D-bows do my head in, tiller help needed

[stuckinthemud]

Ok, I may be thinking wrong, and I don't mind being corrected but I believe a D-bow should have an elliptical curve.  I find I am getting slight set in the outer thirds and end up with arc of circle , should I be using a gizmo? Hows this looking?

[Eric Krewson]

Yep, your handle has to bend more, a gizmo will show you where to remove wood to get this happening. Set the pencil out near the tips where your limbs are bending the most.

[stuckinthemud]

Ok, no problem,  but if the gizmo gets the whole limb bending evenly will the tiller result in arc of circle rather than elipse, or is the elipse generated by how long/stiff the handle is?

[bownarra]

With elliptical tiller I will use a gizmo but as I move along the limb I wind in the center piece. I don't bother with the pencil I use a piece of threaded rod to facilitate this.
You are correct that your bow should have an elliptical tiller.
you should aim for no set in the middle, a little mid limb and the rest out to the tips.
Tiller shape is a factor of how the limb tapers in width and therefore thickness. If you have a thickness taper bend should increase as the limb gets thinner.
Is this a crossbow prod? It looks short.

[willie]

draw an ellipse 1.2 times as long as the ntn length of the bow

Code: [Select]

https://www.mathopenref.com/printellipse1.html
compare to the drawing except for the tips which should be stiffer

leave the handle a bit flat until you get tillered out more.

[stuckinthemud]

Thanks Willie, that makes everything much clearer, the ellipse I had been developing was virtually flat across the centre 1/3, I can see now why I need to get it moving more.

Yes, a crossbow, my better half volunteered my services to a friend of the family so I thought I would use the occasion to try and develop my skills in an area I am weaker at  , and I am struggling a bit with D-bows, a bend through the handle flat bow would have been much more straight forward.  Hopefully this will really improve my ELBs.

The threaded bar is really clever, will have to give that a go. 

I had initially laid in a 2mm per inch taper in width and thickness but this was way over target weight, I think by the time I'm done it will be nearer 1mm per inch with a constant width over the centre 8 inches.

[stuckinthemud]

Currently looking like this 
Currently 69lb at 9" draw, 10mm set where I took the tips too thin at the beginning.  No permanent set but soft set ( that disapears after a few hours) evenly along the entire bow,
Draw weight rises at 13lb per inch, predicted draw weight is 100lb at 12 inches, target weight is 50@12....really need advice on what to do next

[Pat B]

Get the outer 1/3 of each limb bending more and if you have enough meat left bring the bend back towards the mid 1/3.

[stuckinthemud]

Ok, so its been on a huge weight loss programme,  but I've lost some tiller too. Think the tips might be too stiff? Am slightly above target weight but have picked up 1/2 " of set, could do with some fresh eyes on it please.

[Marc St Louis]

Get a bit more bend in the middle

[stuckinthemud]

Better?

[Del the cat]

Better?

Nice
Del

[stuckinthemud]

Thanks Del, 45lb @9". It will draw to 10 but its starting to stack hard at 10 inches. Is there any point in pulling out some of the set when I heat treat it?

[George Tsoukalas]

There is no need to draw it to 69#, even on the long string, when your target weight is 50#.

You are really straining the bow's fibers.

The limbs look awfully thick.

Yes, do get inner limbs bending more.

The term "D bows"  can be confusing.

There is  a D bow where the limbs' cross sections are D shaped. The tiller shape can be anything you want from D (elliptical)  to rounded (C). and anything in between. Personal preference.

Then there is the D shape on the tiller which is elliptical.

To complicate matters further the actual shape of the outer limbs determines tiller shape. So pyramid shaped limbs should have the C shaped tiller to cut down on set while the limbs which are parallel (Meare Heath) to just past mid limb should have an elliptical tiller.

Then there is the  tiller where the outer limbs bend too much resulting in "whip tiller" and needless set. Been there. Done that. Looks like you are there, stuck in the mud.

I've exceeded my time. Going for a walk.

Jawge

[stuckinthemud]

Thank you for your comments, they are much appreciated.  I agree with you completely and have learned more than I expected on this project. When I set out I sort of had this very early Scandinavian crossbow in the back of my mind, though it might be it survived by being useless and thrown away....