Messages

1171

General Mach Discussion / Re: Routing Acrylic!!! Please Help!

« on: April 28, 2009, 12:42:30 PM »

Like it has been said here, you have to use razor sharp cutters for plastics. High helix cutters designed for aluminum work well. The feedrate should be very fast - about 3 times faster than for aluminum with a given cutter diameter and spindle speed. For example, I am limited on my mill to 2000RPM, and I found that a feedrate of 600mm/min with a 6mm cutter gives a nice finish.

Daniel

1172

Show"N"Tell ( Your Machines) / Re: My new CNC mill. JEH Guitars

« on: April 28, 2009, 12:24:50 PM »

Excellent job Jens. But as I commented on YouTube, what about the ball screws? They are exposed to the dust?

Daniel

1173

General Mach Discussion / Re: Can two 36v power supplies be used in series to get 72v?

« on: April 25, 2009, 03:28:01 PM »

I think if he can find a zener diode that can handle this voltage at the current he needs, then it would be the best solution...

Daniel

1174

General Mach Discussion / Re: Can two 36v power supplies be used in series to get 72v?

« on: April 24, 2009, 12:48:11 PM »

Ray,

What determines the current capacity is the capacitors. And as you know, a capacitor can deliver enormous currents. As to the capacitance, you're right - you need the same capacitance twice - one for each half of the rectified signal.

Daniel

Daniel,

Sorry, but that's not correct.  Take a 24V/5A, or 170W, transformer, connect  it to a full wave rectifier and filter, and you'll have a 34V/5A, or 170W,  power supply.  Connect that same transformer to a voltage doubler and filter, and the power the transformer can deliver remains 170W, but you've douibled the voltage, so the current must be halved.  There's no free lunch here.

Regards,
Ray L.

Ray,

Got your point, you were meaning the bulk current capacity the transformer can deliver. But you never walk on the edge of the transformer's current capacity, and what is more important for a motor drive is the peak current - and this one can be quite large with any circuit using that big a capacitance.

With steppers for example, the drive would hardly draw 1/3 the motor's rated current. I have mounted an ammeter on the lathe where I used the voltage doubler circuit and the current never goes above 0.5~0.7A. The steppers are rated 2.1A.

However, as you pointed, a voltage multiplier circuit might not be good in systems that use servos, due to their tendency to draw large currents under load.

Daniel

1175

General Mach Discussion / Re: Can two 36v power supplies be used in series to get 72v?

« on: April 24, 2009, 07:09:28 AM »

Simpson,

You got me confused... are you seeking to get 72VDC from 36VAC or vice versa?

In any case, voltage reduction by using resistors is not good for the purpose of powering motor drives, since the current the drives draw is not constant and there is no way to calculate the resistors required.

Daniel

1176

General Mach Discussion / Re: Can two 36v power supplies be used in series to get 72v?

« on: April 24, 2009, 07:00:56 AM »

Ray,

What determines the current capacity is the capacitors. And as you know, a capacitor can deliver enormous currents. As to the capacitance, you're right - you need the same capacitance twice - one for each half of the rectified signal.

Daniel

1177

General Mach Discussion / Re: Can two 36v power supplies be used in series to get 72v?

« on: April 24, 2009, 12:43:15 AM »

Hi,

Another option to get higher DC voltage is to use a "voltage multiplier circuit". It's a very simple, commonly known, circuit, utilizing  two diodes and two capacitors. I used it on my lathe since I didn't have a suitable outlet from the transformer to get 80VDC. So I got them from 28VAC using this circuit.

Daniel

1178

Share Your GCode / Re: Corrected problem of positive shift not being shown on DRO's

« on: April 22, 2009, 02:43:24 AM »

Hi Lokey,

Just went through this thread, and it looks very familiar to me I had this same problem a while ago. To sort it out, what you're seeing is lost steps upon reversals. An easy test can be made to prove it. You don't need to home your machine and read the dial indicator after that. Leave the same setup with the dial indicator and use incremental jogging in Mach3. Set the increment to, say 0.005", then jog one increment towards the dial indicator (remember the reading), then one increment backwards, then again towards. The dial indicator reading theoretically should now show the same as the first time you jogged towards the the clock. In your case I'd bet it would show 0.0015" off.

If this is the case then your loosing steps on reversals. Your stepper driver expects the direction change pulse to appear in a particular correspondence with the step pulse. Some drivers work on the leading edge of the pulse, but most, I think work on the trailing edge (don't quote me on that). For the last case, for example, it means that a pulse is registered be the driver only at the trailing edge. So depending on when the direction pulse occurs relative to the step pulse, you get an axis reverse in the correct moment or not. If the direction pulse is sent "late" then all the steps(actually one) sent till that moment would still move the axis in the previous direction.

Theoretically, changing the active state of the step pulse should solve the problem. But sometimes it doesn't work (as it was in my case). Sometimes, the break out board messes up things. To check this you'd have to bypass the beakout board and feed the signals form the PP directly to the driver. Assuming your PP output voltage is sufficiently high for the driver.

If you want to learn more on the subject and my path in solving this problem, follow the thread "HELP! Accumulative moves" on Mach3 Yahoo Group, at

http://groups.yahoo.com/group/mach1mach2cnc/message/102773

And also the thread "HELP! Accumulative moves - Good News" at

http://groups.yahoo.com/group/mach1mach2cnc/message/102937

Daniel

1179

General Mach Discussion / Re: Muti-start threading....

« on: April 21, 2009, 03:52:43 AM »

Andy,

I would suggest you to first make sure your spindle speed reading is rock steady. As I said, from your picture it doesn't look to be so.

Daniel

1180

General Mach Discussion / Re: Muti-start threading....

« on: April 21, 2009, 03:00:23 AM »

Hi Rich,

Wouldn't say it's backlash, since you don't see it on single and double start threads. Why would it be there on a three start thread then?

Are you sure the math is correct?

Daniel