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Jeff White

Bow Repair Blownout mortice

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I am wondering the best way to fit a new piece to repair this.  I'm thinking of choices:

1.  Open up the area like  a neck graft and work the new piece in at a taper (like a neck graft)

2.  Enlarge the area to go past the sides of the mortice for better stability.

3  Make a cast of the head and mill out most of the inside and replace. (don't have a mill, but..........)

I can think of a few other options, but I'd like to hear accepted procedures for this type of repair.  Simple gluing in a well fit piece seem folly as it there is no support.  Maybe not much support is needed since the headplate will hold all together and  a well fit plug won't cause a problem?  The bow is a commercial older silver Pernambuco bow (viola) stamped Roderich Paesold.  Pretty nice commercial stick and might not be worthy of doing this right, but is good practice for me on something I haven't done.

IMG_4289.JPG

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Make a new bottom for the head (I'm assuming the Perrnambuco is available), cut the old bottom off, and treat it like a splined break repair (I've done some of those)?  All I can think of at the moment.  :huh:

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This looks like a good candidate for a rounded pernambuco cheval, something like this: https://trianglestrings.com/cheval/. I have done them in the past to replace noses, but recently saw a Dodd that Peter Ratcliff did this sort of thing to replace a piece similar to what you have.  Of course you do not have to do it the full length if you have roundover cutter......it could be more fun than a barrel of monkeys!

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3 hours ago, Jerry Pasewicz said:

This looks like a good candidate for a rounded pernambuco cheval, something like this: https://trianglestrings.com/cheval/. I have done them in the past to replace noses, but recently saw a Dodd that Peter Ratcliff did this sort of thing to replace a piece similar to what you have.  Of course you do not have to do it the full length if you have roundover cutter......it could be more fun than a barrel of monkeys!

In the absence of a milling machine, can I use my micro lathe?  I'm thinking I can put a quarter round bit in the chuck, and attach the bow to the movable table.  Put the bow face, facing the chuck (90 deg from the way I cut a spline in the head).  I could then feed the bow head into the cutter a slight pass at a time.  Not sure it I'm giving you the visual I'm seeing.......  

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23 minutes ago, Jeff White said:

In the absence of a milling machine, can I use my micro lathe?  I'm thinking I can put a quarter round bit in the chuck, and attach the bow to the movable table.  Put the bow face, facing the chuck (90 deg from the way I cut a spline in the head).  I could then feed the bow head into the cutter a slight pass at a time.  Not sure it I'm giving you the visual I'm seeing.......  

Sure, it might be the perfect excuse/reason to get a mortising attachment/palmgren.  Don’t forget to protect your feather edge!! Packing tape, casting compound, or both.  This really could be a lot of fun...better than a roller coaster....

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And another thing, I am sure you know,....matching wood is critical....check the flash with mineral spirits, and the color with the oxidized wood before planing......

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3 hours ago, Jerry Pasewicz said:

Sure, it might be the perfect excuse/reason to get a mortising attachment/palmgren.  Don’t forget to protect your feather edge!! Packing tape, casting compound, or both.  This really could be a lot of fun...better than a roller coaster....

Jerry, What the H.LL is a mortising attachment/palmgren.  I get the feather edge/packing tape.  I wished I could make the edge match to the inside of the bevel/campfer to hide the joint, but it's too far left.  Figure I might match the cutter with the inside of the bevel on one side and the feather edge of the outside of the bevel of the other.....what are the odds I could find a mill that would match that.  What speed is ideal for a metal cutting mill (1/4 round?) on wood?   jeff

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14 minutes ago, Jeff White said:

Jerry, What the H.LL is a mortising attachment/palmgren.  I get the feather edge/packing tape.  I wished I could make the edge match to the inside of the bevel/campfer to hide the joint, but it's too far left.  Figure I might match the cutter with the inside of the bevel on one side and the feather edge of the outside of the bevel of the other.....what are the odds I could find a mill that would match that.  What speed is ideal for a metal cutting mill (1/4 round?) on wood?   jeff

