Scarf Joints
Scarf Joint in timber frame construction, glued & reinforced w/ iron spike.
...Brief History - Scarf joints have been used in wooden-boat construction for centuries, from joining together planks for hull & deck, to making up full-length mast staves from shorter pieces. Even Lloyds of London had specifications for scarfs in boat building, 4:1 for planks, 6:1 (possibly 8:1) for keels, & 12:1 for spars.
Across the water, Japanese woodworkers brought hand cut wood joinery to extremely high levels, including many elaborate designs of scarf joints.
Across the water, Japanese woodworkers brought hand cut wood joinery to extremely high levels, including many elaborate designs of scarf joints.
Scarf length of 8 in gives very good strength in 1 in material (8 to 1 ratio).
Basic Principles - In woodworking, scarfs are often used to extend the length of material. Scarfs are defined by the length of the joint in relation to the thickness of the material — eg, a 4:1 scarf in 1″ thick wood would have a joint length of 4″. A common carpentry rule of thumb for good strength is length should be 8x material thickness.
Experimental Results of Strength & Flexibility - Richard Birmingham [Boat Building Techniques, 1984] states efficiency of scarfs ranges from 65% of the strength of solid timber for a 4:1 ratio & up to 95% for a 20:1 ratio. More recent experimental tests of flexibility & strength [Jean-Baptiste R.G. Souppez, Solent University, UK] show a similar pattern of increasing strength & flexibility w/ high scarf ratios: 100% (12:1), up to 120% (16:1), to even 128% (20:1).
Scarfing Bamboo Rods - Excepting traditional scarf-jointed British rods, & current day "nodeless" rods, scarfs are used primarily to repair fractured bamboo rod sections. Doubtless rodmakers before Everett Garrison capably joined broken sections of split bamboo rods. Modern techniques, however, follow Garrison's published account of his rod making & repair practices [Garrison, Everett & Hoagy Carmichael, 1977]. With a professional engineering background, Garrison examined the stresses on tapered bamboo rods, & developed methods of repairing bamboo sections to withstand these stresses.
Perhaps through his own testing, or from standard engineering practice of designing to 150% of required strength, Garrison designed his scarfs w/ ratios of length to thickness of 20:1 to 50:1, providing 100% or more of original strength.
Perhaps through his own testing, or from standard engineering practice of designing to 150% of required strength, Garrison designed his scarfs w/ ratios of length to thickness of 20:1 to 50:1, providing 100% or more of original strength.
Many scarfing forms are built using Garrison's design, resulting in a scarf length of approx 3-9/16 inches. Depending on the diameter of the rod section being scarfed, the scarf ratio varies from 20:1 for larger mid sections, up to 50:1 for fine tips.
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Scarf Joint Angle & Length To achieve 100% or greater strength, scarf joints in bamboo rods use long tapered joints, w/ scarf ratios from 20:1 up to 50:1. Scarfing forms built from Garrison's design use cutting angles from 2 to 3 degrees, & result in scarf joints 3-1/2 to 4 inches in length. |
Following Garrison [1977], the Scarf Joint is cut between flats 2 & 5 (or 3 & 6), diagonal to the guide flat, and fore & aft casting plane of the rod
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Joint Cross Section
For maximum strength scarf joint is cut across the bamboo section between flats 2 & 5. The glued joint is diagonal to the forward & backward bending plane of the rod (between flats 1 & 4), which evenly distributes stress on the joint during casting & playing fish. |
Where to Scarf ? - Although the obvious location for the scarf joint would be at or near the rod break, judicious adjustment of scarf position can provide a stronger repair, as well as a more visually pleasing rod. Factors to consider are location of break, integrity of cane next to the break, location of guide wraps, & cross section of the bamboo in the break area.
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Tip Sections - While it is theoretically possible to create a strong scarf joint very near the tip of a bamboo rod, small cross sections (0.060" & smaller) make precise cutting & gluing a joint in this area less practical. Often moving the joint one or two guides down from the tip provides more cross section to work with, as well as allowing scarf to be positioned under a guide wrap, providing additional strength and security.
In this example the scarf joint on Hardy 'Fairy' was placed 15 inches down from the broken tip, where 0.126" flat to flat measurement provided more cane for precise cutting & gluing of the joint, & where one end of the scarf could be positioned under the second guide wrap for added security. |
Break occurred 5-3/4 inches from tip on Hardy Fairy, 9 ft, where cane was 0.080". Joint was centered at the 15 inch position, where cane was 0.132" Completed scarf joint is just visible one inch in from each tape mark.
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Completed Scarf Joint in Heddon tip section. Scarf joint is just visible, but will be blended w/ new varnish after guides wraps are replaced.
When a break occurs closer to the ferrule on tip section, the scarf is more straightforward. Here scarf was cut close to the break, between the 2nd & 3rd snake guides, & guides rewrapped in original locations. (See 'Current Repairs' page for more photographs, including guide wraps, ferrule resetting, & re-bluing.)
Mid Sections - Many breaks in mid-sections occur near a ferrule, where bending stress is concentrated at the transition from bamboo to metal. Heddon mid-sections, for example, are commonly seen w/ a break just below the female ferrule. Experience has shown that a well made scarf joint in this area often increases the strength of the bamboo, thereby decreasing the chance of a later break in this area.
Selection of Cane - Being a rodmaker, Garrison carefully built a new cane section of the required length & taper to complete the scarf of a fractured rod. Others w/o the knowledge or tools to build a new section use available cane from 'parts rods' for the new piece. The higher the quality of the cane used, the higher the quality of the completed rod section. Ideally the new bamboo would come from the same maker, & even the same model, as the original rod. If not available, a section of cane from another reputed maker, in good condition, can suffice.
Careful measurement of dimensions of original rod section, together with that of other vintage Granger cane, in preparation for scarfing tip of 1929 'Granger Special' casting rod (tip closest to caliper jaws)
Taper - Of primary importance is the taper of the rod section being replaced. Careful mic'ing of the original broken section & replacement cane pieces insures the action of the scarfed section will be similar to the original rod.
Although node pattern of the new piece may not match the node pattern of the original rod, care should be taken to avoid cutting the joint across nodes. Lastly, color & treatment of cane (eg ammonia treating, flaming, etc) should match between the two sections to be joined, although some color adjustment can be achieved by blending final varnish.
Although node pattern of the new piece may not match the node pattern of the original rod, care should be taken to avoid cutting the joint across nodes. Lastly, color & treatment of cane (eg ammonia treating, flaming, etc) should match between the two sections to be joined, although some color adjustment can be achieved by blending final varnish.
Adhesives
[text to come...]
Invisible Wraps
[text & photos to come...]