Inconsistent Bow Draw Weight...Help

[Little Missouri]

We use purchased wood and fiberglass laminations and we have used the same clamping method for years.  We use pipe clamps every 10 inches along with a metal strip and rubber between the bow and upper form.  The bow is 4 lams with the core being two lams of action boo.  We check the laminations with a micrometer.  Yet, the draw weights of roughed in bows of identical dimensions can vary as much as 10# on a 50# bow.  Inspection of the bows reveals even glue lines and the bows perform well.  Any thoughts regarding the source of the inconsistency in draw weight?  What do you accept for tolerance on wood lamination thickness?  Would the fire hose compression method yield more consistent results?

[fujimo]

one thing i have noticed, when helping newbies make glass bows, is that their fade areas can be different thicknesses, so more or less of the limbs can be working, and i find that affects the DW. doesnt seem to take much to make quite a difference.
 i built a template sander- and that resolved most of the issues, but i still like the newbies to sand at least one riser by hand.

[Little Missouri]

Thanks for that tip.  We have been very careful about the fade outs and manually sanded them to near paper thin, and all but one have been visually a near perfect merge with the laminations.  Could they be too thin?  I will put some of offending bows on the tiller board and look for differences at the fade outs.

[Holm-Made]

I bet it has to do with how the core lams are tapered.  I grind my own of course and try to stop the taper about 5 inches short of the butt.  I try my best to do it the same every time.  If I am not paying close attention and I bring the taper all the way to the butt, the bow will be a couple pounds light.

So if your buying these tapers and different guys are grinding them for you, some may be more aggressive then others and it will be a challenge to hit weight, especially in bows with multiple tapers in the core.

[Roy from Pa]

Are the bows lengths exactly the same?

[kennym]

My speculation would be that if you check all your lams closely and still have trouble, the clamping pressure may be different leaving a fatter or thinner glue line.  10 # is a lot of variation, but it can happen.  I just had one come in 7 light  but I used walnut core and spalted veneers. 

Did you change glass brands or glass thickness?

I just laid a couple pcs of glass back to use later that measured .037" instead of the .040 it should have instead of shipping to a customer... .006" less on a bow , especially on the glass portion will make a big diff.


The Bearpaw glass is very clear but comes in about 3-4# lighter than Gordons.


A lot of possibilities, hope you can nail it down!

[Shredd]

I have a couple of tips or suggestions...   Map out your limbs by drawing them on a piece of paper at brace and different draw lengths using tiller sticks to see how they are bending...  Compare two bows and see if they are bending the same or not...  If they are bending differently it's your fades, lam tapers or limb profile...  Next, accurately mark off your limbs at every 4" and compare thicknesses and widths for any variations... I would also be wary if you use a belt driven sander...  Try to place your lams on the sander where the belt is not connected together...  I would switch to the air hose for consistency when gluing....

  10# is a lot...  On my bows that means .026 to .030" variation... 

[monterey]

I agree with all who suggested careful measuring of the lams and like Kenny said, the glass too.  I have encountered glass that is surprisingly off.  If I carefully measure every component the lay up generally meets my expectations.

I wasn't sure about the effect of the BP glass but recently did some careful examinations of results with Gordon's and BP and I'm definitely coming in lighter with the BP.  It's about 4 to 5 # on my ASL styles.

[Little Missouri]

Thank you to everyone that responded.  I will do some investigating and see what I can uncover.

[TradBowyer]

couple things...are you using the same scale to measure draw weight? I started getting calls about my bows being off in weight so after checking everything I put some steel weights on it and the dumb thing was way off. if you are using 2 different scales maybe one is off. another thing I found on longbows is softer woods like bamboo, cedar, elm will come in lighter than harder woods. I always add about 7# or so to my stack when using these. I hope you figure it out.

[Walt Francis]

This is probably one of my longest responses ever.  Please bear with me and excuse the the mistakes, my fat fingers don’t do well typing on my phone.

First, I have always ground my own lam's.  Always used the same sleds, sanding belts, ect..
When I first started making glass bows I bought several boxes of bamboo flooring and then made over a hundred bows.   After building a couple dozen bows I got it down to repeditive production work. Mic the lam's, butter them up, cook, lay out the pattern with my metal pattern, then grind to the lines.  The bows were almost always 2-4 pounds over the target weight, and close to dead on when final sanding was done. 

Anyway, a few years later I had used up all those boxes of bamboo flooring and needed some new lam material.  I went to the flooring store and bought several more boxes of the same brand flooring, the boxes were almost identical to the old boxes.  The next 6-8 bows I made with the new lam's came in 8-18 pounds heavy.  I was pulling out the little hair I still had trying to figure what was wrong. 

I'd like to say I figured it out on my own, but I didn't.... I'm too dense. A few weeks later we were putting together a video for the PBS and Dick Robertson dropped by one evening.  We took a break out in my garage and started B.S.ing about bow making and other things when he asked how my bow making was going.   When I told him I that after roughout I was getting tolerances within a few pounds until a couple of months before he asked what was different.   After a couple of minutes and a bunch of questions, he had isolated the issue and told I needed to scrap all the old data in my glass bow log, get new core material, and start over.   He said due to the inconsistency of wood he has had to do it many times during his time building bows.  He thought I had been lucky to have have had material so consistent and been that close in weight for that number of bows.

To test his theory, over the next couple of weeks I made a couple of bows with some of the old lam's. They came in at 2 & 3 #'s over before final sanding.  Tracking the new flooring I found it was inconsistent, with a varience 0-8 pounds with the same lay up. I ended up putting that bamboo flooring on a spare bedroom floor where it looks good.

[Little Missouri]

Thanks so much to everyone, there's work to be done on my end.