a
"Bill Schwab" wrote in message
nk.net...
Harold,
snip---

I arbitrarily set the dials to zero, and
settled for allowing half the nominal size of the endmill to establish
the dimensions. That was more than good enough, because only relative
hole positions were important on the part.

For the right and front edges, it might have been (or seemed??) easier
to switch to what you have termed left hand backlash. Is there a
correct way to do that? As it was, for each cut that went against
backlash, I backed up, overshot, and then stopped at the new position.

Unless there's a reason you need the dial to be direct reading from both
sides, all you do is reverse feed when you get to the end. Backlash, in
this instance, isn't an issue. Once you've measured the piece after taking
a cut, you have an idea how much must come off to take the part to size, so
you can then read the dial backwards. What that means is that you don't
expect a dial to give you readings on each end of a part. If, by chance,
you know exaclty how much backlash the machine has at a given point, you
could calculate that into your reading, but it's not done routinely, if at
all. If that isn't what you want to do, then how you detailed the cut is
how it is done. You always go past the target mark, then feed back to it
turning the handle in the direction you've set backlash. Using a DRO
eliminates the worry of backlash as far as dimensions are concerned, but I
wasn't afforded that luxury when I was trained. There was no such thing.
I learned to machine without them, and still work that way.

If you're cutting a window, and you need to know where the cutter stops on
four sides, what I do is measure and ignore backlash. Make a little sketch
of a window, and write the dial locations on each side---and don't worry
about backlash on the third and fourth sides. By measuring, you know where
you are, and can turn directly to your finished size. That's not a big
deal for one piece, but if you have multiple pieces to machine, once you've
marked the dials, you can cut windows in short order. With a final pass,
maybe a couple thou, climb milling, you can go into corners and out without
leaving a trace of undercut.


I suppose the same problem would arise if one wanted right hand backlash
but somehow needed to edge find on the right side of something. So
far, the best I know to do is backup, overshoot, and re-approach in the
backlash-safe direction. For an RT, I assume this will be necessary on
one side in each direction??

Depends on how you apply the table. I think you'll find that there are
times when you may set your backlash in one direction (as I detailed, fore
example) and be able to run your parts without worrying about changing
anything. One thing to remember is if you screw with the dials on the
machine after you've located the centerline of the table as it relates to
the spindle, you have lost orientation completely, so you have to start
over. I highly recommend you learn to deal with backlash as you outlined
it, just by overshooting it than turning back. If you establish that habit,
it's much easier to work, and you eliminate, to a large degree, the
potential for making scrap. Don't be too discouraged if at first you
screw up a little. Most of us do.


Please feel free to answer with something along the lines of "keep
trying to formulate that question".

Not necessary. In my opinion, that was a good question. The answer is
simple. Those of us that work freely with backlash do so because of
extensive experience. Some achieve the same thing instantly, while others
will struggle for some time before it becomes routine. It's just one more
of the things that separate machinists from the guy on the street. Like
any trade, these are the types of things we master in the process of
becoming journeymen.


You made reference to getting by with small table. How does one know
what is small relative to a given part size?

One simple rule is if you're cutting the exterior of a large item and the
table is smaller (envision holding the part by holes located within the
part, such as holes in a sprocket, for example). The mechanical advantage
can get out of balance in a hurry, and it becomes difficult to turn the
table while conventional milling, and down right dangerous if you try to
climb. Climbing on a rotab isn't a good idea anyway, but you can often do
so to good advantage by setting drag with the table lock(s).

My rule of thumb for a rotab is real simple. Anything less than a 12" table
isn't a good idea. I'm likely to get a lot of arguments with that
statement, but if you've ever had to do any serious work on a rotab, you
quickly come to terms with how damned hard it can be to make a setup. It's
often almost impossible to find a place for clamps, for one. The larger
the table, in most cases, the better. Keep in mind you have to handle them
to get them on the machine----so anything larger than a 12" becomes really
difficult. If the table is a compound, it's likely already impossible.
If, by chance, you have to use the table on end, size then works against
you, too. There's no perfect answer unless you happen to own more than
one, each a different size. Bottom line here is each guy feels he can do
everything he needs to do with the size at his disposal. I'd recommend
nothing smaller than a 6" table, but I'd hate to be tied down to one that
small. Maybe the best thing to do is visualize the projects you might
be interested in running, then see how it works out with clamps. The
typical rotab has slots @ 90 degrees, so you do have the added advantage of
a second set of slots as opposed to the typical mill table.

Thanks!

Bill
Better check what I said before you thank me, Bill. I have my own way of
working, and it isn't necessarily the same way as others. It works for
me, but it might not for you. At any rate, let us know what you decide.
Your opinion might be valuable to others in a similar circumstance.

Harold