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Hibari-San
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[*] posted on 10-14-2015 at 05:43 AM
calculating measurements of ribs


Hey guys,

now that I'm building my second oud, I stumbled over some problems.
I made a new mold which is slightly smaller than the first one.

There's a way to calculate the measurements of ribs mentioned in the oud construction manual I own. But everytime I end up scraping and chiseling until one rib fits to the other. (after that, the rib doesn't look like the pre calculated pattern)

Is it possible to exactly calculate the ribs that one could cut them out and they fit actually ? How do the builders with no mold know how wide the ribs should be ? Do some of you think that a computer program could help designing an oud mor accurate ?

Please help me with your ideas and experience. If there's no solution, building of ouds will stay very frustrating for me. :D




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Hibari
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SamirCanada
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[*] posted on 10-14-2015 at 06:35 AM


You are talking about rib width at a certain height? I can help you.

Measure the oud mold at various height: 7cm, 9cm, 15 cm, 19cm, 10cm. 19cm is the highest point of the oud

you have to decide how many ribs you want.

Example, you say 17 ribs. C = 2πr = 2π19 ≈ 119.38052

119.3 /2 = circumference of diameter = 59.69

59.69 / 17 ribs = 3.51cm this is the width of each rib at that height of 19cm.

so what you do then is you establish the width for each of your heights (5cm, 7cm, 10cm, 15 cm, 19 cm, 13cm, 5cm) and you create a "master" rib template which you never use on the oud but you use to measure the other ribs.




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jdowning
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[*] posted on 10-14-2015 at 07:47 AM


Alternatively you can make a jig from the bowl or sound board longitudinal profile and trace the master rib profile from that as shown here:

http://www.mikeouds.com/messageboard/viewthread.php?tid=15437

and here:

http://www.mikeouds.com/messageboard/viewthread.php?tid=8488&pa...

Each rib will still require hand fitting to its neighbour for precise fit (using an inverted jointer plane or sanding board to obtain the correct joint angle) as bowl building proceeds. This method of course only applies to bowls of symmetrical geometry and semicircular cross section which old ouds (and lutes in particular) hardly ever were.

For non symmetrical bowl geometry a solid mold is required on which the rib profiles are individually laid out in ink. Each rib (measured from the inked profiles) then is of unique geometry and must be individually cut and fitted. For this type of bowl it can help to fit dark wood 'lines' between the ribs so that any slight discrepancies in the rib joints will not be noticed.
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Dr. Oud
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[*] posted on 10-15-2015 at 08:28 AM


The ribs must be fitted, so the calculated dimension is not the cut dimension, rather, cut the ribs at least 3mm/18in wider than the calculated dimension for trimming to fit. Cut a few extra ribs wider in case fitting takes too much off the width. For the top ribs, cut the curve on one edge only, leaving the other edge square to trim when assembled.

Fit each rib carefully so there is no stress/pressure when gluing them together. Pushing the rib to fit together will result in a distorted bowel and inherent stress more likely to crack open the bowel when bumped.

Measure the arc length at each step of assembly and compare it to the calculated arc length for the number of ribs assembled, then adjust the next rib to match the calculated arc length. After fitting a new rib to the assembled rib edge, flatten the other/open edge on a flat sanding table or inverted plane and check the flatness on a flat table/board. If the arc length is short, use a wider rib next, if the arc length is too much, remove some of the flat edge to match the correct arc length.

Add some extra width to the last rib to trim the top edge while leaving plenty of rib material to attach the face braces, a little extra depth of the bowel is a good thing.

Separator strips between the ribs provide some stress relief as well as a pleasing appearance.




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Hibari-San
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[*] posted on 10-16-2015 at 11:34 AM


Thanks a lot guys !!!

Samir thats quite a nice way to calculate !! Thank you for the formula !
It provides an accurate measuring method of the rib for every position on the mold.
I felt sometimes like the 4 or 5 measurements taken from the molds cross sections weren't enough. Very helpful !

That jig is very interesting Mr Downing, is it common for lute building ? I never seen it so far... I will definitely give it a try for the next oud !!
Thank you for sharing such a precise instruction !

At last Dr. Oud thats exactly what I am doing right now but it felt like there may be a easier way to achieve the goal of nicely fitted ribs :)
It is described that way in the german manual I own. But measuring the arc lenght is a helpful hint !!

Now I have more than I expected, three ways to calculate/fit the ribs for the body.
I will find my favorite method sooner or later.

Thanks again for taking the time gentlemen ! I'm sure this threat can help many other newcomers in oud building !!

Btw I have 13 of 19 ribs attached on my oud #2,
maybe one day I will have a standard procedure in making bowls as acurate and beautiful like the many luthier masters.