Sorry, I meant milling attachment https://www.ebay.com/i/283560450606?chn=ps&norover=1&mkevt=1&mkrid=711-117182-37290-0&mkcid=2&itemid=283560450606&targetid=541454012852&device=t&mktype=pla&googleloc=9009669&poi=&campaignid=1881505439&mkgroupid=70056983237&rlsatarget=pla-541454012852&abcId=1139336&merchantid=114627101&gclid=EAIaIQobChMIvcOnzIf14wIVyJ-zCh0x6wwAEAQYASABEgImPvD_BwE 

the size of the ball end mill is dependent on the depth of the cut....meaning, you cannot cut down more tha the radius of the cutter.  What is the maximum depth?

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I`ve used the dowel method for various bow repairs  for years , one tip is to make a groove in scrap wood with the ball endmill and use it to make sure sure you get a good fit when turning down the dowel on the lathe before you remove it from the lathe. Its easier than relying on calipers.

 

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3 hours ago, Jerry Pasewicz said:

And Jeff...hide glue and treat with glutaraldehyde. 

Jerry, what's the purpose of glutaraldehyde in this application? 

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I assume that the OP does not have the piece that is broken out because the easiest repair would be to glue it back into place.  

I would square the break and make a square plug to fit.  This can be done by hand using the old fashioned chalk technique though a mill would make the work faster and more accurate.

I would use G-2 epoxy for this over CA glue or hide glue (with formaldehyde), and then remove the excess wood after drying.  I would give it a few days of sunlight to darken the wood, and then start the coloring process to make it invisible.

I did not think hide glue would work with an oily wood, and that the treatment with  formaldehyde or glutaraldehyde would solve the problem of an oily wood.  This is new to me.  I did a literature search but did not find references dealing with glue strength, oily wood, and  the aldehyde treatment.

Mike D

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Having a square joint is a mistake as the angles create natural stress points, no need to create more cracks.  Hide glue will work well as always depending on the glue and the prep.  The advantage is it does not leave a glue line to deal with, and we really should try to get away from large epoxy glue lines, as well as the ever popular crazy glue with wood dust. We really should treat these things with the care and respect they deserve. 

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4 hours ago, DelViolino said:

Jerry, what's the purpose of glutaraldehyde in this application? 

The glutaraldehyde reacts with the glue making it non-water soluble.  This was traditionally done with formaldehyde or formalin, but evidently the glutaraldehyde is more effective..(I am not a chemist so this is my interpretation of chemist talk)  this allows us to do feather edge joints that can disapear as the molecular bond of the hide glue does not need thickness for strength.

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14 hours ago, Jerry Pasewicz said:

5.6mm, so I would need at least an 11.2 bit, right?  That's wider than the head.  I don't think I want to get that invasive (read.....chicken) as I have a micromark small lathe and I'm not sure about setting it up right for a feather edge on the sides of the head.  I was thinking more of a series of passes making a straight sides working into a radius at the bottom.  Harder to fit as I couldn't use a lathe spun rod.....

    This is a mock-up. Sub correct bow and ball end mill bit. I’m thinking I would get too much chatter with this setup. Probably need a milling attachment. 

D2D20F88-663C-41A7-9BA6-25CAC3ED378A.jpeg

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1 hour ago, Jerry Pasewicz said:

The glutaraldehyde reacts with the glue making it non-water soluble.  This was traditionally done with formaldehyde or formalin, but evidently the glutaraldehyde is more effective..(I am not a chemist so this is my interpretation of chemist talk)  this allows us to do feather edge joints that can disapear as the molecular bond of the hide glue does not need thickness for strength.

