When to Reverse in the Mill?

[Ray C]

For any normal tool, you'll always want forward rotation -regardless of the direction of cut. With an end mill (for example) the edge of the flute must cut into the material. If it's spinning in the opposite direction, you'll just beat the tar out of the backside of the flutes and the tool is toast in a split second. For all I know, there may be reverse-spiral tooling -but I've never encountered it. In the case of indexed tooling (cutters with carbide inserts) they too have an edge side that must engage the material in order to cut. -And you can't simply reverse the insert because that would put all the force on the teeny-tiny screw that holds it down and it would probably cut loose in a heartbeat.

Only reason I've ever used reverse on a mill is to power tap left hand threads. As a mater of fact, left hand threads are the only reason I've ever reversed a lathe although you could conceivable flip a tool bit and cut on the other side of the workpiece. Doing that however puts upward force on the toolpost, compound, carriage etc and I doubt that's a good way to achieve good accuracy since, the entire setup was designed to work with downward force onto the ways.

How about to climb mill (or not) in both directions of travel; or am I just confusing things?


M

 

[Ray C]

LOL... All I know is that my toilets flush the same way as my mill spins. Maybe south of the equator things are different there and all the tool bits come left-handed.

Power tapping left-hand threads -and of course, it's handy to occasionally ruin a tool designed for forward operation.
:rofl:

Not you too!!!!!!
I hesitate to point out the problem of which side of the equator you live on. You know, the Coriolis effect.

 

[jgedde]

Have to be kind of careful here. Some mills don't do the quick reverse. I have run more than one that if you throw the switch in reverse at speed, it just keeps going the same direction. No good for tapping. You have to let it coast down a bit, nearly to a stop before it actually reverses. I suspect those are the motors with the centrifugal starting coil cutout switch, but I don't know for certain. Before you try to tap using this method, test it for rapid reversal.

Only three-phase and DC motors are instant reversing. It may work with a cap start motor, but don't count on it being a reliable behavior. This is because above a certain speed the starting cap is switched out of the circuit and the motor will simply continue to run in the same direction.

If the motor is plugged (reversed suddenly) below the switch cutout speed, it may or may not change direction, but you're putting a HUGE current through the starting cap which will shorten its life significantly.

Bottom line, 3-phase or DC motors and only with mills designed for instant reversing.

John

 

[jgedde]

I'm new to this but logic would say that you don't reverse... since the drawbar and other attachment may loosen if you spun a tool in reverse. I really can't think of a use for it. Climb cutting has more to do direction of feed you approach the too;l you can do conventional or climb with the tool spinning in the same direction. Please correct me if I'm mistaken there are some experienced guys in here.

There's a pin that engages the collet to prevent loosening due to reverse rotation. Nonetheless, I've used many mills where this pins has been broken off or is missing...

As far as reverse rotation is concerned, I've heard that many aircraft parts are made symettrical left/right. Left hand mills are used on one side's parts so that all cuts that are conventional or climb (as desired) as they would be on the other side.

John

 

[ranch23]

Only three-phase and DC motors are instant reversing. It may work with a cap start motor, but don't count on it being a reliable behavior. This is because above a certain speed the starting cap is switched out of the circuit and the motor will simply continue to run in the same direction.

If the motor is plugged (reversed suddenly) below the switch cutout speed, it may or may not change direction, but you're putting a HUGE current through the starting cap which will shorten its life significantly.

Bottom line, 3-phase or DC motors and only with mills designed for instant reversing.

John

Thanks for pointing that out, its important.

 

[09kevin]

Here is a picture of a left and right hand end mill, The left hand end mill is on the left. You can see if the spindle was turning forward (clockwise) with the left hand end mill it would be turning in the wrong direction to cut.


It is true that most airplane parts are as shown and opposite, but we use right hand end mills 99.9% of the time regardless if we are machining an as shown or opposite part.

Kevin

 

[hq308]

I've started using reverse a lot lately - the switch stopped working in the forward direction so I reversed the polarity and use the reverse side until I can get out and pick up a replacement switch. :lmao:

 

[4GSR]

The VFD on my mill doesn't like to be reversed instantly at higher speeds. But at very slow speeds for tapping does OK on instant reversing.

You need reverse for left handed drills.

 

[Metalmann]

"So what is the reverse on the vertical mill really for?"




The guys pretty much covered it all. Left hand tooling can sometimes be bought for a song, which helps out the shop economy.

On Ebay, sometimes you'll find LH drills and cutters pretty cheap, buying them new; can be very expensive.

Once you get enough experience power tapping, you'll never go back.

 

[Metalmann]

I've started using reverse a lot lately - the switch stopped working in the forward direction so I reversed the polarity and use the reverse side until I can get out and pick up a replacement switch. :lmao:

When I have that happen, I just take the switch off; bolt it back upside down,.....problem fixed.