Official Luthiers Forum!
http://www.luthiersforum.com/forum/

Bending question?
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=21228		 Page 1 of 1
Author:  Randolph [ Thu Feb 26, 2009 11:19 am ]
Post subject:  Bending question?
I did my first bend yesterday (a tester). I built a Fox bender and covered the inside of the form with some thin aluminum I had laying around. I think most people cover with aluminum foil. It seemed to me that in 15 min. the whole thing wasn't as hot as I expected. How important is it to use foil (ie.. reflective side out?) Would it increase temperature?

The bend went well except it seems that the side is not as tight against the form in the waist section as I would like (tightest part of the bend). I'm wondering if this is because of a heat deficiency? idunno
Author:  Hesh [ Thu Feb 26, 2009 11:29 am ]
Post subject:  Re: Bending question?
Randolph my friend are you using a heating blanket or light bulbs? Also what kind of slats are you using (what are the made of, thickness, etc)?

Thanks
Author:  Randolph [ Thu Feb 26, 2009 11:33 am ]
Post subject:  Re: Bending question?
Sorry. The heat source is 3 clear 150 watt bulbs. I'm not sure what part you are referring to with "slats" :oops:
Author:  Randolph [ Thu Feb 26, 2009 11:41 am ]
Post subject:  Re: Bending question?
One more thing. The side was a piece of mahogany .085 thick
Author:  Hesh [ Thu Feb 26, 2009 11:46 am ]
Post subject:  Re: Bending question?
Ok that helps thanks Randolph.

Light bulb powered benders take some time to heat up and because of this they can take much longer to bend a side. With a heating blanket I can have a side bent in 5 minutes flat. If I recall what the video available from LMI says about using a bulb bender IIRC they took about 30 minutes to bend a side.

Because the bulbs take longer to heat up and IMHO the heat is not distributed as well as say a blanket bulb benders can also result in not so great results or worse.... the d*eaded b*oken side.

Now that I have that off my chest :D you might want to consider getting heating blanket and John Hall, Blues Creek is my go-to-guy for such things.

He also has slats, spring steel or stainless. The slats do two things IMHO: First they provide support for the side as it is being bent and while it is baking in the bender. Second they help contain and direct the heat to the wood in a more uniform fashion.

A typical bending "stack" (what the wood, slats, blanket, etc looks like) can be from the top down:

1) Blanket
2) Slat
3) Wood wrapped in craft paper (what I use) or foil (what I used to use)
4) Slat

There are lots of other possibilities for what the stack looks like too - this is just one example.

So you can see that we have a sandwich of sorts with the wood in the middle. The wood is supported by the slats at all times and even if the heat source is light bulbs (hopefully not those Al Gore light bulbs either.... ) the slats help direct and contain the heat.

If you covered your bending form in foil I would think that this would help reflect the heat back into the bender and not let it rise well into the bending stack. Foil offers no support either and is no substitute for the bottom slat.
Author:  Hesh [ Thu Feb 26, 2009 11:51 am ]
Post subject:  Re: Bending question?
I understand - mahogany can be tough to bend or easy depending on the piece that you have. .085 is a good thickness and I will usually take mahogany to .080 but I am splitting hairs which is preferable to splitting a side.... :D

I do notice that mahogany has more spring-back (what you observed with it not being tight in the waist0. Many of us will bake the side twice to help reduce spring back. I do the bend, leave it in the bender and let it cool completely to room temperature and then turn on the bender again, get it up to the heat that I used to bend and cook the side for a couple of minutes and then unplug everything and leave it in the bender for 12 hours. This seems to reduce spring back.

Others will cook the side longer at a reduced temp.

John Hall has a great toot on his site for bending sides. I am about ready to repost my own side bending toot since it died on the old OLF server.
Author:  Randolph [ Thu Feb 26, 2009 11:57 am ]
Post subject:  Re: Bending question?
Hesh, thanks for all the info. Ok, the foil I was referring to is on the side walls of the form. I am using 2 stainless slats on either side of the mahogany. Make sense?
Author:  Michael Dale Payne [ Thu Feb 26, 2009 2:59 pm ]
Post subject:  Re: Bending question?
For what it is worth, my experience is that 150w are tad on the low wattage end. Back when I used bulbs I used 200w clear with a dimmer for each bulb. The problem with bulbs is that the heat is escaping more than it is transferring till you have the form closed off. Foil or in your case aluminum plate on the inside of form helps to radiate the heat more evenly but bulbs just don't do a good job of providing consistent heat across the entire board. the create hot spots at the waist and upper bout. That is what drove me to adding a dimmer for each bulb. I ran all three at high till the waist reached bending temp. I then bent the waist 3/4 complete and turned it down to 1/2 power, bent the upper bout and turned it down to 1/2 power. Bent the lower bout and then turned it down to 1/2 power and finished the waist bend to draw everything tight.

It really is not about how hot the area inside the form gets it is about how evenly you can spread a constant heat and transfer it to the wood. This is why I moved to silicon heating blankets years ago. With blankets the heat is at the wood and contained between the slats. It is the same temp at one as it is at another.
Author:  Hesh [ Thu Feb 26, 2009 3:48 pm ]
Post subject:  Re: Bending question?
Yep that makes sense Randolph.

Not to be a nag but I just wanted to be sure that someone mentions in this thread that benders have caught on fire before. So please never leave your bender, when in use, unattended and be sure to unplug the thing when you leave your shop.

Thanks
Author:  Randolph [ Thu Feb 26, 2009 4:11 pm ]
Post subject:  Re: Bending question?
Michael, Todd, Hesh, thanks for the great feedback. It definately looks like I'll be investing in a blanket before long. In the meantime the 200/250/200 set up makes a lot of sense. I'll give that a try along with the double cooking.
Thanks again.
Author:  Michael Dale Payne [ Thu Feb 26, 2009 4:31 pm ]
Post subject:  Re: Bending question?
Randolph wrote:
Michael, Todd, Hesh, thanks for the great feedback. It definately looks like I'll be investing in a blanket before long. In the meantime the 200/250/200 set up makes a lot of sense. I'll give that a try along with the double cooking.
Thanks again.


Wire a dimmer on each blulp or at least one on the lower bout and one to control the waist and upper bout. with bulbs if you are not contoling the waist and upper bout buls you get too much heat at the waist and upper bout during the cook and end up scorching the waist. Don't ask how I know this eek
Author:  Randolph [ Thu Feb 26, 2009 4:55 pm ]
Post subject:  Re: Bending question?
Michael, how'd you find out about that scorching thing? oops_sign That's right, I wasn't going to ask :lol: I might as well wire them all separately with dimmers. Seems more control would be better than less, no? Thanks for the hot tip!!
Author:  Blain [ Thu Feb 26, 2009 9:31 pm ]
Post subject:  Re: Bending question?
I'll second the Heat Blanket suggestion.

When I bought my bender from John Hall, I asked him if he could sell me one with bulbs and also sell me heat blankets. Rather than charging me extra cash for all the extra items, he talked me out of the bulbs because he so highly recommended the heat blankets. I went ahead and got the bender without bulbs and got a heat blanket from him supplied with my bender. I can't tell you how easy it was to bend using the blanket. I then heard of others using two blankets so I ordered a 2nd from John thinking at the very least I could have a backup. I now use 2 heat blankets and couldn't imagine it being any easier.
Page 1 of 1 All times are UTC - 5 hours
Powered by phpBB® Forum Software © phpBB Group
http://www.phpbb.com/