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My CNC build
« on: March 08, 2009, 05:59:14 PM »
Hi folks!
I have just registered my copy of Mach3, LazyCAM Pro, and Mill add-ons. I have been using Mach3 to drive my home made Router table. I am using a Mechatronics 4 axis driver board, and NEMA 34 Vexta PK296-01AA stepper motors 24VDC 2a per phase Bi-polar wired serial. I have been through MANY frustrating hours of attempting to tune the motors in Mach3, and switching the DIP switches on the Mechatronics board between Full, Half, Quarter, and Eighth step and STILL cannot get the motors to drive the machine reliably without losing steps. Can anyone help with DIP switch settings, and Step, Velocity, and Acceleration rates which will yield reliable results? For a look at my machine build, please see the MYCNC.PDF Any help would be greatly appreciated!
Re: My CNC build
« Reply #1 on: March 09, 2009, 01:48:13 AM »
Just a quick thought.

You say your stepper motors are 24v... at what voltage are you driving them?

Steppers are supposed to be driven at somewhere between 20 and ive heard sometimes 35 times their acutal voltage.

Offline Tweakie.CNC

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Re: My CNC build
« Reply #2 on: March 09, 2009, 03:25:54 AM »
Nice build and brilliant documentation on the construction.

I think Fernando may be right in that your steppers are not providing enough torque to drive the axis's reliably.

(Just for reference my steppers are 5 Volt, 2 Amp per phase and they are driven at 25 Volts, 1.5 Amp per phase).

Re: My CNC build
« Reply #3 on: March 09, 2009, 06:17:17 AM »
Fernado and Tweakie,
The power supply I am using is a 24vdc 6.3a switched system. I know very little about electronics, but received this one along with the Mechatronics 4 axis board as an ebay purchase. The motors were purchased from a different ebay vendor, and the specifications for them I obtained from oriental motors.com 24 volt DC 2amp per phase Bi-polar with 2ohm coils. They are six wire motors, and the wiring diagram (also from Oriental Motors) showed that for Bi-polar funtion to wire one coil to the A+ and A- and the other coil to the B+ and B- outputs of the driver board. (The center taps are NOT to be connected for serial Bi-Polar function) which is the way I wired these motors. I cannot use a power supply higher than 24vdc, as this is the limit that the Mechatronics driver board can handle. Can anyone recommend an economical solution here? The motors are rated at 420 oz in in Bi-polar mode, and I have learned that Oriental Motor offers other models which are rated as high as 1200 oz in. However they required higher amps (which the Mechatronics board can't handle) and have a $308 EACH price tag. This is more than I can afford right now. Any help would be appreciated. And THANKS for the response!

Offline RICH

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Re: My CNC build
« Reply #4 on: March 09, 2009, 07:29:26 AM »
Lots of things to cover so will rermark at a high level.
Your voltage should be 4 to 20x motor voltage with enough amperage for all the motors.
Take a look at the motor curve for the motors you have and see where you are on the
torque to speed curve as that will give you an idea of just what kind of torque your getting out of them.
As speed increases the torque will decrease and power which is what your interested in will be max
around 1/2 speed and 1/2 torque approx so that gives you another ballpark on where to run them.
 If that dosen't satisfy requirements for your desired speed and power / torque then you need to review
the whole system, which you should have done in the first place, and see what if anything you should do.
The driive train works as a "system", so the weakest link in it can have a big effect. The mechanics of the system
defines system rerquirements. You can analyze your requirements via software from major manufactures
if you so desire, but using practical considerations as compared to similar equipment has great value.

With the above in mind just simply reduce your speed and acceleration and test the axis requiring the most torque
and gradualy work your way up until you start to skip steps. Back off say 30% and see what you  have in terms
of speed. If that is not satisfactory then you need to increase the powere supply to the motors within the capabilites of your drives / board / system. 420 in/oz motors got a lot of torque.

So it is difficult to make anymore specific / usefull comments without addiitional info on the mechanics of your system.

