Stack thickness measured from an existing limb

[Theolithic71]

I admit I'm trying to get a little hacky here...

If I measure an existing bow limb thickness, can I reliably back calculate stack thickness to arrive at a similar weight?  Do I need to subtract any amount for glue lines or compression?

For the record, I'm not trying to profit from this.  Just trying to emulate a design for my own use.

[kennym]

Hi Ted,

The best I can tell is a glue line is around .001 to .002" added.  I always go past the fades an inch so I don't get extra glue thickness and then figure taper back to ends of lams to get butt thickness.

Hope that helps!

[Theolithic71]

Thanks, Kenny!
Is that figure for each glue line?  So a two lam recurve would count 3 glue lines worth?

[BrushWolf]

When I build a limb I measure in several places. My mid limb measurements are usually a few thousands under after glue up. I personally wouldn't worry to much about glue lines. If there's finish on the limb that will add thickness also.

[kennym]

You better go with Mike's advice on this, I haven't been nearly as thorough with mine. Only went off stack written down and compared to finished limb.

The last one I checked out was .008 thicker(by my figuring back toward lam butt) than stack indicated it should be. 4 wood lams and 2 glass on it.

Mike, out of curiosity, is this recurve with 2 lams?

And what grit do you use for lams?  Maybe I use heavier grit.

[jsweka]

I've never done this before, but I think you'd also need to take into account the length of the fadeouts and the total taper rate of the lams.  The thickness you measure outside of the fadeouts would be a little less than the total stack if total stack it measured at the butt thickness of all lams and glass.

[Theolithic71]

I've never done this before, but I think you'd also need to take into account the length of the fadeouts and the total taper rate of the lams.  The thickness you measure outside of the fadeouts would be a little less than the total stack if total stack it measured at the butt thickness of all lams and glass.

Yup.
I figure that measuring total thickness at specific lengths from the center butt will help map out taper rate.

[Theolithic71]

You better go with Mike's advice on this, I haven't been nearly as thorough with mine. Only went off stack written down and compared to finished limb.

The last one I checked out was .008 thicker(by my figuring back toward lam butt) than stack indicated it should be. 4 wood lams and 2 glass on it.

Mike, out of curiosity, is this recurve with 2 lams?

And what grit do you use for lams?  Maybe I use heavier grit.

Kenny,
On you R/D plans, 0.010 more stack makes 5 pounds.  So 0.008 error could cause a fellow to be quite a bit off of target weight! 
So if I figure about 0.0015 per glue line, and get a feel for finish coating thickness, I should get pretty close.

[jess stuart]

I record thickness at the butt, mid limb and tip.  I put the most importance on the mid limb and tip measurements and will let a few thousands either way at the butt go.  If you are going to build a recurve I would suggest .040 glass on both back and belly.  If longbow I defer to those with more knowledge.  I would be overly concerned with the thickness of the glue lines.  You can always adjust on the next one and yes there is always a next one.

[kennym]

If going off my plans, .008 would make about 4# which is easy to get rid of. That is if your finding is same as the bow I measured.  I think I would go with the measure you get and make sure of taper rate, and if it works like the one I checked, shave the 4 #

Way easier to lose 4# than gain.....

[BrushWolf]

You better go with Mike's advice on this, I haven't been nearly as thorough with mine. Only went off stack written down and compared to finished limb.

The last one I checked out was .008 thicker(by my figuring back toward lam butt) than stack indicated it should be. 4 wood lams and 2 glass on it.

Mike, out of curiosity, is this recurve with 2

And what grit do you use for lams?  Maybe I use heavier grit.

I grind with 36 grit. As for stack 2 glass 2 veneers 1 taper and a piece of stable kore

[kennym]

Thanks Mike! Not the grind then , I'm using 40 to 50 grit. Same number of glue lines.

Ted, you better go with Mike! He has measured before and after, me , only after so his numbers have got to be closer!!

[Theolithic71]

Mike, Kenny,

Thanks for the input.  I'll let you know my findings.

Ted

[kennym]