Thread: Bending with thermal blankets

Thermal heating blanket??? Thermal blankets dont need "cranking" up... they just are... do you mean an electric blanket? you know the ones you whack into the power point lay on the bed under the sheet and turn the knob gadget to make the bed nice an warm?

Im confugged

Thermal blanket = silicon thermal blanket. A product of the NASA space program. Ya plug the thing in and it starts to make weird buzzing sound and it gets really hot in a really short time. I'm talking 285 deg F in about 5 minutes. If you stuck the thing on your bed your bed would be charcoal by morning!

Pics appearing here tomorrow!

Okay here we go bending some Indian Rosewood sides for the 12 string. Sides have been worked to just over 2mm thickness and one side has been trued up (this will be top side).

Pic 1: Side bending jig. The jig is based on the one in Jim William's book but I couldnt get the book style press screw to work so I whacked the tower section off the jig and replaced it with two threaded rods and wingnuts. Original heat source was 3 200 watt light bulbs. The box at the front of the jig is the temperature controller for the thermal blanket (its a router speed controller).

Pic 2: The form around which the side will be bent. This form fits snuggly into the bending jig. Material is 19mm MDF with aluminium spreader bars epoxied in. The al foil is to reflect heat of light bulbs and reduce charring of the MDF when using light bulbs as a heta source. On top of the form is the waist caul.

Pic 3: View of the jig from the top. Note porcelain sockets for light bulbs.....DONT use plastic batten holders, theyll melt.

Pic 4: Thermal heating blanket. This is a 120 volt 1080 watt blanket bought from Blues Creek Guitars in the US (http://www.bluescreekguitars.com/). I run the blanket off a 240/120volt 1500 watt stepdown transformer via the temp controller mentioned above.

Pic 5: Rosewood is lightly spritzed with distilled water. Tap water may contain impurities that can discolour the wood. Once wetted down the wood is wrapped in al foil. The foil retains heat and moisture while heat is applied.

On with the show.

Pic 6: Wrapping the wetted down side in al foil. Dont forget to mark waist position and other orientation marks on outside of al foil.

Pic 7 - 9. Building up the sandwich. From bottom to top I have stainless steel slat - wood wrapped in al foil - second stainless steel slat - thermal blanket - spring steel slat.

Pic 10: The side sandwich installed on the bender and ready to go. I use wpring clips to temporarily hold evrything in position untill the waist caul is tightened down.

Last lot of pics:

Pic 11: Blanket is turned on and given 5 minutes to heat up. Target temperature is about 285 degF. I use a meat thermometer tucked into the side sandwich to monitor temp but also test temp by spraying water on spring steel slat. I also listen for the sound of the wood fibres starting to relax. When the water instantly turns into dancing beads its hot enough. I then wind down the waist caul until its about 3/4 of the way home.

Pic 12: Lower bout caul gets slowly moved down to butt end of side. Watch the fingers...that top slat is hot.

Pic 13: Same again for the upper bout caul. About this time the blanket gets turned off (after total of 12 minutes on)

Pic 14: Everything all tightened up. I now leave the side for 1 hour and then turn the heat back on for 5 minutes. I thenleave the jig for at least 3 hours (ideally it should be left overnight).

Pic 15: the 22Kg 1500 watt stepdown transformer supplied by Tormec. http://tortech.com.au/index.php?productID=125

Pic 16: Side out of the bender. A bit of fine tuning with the pipe bender was required at the butt and neck ends of the sides.

Thanks for the pics. They look great. Is this the first Guitar with the blanket? Thanks also for the link to your sources. I think I know what I'm getting for Christmas now.
All the best
Kevin

Yes, first guitar built using the blanket. Previous instruments were made using an electric bending iron. The bending iron will still get a bit of use. Useful for doing fine tuning on a side after it comes out of the bending jig and for bending bindings etc.

