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Hello,

I was bending some light colored wood for laminated sides and on a couple of the pieces I got some scorching at the upper bout. Out of 9 bends, 4 had scorching in the exact same two inch area.

The blanket I’m using is a new keenovo with the built in controller. The old one I had went haywire.

I had set the temp to 150 C. I used a secondary thermistor attached to a different controller for my old bender and was surprised to see the lower bout was at closer to 165C. So then I checked the upper bout and it was 170+.

I checked the advanced settings on the keenovo and the offset was at zero, so something is up with the new one as well. I have to think it’s something I did and not getting two bad ones in a row?

So, I adjusted the max temp on the keenovo until I got a consistent 150C at the upper bout. The setting on the controller was now at 118. I checked the temp at the waist and it was 136, and then 125 at the lower bout.

Would you expect these differences across the blanket?



I wonder if it’s something I’m doing to burn up these controllers?

Brad


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Even a faulty controller should not cause that, although offsets may be dodgy as you found.
Theoretically, differences like that may be down to variations in the heating wire thickness, thinner in the hotter areas (quality control?) or an increase of wire/area.
John Hall says he has observed that scorching is caused more by time, not so much heat.
He's talking about using 375 F, 190 C for bending all his sides, but only briefly. (I chicken out at 160/165)
See his posts here http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=51516&p=678218&hilit=+bending#p678218
FWIW, (I also use Keenovo blankets) I much prefer a shop built separate controller/thermistor set up with thermometers as cross checks.
I see as much as 5 degrees C variations on the 2 thermometers to my setting, not enough to worry me.
Before I bought mine, I had seen many posts on forums where their built in controller offerings were extremely bad.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Just as an afterthought, an increased temperature will probably be normal at the waist due to the insulation value of a waist caul. All other areas are loosing heat to the environment fairly consistently..

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

it seems like you may be waiting too long to throttle back the blanket after the bend is complete. In particular, rosewoods don't require much encouragement to bend, and even mahogany and anigre will offer you near zero spring-back with peak temps just over 300 deg F, which is well below where scorching is likely. Being a bit of a process person myself, bending 'by-the-numbers' seems intuitively attractive, but with the variation in bending stack temperatures appearing to be wide enough to result in scorching, you might consider defaulting to the more conservative approach described below.

We use ProTherm 5 watt/square inch blankets with a router speed controller (Blues Creek sells the blankets as well as the integrated timer and router speed control). With the speed controller on full, we start the bend as soon as steam is seen rising from the bouts and once the bend is completed - usually 8-10 minutes after turning the blanket on and no more steam is visible rising from the stack - the controller is set to produce 220-230 deg F for 30 minutes of drying. With SuperSoft 2 use for figured or difficult-to-bend woods, we don't see any spring-back or issues with tight Venetian cutaways (note that much like the ancient Greeks, Mr. Hall uses Windex ammonia-base window cleaner to achieve the same effect as SuperSoft 2).

Blanket rate of heat rise is important - too slow and all the moisture will be slowly cooked out of the stack before the bend is completed. Blanket rating of about 5 watts per square inch seems to be the sweet spot for the task, and that seems to be what most luthier's tools vendors offer. While I have not used a higher wattage rated blanket (7.5 or 10 watts/square inch rating), I suspect that working through a complex bend, such as a Venetian cutaway, might take enough time such that the available liquid water is cooked off before the upper bout and waist could be set.

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Thanks, Woodie. Good point about the watts per sq inch. I didn't realize there were two offerings from Keenovo, an 800W and 1200W. I bought the 1200W (both times based on my receipt). It states in the description:

https://www.ebay.com/itm/6-X36-1200W-12 ... 1581026435?

Blanket rated at 5.55 watts per square inch with SUPER fast and even heating.

I do agree, it gets ripping hot in no time.

I'll take some pics later of my setup because I think you and Colin might have hit on what is the issue in my process.

Appreciate the input.
Brad

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Didn't realise the ancient Greeks had Windex, let alone bent guitar sides.
Maybe I read that wrong...

