12X12in vmc build confused about servo or stepper

[jasminder]

friends,
I have started a mini vertical mill with upto 12X12 of x and y axis travel. Here is what i have:

25mm ballscrews from linear motion bearing (yes they are rolled but i loved their zero backlash, but i will be using double nuts. i have used 16mm version in my mini lathe and had no tolerances issue)

15mm thk lm guides from ebay for all the three axis.

But i am stuck with the electronics. Servos are recommended for a mill but very costly as compared to the torque ratings. here is what i am looking at
3 Aixs Nema 34 stepper motor DUAL shaft 1600oz.in CNC - eBay (item 190466647208 end time Dec-09-10 17:11:05 PST)

Servos with similar power rating will cost me 2,000 and kill the budget. cheaper servos (~ 300W) are available on ebay but they are too weak as compared to the steppers that are 1600 oz.

Any good VMC with steppers out there? The mill also have ATC. but lately.
Casting body is almost ready. waiting for lm guides and balscrews.

suggestions are always welcomed. I want to machine within +-0.0005.

jasminder singh

[jasminder]

hello friends,
i am wanting to add threading support to my cnc lathe. I am thinking about a 2hp delta VFD. But i don't want to spend ~100$ for just controlling it so i have simple plan. See if it can work.

The VFD needs 0-10 volts to run the motor and mach can output 0-5V pwm signals. because PWM signals cannot be converted to voltage easily, i have simple plan of using a ir led and receiver. The led will get pwm signals from mach and dim on less speed. The receiver will output high voltage when the led will be getting higher pwm signals (obviously a resistor will be in the circuit).
In addition to a controlled VFD i will get optoisolation from the PC to the VFD (everything safe). I have controlled a DC motor with mach PWM already so i guess it will work on VFD too. The circuit will cost less than 2$ and i won't have to wait for the cnc4PC card from US also.

Any comments?
jasminder singh

[dar303]

Hmm, the usual way to convert pwm to volatge is a low pass filer consisting of basically a resistor and a capacitor, even cheaper than $2!
Also, you may get pretty bad frequency response using the IR-led/receiver if it is not matched to the frequency of the pwm.
If you need optoisolation just use a good quality optocoupler befor your pwm-voltage converter.

ah, someone* has already done it for you, look here!


*(from http://www.cncathome.com/vfd.html)

[jasminder]

Hmm, the usual way to convert pwm to volatge is a low pass filer consisting of basically a resistor and a capacitor, even cheaper than $2!
Also, you may get pretty bad frequency response using the IR-led/receiver if it is not matched to the frequency of the pwm.
If you need optoisolation just use a good quality optocoupler befor your pwm-voltage converter.

ah, someone* has already done it for you, look here!


*(from My CNC Adventures)

thanks dar303,
for the schematic. Unfortunately this 4011d is not availabe here(didn't see it on ebay either). I have a pair of ir transmitter/receiver so i will give it a try if it works. Or definately would buy a 4011d from digikey or radioshack (import to india) . I don't think that frequencies will do anything to it. If the output rpms are not equal to input rpms, the pwm will change and light will dim or brighten.
It will be a very fast feedback system (I know the schematic given by you will work better) Still i wud like to try it with what i have. any online source for 4011d or any other working filter will help. Trust me i am very bad with electronics also.
just learned how to program avrs somehow.
thanks,
jasminder singh

[Ark1]

friends,
I have started a mini vertical mill with upto 12X12 of x and y axis travel. Here is what i have:

25mm ballscrews from linear motion bearing (yes they are rolled but i loved their zero backlash, but i will be using double nuts. i have used 16mm version in my mini lathe and had no tolerances issue)

15mm thk lm guides from ebay for all the three axis.

But i am stuck with the electronics. Servos are recommended for a mill but very costly as compared to the torque ratings. here is what i am looking at
3 Aixs Nema 34 stepper motor DUAL shaft 1600oz.in CNC - eBay (item 190466647208 end time Dec-09-10 17:11:05 PST)

Servos with similar power rating will cost me 2,000 and kill the budget. cheaper servos (~ 300W) are available on ebay but they are too weak as compared to the steppers that are 1600 oz.

Any good VMC with steppers out there? The mill also have ATC. but lately.
Casting body is almost ready. waiting for lm guides and balscrews.

suggestions are always welcomed. I want to machine within +-0.0005.

jasminder singh

I purchase 72 volt servos on ebay ( Keling ) that are about 900 inch ounces of torque. Price was $135, 90 volt 1100 inch ounce were $155, With a 3 to 1 belt reduction that would be 2700 Inch ounces. Do you have to driect drive your axis?

[jasminder]

I purchase 72 volt servos on ebay ( Keling ) that are about 900 inch ounces of torque. Price was $135, 90 volt 1100 inch ounce were $155, With a 3 to 1 belt reduction that would be 2700 Inch ounces. Do you have to driect drive your axis?

dear ark1,
it is not a good idea to use direct drive with servos. With steppers it is the best idea. Servos have better torque on higher speed, so pulleys as you said 3:1 is best. You said 135$ for servos. what did you get in 135 a servo with drive or just a servo?
If it is just for servo then i guess servos will kill my budget. 135+135+135+(gecko drivers) 150+150+150+(ps) 200= 1055$ +shipping
opps thats the double i can pay.

lets see if the luck is in my side this time. What u used for drivers btw
jasminder singh

[Ark1]

Just the servo from Motion Tek. I had to purchase the belt pulleys $60 and then the Gecko 320X $114 plus shipping. There are a lot of plus when it comes to dollars.

Ark1.

TonyK.

[mactec54]

jasminder

The servo system/package from Dmm will do the job for you & the cost is very good as well

Motion Control Products - DMM Technology

[jsheerin]

Servos have better torque on higher speed

No, servos have more power at higher speed. Power = torque * rpm. If you look at the torque curves for most servo motors the torque is typically flat up to some rpm and then drops off a bit. So if you never run the servo at full speed, you won't get the max possible power out of it.