Ozbow

Just gluing up another bow and was planning on using 3 bamboo laminations each at 2.2mm (86thou). Unfortunately I noticed they all had splits in the orginal glue joints so decided not to use them. As it turns out I had 2 lams sitting there at 3.3mm (129thou) so have decided to use them. These will be the thickest that I've used so far. I have previously used 2 x 3mm (118thou) in this bow design with no problems but feel that I'm about at the limit with 3.3mm. All the lams are also tapered at 3thou/inch and the bow will be 62" long with 18" riser making each working limb 22" long.

So whats the thickest individual bamboo lamination you've used?

just from memory without consulting my torn and tattered 'recipe' book I have gone to at least .120 and that is with a .001 taper too. i don't think the individual thickness is such a big deal as getting the right total amount.

Muz,

Muz1970 wrote:......I noticed they all had splits in the orginal glue joints so decided not to use them........?

If the offending glue line falls inside the limb plan I work the epoxy into the split and continue the glue up as normal.

As you can see from the following there is little difference between three and four laminations. I run the belly lamination around .090” to make it easier for the lamination to conform to the handle.

Incidentally the bows are 68” n to n.

Daryl.

Was going to do that Greybeard but would have got fiddly with the fine tapers. Got no problem using the 3.3mm lams. Just got the feeling when i flexed them in an arc on the bench last night that there getting close to their limits. For example, I don't think my 66" bow with a stack of .525 would do well with 2 lams at around .195 hence why there usually 3 or 4 lams. Just wondered what was the thickest people went too.

What is the acronym RCT? = R??? Core thickness

Cheers Muz

Raw? Maybe

RCT is a term that John Clarke of Woomera longbows in his book on bow making alludes to. Forget the first word but the following 2 words are core thickness. He had an elaborate mathematical formula for determining bow weights.I have never personally used the system.

Muz1970 wrote:........What is the acronym RCT? = R??? Core thickness..

Muz,

RCT refers to a measurement taken from the centre of the handle to mid point [approx.] in the length of the limb. To keep things simple I chose 18" as it suits the style of longbow that I produce.

The details of the second bow shown in the table are as follows;

Thickness at centreline .379, 18" from centreline .357" and at the tips .272". I was using butt thickness as a reference point but now prefer the RCT method.

Daryl.

Yeah ok. Cheers

I just measure my total stack at the point where my riser fades finish. When I taper my laminations they only start to taper at the fades also.

Morning All.

There are a couple of different ways you can design/predict things.

A method is to build something, compare it to what is desired, if it matches good if not modify it and repeat. Eventually you, hopefully, come up with design/dimensions that work. If you want something different you start all over again with a level of pre-knowledge that you have experienced and retained.

Another method is to develop a model. Compare the model to actual, ie build a prototype, modify the model, test again and re-iterate until the model can be used to predict the way the product will perform.

Developing a model to predict performance is, in the long run, a more efficient way of doing things.

RCT is a simple way of predicting the height/thickness of a stack of laminations to give a bow of a desired poundage.

If you building a bow of John Clark's design you can use the data to determine the stack dimensions to give a bow of the desired bow weight.

You can alternatively collect the required data and use the same process for other designs.

RCT is just a tool available to the bowyer that enables the bowyer to build bows to a desired performance.

Muz, RCT is an acronym for Reference Core Thickness. John Clark's definition includes some amount of allowance for "butt stagger" when joining lams near the middle of the bow. I don't have butt stagger because I make one piece (full length) laminations. My modified interpretation of RCT is the thickness of the core at the reference length, which is the point midway between the fadeout and the string nock. The reference length varies with NTN and riser length. It represents the "average" thickness of the core at the mid point of the working section of the limb. I also use a reference limb width at this point for second moment of area calculations (i.e. Ixx).

This is useful in further calculations that can be used to predict the required stack after having made one example of a particular style. Although the theory used for structural beams is based on assumptions of small deflections, angles and rotations, it is the only method available (other than making lots of bows and recording results or finite element analysis) of predicting required core thickness. It also utilises material properties such as elastic modulus and can account for composite beams such as we make with a wood core and fibreglass outer layer.

As you have indicated, it is easier to apply the taper only to the section outside the fadeout, which also avoids very thick lamination butts. So far my thickest lamination has been about 140 thou at the fadeout. I think the maximum is largely governed by you design and the need to apply a small radius bend to the lamination (e.g. in the riser ramps. The larger your minimum radius the thicker you can have your lams.

Sorry about the long winded rant, hope something in there helps, cheers... Rod

Thanks for the explanation gents, have definitely been doing the later as far as predicting weights.

Cheers

Hi Muz,

I'm only an amateur bowyer, but I have used 120 thou, red elm laminations in a 66" D/R design with no noticeable durability problems. I decided to use three laminations of higher thickness so I could save a lam of the timber for another bow as well as minimising lam grinding time.

I found that my bow weight was as expected so long as my overall lam thickness and overall taper rate was the same, despite having a different number of laminations.

This bow is the only bow where I have used 3 timber laminations instead of 4 and it holds less reflex, I assume as a result of the thicker laminations wanting to flex back after being removed from the form. The tips are around 3mm further back than my other bows when unstrung. The bow is still comfortable to shoot however, and I don't notice any difference in performance.

Cheers,
Scuzz