Tip Overlay Problems - suggestions?

[mwosborn]

This spring I decided to try and make myself a glass bow off my own design.  Forward handle D/R longbow.  Things have gone well for the most part - however I am having issues with the tip overlays.  As you can see by the pictures, the wood overlay gave way.  I have made a couple of bows off this form and both have had this happen.  On the first bow I replaced the overlay with antler thinking that the wood was just weak.  A dozen shots later, the antler gave way too.

I am not sure of the cause, but this is what I am thinking...

maybe the angle of the string grove is too great putting pressure on the overlay when the arrow leaves the string?

maybe the string grove is too deep?

maybe grain running wrong way (at least in the wood overlay)?

maybe the design of the bow has something to do with it?

My question is what do you think is the cause - so I can fix the problem so it doesn't happen again!

I have put several overlays on my selfbows and have never had a single one fail.  My only other glass bow was a bingham design hybrid longbow and I had no problems with it.

First 2 pictures are of the first overlay after it gave way.  I do not have a picture of the antler one.

Second 2 pictures are the current tip.  I changed the angle of the string groove to almost straight up and down.  Had to also change the shape quite a bit because of the old groove.  This tip is almost a "post" style now - but it seems to be holding ok - maybe a hundred shots through it.

BTW - the bows are not finished yet - what to resolve the issue before I do final sanding and spray on finish!

Thanks for any suggestions!


   IMG_0548  by  mwosborn1 , on Flickr

   IMG_0549  by  mwosborn1 , on Flickr

   IMG_0561  by  mwosborn1 , on Flickr

   IMG_0562  by  mwosborn1 , on Flickr

[jess stuart]

What type of string material and how many strands?  Looks like the string groove angle is to abrupt, putting added strain on the overlay.  The second set of pictures the angle is better but, still to abrupt in my way of thinking.  Of course pictures can be misleading.  What species of wood did you use on the overlay?

[mikkekeswick]

Walnut is fairly weak to start with!
The main problem though is that your string angle is way to steep. Look on the side view photo, see how yours is somewhere around 80 degrees (going off the belly), you should make it more like 40 degs.

[mikkekeswick]

So I drew the black line where your string /groove should be. Also note the shoulder at the top of the groove. The string shouldn't be sat on the tip of an angle. When the bow is braced your string should run down the groove and there should be hardly any angle where it leaves the bow. Also look at how the string angle changes when you draw. You should round out the base of the nock groove (on the belly) so that the string is free to move as you draw.
One more thing is to use a wider nock file. I rough mine out with a tile saw. then use three progressively bigger chainsaw files to widen the nock to it's finished width. As you go up in the sizes it's real easy to correct any wobble on that first cut.

[mwosborn]

Thanks for feedback Mike.  If you look at the first 2 pictures of the overlay that broke, you will see the string angle as you suggest.  That is how I have always done my overlays on my self bow.  

After it broke off (twice), I filed it down and re-did the overlay as you see it in the last 2 pictures (the picture that you drew on).  This is the one that is holding up.

The string is D97 15 strand.  Bow is right at 50# @28.

[BenBow]

I would also deepen the groove so that the string rests on the phenolic and not on the wood. The phenolic is much stronger and also leaves the wood thicker giving it more strength

[Al Dean]

It looks like your grooves are too narrow causing extra strain on wood.

[pinky]

Thanks Mitch, I'm Glad this thread came up. A day late for me, I just redid the tip overlays of a CraigsList find. The previous owner had warned me that the bow needed some attention including the bottom Elk antler overlay. The bottom one lifted off easily with my thumb nail, the top came off to easily too. I added a little Walnut to dress up the bow, and a little more angle to the grooves. I searched for a little info before I started. this thread would have helped with some questions.
 
 
 

Just fixing,  not building yet. One of these days.
Hoping to test this bow out today.  

[mwosborn]

Al - how wide do you make your grooves?  I am using a 5/32" chainsaw file.

Mitch

[Al Dean]

I think I use a 5/32 to 1/4 tapered rat tail file.  So my grooves are 3/16+.

[Robertfishes]

I like to use multiple layers of phenolic and fiberglass with a 020 wood veneer on top. The tip pictured has phenolic, red glass and a thin venner. I use a 5/32" chainsaw file to cut thru the veneer and 1/2 way thru the top piece of phenolic.  

[mwosborn]

Looks nice Robert - thanks for the pic!

Sorry Pinky - I have been frustrated with this problem for the past week or so - should have posted earlier - but I can be stubborn sometimes  

[bowberry]

I've been building self bows with self nocks so this has not been a problem.
Just last week I tried doing a deer antler overlay. I glued it and notched it so I thought it looked nice,but my glue was not setting right(old glue??) also I was frustrated with my clamping.
So I bought new epoxy, cleaned the pieces up and reglued. to hold it in place I strung the bow with a spare string I had.( 50# bow should hold it).
To my dismay the overlay slid about 3/16 toward the handle and settled into place. The wrong place.
stay with me ,theres a point here.
 I rethought the geometry, threw that overlay away and made a new one. this time I roughed in the notch and put a string on it before I glued it. a couple of adjustments with a file had the overlay staying right where I wanted it,(without glue).
The point is that all the previously unseen forces were revieling them selves to me, and I will use this aproach on all my future overlays. sorry so wordy.
Scott.

[mwosborn]

Bowberry  - I think you are right, lots of different forces acting on the tip of the bow during the shot.  These overlays all appeared to break due to a force toward the handle.

[LittleBen]

I don't think anything beats e ridiculous toughness of a phenolic overlay.

[bamboo]

I've gone to using G10 for overlays