Primitive Archer

Main Discussion Area => Bows => Topic started by: DC on November 16, 2019, 01:36:56 pm

Title: Heat treating/ limb thickness
Post by: DC on November 16, 2019, 01:36:56 pm
I'm trying to improve some on my best bow. The top limb(in the picture) is the existing bow and the bottom is the blank I'm working on. It's boo backed Yew and the new one will be too. I want to do a really deep heat treat and I think I'll treat it from both sides before gluing the boo on. In order to get a nice even treat I think the blank should be as thin as possible. In the picture the existing bow's yew is about 1/2" thick and the blank is about 3/4". The new bow is 2" shorter at 62" and will(hopefully) have a bit more reflex so there will probably be a little less than 1/2" yew on the new one. Figuring back from the old bow and allowing for wood vagueness, how much do you think I can reduce the thickness of the blank? Or how thick should I leave the blank for conventional tillering? I want to leave enough so I can have room for tillering. I don't want to try and hit the weight right out of the glue up. I ain't that talented ;D ;D

Title: Re: Heat treating/ limb thickness
Post by: DC on November 16, 2019, 01:48:16 pm
Maybe another way of looking at this is how deep does a good heat treat go? And if I heat from both sides how thick can I get thoroughly heat treated?
Title: Re: Heat treating/ limb thickness
Post by: aznboi3644 on November 16, 2019, 02:06:36 pm
Have you read Mark St Louis’ chapter in TTB vol. 4 on heat treating?  He does state that a good heat treat goes deep into the wood
Title: Re: Heat treating/ limb thickness
Post by: IrishJay on November 16, 2019, 02:10:40 pm
I'm far from an expert, but in my experience conventional heat treating doesn't go very deep at all. I'm working on one right now that I have been toasting periodically throughout the tillering process in an attempt to control set and keep the profile I shaped into it after floor tiller.  Each time I put it on the tree I end up scraping through the toast anywhere that I take off more than 4 or 5 scrapes. My bow isn't backed so I just retoast it and then wait a few days to put it back on the tree.

Wood is an excellent thermal insultor, so with a heat gun you're going to blacken the extrior before you get 1/4" penetration of the heat.

I think if you want to get an even heat treat the whole way through your blank you're going to need to heat it at a much lower temp than a heat gun produces for much longer time. Think of a piece of meat 🍖 if try to cook it fast over high heat youll burn the outside without getting the inside fully cooked.

I'd say you'd need a kiln. Start it at 350~400 degrees and let it go 24hrs or longer to get the whole blank heated evenly. Then increase temp 50~100 degrees and wait. Wash rinse repeat until you get the whole deal up to 800~900 degrees. Then verrrrry long wait for the MC to normalize enough to work it, because its going to be drier than a popcorn fart when it comes out of that kiln.
Title: Re: Heat treating/ limb thickness
Post by: DC on November 16, 2019, 02:24:42 pm
I've thought of the kiln thing but that's more than I want to do. I think 400-500 would be the final temp. 800 or more is past the ignition temp of wood I think.

I also suspect that I have tillered through the heat treat a few times but more like 1/16" or more, not just a few scrapes.

PS let me modify that last bit. I have had occasion while tillering when a heat treated bow seemed to suddenly start taking more set for no obvious reason. I have suspected that I have tillered into the softer wood. Just because you have scraped off the dark wood doesn't mean you have gone through the heat treat. It goes deeper than that.
Title: Re: Heat treating/ limb thickness
Post by: IrishJay on November 16, 2019, 02:36:31 pm
Good call on the ignition temp. Just looked it up, auto ignition os almost immediate at 700, will auto ignite over time at 450.
Title: Re: Heat treating/ limb thickness
Post by: Stick Bender on November 16, 2019, 03:07:20 pm
I just heat treated & corected some serious lateral bend  over the last week but I only treated to 375f using a infrared thermometer and held the temp for atleast a minute the limb average was 1/2" I don't know how deep it goes but deep enough to hold the lateral correction & reflex ,Im sure some of the reflex will come out after tiller but exspect the lateral to stay , in your case if you could put a ruff pre taper based on your previous builds you could probably go deeper ,wood usaualy plastisises at 350 F  so 350-375 F is probably good ,but Im sure every wood is slightly different ,Im not sure testing and cuting a test peace would tell the whole story !
Title: Re: Heat treating/ limb thickness
Post by: Eric Garza on November 16, 2019, 03:33:04 pm
Do conifers benefit from heat treating? It seems to me that I read somewhere that hardwoods are the ones that benefit from this, more so than softwoods. It seems to me that the resins in conifers would cause them to ignite well below their wood's spontaneous ignition point.
Title: Re: Heat treating/ limb thickness
Post by: DC on November 16, 2019, 03:35:55 pm
I have an infrared thermometer. I usually treat to 400f+ and I keep the gun 3 1/2"(2x4) away. I do my heating on a caul though, so I can't take the temp of the back. Maybe if I cut a notch in the caul big enough to get the thermometer in. Hmmm I have some scraps :D

