OT Bridgeports

Dave H (MI) · Nov 13, 2021

I have watched a lot of videos about these old mills...mostly to kill time. I see them around but know nothing about them and wanted to learn I guess. Anyway...the ones with the variable speed control where you turn a dial on the side of the head. All the videos say NEVER TURN THIS DIAL UNLESS THE MACHINE IS RUNNING! They never say what will happen if you do. That's my question. What will happen if you do? Anyone know?

REALLY know?

Dave H (MI) · Nov 13, 2021

Dial? More like a round crank type handle...whatever you want to call it.

timcasbolt · Nov 13, 2021

(quoted from post at 13:37:09 11/13/21) I have watched a lot of videos about these old mills...mostly to kill time. I see them around but know nothing about them and wanted to learn I guess. Anyway...the ones with the variable speed control where you turn a dial on the side of the head. All the videos say NEVER TURN THIS DIAL UNLESS THE MACHINE IS RUNNING! They never say what will happen if you do. That's my question. What will happen if you do? Anyone know?

REALLY know?

'm not positive about this, but I believe either you can damage the belt or get it out of position requiring some disassembly to realign.

tim s · Nov 13, 2021

It will Pinch the belt...

Dave H (MI) · Nov 13, 2021

Thinking there is more to this than pinching the belt. They all give a pretty stern warning. I'm thinking the belt is just the snowball you start rolling down hill. Maybe something worse will happen down the hill...

Gary Reif · Nov 13, 2021

I've operated one of these for many years and they work like any belt drive variable speed drive on the older combines and other farm equipment. What they do is squeeze the sides of one belt pulley to force the belt to ride up while pulling the belt deeper in the driven pulley which of course changes the spindle RPM's. If the pulleys are not turning it is very hard to get the belt to ride up on the diameter of the pulley. Hope I didn't make this to confusing.

chuck machinist · Nov 13, 2021

The belt rides on a split pulley, and when you turn the knob you are spreading the two pulley halves. Its not good on the motor if you run it all the way out to high speed.. It is an excellent machine, and well maintained. Chuck machinist

Janicholson · Nov 13, 2021

Variable speeds on these (and other machine tools are exactly like the transmission in snow mobiles. There are two pulleys and a belt. Both are large 8 inch or so in diameter, and split so that they can change width (axially). One pulley is spring loaded towards pinched together, making the belt run on its outer diameter. The other pulley width is controlled by a throwout bearing like arrangement that uses a screw and crank to adjust the width mechanically. The belt is sized to be 1/2 way up the diameter of both pulleys when the speed ratio is 1:1. The crank is turned to set the speed by making the width adjustment to suit the speed required within the range allowed by the pulley sizes. Speed reduction has the driven pulley big and the drive pulley small. if not running, the action causes the adjustable pulley to open up loosening the belt and possibly causing no drive. If adjusted the other way, it pinches the belt and can cause the pulley or mechanism to be under strain it was not designed for, possibly breaking it. I have 4 machines with this drive, and thousands of students have operated them none have broken. All have large signs dictating the procedure, and demos are given as to what and why. Jim
Variable speed drive

Janicholson · Nov 13, 2021

See my comment below. The Bridgeport mill, and its clones from other quality companies, are solid machines with no real grief, Abuse and poor maintenance are the only factors that ruin them. Almost all manually operated mills have backlash in the axis screws. A few have ball screw systems with no slop, but rare, The normal backlash (thousandths of an inch of lost motion when the axis wheel is turned) is between 10 and 25 thousandths. Backlash may be judged best in the center of the X and Y travels. that is where screw wear is going to be greatest. Normal milling is pushing the work into the oncoming cutter edges. All milling on a manual machine should be done in that direction. Climb milling can pull the work into the backlash and break tooling or ruin the part. Normal milling is the same as using a table saw on wood. Climb milling is like feeding wood into a table saw from the opposite side. Not good. Jim

paul · Nov 13, 2021

Well after the belt pinches and you continue to crank on it, something has to give. Either you stop cranking, or something breaks.

So, the advice is - dont do that!

Paul

moresmoke · Nov 13, 2021

Because it says so!

Bob Harvey · Nov 13, 2021

I have a Shopsmithh, that is a precision machine and variable speed. There is no warning label, but I just haven't ever done it over the last 30 years, based on a 'smidge' of ??? Would never do it because !

Dave H (MI) · Nov 13, 2021

You sound like my doctor.

Dave H (MI) · Nov 13, 2021

No, actually, I have a combine and repaired something like that once long ago. I get it.

Dave H (MI) · Nov 13, 2021

Excellent info Jim! Must have been doing my homework because I understand everything you are saying. That doesn't always happen for sure! Thanks!

560Dennis · Nov 13, 2021

Pinch belt in the sheaves, turn it on and it will snap with the preload

tim s · Nov 13, 2021

Yep....

DMartin9N-2N · Nov 13, 2021

Good evening, Dave H: I am just browsing through the various threads tonight, the discussion on variable speed drive sounds like a John Deere variator. I think many machines have used the same principle over the years.

Dennis M. in W. Tenn.

mikeb789 · Nov 13, 2021

You can also get the older bridgeport that has stepped pulleys like a drill press has.

timcasbolt · Nov 13, 2021

(quoted from post at 22:39:43 11/13/21) You can also get the older bridgeport that has stepped pulleys like a drill press has.

nce you've used a variable speed head, you won't want to go back. Especially if you're using a phase convertor.

Fritz Maurer · Nov 13, 2021

Why ? How does a phase converter affect either
one?

timcasbolt · Nov 13, 2021

(quoted from post at 23:23:03 11/13/21) Why ? How does a phase converter affect either
one?

he machine really groans when you start it up at a high speed. I've used them with a static convertor, a rotary convertor and at work where we had regular 3 phase. It's worse with a convertor. With a variable speed head you can start it up at a slow speed and crank it up after it gets going. I don't know if the groaning hurts anything, but I try to avoid it as much as possible.

M-MAN · Nov 14, 2021

These variators have been around a long time. In the textile industry we used them to control the warp let off speed. In the late sixties they were added to the front side to control the cloth movement (ppi). In the latter we used a metal chain that looked like an old engine timing chain. It ran in oil. Now a day's they are being used in automobile transmissions.

OT Bridgeports

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