Heating Blanket and Temp Controller on the Cheap(er)

[Darryl Young]

A PID controller would be just the ticket for your needs. Regardless of how fast the blanket heats up, the PID controller, available from Amazon, along with the rest of the components, would get the blanket to the right temperature, and hold it there for as long as you like. .......bert

If it is a real PID controller, then it needs to be "tuned" for the application. If the tuning parameters aren't set right, then you can overshoot your target temperature or get in a "resonance" situation where the temp yo-yos back and forth across your setpoint. So just having a PID controller doesn't guarantee the temperature will be maintained as expected without some tuning parameter adjustment. Some of the higher end controllers have an AutoTune function to set these parameters.

Also, the timer John mentions is a safety to shutoff the controller in case someone forgets it is on and walks away. When the timer expires the unit shuts off no matter the temperature. So the timer isn't used as an indirect method of getting the wood up to a particular temp.......it's a safety shutdown that removes power to the controller/blanket so it's not left on for prolonged periods creating a fire hazard.

As for the speed of getting up to temp, I think the issue is not only the max temp but also how long the wood is at elevated temp. I don't have enough bending experience to give a lot of detail but it seems the wood "wood hardens" (for lack of a better word) after sitting at elevated temps. I realize this is not the appropriate technical term but it makes the point. I placed a spruce top in an oven at about 225F for an hour or so once (forget the exact time). I didn't put it on flat spacers when I removed it and it cooled with an arc across the grain. Now, no matter how I heat it I can't remove that arc. So that wasn't a case where too high a temp was used but you can no longer bend that top......at least not side to side.....and have it remain flat (didn't try bending it along the grain).

[tippie53]

wood is stupid and will bend and set a bend at given temperatures. There are many techniques of bending. Many misrepresentations on bending. I have been a a consultant on a bending for 2 major manufactures.
Heat control is all you need. How you control it is up to you. Heating too slow can cause more problems than heating too fast . Wood cells will become set after heat and duration. The higher the heat the faster the set.
I am a low tech guy but I can appreciate the technical gizmos.

[Dave Sayers]

I will always consider all the available evidence from experts, study their videos and notes and then I will do my own thing as usual.

I have decided that the temperature controller I will use with my bending blanket jig will be a carbon/oxygen/water based timing device capable of carrying out multiple functions and independent thought. In other words I will forget about timers and carry on standing by the machine monitoring the temperature and switching it on and off if necessary. I mean, it only takes a few minutes to bend a side.

[tippie53]

Different wood species may need a different temp. Most woods will bend and set well at the 275F to 325F range. Figured woods like up to 375f but less water. The more you bend the more comfortable you get. I also use about .080 as my bending thickness.
Too thick can cause a lot of problems.

[Darryl Young]

.......In other words I will forget about timers and carry on standing by the machine monitoring the temperature and switching it on and off if necessary. I mean, it only takes a few minutes to bend a side.

You will likely be fine. But hey, life happens. Sounds like something I would have said before I had kids <smile>

[btberlin]

For the benefit of others who may be reading this thread, and the comment about tuning the PID - all the ones i saw listed on Amazon have an autotune function. On mine, its a few button presses to get the PID to autotune. After that, it seemed to work quite well, with little over or undershoot around the set temperature (about +/- 1%). But hey, you are entirely correct - it doesn't take long to bend, and standing and watching it works well too. Robert O'Brien mentions in his DVD that he learned to bend on a pipe filled with hot coals.

Bert

[songman101]

Hello Bert
This is quite a project isn't it. I can see the economical side of this issue. Like john mentioned "one needs good tools" Yet is it better or as good as John Halls unit he speaking of? please reply the new newbie Stephen

[tippie53]

if it works that is what matters. Making a guitar is a process and if your process is in control you will have a good outcome.

[songman101]

Hello John

Well spoken I agree, everything needs control, their are really no short cuts to doing a real nice job is there? I will eventually get the proper blanket and heater with the benders in your videos .By the way those videos are invaluable and greatly appreciated by newbies like me .thanks John,, new newbie Stephen

[tippie53]

glad you like then
we do like to help
jh