PS Or just use a test piece that doesn't need a caul Duuhhhh ;D ;D

Eric- I have treated Yew to 425 before and it does help. It starts to turn dark just over 400.
Title: Re: Heat treating/ limb thickness
Post by: PatM on November 16, 2019, 03:56:44 pm
I've cut up a  few heat treated bows.  The heat goes very deep if you take your time.  In fact the cut cross section will look remarkably similar to a good sapwwod heart ratio or typical backed configuration.    The color change being noticeable to a depth of about 3/4 to 7/8 of the stave thickness.
Title: Re: Heat treating/ limb thickness
Post by: DC on November 16, 2019, 04:07:47 pm
I just did a test on a 7/16" piece of Yew. After about 15 min in one spot the belly was 425-430° and the back was 205-210°. Interesting thing is the back got grey at about the 7-8 min mark and it never got darker. The belly temp first hit 420 about then and the back was only 160ish. That must be the limit of my gun so maybe going by the color is not the best idea. I'd hate to have to start timing this. It's boring enough as it is.
I'll try a thinner piece
Title: Re: Heat treating/ limb thickness
Post by: Stick Bender on November 16, 2019, 04:19:26 pm
I was thinking that to ,maybe color doesn't tell the whole story maybe the wood is effected beyound the color ?  Really only one way to know make the bow and see especially sense you have a similar one to compare to !
Title: Re: Heat treating/ limb thickness
Post by: DC on November 16, 2019, 04:43:27 pm
A thinner piece wasn't a lot different. I used a 5/16 piece. The back got about 10° hotter. What is surprising me is that this time I left it run for over a half hour and it never got more than grey. That's at 4". If I move the gun down to 1"(to see if the air is hot) it chars immediately. I guess at 4" from the gun the air has cooled to close to 400°
Title: Re: Heat treating/ limb thickness
Post by: Stick Bender on November 16, 2019, 05:14:43 pm
Don sense your trying to heat a lam vs stave where your actualy trying to heat each side I wonder if you put some heat reflecting material like tin foil under the lam if it would bring the temp up on the opposite side ?
Title: Re: Heat treating/ limb thickness
Post by: IrishJay on November 16, 2019, 05:42:31 pm
Don, have you considered a spit type setup using fire or hot coals?
Title: Re: Heat treating/ limb thickness
Post by: DC on November 16, 2019, 07:00:08 pm
I tried another test and this time it wouldn't even char the wood at 3" so I ran up town and got a new heat gun. Now I have to rebuild my gun holder because the new one is huge and has all kinds of features that I don't want or need. Stupid progress ;)
SB I have a bunch of reflex in it and I would lose that if I didn't have it on a caul.

IJ One day I will because I think that's the best way but today it's blowing a gale and it's supposed to rain a couple of inches.
Title: Re: Heat treating/ limb thickness
Post by: Mo_coon-catcher on November 17, 2019, 05:53:30 am
When I hear treat I go slow and constantly move the gun. This give the heat time to travel through the cell layers of the wood before more heat is applied. It’ll take 30-50 min per limb depending on the wood type and how well heat travels through. But by the time I’m done the back is hot enough I can’t hold my hand on it. When I do it this way a second heat treat does nothing. The first heat treat I do after long string tillering to about 2/3 draw and as soon as I even out first full brace, but before I pull it back any at full brace. Especially with black locust (main wood I use) the limbs are about 3/8” thick at this point and this wood seems to transfer heat very well, so it doesn’t take long to get a good heat treat. Sometimes as quick as 15-20 min per limb. I’ve tried a second heat treat after tillering to full draw with no effect on weight or tiller. But the first heat treat takes my draw to point for a weight from, let’s say 20” draw at 50# down to 10-12” at 50#. It could just be the way BL reacts to heat,  it I have the same experience with hickory doing this. Now if I rush the heat treat the limbs do benefit from a second toasting later in the tillering cycle.

Kyle
Title: Re: Heat treating/ limb thickness
Post by: DC on November 17, 2019, 11:44:45 am
Thanks Kyle, that helps. My old arms won't take an hour of holding a heat gun so I'll have to stick with a jig and spot heating but the numbers still work. The new gun doesn't seem to work any different than the old one so I may return it. I don't know why this wood doesn't want to char. I treated one limb last night and had it up to 435°f and it was just light grey.
Title: Re: Heat treating/ limb thickness
Post by: IrishJay on November 17, 2019, 01:19:57 pm
Don, have you tried a coat of veggie oil? Seems to help with penetration/browning.
Title: Re: Heat treating/ limb thickness
Post by: DC on November 17, 2019, 01:43:56 pm
I'm not convinced about that. Just with my messing around here it makes me wonder how people know it improves penetration. It's not an easy thing to test. Combine my doubt with not wanting to contaminate a future gluing surface with oil. I dunno :D
Title: Re: Heat treating/ limb thickness
Post by: SLIMBOB on November 17, 2019, 02:03:54 pm
Always used oil in my early days. I never use it now. The only difference I notice, is much less clean up.
Title: Re: Heat treating/ limb thickness
Post by: IrishJay on November 17, 2019, 02:25:34 pm
The biggest benifit I get from using the oil is a more even toast, with no "hot spots" where I accidentally cook a spot too dark. I think this is because the oil conducts heat well, so even though the gun is hitting one particular spot the oil helps conduct the heat along the woods surface.
Title: Re: Heat treating/ limb thickness
Post by: SLIMBOB on November 17, 2019, 02:43:52 pm
I’ll give that a maybe, but I don’t see it. Not on heat treating. For bending hooks I think there is a stronger argument, but even then I have gotten away from it. Just to much clean up for glueing and applying a finish. That’s just me though. If someone else sees a benefit worth the effort I say stick with it. I just quit using it 10 years ago or better.