Belly Tempering, a Test

[Del the cat]

Hmmm, I've read about tempering and I've just finished a Hazel bow. I didn't want to risk tempering it as I'd not done it before. I wondered if it really worked so I did and experiment, I don't claim it's a perfect experiment, and I'll let you draw your own conclusions.

I made a representative bow limb from an off cut of the same wood, I clamped it firmly in the vice and marked where the tip was on a piece of board placed behind it, I then hung a 24lb weight on it and marked where the limb deflected to, I removed the weight, it returned to the start position. (OK I could have exercised it more...but I didn't)
I got the heat gun on it, up and down, until I got the slightest hint of discolouration, as I did this I noticed the tip move up by about 1/4" and then came back down! The limb was very hot to the touch on the underside (belly).
I allowed it to cool, the rest position of the tip had moved down by a millimeter (pretty insignificant).
I hung on the weight, It deflected significantly past the original deflection point and when I removed the weight it didn't return to the start point (about 1/4" short of it)
Maybe it was still warm? Maybe I hadn't tempered it enough?

I heated it more until it was very slightly browned all over the underside (belly). I allowed it to cool for a good hour. The results were similar to those above E.G longer deflection than the original test and it didn't recover.

This was a crude test with one piece of wood, draw your own conclusions.

I would respectfully sugest that anyone considering tempering should conduct their own tests.
I will post pics and dimensions if the topic takes off and people are interested.
I won't be tempering my Hazel bow.
Del

[sonny]

Have you read Marc's chapter in TBB 4 about heat tempering ??
If not I suggest you do so....

note: The above likely sounds as if I'm beating you up. I can assure you that is not the case.
 

[Del the cat]

No I havn't, and no prob with your post...I'm positively asking for comments.

I'm just reporting my results and wondering if others have direct measurable data.
Does Marc do tests to show the effect of tempering?
I'm always keen to listen to stuff, but I always like to verify for myself.
If I'm missing something , I'm keen to find out...

Out of interest here's some data.
Limb material seasoned Hazel
working limb length               365mm
mid limb width / thickness      32mm / 7mm
Deflection, untempered         125mm
Deflection tempered              138mm
Difference between rest position before and after deflection before tempering 0mm
Difference between rest position before and after deflection after tempering 3mm
Del

[Dano]

Nice test, doesn't prove much in my way of thinking. Most guy's will temper a bow clamped to a caul with some reflex, and most everyone that's done it has had good success, gaining a few pounds of draw wieght. What was it you expected to get out of tempering?

[Woodland Roamer]

I always temper with the bow clamped in some reflex as Dano said and I wait at least 24 hours or more before bending the bow any at all. I've had very good results tempering Osage, Locust and Hickory this way and do it to almost every bow I make.

Alan

[Del the cat]

Ah, this is interesting...
If the bow is being clamped during heating, then it's more like heat bending or steam bending as I see it.
Maybe it's the explanation of 'tempering' which is missleading?
I was lead to believe that it's supposed to harden up the wood and improve the compession strength, which would have given less deflection after tempering in my test.
Del

[Pappy]

I always do them on a form and wait a few days before I bend it to let it rehydrate.It will usually pick up at least 6/8 lbs and it will help it hold the reflex.I am sure it will help if you just heated it not on the form .I do it so the reflex is even. Never really done any test except on bows but I have done a lot after doing it and even more before doing it and can tell you it dose help.
   Pappy

[TBod]

Ah, this is interesting...
If the bow is being clamped during heating, then it's more like heat bending or steam bending as I see it.
Maybe it's the explanation of 'tempering' which is missleading?
I was lead to believe that it's supposed to harden up the wood and improve the compession strength, which would have given less deflection after tempering in my test.
Del

I'm with you there except:

For example if you look at Gordon's Hazel buildalong. When he temper he let's the wood get brownish. And when he heat bends using dryheat he stops before it Gert's brownish. Maybe thats the difference between heat bending and tempering

[Ryano]

Heat tempering does harden the belly and make it stronger in compression. All you need to do to see this is try to work wood with a scraper and sandpaper after it's been heat treated. It totally changes the way the wood works. It changes the properties of the wood big time. Most people don't heat the wood enough to make much of a difference, in order for it to work well it needs to done hot enough and long enough for the heat to deeply penetrate the belly wood. If the belly doesn't look burnt you didn't even come to heating it enough. Further more if your just scorching the outer layer of belly wood and then scraping it away during tillering its not going to do much for you. 

[Del the cat]

Heat tempering does harden the belly and make it stronger in compression. All you need to do to see this is try to work wood with a scraper and sandpaper after it's been heat treated. It totally changes the way the wood works. It changes the properties of the wood big time. Most people don't heat the wood enough to make much of a difference, in order for it to work well it needs to done hot enough and long enough for the heat to deeply penetrate the belly wood. If the belly doesn't look burnt you didn't even come to heating it enough. Further more if your just scorching the outer layer of belly wood and then scraping it away during tillering its not going to do much for you. 

OK... So, If I repeat my test but get it really scorched you think it will deflect less ?
I shall have to give it another try.
Del

[NOMADIC PIRATE]

You can have excellent tempering jobs with barely a change in color, ...and you can have terrible tempering jobs with the belly getting allmost black.

 it's all in the tecnique,...and patience, and as mentioned above, tempering surely works, wood gets harder and keeps more of the shape,...that again if it's done right 

[PeteC]

I temper all whitewood bows.It enables one to significantly improve performance with average woods ,with a simple procedure. God Bless

[radius]

gotta be careful though...i recently tillered out a decrowned yew bow and heat tempered it to counteract the set.  Didn't bend it into reflex, just straight.  Last night was four nights after the heat temper,first i touched it.  It drew 52 # @ 28 before, i got it to 52 at about 24 last night, and then it broke into 5 pieces when i went to see if it would still bend to 28.  I think it got to 25 or 26 before kablooie.

[Dano]

I doubt it was the tempering that caused the destruction of that bow Scott.

[sailordad]

alright then,i have a hick bow that is all but finished(just needs a finish).
i have thought about heat tempering the belly,but dont know the proper procedure.
so what do i do to do it properly.
i have a sweet ass heat gun,a form that will let me clamp it down about 2".
so how far from the limb,how long on each limb and how hot on the gun.
and then i need to ask,will it change the tiller?