Sinew backed crossbow prod tillering. Update and questions.

Hi there. 

I have let the sinew on my first crossbow prod dry for 5 months now and have started tillering my prod. Thanks to everyone on my first post. I went for dimensions based on what you guys were saying and so far so good. A few questions. This is my first go at this so can't expect perfection. I have a youtube link here showing where I am at right now minus a few mm.

https://www.youtube.com/watch?v=7gx6zvHRrE8


Bareing in mind I have a loading mechanism that is able to draw 700lbs to a 12 inch draw I would like to see if i can keep the power on the prod as high as I can.


So far I have the following dimensions and weight in my tillering process.

The prod has 3 layers of sinew with animal glue. 

My prod is:

knock to knock 775mm (30.5")

width -46mm at the centre tapering to 38 at the tips

Thickness is varying at the moment based on what I am seeing is bending but roughly speaking the centre is 25.5 tapering  to 20 mm with the thinest part being 19.5

So far I have a draw length from it being unstrung (brace height not included) of only 4.5 inches. At this length the draw weight is 220lbs.

I am aiming to achive a brace height of 3" - 76mm and a draw length of 8" - 203mm

My questions regarding how I proceed. 

It was suggested in my first post that 18 mm at the tips tapering up 1 mm every inch to the centre would give me 220 lbs at an 8 inch draw. My concern is that if I proceed to this thickness I will start getting into the sap wood on the side of the yew as the back of my bow is quite arched. see video. Is this a problem? 

I have one central knot in the middle of the prod and dealt with this on the back by layering extra sinew and leaving more sap wood around. Is it safe to go straight through it on the front of the bow? currently it is flat and in line with the limbs but should I leave more wood around it as I take more heart wood off?

The rest of the process i am relatively familiar with as I have watched many tutorials. I am taking my time and watching to see where I might be over stressing any parts. 


So far During the tillering process there was a audible crack but we thought it might have been a bit of sinew as the wood was in tact and when we drew it even further no more cracks were heard. Is this normal?

thanks!

Ed

Hi Ed, fantastic to have an update on this project!  

If you end up reaching the sapwood this is not a problem, the most you will need to do on your bow is to slightly radius the edges, no more than 3 or 4 mm, sharp edges act as stress raisers, you just need to blunt them off. Yes, this will mean cutting into the sinew, so don't overdo it.

I suspect that you will be able to totally ignore the knot in you bow. but you NEVER leave extra thickness, only extra width as the grain flows around the knot.


Re dealing with knots, you should do what the tiller tells you and gradually rasp them down as the tiller shows you any high-spots or stiff sections.  You should find the knot will get smaller and they often disappear completely. If a knot has a rotten black edge you can stabilise this with runny super-glue.  If the knot is really bad you can rasp out a section and patch it with clean heart-wood, that is fit a "belly-patch".  Leaving a little extra wood around the sides of a knot is seen as good practice IF the knot is near the edge but you can end up leaving too much, bit of a balancing act, do not leave extra wood on the belly of the bow, this will be a high spot and ruin the bend of the bow.

By way of an example, this is a lightweight 40lb crossbow I am currently working on, 29.5 inch nock to nock, 14mm thick at centre, 9mm at the tips, so a 5mm taper along a 14 inch limb, or 1mm to 3 inches.

Anyway, this knot shows some of what I was talking about, with a slight extra width and no extra thickness.  In fact, the belly is hollowed out on the knot-side to keep the thickness correct, the photo doesn't really show it but the belly follows the grain as it rises and falls and has not been cut through.  The knot has been almost entirely cut away and the rotten bit filled with superglue and sawdust.

Thanks for this and thanks again for earlier. So the knot here as shown in the picture is quite big and in the middle of the width and length of the crossbow. Dead centre of everything. There are no other knots apart from a very small one. I think I understand your comment about how we should hollow out the front of the bow where the knot is and follow the grain to keep the thickness the same. However from what I have seen it looks like the very centre of the prod. The bit that sits on the stock often doesn’t bend at all. Is it okay therefore to keep it as the picture shows Or should I follow the Grain into the knot and let the centre bend as well

The knot at the very centre of the prod, the knot that rests against the tiller can safely be ignored, it doesnt bend.  The litlle knot in the bending part of the limb will need to be watched as that section needs to be working in harmony with the rest of the limb.