If youre going for the blanket option then cost it all out. The step down transformer cost $Aus385 but it means I have access to a greater range of blankets and associated devices. There are 240 volt blankets available from alternative sources and it probably wouldnt be hard to source a 240 volt router controller.

Both sides have had the ends trimmed and have been fitted into the exteernal mould and had the butt and neck block glued in.

Hey martin, I have a similar setup but I got the Blue creek bending machine.
The first few sides i used it for were nice but had a few ripples in them. I wansn't happy with the tension on the top slat which slips up and allowed the sides to ripple.
I read Mike Doolans web site and advise on side bedners and combined with what he does and ideas from the taylor factor videos and other sources i modified my bender and now get great results with no ripples or cracks. i have just bent my first cutaway on an OM with no problems.
Here is my setup. I have rivited the the two SS slats together at the heal end and attached a plate with holes to attach springs. At the other end the plate is only attached top the the top slat to allow the two to slide around the curves.

Photo #1 shows the heal end of the sandwich and if I was doing a real bend it would be slat, wetted wood wrapped in damp craft paper, heating blancket, top slat. I have started to profile my sides close to finished taper before bending so i need to reference the waist in my sandwich. When it goes into the press I bring the waist ram down a bit to hold everything together and align the waist.

Photo #2 once the waste is aligned i bend the lower bout as usual then attach the springs as show to hold the heal end down.

Photo#3 after the upper bout is bent I attach these two turnbuckles and tighten them up a bit. Just enought to take out any slack.

Photo#4 with both ends secured, when i bring the waist ram all the way down it pulls to sandwich together very tightly, the springs holding it at a good tension. The slats are as tight as a drum and there is no where for the wood to move. The last two sets i have done like this have come out perfect.

Photo# 5 Shows a cutaway I bent using this setup the other day.
Cheers
Dom

Thanks for the info on the mods to your jig Dom. One thing that needs fine tuning for me is the heating time for the blanket. I used Hesh's tutorial on the OLF forum as a guide for my procedure but am finding Ive needed to extend the time I have the blanket turned on.

The instructions i got with the bender seem to work fine. I put the sandwich in the jig and give it 10 minutes on the timer. About five to warm up and the five to slowly do the bending. Then I have found that once its bent I give it another 5 minutes. Then take it out the next morning when cool. This was with EIR.
I do my bindings in there also, just tape them up and wrap in paper and bend at the same time as the side.
Dom

Thanks for the info and pictures Martin, and Dom! I'm finding it very useful to see how you're doing it.
I've been using the heat blanket for side bending for the past six months and find it very good on the whole.
The only tricky bit for me, has been accurate temperature measurement at the startup. I'm using a thermocouple and I find there is a delay as the blanket heats up, so for the first half a minuet the blanket is hotter than the thermocouple. Its not for long, but long enough to scorch the sides. So I find i have to manually control the temperature for that time by switching the thing on and off.
I haven't been able to find a way to stick the thermocouple to the blanket, so have taken to trapping it under the caul that screws down to bend the waist ( I'm using a similar bending jig to the ones pictured )
It seems to work pretty well like that for me, but its interesting to see how you guys approach it
Cheers!
jack

Jack,

Im using an eletronic meat thermometer but getting an accurate idea of temp is a problem. Judging by the way the water is vapourising off the top slat Im sure the actual blanket temp is higher than showing on the thermometer. The thermometer is tucked between the top stainless steel slat and the wood.

Hi Martin, thats interesting. Now that you mention it I wonder if my setup is running hotter than I'm measuring all the time as well. When I first looked into it, I read that a temperature around 140- 150 deg C was about right. In practice I found that too hot and got much better results at around 110- 120 degC. So I wonder if I'm measuring under all the time, not just at the startup.
Interestingly I discovered that bending the sides too hot didn't just scorch them, but also was less effective as they sprung back a fair bit. I've since found that running it cooler, and for a bit longer, with the heat for about 15-20mins, has been very succesful with hardly any spring back. Don't really know what to make of that...
Jack