Bridgeport Manual to CNC conversion...

[jamesgates1000]

Why don't you trade your Bridgeport in for a Bridgeport interact and convert that to Mach3?

[Davek0974]

Why don't you trade your Bridgeport in for a Bridgeport interact and convert that to Mach3?

Something like this???

http://www.ebay.co.uk/itm/Bridgeport...sAAOSwYmZXD2Iq

[jamesgates1000]

Looks like a pretty good deal, I have used them before with the heidenhain control and they are good machines. Best wait for some advice from the others regarding suitability for Mach3 conversion.

[Davek0974]

Luckily, I got the tape measure out last night.

I can't fit anything bigger than the standard Bridgeport in! That only fits because the head is between the roof trusses, so that rules out practically all others I think as even an Interact is 90" tall which i think is 12" more than a standard machine.

So my options now seem limited to converting my own BP, buying another BP and converting that then selling my on on or not bothering.

[Davek0974]

Just had a lengthy chat with my supplier of bits and they seem to favour heavily towards 8NM stepper-servo's for this task. The specs read well, basically a closed loop stepper and DSP drive with variable current etc. Price was about £250 per axis for motor and drive with cables.

Being stepper also means i can use my existing knowledge of BOB's and USB motion controller etc.


More options to throw in the pot.

[Davek0974]

Knowledge gap...

Stepper-servo motors, I know I need the drive plus a BOB plus a motion control - done that before and it works.

AC servo motors, I know I need the drive amp plus ? - what goes between the drive unit and the PC?

Just trying to price up, the price is identical between 8Nm stepper-servo motors and drives OR 750W servo motors and drives - I have no idea how to choose between one or the other, one I know the other I do not.

I am advised that 750W is ideal for the job so thats one decision, also the 8Nm step-servo is ideal it seems.

Anyone want to fill the gap?

I know the servo is rated to 3000rpm and the stepper to maybe 1000rpm?

The stepper torque is way down where it's needed but the servo is where, linear spread?

Looks like a ballscrew kit is going to be around £1300 so thats a fair chunk of cash in one hit, the rest I need to get right first time.

[JAZZCNC]

Ok well in theory there's no difference in requiremnets between Servo and stepper for connecting to PC. However in practise there is specification difference.

Servo's require much higher Pulse frequency and much cleaner signals. Now your average £5 Bob doesn't cut it and the Parallel port is just doesn't cut the mustard.

The speed and resolution you get from Servo is dependant on the encoder fitted. The encoder also determines the pulse frequency required.
For instance most Servo come fitted with 2500 count encoders which are quadrature encoders so this number is *4 so 10,000 pulses required for one revolution at full speed. If 3000Rpm that's 30,000,000 pulses in 60s. Break this down to seconds and that's 500,000 pulse per second or 500Khz required to achieve full motor RPM.
Now PP is only 25Khz or at realistic best 45khz 45,000 pulses. At best this means you'll only get 270rpm out of your 3000rpm motor.

Now most servo drives provide a pulse multiplier so one pulse becomes 2, 4 etc depending on amount set. This however comes at the cost of Encoder resolution which gets reduced by the same amount. IE: 1:2 Multiplier 10,000 = 5000ppr 1:4 2500ppr etc.
Not problem your thinking I don't need all that resolution anyway but it's not just resolution that suffers, so does smoothness of the motor and really the whole point of servos is for there superior Resolution and smoothness along with linear torque.

So bottom line is if using servos then really you need an external motion control card that gives you required pulses of good quality.

On more positive note there are other advantages to servos than just resolution and linear torque. 3000Rpm often is too fast so you'll use ratio which increases torque so smaller cheaper servo can be used.
Torque is linear and the rated servo torque is when at full motor speed unlike steppers which is when at stand still and drops quickly when rpm rise. Large steppers drop very quickly and not much use after 900-1000rpm.
Servo drives are powered directly from mains so NO PSU required.

If I was converting a mill then I'd use servos because they are much stronger than Steppers and smoother in operation. Torque is linear so much stronger when your pushing hard and deep.