There are others thing you can use to cross link hide glue such as this. The resultant glue is supposedly more water resistant than when treated with glutaraldehyde, and has a higher tensile strength . But i believe it takes longer to cross link and needs heat of around  50 Centigrade. Around 5% of the weight of  dry gelatin  made into solution with water.

https://www.amazon.co.uk/BULK-POWDERS-Acetyl-L-Glutamine-Powder/dp/B00IZD3BKK/ref=pd_lpo_sbs_121_t_1?_encoding=UTF8&psc=1&refRID=7R9QCDP4E1X68S62RKSA

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45 minutes ago, Jeff White said:

5.6mm, so I would need at least an 11.2 bit, right?  That's wider than the head.  I don't think I want to get that invasive (read.....chicken) as I have a micromark small lathe and I'm not sure about setting it up right for a feather edge on the sides of the head.  I was thinking more of a series of passes making a straight sides working into a radius at the bottom.  Harder to fit as I couldn't use a lathe spun rod.....

Sounds good.  Or you can stack chevals...

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49 minutes ago, fiddlecollector said:

There are others thing you can use to cross link hide glue such as this. The resultant glue is supposedly more water resistant than when treated with glutaraldehyde, and has a higher tensile strength . But i believe it takes longer to cross link and needs heat of around  50 Centigrade. Around 5% of the weight of  dry gelatin  made into solution with water.

https://www.amazon.co.uk/BULK-POWDERS-Acetyl-L-Glutamine-Powder/dp/B00IZD3BKK/ref=pd_lpo_sbs_121_t_1?_encoding=UTF8&psc=1&refRID=7R9QCDP4E1X68S62RKSA

Thanks for this!

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4 minutes ago, Jerry Pasewicz said:

Sounds good.  Or you can stack chevals...

By that, you mean, put a round one in to fill the bottom and then do it again, milling out 1/2 of that, well, with a round bottom much more.  Jerry, note that I added to my earlier post and put a pic of the mock up.

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I went with plan 1 -1/2 and it's held up for 3 or 4 years now.

Neck-graft-ish but chiseling below the base--and for the full length--of the existing mortise to expose three large surfaces for long grain gluing. Then re-carve the throat and the mortise.

Extremely important to support the sides of the head before beginning. I glued some maybe 5 mm thick basswood sticks I had left over from when I used to use them for spreaders. They worked fine and were easy to file away later.

I probably have some pics if you opt for the low tech approach.

BTW someone mentioned just gluing the blown out piece back in place. The reason I had to do the graft was because someone before me had tried the simple glue job and it failed the first time the hair was tightened. There's just not enough gluing surface for such a highly stressed area.

When Jerry mentions roller coaster exciting, the picture that comes to mind is your (what you thought was) adequately supported bow head jumping out of place as soon a machine powered force is placed upon it and mayhem happening before you can hit the off switch. A mill head is going to exert more driving force than say a rotary saw doing a spline. It's a tricky shape to secure.

Good luck.

 

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5 minutes ago, Jeff White said:

By that, you mean, put a round one in to fill the bottom and then do it again, milling out 1/2 of that, well, with a round bottom much more.  Jerry, note that I added to my earlier post and put a pic of the mock up.

Yes, that is what I meant.  The mock-up is the right idea as long as it is in very solid, you protect the feather edge, and use 4 fluted ball end mills, not a drill.:D

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51 minutes ago, bengreen said:

 

When Jerry mentions roller coaster exciting, the picture that comes to mind is your (what you thought was) adequately supported bow head jumping out of place as soon a machine powered force is placed upon it and mayhem happening before you can hit the off switch. A mill head is going to exert more driving force than say a rotary saw doing a spline. It's a tricky shape to secure.

 

He mentioned that before my "unsecure mockup".  I was thinking it wouldn't be that much force if I went crazy light on the passes, but I haven't every milled wood (just spline sawing) and don't have a clue.  Thanks for the caution, I get the feeling Jerry will squash my mock-up.:wacko:

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1 hour ago, Jeff White said:

He mentioned that before my "unsecure mockup".  I was thinking it wouldn't be that much force if I went crazy light on the passes, but I haven't every milled wood (just spline sawing) and don't have a clue.  Thanks for the caution, I get the feeling Jerry will squash my mock-up.:wacko:

No so much squash as further support.....we all like support.  Just keep in mind that if the head moves, nothing good can result.

 

 

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