Re: [CAD_CAM_EDM_DRO] Climb milling was feeds and speeds.

OK, after a little thought (after Jon's explanation) I figured it out.
Say I have a .10" slop. I crank the hand wheel clockwise and I'm
eventually drawing the lead nut toward the handle. All is cool.
If I stop however, I can still grab the table with both hands and give
it a tug and it will still move towards the handle by almost as much
as the backlash.

This is what can happen climb milling. While I'm drawing the table
toward the handle, the cutter can take a bite and move the table still
more towards the handle by as much as the backlash. Bad.

Thanks. Needed a little nudge.

-G

> Woody wrote:
>
> > Jon Elson Wrote:
> >
> > >
> > > You can't climb mill safely on machines with sloppy leadscrews.
> > >
> > OK. This is something I was taught in High School (ok I was taught
> > NEVER to climb mill)
> > But nobody ever told me WHY. I THINK I can understand the change in
> > forces in the system but not exactly how they relate to a sloppy

leadscrew

> > and why a tight ballscrew is OK.
>
> The rotation of the milling cutter will draw the work into the tool. If

the

> leadscrew is tight, and there is a hand on the handle (or a CNC control)
> to keep it from accelerating into the cutter, then things are OK. With

(manual)

> ballscrews you must not take your hands off the handles, because the

ballscrew

> is not self-locking, as the 5 TPI Acme screw is, and it will offer no

resistance

> to the work leaping into the cutter.
>
> Even with a CNC machine with sloppy screws can't be trusted to climb mill
> safely. Either manual or CNC, if there is more backlash in the screw than

an

> acceptable chip load (feed per tooth) then the work can jump into the

cutter

> with disasterous results, such as the cutter exploding, the work being

ripped

> out of the vise, the machine knocked out of alignment, spindle stalling,

or

> all of the above at once (which I have seen myself!)
>
> With small backlash, even if the work jumps into the tool by the amount of

that

> backlash, it might only cause a small surface finish change, as long as

the

> backlash is less than an amount which the cutter can safely handle on one
> cutting tooth. That could be .010" or more for a 1/2" end mill on a

Bridgeport,

> to .001" on a 1/16" end mill on a Sherline. (On the smallest end mills,

the

> cutting forces are not enough to pull the table against friction, so they

are

> rarely a problem.)
>
> I always climb mill a finish pass, at least, even with a manual machine,

because

> the surface finish is much better. You have to be aware of what cuts will
> produce enough force on the table to possibly overcome friction.
> Now that I have a pretty tight CNC machine, I do almost all work in
> the climb direction.

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