Bridgeport upgrade

Another of the jobs that have been backed up waiting for completion is the motor upgrade for the Bridgeport clone (milling machine). Last year John Stevenson sourced a motor and adaptor plate for me to update my machine, subject to some finishing machining. The standard 4-pole motor is rated at something like 1.5hp / 1.2kW at 1400rpm. I'm driving it with a standard VFD, which allows me to overspeed the motor by 30-50% or so. Obviously by doing so, it is operating in the field weakened region (it's run out of volts), so can't develop the rated power. 

With a motor like this, the machine is possibly a bit underpowered for modern, high speed carbide cutters, so for some time my feeling has been that a higher power (how about 2.2kW?), 2-pole (2800rpm) motor might be a useful upgrade. John has been refurbishing and upgrading Bridgeport milling machine motors for many years, so I was pleased when a surplus motor came available.

Fast forward to today. I pulled out the adaptor plate and bored out the hole to size (it was plasma cut and required finish machining), chamfered the edges, drilled the 4 motor fixing holes in the milling machine (physician heal thyself etc), then removed the standard motor, ready to swap the pulley across. 

It's fair to say that this adaptor plate is about the largest diameter I could accommodate in the Bantam. With the carriage up next to the headstock, the clearance was there but you had to look closely.

First part of the job done - fits the motor and the bolts are in the right place. Time to fit and connect it up....

My machine is a Taiwanese clone, so uses a mixture of metric and imperial dimensions. Although John's motor was supplied for a pulley head machine like mine, it seems that mine differs from a normal BP. My motor had a 22mm shaft and 7mm key. His motor had a 19mm shaft and 6mm(?) key. Bugger. I've had a few minor upsets with this machine but this is not the end of the world (that's next week).

You can see the diameter difference here (and the different keyway width). The shaft lengths are different but that's not an issue:

So, out with the rotor, off with the bearings and get ready to build up the shaft diameter. 

The plan is:

• Use MIG welder to lay down steel, building the diameter up to >22mm.
• Support the rotor in the lathe using a fixed steady.
• Machine the shaft down to 22mm.
• Mill a 7mm keyway (will require me to refit the old motor - or use the CNC machine in manual).
• Drill and tap the shaft end for a retainer bolt to secure the pulley, as per the original.

Had a chat with John last night to see if he had any handy hints. Main suggestion that sticks in my mind is to run the weld beads around the circumference, rather than as longitudinal strips - to minimise banana distortion. I'd planned to do that but it's good to hear confirmation.

The process John uses is shown in this thread on the ME forum.

It's all set up now - what could possibly go wrong?

To be continued....