Ball Screws in Bridgeport mill?

I went to an auction recently and picked up a set of screws that turned out to be ball screws for a 9x42 Bridgeport.

So, do they have use in a manual mill and are they in any way better than regular drive screws? Or are they for CNC conversion only?

Unless I'm mistaken , the big advantage to ballscrews is that they have virtually zero backlash . I don't know any reason they can't be used in a manual mill .

I thought that the problem with ballscrews is that they can start rotating under pressure. So if you do, say, a climb cut, the endmill can pull the table towards the endmill.

Yes, ball screws do not provide much resistance. If you mistakenly leave the Y axis lock loose, (when moving the X), it can move the Y. Climb milling is also a problem.

Ball screws have been retrofitted on machines that were primarily used for precision boring many holes on long parts. The ball screws can have better tolerance over long lengths, and are less prone to wear (when properly lubricated).

Having had to use a CNC'd manual mill in manual mode, there is one huge disadvantage. Ball screws have so little friction that it is easy to drive X and/or Y axis' from cutting forces. First time I tried a decent climb cut I nearly had the handle torn from my hands, what saved me was the cutting forces moved the Y axis, coming most of the way out of the cut. Even a shallow climb cut in X can have the Y axis creep out of position. You have to lock which ever axis you do not want to move. I thought the zero backlash would be neat, but overall it was a real pain.

Jon

The difference in friction, rolling friction of the ball screws and the sliding friction of the acme screw is the problem using ball screws on a manual mill. The acme screw was designed to not rotate under load and therefore will hold a vertical load in place or from pressure from cutting forces. The disadvantage is the acme screw takes more power to turn it and also with the rubbing rather than the rolling friction of the ball screws, the accuracy of the acme screw is decreased much faster because of the wear in the nut and screw. If you use ball screws in a machine the axis must have some type of braking system on them or they will move. This a reason why when you move a cnc machine you must lock down all the axis or when the vehicle accelerates or brakes the axis will start moving and may cause considerable damage.

John

OK, thanks. I think that with a DRO, backlash is not even remotely a problem, so I should not even consider ballscrews.

Most machines with acme screws use two nuts with a way to move the two nut either closer or farther apart to take up the wear backlash component. Most bridgeports have two nuts or a double nut on the x axis. Almost all bigger machines have them.

John

You probably already know this, but the backlash can still be a problem, since the cutter can still pull the work toward it if you don't lock the "other" axis. This shows up when milling the four sides of a pocket for me. Maybe my mill is just too loose.

Pete Stanaitis

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Ignoramus25214 wrote:

And now I know a reason not to use them in a manual mill !

That is why you don't normally want to climb cut with a sloppy machine. You can lock the moving axis enough so that the cutter cannot pull it but then you will have to put more effort in turning the wheels and in turn put more wear on the parts.

For anyone that does not know what climb cutting or conventional cutting is, the operation is determined by which way the cutter edge approaches the work. Climb cutting pulls the work into the cutter and gives you a better finish. In conventional cutting the cutting edge pushes into the work and in turn takes up all the backlash and reults in an even feed since the cutter cannot be pulled into the work and cause the carriage to jump and in turn possibly break the cutter.

John

I agree in general.

My personal machine does not give me any problems with climb cutting.

You can also remove backlash prior to climb cutting. (move screw to the side that is being pulled by the cutter).

Simply anti-backlash they are.

Mart> I went to an auction recently and picked up a set of screws that

Nice find!

Stepper or servo motors. Note that the shanks have two spaces for keys -- one for the timing belt gear which drives it from the stepper or servo motor, and the other for the handwheel (with a spring-loaded folding crank handle, so you don't get beat to death when the thing starts up unexpectedly. :-)

The problem with ball screws in a manual-only lathe is that they are too easy to reverse drive. Replace your current leadscrews with these, and you can grab hold of the vise on the table and pull it to where you want it -- driving the screws from the table's motion.

A stepper naturally holds in specific positions -- like detents. A servo with no power is a drag on the screw, so you can use the machine in manual mode if you so desire.

Note that the X-axis ball screw used by Bridgeport in the Series I (BOSS-3 and later) does not rotate. Instead, the ball nut is mounted in a pair of opposed high-precision preloaded bearings, and driven by a cog (timing) belt. This way, you don't have the screw whipping during rapid motions. But -- you also don't have manual control of the machine at all (other than with the jog switches).

As for benefits of the ball screws -- consider near zero backlash, and very long life, as well as the low torque needed for the motor to move the table.

If you set up an adjustable clutch on each screw, you *could* use them for a manual machine, I guess. But normally that is too expensive -- unless you luck into a set of the screws like this. :-)

Oh yes -- also the Bridgeport Series I BOSS-3 and later have a large diameter hollow ball screw around the quill, and the nut is driven by a motor outside the headstock housing. This way, the Z-axis force is truly concentric with the quill, unlike the aftermarket adaptations which I have seen, which replace the depth stop screw with a ball screw, which provides the force off center, and I expect hastens the wear of the quill and housing in certain directions.

Enjoy, DoN.

I paid $35 for the lot. The lot included these two aforementioned feed screws and seven Bridgeport drawbars (one of which I now use on my mill, it has a taller hex part). Talk about deflation.

OK, I think that I can visualize it, after installing a Z axis power feed.

I think that this is definitely for a CNC application only, given what I know by now. I had a brief moment of insanity where I considered replacing my screws with these.

They are sometimes used on manual mills, subject to the free-wheeling others have mentioned. The mechanical efficiency is something like 90 percent for ballscrews, versus 50 percent for Acme leadscrews.

They are expensive and hard to find. Don't hold on to them if you're not going to use them.

Friction. Very low friction. To use them, you will Always have to lock every axis you are not operating

Gunner

"They couldn't hit an elephant at this dist..." Maj. Gen. John Sedgewick, killed by a sniper in 1864 at the battle of Spotsylvania

All, I have read all these comments in this thread and I not going to say they're wrong, but I do have a different tilt on the subject for what its worth.

Ball scews have their place in CNC and NC machines, but you never see them from the factory in manual machines and there are many reasons for that. The major reason is that they are not required and they really do not add value over an ACME screw and follower that is not worn out. Ball screws have a significantly faster pitch than the ACME variety. This makes the machine much more sensitive to tool load moving the table inadvertently. Most of the readers of this NG have no requirement for CNC, as those machines are generally used for long production runs. Now NC is a different story. Someday, when I grow up, I will buy one for myself. It would be nice to be able to do work with compound curves and scrolls. If you want to do that stuff, you should go out there and buy an NC machine, because there are a lot more differences between a manual machine and an NC one than just ball screws. If you try to make an NC machine out of a manual one, it will very likely never be up to snuff. I think you will find and it has been my experience, that buying the right machine is less expensive than trying to convert a manual one. Steve

Not to hijack your message or anything, but I wanted to say that I received that MT5 reamer and started cleaning up the spindle bore.