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Bridgeport Series 1 MDI is a vertical knee mill made in the early 80s, capable of fully automated CNC operation, semi-automated MDI (Manual Data Input) work, as well as traditional manual machining using mechanically coupled handwheels - or any combination of the three within a single job. Factory-fitted mechanical handwheels are a very rare feature in CNC milling machines, which makes this this model rather unique and extremely useful for learning and prototyping.
The mill is made almost entirely of cast iron, weighs about 1500kg and provides a 457x305x127mm (X/Y/Z) working area, adjustable along the Z axis within further 365mm by moving the knee. Major cast parts are shared with the Bridgeport Series 1 CNC mill, which is equipped with a much more advanced control, but lacks handwheels for manual operation. Other mills sharing all or most major parts include Bridgeport BOSS, Bridgeport EZ Trak and early Bridgeport Interact models.
The spindle is driven by a 1.5kW 3-phase induction motor controlled with an INVT GD10-R2G2-4-B 2.2kW VFD and a Reeves-style continuously variable transmission system. Tools use a QuickChange 30 mount, which is a modified DIN 2080/NMTB 30 taper. The milling head/spindle assembly as a whole is a CNC-specific variant of the Bridgeport 2J2 milling head, mounted on a rigid ram, with a permanently coupled ball screw quill in place of the lever-operated classic Bridgeport quill.
The X (table), Y (saddle) and Z (quill) axes are driven by SEM MT30H4-44 permanent magnet brushed DC servo motors equipped with built-in tachometers and fitted with coaxial Heidenhain ROD 450B 125line/rev sin/cos encoders. The servos drive 5mm/rev ball screws through toothed belts at a 2:1 reduction ratio. Notably, the X and Z ball screws are stationary and the nut, integrated with a belt pulley, is being rotated around them. Additionally, the Z ball screw is a part of the quill, with the spindle encased within it.
The servos are controlled by a single Bosch Z15-1A SCR-based servo drive card operating in velocity regulation mode, with a PID feedback loop monitoring the tachometer output. A single servo drive imposes an obvious limitation: only one axis can be driven at a time, the two others being held in place with solenoid-actuated disc brakes to keep the ball screws from being backdriven. The Bosch board is due to be replaced by a set of modern servo drives.
The mill is controlled by a Heidenhain TNC 131 control module, based on the Intel 8085 CPU. TNC 131 is intended to be used as a Manual Data Input control, but can accept and output program data over RS-232 as well. It is, unfortunately, limited to straight cuts, being unable to interpolate arcs or even diagonals, which is why the machine isn't fitted with more than one servo drive board.
For manual operation, a handwheel on each axis can be engaged at will. Doing that couples it to the axis drive, cuts servo power and releases the disc brake on the axis. As the brakes are the only means of preventing ball screw backfeed with no servo power, it is not a good idea to engage all handwheels at once. Instead, they should be engaged as needed and disengaged immediately afterwards.
Additional features include a Bijur Delimon automatic oiling system, coolant reservoir and flood coolant pump.
TNC 131 requires a standards-compliant RS232 link with hardware flow control (lack of DTR/DSR will prevent any communication), use a good USB-to-serialconverter. Prolific PL2303 has been tested and confirmed to work properly.
import serial # Open serial port s = serial.Serial('/dev/ttyUSB0', 2400, parity=serial.PARITY_EVEN, rtscts=1, dsrdtr=1, bytesize=serial.SEVENBITS) # Send file to TNC s.write(open('file','r').read()) # Read TNC memory while l = s.readline(): print(l)
Transfer needs to be initiated from the TNC, for details see the TNC 131 Operation Manual, chapter E4.