Hand held cutter head for the PWM hotwire controller

I have made a USD 2,- handheld cutter head for the PWM controller desribed in my previous blog post. The handheld cutter is nice for trimming off edges and so on. The spring keeps the wire tensioned. The final version don't need a series dropping resistor as the PWM driver stage uses two FETs.

The controller has also has been tested on a vertical setup for cutting spacers to my KFM3 depron wings (below):

The cutter gives nice clean cuts for vertical cutting operations (below). Please note the fan that sucks gases out from the workpiece. This is very important because blue and red foam gives off fumes that are very toxic when they are melted.

The PWM controller outputs pulses and is configurable both in duty cycle and several other parameters (below).

Based on interest, I may make a kit available.

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Comment by Morli on July 15, 2010 at 12:31pm
Pls do. looks interesting. I was using regular home brewed hot wire bow for cutting my foam cores. So what is the advantage in using a PWM controller for the same task.
Comment by UFO-MAN on July 15, 2010 at 12:46pm
Hi Morli. The advantages are several. First of all the current can be precisely controlled and it is possible to measure both wire temperature, wire resistance and adjust the pulse width (average power input) so that the cutter speed can be kept constant regardless of material type and thickness. It is also possible to send PWM commands to the wire cutter from a CNC controller via a serial or USB link. Also, safety measures can be implemented in a microprosessor controlled cutter. I know severl ppl are using hopbby railway transformers, but that is a very old fashioned way to do it. This controller will work very well off a LiPo cell, Lead acid cell, NiMH cell, any 12V trafo and even 24V DC sources so it is very flexible.

Comment by Morli on July 15, 2010 at 2:38pm
thanks for the info, so are you going to publish the circuit and design for those who have the ability and resources ?
How much do estimate the cost of each kit will be?
Comment by UFO-MAN on July 15, 2010 at 5:21pm
Here is the simplified schematics. I have modded the current version with one more IRFZ44 in paralell with the one on the schematic. This enables no series drop resistor to be used and shorter wires to be driven w/o thermal runaway problems. I might change the FET to a larger one later.

I havent decided if there will be a kit yet. Depends on interest. However you can easily make one yourself. Its arduino based.
Comment by Greg Fletcher on July 15, 2010 at 9:14pm
You can get a 90 A , ..5 mohm shunt from sparkfun with an amplifier, for 20 bucks+
0-3.3 V output :) You could know the current & voltage on the wire. Set of an alarm if peak amps are to high.
For a short wire you need to use less voltage to limit max amps. I think I will order some parts soon. I could use one of these things. look here
Comment by UFO-MAN on July 16, 2010 at 4:12am
Excactly. You could also measure the voltage over the wire since U x I = P and the battery voltage may drop due to internal resistance and lower cell charge (if you use a battery). Alternatively you could also use a drop resistor of very low value and measure the voltage over that instead of the hall effect device. That would probably be cheaper. On the arduino, there are several A/D inputs you could use, just make sure to not exceed the max input voltage. A divider and a clamping diode could take care of that.
Comment by bGatti on July 16, 2010 at 5:05am
I'm not sure you can measure voltage over wire - because wire resistance changes under temperature - and temperature changes under use.
You might should measure both current and voltage drop on wire, and try to stabilize the wire temp.
- to be honest - I don't know ow to stabilize the wire - I use adjustable voltage power supply with current cut. Seems ok-ish - I still have trouble on EPP.
Comment by UFO-MAN on July 16, 2010 at 5:25am
Well, yes you can measure voltage over the wire. And if you multiply that with current (if pwm, peak current x duty cycle) you have calculated the power. Resistance changes allright, and that is excactly why you want to measure both voltage and current. You can determine wire resistance with R = U/I or R@peak = Upeak / Ipeak. The resitance of the wire changes as a function of temperature. It is power over time that determines the energy given off to the wire and then energy from the wire to the material you are trying to melt. However I am not sure sophisticated feedback controls are really necessary.
Comment by bGatti on July 16, 2010 at 8:19am
I got the impression my EPP cuts would be improved if I could vibrate the wire - anyone tried this?
Comment by UFO-MAN on July 16, 2010 at 9:03am
Hi. No that I haven't tried. However, wouldn't vibrating the wire give a cut that is not clean?

If you feel that you struggle to cut thru the material, it might be that you run to little current with the voltage you have available from your voltage source.

One test could be to shorten the heated part of the wire (put the electrodes that connect to the wire as close as possible to the piece that should be cut). Alternatively you could go to a thicker wire (less resistance). The best would be to try to increase the voltage with the setup you have. In case you have a PWM source, making the on time higher, would do the trick (provided the on time is not maxed out).


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