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Bow graduation numbers

 
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wm_crash
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Joined: 24 Feb 2013
Posts: 140
Location: Wilmington, DE - USA

PostPosted: Sat Nov 15, 2014 12:18 pm    Post subject: Bow graduation numbers Reply with quote

Howdy,

So after a rather longish break, I am thinking again about bows. The plan is to have someone make a holding jig that will hold the bow for the roughing of the 8 sides. And when I say roughing, I mean 0.2mm away from finish size. In my books, the 0.2mm will be for final sanding.

I have already worked out math for the jig, but I am still trying to figure out the graduation of some good quality bows. I don't have anything of that nature in my arsenal. I have numbers from Jim from a while back. If anyone else would like to help, I would be most thankful. What I would need are front to back measurements in mm on every inch starting from under the head to the end of the bow.

Anyone willing to help, please let me know.

I will post another thread later in the day with my jig in case there is interest.

thanks,
Cosmin - because wife just told me that going as "the friendly hooligan" is getting old Smile
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wm_crash
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Joined: 24 Feb 2013
Posts: 140
Location: Wilmington, DE - USA

PostPosted: Sun Nov 16, 2014 1:52 am    Post subject: Reply with quote

Alright, so I promised I'd describe the jig I have in mind for the rough shaping of a stick. The idea was to start with a piece of 3/4 mdf, and have a set of very precise varying depths dados cut in them. The bow blank would be pushed into these dados successively,and it would be planed flush with the template. The nature of the dados would made it so that the stick is shaped as per one's desired numbers.

Since a bow has 8 facets, in theory 8 dados are needed. But there is some duplication and only 6 are truly needed. See attached image for reference. It contains the grooves as well as the cross section view. First groove takes care of the back, second groove is for the belly,then one side, then the other side, the next groove starts the octagon,and it can be used for two octagon sides. Last dado is for the last two facets.

The idea is that the stick must be pushed to bottom out in the grooves, so that the planing gives you the right dimensioning.

This is an oversimplified version, because there are some items that need to be taken into account:
1. front to back does not measure same as side to side
2. you need to leave some room for final sanding
3. you can't plane FLUSH with the template because you will ding it, so you have to plane a set amount above template

First two items are solved by careful math. I have an Excel sheet that calculates the grooves depending on desired final parameters.

Third item, where you need to plane a set amount above the template. You can use a block plane with UHMW tape as skates. Or you can use a Lee Valley string inlay scraper which can be set to a precise clearance using a feeler gauge:

http://www.leevalley.com/US/wood/page.aspx?p=69877&cat=1,43314,69873

And now the jig image:



cheers,
Cosmin
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whatwasithinking
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Joined: 26 Jan 2013
Posts: 221
Location: Washington State

PostPosted: Tue Nov 18, 2014 12:53 pm    Post subject: Interesting... Reply with quote

Hi Cosmin,

So now you've become an "unfriendly" hooligan? Understandable, if you're trying to make bows. Richard Wilson says bow makers over the age of 50 are no longer sane. I think you need to be a bit sketchy to even want to try, though. Wink

Is this based somewhat on the Kun and Regh book? Seems like there might have been a jig a bit like this, but I can't remember off hand.

Would this be only for roughing out a stick, or are you planning to use this for final graduations? I can imagine it working for roughing out, but not for finishing a stick. But if you're just using this for the ebouchage, then why not keep it really simple? Just plane the blank down until it's a couple of mm fat in each dimension, for the back 2/3 of the stick. Then taper the front 1/3, and camber it. Maybe the jig would help keep the octagons regular, but I don't know anyone who does this. 'Course, I don't know lots of people. Oh, I take it back. I remember that Frank Henderson did something like this in his book, but I don't think it was for final graduations. I believe it was to keep the octagon regular.

I think there are two variations on the formula for the Vuillaume curve, if that's what you're thinking of. I've found that the Woolhouse version is a bit closer to the bows I've actually measured: -6.17+5.08*LogX, where X is the distance behind the head.

But Fetis used a slightly diferent formula: -5.26+5.14*LogX

These formulas are for violin bows. You just have to start a little fatter for viola, cello, or bass.

The actual finished sticks I've measured only approximate these numbers, and some deviate a great deal. I record stick data on large rigid poster board, upon which I trace outlines of the sticks. I record height and width of each stick, but not at regular intervals. Instead, I use the Vuillaume curve to mark locations where the sticks should be growing in depth by about 0.5mm. These measurements coincide with the compas I use, that has notches every 0.5mm, with widths from 5mm to 14.5mm. Don't forget that your sticks are probably going to end up with dimensions of height and width that are different from each other, as you move down the stick. This is as it should be.

The board looks a bit like log paper. There might be a better way to do this, but that's what I'm doing.

Not sure I answered your question, and of course you should take my comments for what they're worth. I'm no expert.

