Sounds like too much load on the motor. Did you grease/oil the gearbox? Did you check play in the bearings? I’ll bet it’s either the grease hardened up or sludge in the oil depending on which it uses, or the bearings are gone and need replacing. You could have a failed motor or drive but thats easy to check. With it at constant speed and some load (day 50% of speed) check voltage both AC and DC on the motor leads. If you got more than 5-10 V AC and I’m being generous here the drive has something wrong. DC Volts should be about half of nameplate

 

I checked the readings today while I was there.

Min speed= 5vdc
max speed=25Vdc

AC voltage=10Vac

Motor windings=.9ohms got to be normal, as the knee motor has the same resistance readings.

 

maybe bad brushes on the motor

 

Can you turn the hand feed crank easy, if not have them clean and oil the lead screw and ways. Last one I worked on had a bad motor, brush were oil soaked.

 

Any time that a DC motor gives trouble the brushes are the first thing to check.
Usually though, bad brushes will cause speed variation from set point or motor stalling.

I would guess that there is some mechanical problem loading down the motor. If possible, run the motor and do what is essentially a calibration. Actual current will depend on the actual load.
25% = 22.5 VDC = x current
50% = 45 VDC = x current
75% = 67.5 VDC = x current
The existing problem will probably not allow 100%.
The current increase, at each step, should be linear with the voltage.

Usually, but not always, a DC drive will either fail and not run at all or fail to 100% output. (This is why you always install a line side contactor.)

As best that I remember, the old Bridgeport mills had the DC drives as an integral part of the control board, so improvising a Dart or KB drive may be a challenge.

Good luck to you Sir.

 

Consider restoring your mill.

Tons of astounding Utube videos show how it's done.

 

There are great Bridgeport forums online. They also do not require you to have been a machinist for 4 years with a signed letter from your parents to post...lol

 

25 VDC At max speed tells me it is in current limit.

 

Tearing a DC motor down and servicing it is no big deal. Its probably full of dust from the brushes, Shinny or burn brushes and a dirty commutator. I believe the one you have is also in a oil bath at the front so oil may have seeped past the seal making a real mess inside.

 

10 VAC? Sounds like an SCR is gone or the drive electronics is. If you believe it’s just overloaded disconnect the motor mechanically and run again. If it works fine and AC is gone, it’s mechanical. By the way only use a true RMS meter for drive work, preferably with a low pass filter mode. Cheap meters give bogus readings on drives.

Look at KB Electronics. They will have a drive and a troubleshooting section. Or check the Bridgeport forums. Or look for a local industrial wood/metalworking shop. There is a good one off the 285 loop in Southwest Atlanta and one right off the highway in Dayton. Elsewhere I don’t know.

I mostly work on shunt wound DC motors (like a 500 HP one last week) and I get all the DC troubleshooting calls in our shop. They’re easy up to a point.
As long as you have a multimeter that reads say 10 megaohms with the drive powered off and motor disconnected you should read 10k to about 1 megaohm on the drive leads. Flip leads to test both ways. Higher than that or very low readings indicate a failed SCR (shorted or open) which is what I suspect. If you have a scope meter you can confirm by looking at the output...should be steady DC with no AC at all which will easily burn up a DC motor. Remove the inspection covers and check brushes. Also check commutator. Helwig has a really nice chart showing you what a healthy one looks like and what to look for. This is the basic first step on DCs something you don’t do on ACs.

If you need a drive, KB Electronics makes really good small ones. If it’s the motor any decent motor shop can rebuild them. Shop around and expect to pay way too much because other than brush work, rewinding a DC is a pain. It’s cheaper to buy a new motor, If that’s the case and you have the skill convert to AC, do it! A small micro drive and an AC motor is less than the cost of a DC rebuild if it needs rewinding.

Hard to troubleshoot if I’m not looking at one. Does it have 2, 4, or more leads? Any labels?

 

This particular drive, drives 2 separate, identical motors, at different times.

Via a switch on the spindle control box.

Table-Off-Knee

Running table motor at high speed, blows SCR input fuse.

Knee motor works fine.

You gave me some great ideas, I just dusted off my Fluke 87.

This might be fun.

 

Here are the results.

This is the drive in question

Low speed reading, higher speed and AC measurement, no motor connected.

 

So then I opened another Bridgeport there to verify my suspicions that the drive might be bad.

This one has what appears to be a KB DC drive in it.

 

Is this AC measurement normal, on a DC drive because it's not a smooth DC?

 

PWM DC and your meter is reading the fly back. On a old needle type meter it would look smooth. (i think)