Messages

611

General Mach Discussion / Re: Power Supply Unit

« on: January 30, 2011, 08:37:20 PM »

Yes, you're absolutely correct.  Capacitors in series combine the same way resistors do in parallel.  Similarly, capacitors in parallel combine the same way resistors do in series.  So, two 10,000 uf capacitors in series will give 5000 uF,in parallel will give 20,000 uF.

"That said, I'm not sure total capacitance even matters that much in a power supply." - It matters VERY much, if you don't want your output voltage to sag during the low phases of the AC input.  Too little capacitance will result in output voltage ripple.  The higher the current, or the lower the capacitance, the larger ripple.  On a stepper motor, that will lose torque, and top speed, and could, in extreme cases, cause the stepper driver to misbehave.

Regards,
Ray L.

612

General Mach Discussion / Re: Power Supply Unit

« on: January 30, 2011, 07:11:37 PM »

That's really not a good practice with electrolytics, unless you provide a LARGE voltage safety factor, which kinda defeats the purpose.  The tolerance on the capacitance of large electrolytics is typically very broad (like +/-30%), which means some will charge MUCH faster than others, which means some can have considerably more voltage across them than others when connected in series.

Regards,
Ray L.

613

General Mach Discussion / Re: Cutting inside or outside the line??

« on: January 26, 2011, 12:22:25 AM »

Looks like the arrows are  depicting the direction of table movement, not the cutter.
In that sense, it's right.
A good illustration for a manual machinist ?

That would make sense, except it's more conventional to think in terms of tool movement....

Regards,
Ray L.

614

General Mach Discussion / Re: Cutting inside or outside the line??

« on: January 25, 2011, 11:52:03 PM »

That second drawing has all the climb/conventional directions backwards....

615

General Mach Discussion / Re: Mirror image

« on: January 25, 2011, 11:49:20 PM »

Keep in mind the directions indicate the direction the TOOL moves, not the table.  The long axis is typically X, and runs parallel to the front of the machine, with Y running front to back.  When looking at the machine from the front, X+ will move the tool to the right (or the table to the left), X- will move the tool to the left (or the table to the right). Y+ will move the tool towards the rear of the table (or the table towards the front, away from the column), and Y- moves the tool towards the front of the machine (or the table towards the rear, towards the column).  Z is the vertical axis, and Z+ moves the tool up (or the table down), Z- moves the tool down (or the table up).

Regards,
Ray L.

616

General Mach Discussion / Re: Stepper driver chips

« on: January 25, 2011, 09:45:43 AM »

Ray,

You can blow up anything if you operate it outside the manufacturers specification. Blowing up several Alegro's could be classed as careless. 

Tweakie.

Tweakie,

Twice, after operating perfectly for years, one of my three 3977s went up in smoke in the middle of a job.  Not doing anything it hadn't done 1000 times.  The problem is the 3977 has virtually no self-protection against even the briefest transients.  Other drivers, like the Toshiba, and Geckos, are FAR better at protecting themselves against unavoidable transients.  Search on Xylotex, which also uses the 4977, and see how many others have had the exact same thing happen.  I was operating well below rated voltage and current limits, and well heat-sinked.  I don't know why they died, but they did.  And when they do, they typically take out the PCB and several other components as well.

Regards,
Ray L.

617

General Mach Discussion / Re: Stepper driver chips

« on: January 24, 2011, 10:19:44 PM »

Hi Tony,

Although they both perform a similar function they are not interchangeable. The 3717 is essentially a single phase device and you would need two chips to drive one bipolar motor. Although they are OK for printers and scanners etc neither are really suitable for CNC use because the drive current is just not sufficient. (Most bipolar stepper motors require a minimum 24 Volts at 2 Amps to provide sufficient torque to drive a CNC). There are better Stepper Motor chips available – take a look at the Alegro range for starters.  http://www.allegromicro.com/en/Products/Categories/ICs/motor.asp

Tweakie.

Or, better still, look at the Toshiba TB6560HQ.  They are much higher performance than the Allegro, and much more robust.  I've blown up several of the Allegros.  And, even at their best, the 30V/2.5A limits are rather, well, limiting...

Regards,
Ray L.

618

General Mach Discussion / Re: incresing speed on stepper motors

« on: January 23, 2011, 06:18:42 PM »

That's ok though. that's why i am posting - I am ok with replacing those items it's just getting and understand what I need so I do it one time and not multiple.

For when I do upgrade -
How would I hook the motors to be single coil?

I am making notes of all of these things.

Don't.  Pick your motors and drivers, and design your power supply, so you can run bipolar parallel.  If you do it right, that will give you the best overall performance.

Regards,
Ray L.

619

General Mach Discussion / Re: incresing speed on stepper motors

« on: January 23, 2011, 02:46:05 PM »

You really need to start by defining what performance you're looking for, then characterizing your machine to determine what's needed from the motors/drives to achieve that.  In other words, you need to actually DESIGN the machine, not just bolt and wire together a bunch of random parts.  You need to know your max desired rapid speed, your max desired cutting speed (which is heavily affected by cut depth/width/material), and you need to MEASURE the torque required to move each axis, and the calculate or measure the inertia of each axis, when carrying maximum load.  From that you can calculate the required motor performance, which allows you to select proper motors, drivers, and power supplies to achieve the desired result.  Simply bolting on parts without doing this design work is not likely to do much more than waste a whole lotta time and money.  There are no shortcuts to getting a reliable, high-performance machine.

Regards,
Ray L.

620

General Mach Discussion / Re: incresing speed on stepper motors

« on: January 23, 2011, 12:20:24 PM »

Your motors are also rather high inductance, which will affect speed.  Particularly with the G540, you might well be better with lower torque motors that have lower inductance, like one of the 2mH-ish 380 oz-in motors.  The ones you have would work best on a much higher supply voltage, which neither your current drivers, nor the G540 will drive to their best performance.

Regards,
Ray L.