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BMTECH
11-13-2006, 03:28 PM
Iv just got a hot wire /CNC set up running but the cutting speed is slow it is running a HCNC board (the old type with resistors) I'm running it from a battery can I increase the speed buy upping the voltage to 15v's

Any help will be appreciated.



Rick

BMTECH
11-23-2006, 09:17 PM
OK I think I answered my own question I now have a power supply that will go up to 24volts,I started at 13v & its up to 16v now & it is defenetely faster.
I can get the step speed up to 4,4mm/sec where before it would stall the stepper motors & just make noises.

Now I just need to get the power supply for the cutting wire to output more current with the wire I'm using.

Rick

Foils
11-29-2006, 04:41 PM
Be careful there.. the ballast resistors would have been sized based on the current spec for the steppers. If you change the voltage, you will need to re-configure your ballast resistors to avoid cooking your steppers.. i.e. if at 12V you had resistors configured to limit your current to 0.2A [NOTE: I picked 0.2A arbitrarily - check your motor for it's rated current, do NOT use 0.2A just because I used it here..], you would have a total resistance (motors, and ballast) of 60 Ohms. If you then increased the voltage to 16V, in order to retain your current limiting at 0.2A you need to increase the total resistance to 80 Ohms. The power consumed by the stepper adheres to the formula I^2 * R - the resistance in the stepper coils doesn't change with the increased voltage (the inductance does, but that's a whole other discourse..) so in order not to provide too much power to the stepper we need to limit the current. There's a section in the manual that came with the HCNC board that describes how to work out the numbers (I think).

If you run steppers over current for long enough, you can de-magnetise the permanent magnets on the rotor, so you'll get progressively less torque, and eventually just have to trash them because they won't work anymore.

The reason that the higher voltage allows the steppers to operate faster is that the coils in the motor can generate their peak magnetic strength faster. This is where the Chopper board is better as it varies the voltage in response to the coil current (which does change due to inductance as the magnetic field develops) and allows the coils to reach their peak strength faster. (as well as not wasting all that power in the resistors).

Also look at which mode you're running the board in - try using the wave-drive setting, and enabling half steps as this allows the motor to be more smoothly powered, This does generate a bit less torque than the alternative, but you shouldn't be carrying a big load in this application anyway.

I am using the same HCNC board at 12V and can get a consistent 6mm/sec driving an M6 threaded rod without missing any steps. Startup and slowdown velocity ramping helps too, going from 0 - 6mm/sec instantly is a bit of a tough call for any motor! Depends on the software your using whether or not this functionality is availiable.

BMTECH
11-30-2006, 05:26 AM
I will try the wave_drive settings & the 1/2 step as well,Im using M6 threaded rod & cutting with the GMFC software.

The steppers draw about 4 amps running on 16 volts ,they are 32oz units from HCNC.

What software are you running?

Foils
11-30-2006, 09:27 PM
I have been running GMFC as well. Not 100% sure of the spec for my motors, my first machine used motors out of scrapped deskjet printers, and my current edition is running on a matched set from 2 retired photocopiers - it still amazes me just how much a dozen beers is worth to some people!

Both setups were happy to run at 6mm/s on GMFC. I've had my current motors up to 10mm/s using DeskNC through DOS, but I wouldn't trust it not to loose steps at that speed. Besides, the kerf gets crazy when the wire is hot enough to cut via radiance at that speed. Typically I cut at around 3mm/s in blue foam for the best results. I'm using Nichrome wire on a timber and piano wire bow - plenty of spring in the setup to account for the nichrome stretching as it heats, and it takes a lot less power to drive it than stainless wire.

I've had a few months away from cutting, but will give FCut a try when I get organised again - Moreil seems to be doing good things with it.

BMTECH
11-30-2006, 09:37 PM
I am using the same HCNC board at 12V and can get a consistent 6mm/sec driving an M6 threaded rod without missing any steps. Startup and slowdown velocity ramping helps too, going from 0 - 6mm/sec instantly is a bit of a tough call for any motor! Depends on the software your using whether or not this functionality is availiable.

Are you running with the ramping on HCNC or is that on some other software?

I will be trying to get some RENE wire soon to give that a try, The kerf is very wide with the ss fishing leader .


rick

Sleedo
12-05-2006, 03:50 PM
Are you running with the ramping on HCNC or is that on some other software?

I will be trying to get some RENE wire soon to give that a try, The kerf is very wide with the ss fishing leader .


rick

I'll be interested to hear your results, I feel that the kerf is too large as well with the leader wire. If the RENE produces a much smaller kerf I will have to dust off my wallet and place an order. Funny thing is I just made an order with Ed for a fuselage but wasn't thinking about the wire.

Foils
12-05-2006, 10:54 PM
Last I remember, I was cutting with a 800mm length of Nichrome wire, and had 1.6mm kerf at V/2 (around 1.5mm/s) and still radiant cutting at V (3mm/s) but only just! I've been running a _very_ old electric blanket transformer. It has big windings, and can handle huge current, but puts out A/C, and I haven't got any computer heat control setup yet, so getting it right is more art than science!

Re: Ramping - I think that was in DeskNC, I can't remember any settings in HCNC to control the ramping.

Let us know how you find the different drive mode.

BMTECH
12-06-2006, 05:47 AM
Re: Ramping - I think that was in DeskNC, I can't remember any settings in HCNC to control the ramping.

Let us know how you find the different drive mode.

I will keep you up to date but it might be a while :confused: lots of home duties


Rick

OzDragonflyer
12-13-2006, 11:18 PM
Just finishing off a homemade interface for GMFC.

Interestingly, if anyone's contemplating using a microstepping drive with 1/8 ustep or greater for gmfc.. don't!

The max speed from gmfc is entirely dependent on the timer values (for those who didn't know) So at 4khz, your max speed will be 4000 pulses per second.

so at full step (200 steps per rev).. 20 revs per sec
1/2 step 10 revs per sec
1/8 step 2.5 revs per sec.

So for any decent speed at 1/8 step, gmfc has to be driven by at leat a 10khz timer. Forunately, it accepts this happily, although I guess on slow PC's it might complain a bit.

Pwm frequency of the hotwire is also directly linked to the timer freq, so changing the timer rate, will also affect any heat settings.

Just some info for anyone wondering why they cant get huge speeds from their motors compared to other software like mach2 etc.