hornbow project numpty questions

[stuckinthemud]

Ok, so, when processing horn, you need a strip of horn about 6mm thick, is that measured from the top of the crown, or the edge?. Also, do you run the thicknessing cut right to the end of the horn or do you cut off the tip and save it for other projects? Do you cut a few mm oversize and use abrasives to get to the final thickness?  Finally, some sources advocate a sacrificial jig to mount the horn on before band sawing but is it ok to just use the saw fence? Thanks, Andrew

[Pat B]

Just guessing...I think you want the horn "lam" to be even thickness through out when you cut it from the raw horn then grind it to it's glue up shape and size.

[stuckinthemud]

Hi Pat, yup, I have back strips to process, each one going from 2mm thickness at the wide end, to full thickness about 6" from the tip. I think I'm supposed to make a second cut?

[mikekeswick]

Hot melt glue and a piece of ply are what to use to keep the cut true. Do not use the fence. A 3/8th blade about 10 tpi with a good set to it is what you want and free hand the cut.
Try to envison on what plane the longest, widest, straight strip is.
A horn strip does not need to be 6mm thick. 6mm thick horn would be suitable for a bow of around 100# If you are aiming for something around 50# I wouldn't go any thicker than 4mm. You want to have the core as thick as possible as wood is the most stable and stiff material in a composite. Sinew thickness is more or less set at 2 - 3mm over the center of bending sections so you can really only alter the core to horn ratio. More horn will have less string follow but will be harder to stabalise. Typical core woods will be around 0.65s.g. horn is more like 1.3 s.g.
It is best to cut all the way to the tip, even though it narrows too much to be in the finished bow, it is useful for clamping later when flattening and prepping the strips for grooving.
Yes cut oversize on the bandsaw. Then use, rasps, files then 40 grit to get perfectly flat. At this point you will see that you will have a tapered strip anyway. Once you have it perfectly flat use a very sharp scraper to get them completely smooth. At this point you should not touch the strips with bare hands(important!).Then groove.

[stuckinthemud]

Thanks Mike, that's exactly what I need to know. Target weight is between 40 and 50 for my first one as I  shoot 45# max at the moment. Probably look for 70 from the second one but that's a way off yet.

[stuckinthemud]

I cut the horn plates a long way too thick (11mm)so I'm gonna be a long time rasping them down to thickness; my question is, the concave hollow section at the end of the horn, what do I do about it?

[mikekeswick]

Just keep grinding/rasping until it is flat. This is the part where your lovely long horn becomes shorter! the internal concavity is one of the reason why the Turks (and others) used concave horn/convex core.
You can use the bandsaw to make further cuts on the thick proportion too.
You need to stop and continually check you are removing material on the same plane through the whole strip.
I use my edge sander to get the strips really close. A standard belt sander with 36 grit paper is very useful.

[stuckinthemud]

Thanks Mike, hollowing the horn is an option I'm fine with as my mate with the power tools is an hour away. Presumably where the horn tapers the depth of the hollow reduces?

[mikekeswick]

You are better off making it flat throughout its length.

[stuckinthemud]

Ok, I've filed the horn into 5mm plates and straightened them with steam, they are 18.5" long including a 4" taper and a 4" concave section that i guess I could use for the handle. This leaves 10" of  32mm wide parallel sided working section??

[stuckinthemud]

Any tips on getting the horn plate perfectly true?

[mikekeswick]

I always grind away material to get a straight, untwisted strip. You need to stop regularly to give it a good eyeballing for straightness. It is tempting to follow any minor twist to get a longer/wider strip and heat to straighten but unfortunately the twist will come back and lead to a twisted bow.
Again as heat is used for a large part of the tillering this heat will lead to any earlier heat corrections going back to their original twisted shape. Indeed this can happen when the horn is heated for the gluing process. You need to get it to 50 degs Celsius.
These sort of issues show why every step has to be perfect or else the house of cards starts getting unstable! 

[stuckinthemud]

Yup, this learning curve is huge!

This is where I am, the heat correction was for the curve of the horn rather than twist, still not totally straight though. The photo is deceptive, there isn't any twist as such, should I cut to size then finish grinding? The bottom piece is 40mm wide, the top one is slightly narrower but is a 1/2" longer at the tip end

[mikekeswick]

They look pretty good.
I would continue removing horn on this side to remove the concavity totally first. Then start working on the other side to get it completely flat and thicknessed. Then once it is perfect in thickness remove any extra width down to the size you want.
Remember that extra width is no problem. it is actually very useful to have extra width during the glue-up to the core. Once the horn is glued to the core and dry you can then reduce the core/horn for sinewing.

[stuckinthemud]

Thanks Mike, will do. Going back to the hollow limb section. If the olden-days buyers hollowed their horn plates, in addition to getting greater length from a given horn, might there also be a mechanical advantage to using this form - flexing tubes is much more difficult than flat plates, so you might get a stiffer/quicker bow.