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I think they were bending bozouki ribs with their Windex, but that is certainly not all it was used for.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

I would expect some temperature variation at different places on the form, but not as much as you are seeing.

Can I ask you how are you measuring the different temperatures on the heat blanket? Could this be an instrument issue? Or parts of the form could vary the heat sink affects?

Flipping the blanket end for end and repeating the measures my help tease out form related heat sink effects and inconsistencies in the blanket construction.

_________________
Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

I had a blanket that developed a hotspot after a few years. After scorching two sides in the exact same spot I threw it out and bought a new one. Never had the issue again.

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http://www.Harvestmoonguitars.com

I bend with the blanket to the inside of the rib. Scorch marks that won't sand out easily can live on the inside of the guitar.

physics lesson 101
Heat will vary for a number of reasons . The quality of the blanket , the Contact area , Moisture . The moisture will work like a refrigerant and show a lower temp at the wood than the blanket. I am not sure where you are placing the thermometer but I place it between the blanket and the slat never on the wood. I want to know the hear available to the wood. If you have air between the blanket and the slat your loosing heat to the atmosphere and not going to the wood. How much pressure and surface area of the heat source contacting the slat will influence and lastly how much heat transfer is occurring so don't get too wrapped up in the heat all over.
Wood will start to bend usually in the 220 F to 270 F range. I take the heat to about 375 F then go down to 250 F for about 15 min Scorching happens by temp and time. Under 300 scorching is slim unless you cook it there a long time.
I had tried this brand of blankets as they were so much cheaper and I found the heat was not consistent . Also what wattage did you use? 5 watts per inch can hold the heat much better than 2 1/2 watts per sq in.

Dial in your process and keep notes , you will soon get through the learning curve.

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

I should clarify a few things...

Sorry for the long post, I just want to make sure all the details are here.

This blanket (like the one before it) worked fine for quite a while. It was only the recent bending of paulownia for laminated sides that I noticed the scorching. So I decided to test the temps.

The test I did above, which produced differing temps at the upper, waist and lower, was with just slats and the blanket. No wood or moisture in the mix. I’m testing with a temp sensor attached to a controller and comparing it to the keenovo controller.

What woodie mentioned in the beginning made me think it’s something in my process that is maybe burning up these controllers. I’ll try to outline below using pics of what I do.

Starting out I put the sandwich in and tighten the waist a few turns. Once I see steam I take the waist the rest of the way down. Now notice that when the waist is all the way down I have yet to clamp the ends closed. This is a practice I started a while back. The idea was to keep the bout from drying out while I was bending the waist. Now I’m wondering if have this unclamped is causing the controller to go haywire?

So I start like this (note the temp sensor is in the waist area now):


Once the waist is down I clamp the ends of the lower bout and wait for it to start steaming:


Once the steam starts I move the temp sensor and bend the lower bout:


Next I clamp the upper bout:


Once I get steam I move the temp sensor and bend the upper bout:



Note: the temp sensor is a backup, the controller for the keenovo is what is controlling the blanket.

Also note: After just buying the controller I didn’t have this issue. So I’m wondering if I’m somehow damaging the controller because of leaving the ends unclamped while heating?

The reason I moved to the keenovo to start with is how fast it got to temp, but a side effect is I then felt I had to move super quickly to keep the water from cooking out of the wood too fast. That is why I started doing the unclamped ends process above, to buy time.

Any thoughts?

Brad


Sent from my iPhone using Tapatalk Pro

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Thoughts -
I have read that operating the blanket in open air is not good for it.
I'll quote from Kenoovo's page:



Try clamp both ends from the start, remove as needed to bend, this ensures a good heat sink to dissipate heat from the blanket.
I actually use 2 sets of 2 foldback clips above the waist and 3 below, 10 in total.
I will also mention that because heat rise is so quick I have started to set my controller to 120/125 degrees initially (steam starts), bend the waist (my screw thread is slow), then set controller to 160 degrees, do lower bout, then upper bout.
My temperature sensor is very fine ( the 2 wires twisted together) and stay in the same place, inch or so below the waist tower.
Reduces the pucker factor as some say.
I hope others pitch in with suggestions.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.