Bridgeport CNC Conversion Recipe?

[Eric]

I am planning on converting a Bridgeport Series 1 J-head manual mill to CNC which is mainly for personal use on aluminum and mild steel (rarely do I machine SS). Since I have never machined using CNC before, I am not familiar with how fast the feeds are supposed to be. I keep hearing everyone talking about increasing their IPM and I am wondering how fast do I really need my machine to be (I have lots of time since this is not for production work). I am a wheel turner and I probably don't take deep enough cuts or fast enough cuts.

I am planning on doing the X-Y first and then worrying about the Z-axis later.

Mach2 = 45000 pps max
0.2" ball screws
2:1 pulley ratio
1.4KW AC Servo motors (496oz-in cont., 1600 peak torque)
2000 line count encoders = 8000 pulses per rev (quad)


IPM(max) = (Max Pulses/sec) x (screw pitch) x (pulley ratio) x (60 sec) / (encoder count)
45000 x 0.2 x 0.5 x 60 / 8000
= 33.75 IPM max

Is this fast enough for X and Y?

Is a 2:1 pulley ratio needed, or would 1:1 be better?

What are the recommended tooth counts for pulleys (16mm motor shafts)?

Thanks for any input.

--Eric

[balsaman]

What sort of drives will you use with those servo's? Can you use lower count encoders? I would say 33" per minute is not "fast enough", especially for rapid moves.

Another Eric

[NeoMoses]

In aluminum, on a bridgeport, you could be machining at 30-50 IPM or so with good carbide endmills. I would think 33 IPM might be a little slow, especially when considering 'rapids.' You might be better off with a 4000 count/rev. encoder, which would give you 20,000 counts per inch, or a command step of 0.00005". Not too shabby, IMHO. That would give you a max speed of around 67 IPM.

[Mariss Freimanis]

Keep in mind your 1,400 Watt servo motor would deliver only 124 Watts to the load at 337.5 RPM (Watts = 337.5 RPM * 469 in-oz / 1351), or only 8.8% of its potential.

To get full power from your motor, you would have to run it at 4,033 RPM and gear that down to a speed you can use. At 4033 RPM, the maximum encoder line-count would be 167. Pick a 200 line encoder to get close.

If you pick 180 IPM (900 RPM on leadscrew), the gearing would be 4033 / 900 or about 4.5 to 1.

The thrust would be 4,130 lbs at 180 IPM (Lbs = Watts * 531 / IPM = 1,400 * 531 / 180).

Mariss

[Eric]

I am using some Rockwell Automation F-4030 Sinusoidal AC Servo Motors with their Ultra 100 series 2KW drives. These motors have built in optical 2,000-line incremental encoder so I really can't change those. These are continuous torque motors which don't fall off until 4000 RPMs (if you look at their published torque curves).

I just found out that they have an electronic gearing option when using the Follower Inputs for step/direction mode. I think I can set it up with 4:1 so the motor is moved 4 encoder pulses for every incoming step pulse. This may help me get the motor spinning faster and get the IPM

[arvidb]

Power is proportional to torque times speed.

With SI units:
P [W] = M [Nm] * ω [rad/s]

or as Mariss said, in imperial units:
P [W] = torque [oz-in] * speed [RPM] / 1351

So if they are constant torque, when running at 10% of nominal speed you get 10% of max power, and so on.

Arvid

[mactec54]

Hi
The motors you have Ac servo what RPM do they run at, This and the Ball screw pitch will determine the speed that you will run at. The encoders have
nothing to do with how fast your machine will run, they control postioning and
accuracy that you will be able to cut the higher the resolution the better as long as your control and soft ware can handle it.This 1.4KW motor is more than
enough to go real fast I run 850w Yaskawa and can get 6ooIPM Rapid and cut
at 200 to 450 IPM for engraving At 600IPM the ballscrews are turning at 3,000 RPM which is the max for this motor you need a nice setup to achieve this
I will be posting some photos of this machine soon.