Bob
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wm_crash
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PostPosted: Tue Nov 18, 2014 6:24 pm    Post subject: Reply with quote

Howdy Bob,

Sanity has been tossed out the window at some point during my teen years. I'm not even trying to find it again.

Kun and Regh have some template approach as well, but theirs is a bit different (i.e., more complex and more involved). They would be more reliable as far as producing specific dimensions, but they are somewhat more of a headache to produce.

My intention was to produce a stick within 0.4mm of final diameter (regardless which faces you use to measure the "diameter" of an octagon). That would be my sanding margin of 0.2 mm on each face. Not sure I can call 0.2mm an adjustment margin.

I did not know those log formulas, I measured some bows I had around here and I went with an average tipped mostly towards the one bow I kinda liked. I mostly discounted any measurements from crappy bows made from light density wood, and hence overly fat in order to get to a reasonable weight. I punched the log formulas in my Excel, shifted the Woolhouse numbers by 175mm and the Fetis by 109mm, and I am kinda close.

As far as differences between height and width, I went with a target of 0.3mm all along the stick (except for a few inches at the end where the sick is a perfect octagon) which is what I saw in Bolander.

All that being said, I can see how this could be a success in woodworking and a total failure in bow making. I was tickled enough to try. The guy cutting the template for me is caught in a guitar build, but he said he could probably do the number crunching and cutting this year.

cheers,
Cosmin
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whatwasithinking
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PostPosted: Tue Nov 18, 2014 6:56 pm    Post subject: Reply with quote

Okay, I think I see what you're shooting for. But are you planning to camber the stick before proceeding with the tapering? Or are you tapering it before cambering? If you intend to get down to within 0.4mm before cambering it, then you might have some difficulty if the stick twists during cambering. You just won't have much leeway to make adjustments. I don't know exactly how Bolander did it, but I think he gave himself a bit more wood to work with before coming that close to final graduating. If Ed chimes in, he might be able to expand on that.

I think some very experienced makers start with a pretty skinny blank, so they take the stick down a bit further than 2mm fat before cambering. I actually had one maker tell me that he knew makers who do this on a regular basis, and he wasn't smiling about it. Maybe when I've made several hundred bows, I'll decide I can get away with it...

On a violin stick I recently cambered, it twisted maybe 30 degrees during cambering. It did rotate back a little after I reheated the entire stick. Fortunately, I'd left it about 2mm fat, so there was enough room to plane it straight. I guess you could twist it back with heat, but sooner or later it will probably twist back to where it wants to be.

But maybe I completely misunderstand what you're doing. If you're attempting to follow the Bolander book, which is what I started out trying to do, you might find that important steps are left out, or at least not emphasized. He seems to taper the entire stick before cambering, but that might be more trouble than it's worth. And that's not how I was taught to do it, either.

Bob
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wm_crash
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PostPosted: Tue Nov 18, 2014 9:17 pm    Post subject: Reply with quote

There would be no cambering before the shaping part. I have not decided on how that would happen, but either manually or Kun & Regh style is an option. The outside grill is long enough to accommodate a stick clamped to a 3/4" MDF template for example (preheat the stick, strap it to the template, and let it cool down). I guess any attempt to twist would be prevented by the template.

The original intent was to quickly get the shape without fussing a lot with the calipers. I was expecting slight deviations from arrow straight at the end of the shaping, which could be handled during cambering. Time will tell - I did warn that sanity has left the building, right? Smile

cheers,
Cosmin
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whatwasithinking
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PostPosted: Wed Nov 19, 2014 2:36 pm    Post subject: Reply with quote

Cosmin,

Well, that sounds like an adventure. I don't know how you'd stop a stick from twisting when heated, if that's what the wood was going to do to release the tension inside. And you might need to camber it in stages, to keep from breaking your stick. Can you adequately control the temperature of your grill, and keep it consistent across the stick? The mdf might char before the stick does, but I don't know. Let us know how this works out!

Bob
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wm_crash
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PostPosted: Thu Nov 20, 2014 11:41 pm    Post subject: Reply with quote

My thoughts were that cambering is a bit like air drying lumber (don't shoot yet!). Say you have a sweet gum, and you take it down, mill it into boards, sticker them and come back in 2 years to find a bunch of curly fries. On the other hand, take those freshly sawn and stickered boards and strap them to a flat slab of concrete. Once they are done drying, they will stay flat.

So, let's say I have a curved template to match the camber (and a bit more to account for springback), if I strap the blank to the template and tie it with hose clamps very few inches, I am hoping that will prevent twists during the cooling. The way I see this, it would really not be any different than what Kun and Regh have, they just use some fancier clamping devices and dedicated ovens.

The grill is can be dialed within a good 20F to 30F of desired temperature, but it takes some time, can't just crank it to a specific number.

Can't wait for someone to ask "why does this bow smell like grilled chicken?".

cheers,
Cosmin
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