View Full Version : bridgeport milling machine problem

09-07-2007, 12:49 AM
Hi guys - i noticed something funny about one of the bridgeports at school. I dont have much expierence with full size mills, so bear with me here.

Its a grey (boy that narrows it down doesnt it :D ) bridgy of medium stature and a variable speed drive. Its fed by two power sources and has a large electrical box that gets very hot when left on - it must be stepping down voltage for something...

Anyhow, with this variable speed drive there is an electrical control that says "spindle speed: hi low" which doesnt do anything besides shut the spindle off, and a spindle speed knob which also has no effect. Probably just never hooked up or something. Then there is a dial that changes speeds kinda like the variable speed dp and whatnot.

But here's the funniness - when you crank the wheel to a faster rpm the rpm reader drops lower. Its an "analog" rpm gauge. I say gauge instead of tachometer because its really just a sheet metal wheel with different rpm printed on it and as you crank the handwheel the sheet metal wheel turns and the rpm line up with a black reference mark to give you an idea of the rpm. But like i said, when you crank it to a faster rpm, the sheet metal wheel moves the wrong way. Whats going on?

Sorry i dont have more information for you guys - maybe someone knows what i'm talking about.

09-07-2007, 01:04 AM
The electrical switch is really just a Forward/Off/Reverse switch. "Low" being "Reverse". The lever on the right is what actually selects the backgear (i.e. "Low Range"). And since the back gear has no idler, it reverses spindle rotation relative to the motor shaft rotation. Hence, the electrical switch that simply reverses the motor when in "Low".

09-07-2007, 01:09 AM
Oh, and the other stuff, sounds like you may be talking about the power feed speed selector, which only affects the speed of the power feed on the spindle. That is like a 3 position thing on the left side of the head below where the switch is usually mounted.

Not sure on the "crank to faster, rpm goes down". I never paid attention to mine, I just crank it into the right general rpm range, then fine tune as needed. But your right, it's just an analog "gage" that help estimate relative speed changes based on it's rough calibration.

09-07-2007, 06:02 AM
If the spindle speed increases as the dial speed decreases, it may have been stripped and reassembled wrong.

The speed knob is connected to the vari-speed mech by a small chain-over-a-barrel type linkage, if its on backwards, that might cause the problem.

Download a manual and check it out.


Jim Caudill
09-07-2007, 12:23 PM
What Dave said, the chain that connects the crank roller to the thrust plate is looped over the wrong side. It can proobably be corrected by simply undoing the bolts where the rpm/crank roller is attached to the housing. By cranking the handle and manipulating the assembly you can get it to unwind and the wind it up on the other side.

09-07-2007, 12:26 PM
Thanks guys - i know its been stripped down because it actually came via pick up truck in pieces from a boeing factory. It was actually a donation from boeing so even though it has its oddities, it was a good deal :)