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Hibari
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jdowning
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[*] posted on 10-16-2015 at 12:04 PM


The jig can only be used for symmetrical bowl geometries that is semicircular cross section and bowl profile the same as sound board profile. I do not know of any old surviving lutes that match that criterion - usually the cross section is either a flattened profile (particularly for the very large lutes) or deeper profile for the later period lutes. Also bowl sections tend to be relatively deeper at the neck block (this provides a stiffer structure more resistant to bending under string tension.
I have seen the same non symmetrical bowl geometry in some of the old surviving ouds. Check out my work on oud/lute geometry posted earlier on this forum for examples.

http://www.mikeouds.com/messageboard/viewthread.php?tid=11186&p...

As I mentioned before this traditional style of bowl can only be constructed using a solid mould. There is no easy way - just practice and experience.
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[*] posted on 10-17-2015 at 09:01 AM


There's been no mention about how to generate a flat pattern for the ribs. I suppose that using a solid mold with lines drawn on for the joints would require a tracing paper pattern for each rib, then transferred from the paper pattern to a flat rib blank.

For symmetrical ribs, I've used a flexible steel rule bent in a curve around 3 clamps or nails, with the widest part at the distance from one end. Remember to allow for the small circumference at the neck block. This pattern is cut in a Masonite/plywood pattern, then used to trace each rib flat pattern. Always cut a little extra for trimming and a few extra ribs in case you break some during bending The top rib is cut on one side only, leaving the top edge square, to be cut after fitting the last joint. I make the top rib wider with material added at the neck and tail block. This leaves a wider top rib, helpful when holding the oud and providing a deeper bowel that provides deeper bass resonance.




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[*] posted on 10-17-2015 at 03:45 PM


For the asymmetrical bowl (flattened or deeper in cross section etc.) requiring a soild mold, once the first central rib has been shaped and glued in place to neck and tail block the other ribs are then cut and shaped individually to match as the bowl building progresses. The geometry of each rib is determined by direct measurement of the rib profiles marked on the mold transferred to each rib blank. For a flattened lute bowl section only the first central rib will be symmetrical in geometry - the others may, in extreme, be crescent or even 'S' shaped!

Canadian luthier Michael Schreiner's blog is about historical lute and guitar building - with lots of detailed images of historical instruments - but has much practical information that might be of value to oud makers as well. He deals briefly with asymmetrical bowl mould construction and rib shaping here:

http://schreinerlutesandguitars.blogspot.ca/2011_09_01_archive.html

...... as well as in his blog of May 2012.

The tool that he uses for marking out the rib profiles on a lute bowl mould is a plastic 'Blending Curve' available from Lee Valley, Ottowa - see attached scanned image from an old 2013 catalogue. I find this tool very convenient for drawing odd shaped geometrical profiles.

Details of lute rib measuring from a mould and shaping (on the mould) with small planes and files is also described by the late Robert Lundberg in his 'Historical Lute Construction' book.

All easier said than done!

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[*] posted on 10-17-2015 at 11:17 PM


The flat pattern for the ribs can be treated mathematically... both the symmetrical and the asymmetrical cases
Working through this myself I found it quite challenging, but probably not something that was beyond the historical luthier?
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[*] posted on 10-18-2015 at 02:57 AM


Even if a luthier was able (or motivated) to mathematically calculate and plot the individual flat rib profiles, transfer the patterns to wooden rib blanks and then accurately and smoothly cut out each rib, he would be no further forward. This is because each rib has then to be hot bent to the exact profile of the mould and the edges then planed to be a precise fit at the correct angle.

There is no easy, fast track, way to making a bowl. The most direct and simplest way is always the best. Facility and speed comes with practice in making many bowls - a skill that a luthier, who is trying to make a living from his work, must master.
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[*] posted on 10-18-2015 at 06:59 AM


Hello, I have just joined your very interesting forum!

I have built a couple of lutes, and I think you are addressing here one of the main issues one immediately encounters when starting out


My two cents worth is that calculated 2d profiles for the ribs are useful, but you should not straightaway cut the ribs completely down to the profile. Make sure as Dr Oud says to leave a little room for adjustment and filing as you fit them (with the help of the jig)

Exact calculation is more subtle than SamirCanada's proposed method. But it is very much the kind of calculation that is offered by the standard CAD packages. The very popular Rhino3D program is very good for straked shell structures in general, of which the lute bowl is an interesting example


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[*] posted on 10-18-2015 at 09:57 AM


This article on building lute bowls with flattened section published in FoMRHI 35 years ago may be of interest. See Comm 303.

http://www.fomrhi.org/uploads/bulletins/Fomrhi-021.pdf

Note the non symmetrical rib shapes in 1.3
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[*] posted on 11-10-2020 at 07:25 AM


I came across another relevant FoMRHI article which may be of interest on this thread, it describes how to mathematically calculate the shape of the rib (for non-flattened bowls)

Comm 799 A method for the construction of the rib template for a lute, by D. Kershaw

http://www.fomrhi.org/uploads/bulletins/Fomrhi-047.pdf

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