Violin Forum/Message Board

[jethro]

Hello again all....
Have been working on fitting me back joint of the really nice back I got
from Leif a while back. I started with my usual method. Cut the back
faces to be dead flat with plane. Jointered the edges 1 or 2 very light passes. Clamped back to back in wood vice and staryed planning with big Lee-valley. I can get to dead flat pretty easy. Then I start trying to get
a perfect , even , tapered spring. I can get the spring with some difficulty
to be a nice taper to about 1 inch from the ends. Problem is that while
cutting the spring somewhere I'm goofing up my plane work. Cuttiing the spring causes the faces to go from flat transversly to being in two seperate planes about a fraction of a degree appart. When I put the faces
together I can see and feel a slight rocking- like opening a door. two seperate distinct faces. This means somehow when I am cutting the spring in I am accidently cutting the diagonals and not down the middle.

Then I tried shifting the plates -one after the other- to a shooting board,
a couple of swipes off each plate and re testfitting. I found the shooting board did reflatten the face at least on the high end points------ but --
I also saw that the shooting board cut and the cuts in the vice back-to-back ARE NOT at exactly 90 degrees- that is not the same 90 degree
angle. So it appears I can't mis shoting board and back to back type
cuts.
So I have been trying shooting board only cuts. I can get the faces
dead flat with a little effort. Once flat I attempt to cut spring. I have
noticed that when I do a spring cut I allways get more cut on one end
of the work than the other. It feels like a very slight grain angle change.
My spring cut looks like the top of an airplane wing - deeper and steeper
on one end and shallower and lower angle on the other. I start the cut
about 1 inch in and end 1 inch from end. I try to increase the inward
pressure on the blade while cutting in the central part and letting up at
the ends. I'm having a dog of a time getting the spring evenly tapering
and symetrical. Can yall see a weak point in my tecnique here ??????

( Used to be when I had then in the vice back to back I would try to get
spring by leaving the depth adjustment on the plane the same and just
angle the plane just a little crossways. That did get me spring but it
usualy also got the faces rocking on each other like there was a diagonal
cut- not a straight one. So I'm not skewing the plane anymore )

I have worked on this problem for 2 months now- with not much real
sucess. Any thoughts would be appreciated !

Signed: Frustrated !

[Oded Kishony]

Hi Frustrated.

A couple of points. If you are able to get the two faces perfectly flat then perhaps you should consider a rub joint rather than a clamped joint. Since a rubbed joint does not require springing.

But if you want springing then after you have the two faces perfectly flat, remove one half from the vise and place the other one on top, the two joined halves together. Pinch the ends and pulling downward on each end make sure that your surfaces are dead flat. You can put a strong light behind the joint , bobbing your head up and down, try to see if any light leaks through.

Now pull the blade of your plane all the way back until it no longer cuts. Then slowly adjust it forward as you run the plane along the edge of the half still clamped in your bench. The plane should just remove a whisker, sawdust.
Now place the two halves together again with a strong light behind the joint and look for the leaking light. Again pinch the ends and try to get the halves to rock against each other.
If you're having a problem at this point you can chalk up the half in the bench and rub the other surface over it, this will show you where the contact points are located. I sometimes will use a scraper to remove high points if my plane is uncooperative.

I think the problem you're having is that you are removing too much wood when you're creating the spring. The amount of spring is tiny-just wide enough to see a sliver of light between the surfaces

Oded Kishony

[Jack H.]

Jethro, I may be of the old school, but once you get it 90 and dead flat I glue and clamp.
If the wood is well seasoned adn the glue is frsh and strong there really should be no problems.
I do not understand the necessity for a spring joint, it just adds tension to the violin on a joint that does not need tension. THis is not to say that those who do it that way are doing something wrong, it is just not the way I do it.
If you can do it well, then go for it, if it si something you want to do then learn it. I wish I could help you out more, maybe take 2 backs, spring one adn then do the other flat, and I am talking flat to flat, no littel black lines anywhere, the kind of flat that when you try to move them apart you feel the vacuum holding for just a split second.
Good luck.

Jack

[jethro]

Oded and Jack :
First of all, thank you for taking time out to relate your experiences !
It was my impression from many here that spring is best and that it
protects the ends of the joint from stress later on. ( I have had 2 sets of plates open up at the ends about 1-2 months after glueing- both with very
light spring- both had an invisible glue line a few days after glueing when
I planned the joint and looked under microscope)

We have 100 degrees F here and 100% or over humidity. Could be that the plates are absorbong water and slowly swellling differentialy more in
the middle.

Oded- I have been using the rubbed joint, just with light spring. I make
the spring and put the haves together in front of a 150 watt spotlight in
the fully darkened shop. Easy to visualise the gaps uniformity.

When I am flattening the faces I get a shaving off about 10 inches long
and it is about like toilet paper ( transparent) . When I am trying to cut
spring I am getting about 1/2 or 1/3 third that much thickness and it is usualy an intermittant shaving.

Another detail....
my shooting board is 3/4 inch hardwood plywood. Is there a slicker
flatter surface I could use for the side of the plane to ride on ?
How about a glass shelf ?????? 1/4 inch by 12 inch by 30 inch ??????
I think I have a piece like that......

Oded, one of the problems is that about 1/2 the time I plane by hand
having the wood in the vice ( either back to back or singley) I occasionaly
get the faceted dual surface that rocks. I am trying to eliminate this
by using the shooting board setup !

I am not sure I can plane any better than this by hand !

I have tried a perfectly flat joint and watched it swell up in the first 20 secconds after applying the glue and the ends open up about 1/32 inch
in that time from the differential swelling from the glue.

Not sure where to go from here... may experiment with the glass plate
idea.

Have you ever measured the center light gap for the spring ?
How does yous compare to a transpaernt shaving ?
mine is uusualy 1/2 or less than the thin shaving.

Tim

[violinarius]

If you want to have a gap, then switch to a shorter plane, once you have the surface flat.

Start off square with the plane and then swivel the plane as you approach the center of the joint. As you pass the center, swivel back to a straight forward angle on your plane. Finishing with the plane square to the joint at the end.

Practice on cheap wood first.