Poundage and tiller consistency tip

[Kirkll]

There have been a lot of guys that get frustrated having draw weight vary from one bow to the next using the same stack height and similar materials. What really throws you off is having a lot of tiller difference on a set of limbs that has to be dealt with.

Of course you always need to take great care in milling your laminations as closely as possible, and check your total stack heights prior to lay up. But typically it’s our glass variation that throws things off.

Unfortunately the strength of the glass and thickness varies from one batch of glass to the next. Not a lot, but enough to throw your draw weight off by 6-8 pounds pretty easily. And if the tiller is off too, it can result in building another set of limbs.

I’ve seen glass as much as . 004- .006 difference in thickness from one end to the other in 6’ lengths. But typically they are fairly close from end to end, and vary more from one piece to the next one. This is especially true mixing up glass you have had in stock, with a new batch you just bought.  Or from one end of a roll to the end of the roll can vary a lot using Bear Paw glass. And one roll to the next can vary a lot.

I did a little film clip here showing a method I’ve used that helps a lot on consistency in limbs draw weight and tiller match without sanding the glass at all prior to lay up.

Sanding glass perfectly in a drum sander has its own issues, and I wouldn’t recommend it. I played with that years ago and it’s not worth the time and frustration.

Hope this little trick helps you.     Kirk

https://photos.app.goo.gl/9HtC12heNSu9w8xU8

[Crooked Stic]

Yep just like building a one piece same glass front and belly.

[Buggs]

Thats a good idea for sure, any attempt to maintain consistency is worthwhile. Do you still check thickness from end to end on the individual glass pieces?

[Kirkll]

Only time i'll go back and check for thickness variations in glass is if my tiller is coming in more than 1/4 to 1/2" with identical width profile.

 I never measure my stack height  at the butt. I get everything milled and measure at 10" up from the butt without wedges. I flip the stack and measure both sides for uniformity. Sometimes i'll get slight variation from one edge to the other, and flip my core lams over to adjust....  Typically i have good success doing this.

Milling your lams as precise as you can pays big dividends.  Running them through the drum sander one at a time at the same location and flipping them over every pass is worth the time spent. Grinding two lams at a time side by side saves time, but the finished product is not as uniform.....

Crooked Stick made a comment years ago that has always stuck with me...
 
"Some times you gotta put the sneak on these lams..."  he was referring to using multiple passes and flipping the lams to get the right thickness uniformly.....    Kirk

[Crooked Stic]

Yes sir know your machine and neak up on them 
It's Stic without a - k

[onetone]

Thanks Kirk 

[Buemaker]

Thank you Kirk.

[simk]

Thanks Kirk. Watched your video and found it very intersting and mucho simpatico! Interesting to see how a industrial product like glass can vary. For the same reason I never mix my wooden laminates when cutting them and always and only glue sister laminates onto my bows.

[Kirkll]

Years ago, back when i was playing with carbon prototypes. I was using a thin clear glass over unidirectional carbon on the back of the limbs. I contacted Gordon Glass and researched how they manufactured the glass, and how sanding it to a thin thickness effected the glass.

There are layers of scrim in the glass that you don't want to sand into. It compromises the structural integrity. I cannot remember the exact lay up details that they use when laying up their glass, but sanding .040 down to .020 is not something you want to do.... Its best to order thin glass and keep sanding to a minimal.

Kirk

[Bryan Adolphe]

 Thanks Kirk that makes good sense 

[Buggs]

There are layers of scrim in the glass that you don't want to sand into. It compromises the structural integrity. I cannot remember the exact lay up details that they use when laying up their glass, but sanding .040 down to .020 is not something you want to do.... Its best to order thin glass and keep sanding to a minimal.

Kirk

The manufacturing process is called Pultrusion. Hundreds of spools of fiberglass roving are threaded thru a heated die, after previously being pulled thru a resin bath. It comes out the other end as a continuous feed, cured product. The scrim is part of the initial arranging of the fibers and its purpose is to prevent the print thru of the underlying fibers on the cosmetic side of the composite. They are all sanded to final thickness, so the sanded side can be reduced further without compromising integrity. You don't want to do much sanding on the finish side, ever, but the other side is fair game.

[Crooked Stic]

Also is there glass with no scrim?

[kennym]

I haven't talked to Gordons directly but it's my understanding that they make no glass with scrim now , only UL

You could look at a light with ULS and see the scrim in it.

I built a bow that I tapered the glass to see if less tip weight increased performance, it didn't . Then wondered if I cut too many glass fibers tapering. LOL  As far as I know it's still going.

[Buggs]

I haven't talked to Gordons directly but it's my understanding that they make no glass with scrim now , only UL

You could look at a light with ULS and see the scrim in it.

I built a bow that I tapered the glass to see if less tip weight increased performance, it didn't . Then wondered if I cut too many glass fibers tapering. LOL  As far as I know it's still going.

Thats a great observation about the tapering. I have wondered how much more performance could be had from glass with continuous full length fibers, rather than all the cut fibers on a bow with side taper. I also suspect ASL performance is saved somewhat by the fact that they have less sidecut and more full length fibers.

[mmattockx]

I have wondered how much more performance could be had from glass with continuous full length fibers, rather than all the cut fibers on a bow with side taper.

It makes no difference at all as long as the epoxy matrix and core are strong enough to keep the FG strands attached and in line.


Mark

[Buggs]

Sorry for the crude rendering

So mmattockx, you saying that there would be no difference between these two profiles?

[mmattockx]

So mmattockx, you saying that there would be no difference between these two profiles?

As long as the FG lam doesn't suffer a failure then that is correct.


Mark

[Buggs]

Just so we are talking about the same thing, my "pro" diagram is depicting one fiberglass composite, with the unidirectional fibers terminating in a point and the other would be a bows finished side profile with standard glass and parallel strands.

[Kirkll]

I haven't talked to Gordons directly but it's my understanding that they make no glass with scrim now , only UL

You could look at a light with ULS and see the scrim in it.

I built a bow that I tapered the glass to see if less tip weight increased performance, it didn't . Then wondered if I cut too many glass fibers tapering. LOL  As far as I know it's still going.

I noticed there wasn’t any offering of ULS glass at Binghams any more. How long ago did Gordon’s quit running the scrim glass? If I remember right…. The uls glass gave us a wee bit more poundage, and was suppose to help help with limb stability too. Honestly….. I never noticed that much difference myself, but always ordered my clear glass in the UL type for clarity reasons.

This clear glass,  and glass prices in general have increased substantially….  I need to raise my price on clear glass over veneers, but haven’t done it yet.   Kirk

[kennym]

I think they discontinued the uls about a year and a half ago .

But anymore when I try to think when something happened I have to add a few years to my guess...