Re: My CNC build
« Reply #5 on: March 09, 2009, 11:46:25 AM »
I have not weighed the Gantry assembly of my system which includes on it the Y axis drive, as well as the Z axis drive systems, but I estimate it to weigh around 180 lbs. Obviously, the main axis I am having problems with, is the X axis. (Since it carries the load of EVERYTHING else!). I am using ACME lead screws 1/2" diameter 10 TPI with Lovejoy motor couplings on the X and Y axis, and a .375" diameter 5TPI ballscrew on the Z axis. (This was a pre-assembled purchase which included a NEMA 23 stepper motor from ebay). All 3 axis are wired Bi-polar series. The spindle I am using is a Porter Cable 690LR which I mounted on the Z using two 2" thick x8"x8" blocks of 6061-T6 aluminum which I bored the required 3.4" hole on my southbend lathe in order to clamp the PC Router onto the Z axis slide plate. Following problems with the Z losing steps, I eliminated the top clamp block to reduce weight. The Gantry assembly is constructed of 2"x2"x3/16" angle steel (see the MYCNC.PDF file posted at the beginning of this thread).  I Guess that it is just NOT possible with the motors/driver/power supply system that I am currently using, as I said before. I have spent many frustrating hours trying different step rates, velocities, and acceleration rates trying to find a satisfactory "sweet spot" in which the motors could drive the machine without losing steps. It appears that I will have to wait until funds are available to invest in higher torque motors and drive system. Also, I have noticed that temperature seems to play a major role in the problems I am experiencing. When it is below freezing in my shop, is when the greatest failure rates occur. Now that the weather has warmed up, it seems to have improved somewhat. But the lost steps still continue. I have successfully managed to carve out my name in a block of plywood, and rewound the Gcode (which I wrote by hand for the first test, after mapping the lines which form the letters of my name on a piece of graph paper, and assigning x, and y coordinates- think BASIC geometry and writing a text file from this map to import into Mach3). The repeatability was perfect! But that was at a time when everything seemed to be functioning perfectly (it was a warm day at the time). Sorry for the long post, but just wanted to give as much relavent info as I could. Thanks for your help!
Re: My CNC build
« Reply #6 on: March 09, 2009, 11:56:07 AM »
well from what i can say, the power supply and drives are limited for those stepper motors,
10TPI is a lot of gearing reduction and still you are not able to move reliably.

Although 10tpi qould require high rpm from the stepper to produce acceptable speed rates. You would need 500rpm to get 50ipm which i doubt you can get with noticable torque at those voltages.
Re: My CNC build
« Reply #7 on: March 30, 2009, 04:34:15 PM »
Re: My CNC build
« Reply #8 on: May 22, 2009, 12:26:59 AM »
By using a 6 wire motor in 4 wire mode you are putting the motor windings in series. This will make the inductance more than twice as high and the top speed will be way less than 1/2 a bipolar wiring.

With a 24V supply and 10 TPI you will have a vary  slow machine.

I have a Metronics board and it works OK but the 2A 32V max is a real performance killer.

The lowest cost cure might to be to get a unipolar driver board They are around $100.00. It will give you twice the speed and more amps at the low end.

I you have the money you should look at the GeckoDrives. The new G251 is a nice small driver that will run at 3.5A at 50V.

New 4 wire steppers will run twice as fast as your current 6 wire and will make more power. This stepper should give god speed and will run at 2.5A OK.

Re: My CNC build
« Reply #9 on: May 23, 2009, 04:15:18 PM »
I have since purchased a Gecko G540 driver board which supports up to 3.5a and 50vdc.  This driver did produce an improvement in performance using the 24vdc 6.3a power supply.  (But is STILL stalling/losing steps at times).  I am considering a higher voltage power supply, to use with the VEXTA PK296-01AA motors. I looked at the KL23H276-28-4B motors you recommended at Keling, but they are NEMA 23 motors, and my machine is built using NEMA 34 motors. I have read on this forum that 1200 oz in motors present performance problems, so I am considering the 960 oz in motors instead. However, having made the investment in the new Gecko drive (over $300) I am limited to 3.5a 50v motors.  Can you recommend a power supply/NEMA 34 motor combination which will work well with the Gecko G540 driver?