resistance went back to being
light. I think I might start using not so fancy back wood for a while to
make the planning a little less sensative.

Funny how you can never get away from good footwork !

Tim

[Jack H.]

sorry for the misread Jethro,
could have opened at the ends because of the flat part that you left.
I do not use spring joints. Repairing and restoring instruments for the last 14 years has given me insight into what instruments need and don't need. One thing I have learned is that less tension is better!
I do not use spring joints.
In my work every thing is dead flat before gluing.
I have just seen too much damage to instruments from being built with too much tension. Center joints on the back are a real pain to re-glue once they pop open after x years.
Do you have a good caliper? also a must to help judge how thick the shavings are.

[jethro]

Jack :
I started in the beginning using flat joints. When I would put hot glue
in the joint and ste them on top of one another and rub the joint and then
stand back and watch..... I would watch as the center 85% would swell
up more than the edges and after 1 or 2 minuites the edhes would be appart and the center 85% still touching. I guess if I had 3 or 4 inches of
extra wood at the ends I could sacrifice I would thing the joint might be OK.

I agree with you that I would love NO stress in that center joint. I think the idea of the spring joint is that the stress (after a long dry time) ends
up in the center portion but not at the end portions where a crack would
naturaly want to start (and then run inward ) My experience with flat
joints OR spring joints with NOT ENOUGH spring is that after a long dry
time when all the water from the glue is gone , I am left with a joint where
the center 85% is stress free but the last inch or so is under great stress
outward. Seems like sooner or later it will open a little.

Seems like using extra long wood with a flat joint would work well.
( where you would cut off the parts on ends leaving only the center
which was not under any stress.) I could be thinking too hard again...

I was a machininst so I have lots of measureing tools !

I was thinking about measureing the maximum width of the spring gap.
I wonder if I could use Plastigage in it ????? (I'm REALLY thinking too
hard now !)

Strange I have never had problems with spruce joints. I think it is because spruce is more compressable if you clamp it. and maybe
it is easier to plane you don't get hard spots like maple.

I am practiceing restoration and repair on an old violin that my cousin
gave me. I think its about turn of the century german built with no
corner blocks and really crude graduation on the top plate.
2 cracks top to bottom in the top. Replaced the top and bottom blocks.
Neck reset next. glueing 2-nd top crack. Will remove integral bass bar.

What would be good to reinforce inside of top plate with - it has some deep
gouge marks in the too thin spots. Someone suggested white glue on
cotton cloth worked well . Can you suggest something for that situation?

Strange the back is nicely and smoothly graduated and the top looks like
a drunken dislexic 6 year old did it with a chain saw while wearing a blind fold.......
Maybe not the oregional top ???????

Tim