Harold,

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.

Let me try to paraphrase. You are saying that I can count off the
desired size plus tool radius plus some wiggle room, take a cut,
measure, and then back up based on relative dial readings. Fair?


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.

I wouldn't expect that to work - that by no means implies that it would
not work.


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.

So far, I have found that I can see +/- 0.1 inch, and the dials do the
rest. It is true that I sometimes have to switch from a 0.5 inch
endmill to a 3/32 drill bit in order to be confident of the center of
the cutter, but that's really not that hard to do (have I said anything
good about ER collets today?g) and beats adding to my scrap bin.

For those not convinced by the "I can get by w/o it" argument, you might
better relate to my desire to get other tools, an RT, boring head and
bars, etc., not to mention more woodworking tools.

Another thing that bothers me about a DRO: they can skip. It sounds
like something that will start happening just about the time I grow to
depend on it. Living in FL, lightning is a concern. A direct strike
could knock out my motor, but a DRO would be much more likely to get fried.

There is also the problem of moving raw materials (mostly wood).
Renting trucks is a pain, not to mention that I do not like to drive
vehicles that do not belong to me. A pickup is probably in order. I
like my car too much to work on it myself, but a bomb of a pickup will
give me freedom to dabble in simple repairs (all together now - but NOT
THE BRAKES!!!!g).

Speaking of pickups, recommendations for make/model/year are welcome.
Gas mileage is not really a concern, because I will not drive it much.
Handling plywood will be a common task. Total cost to keep it reliable
and safe is a factor, and being friendly to an inexperienced shade tree
mechanic (with support from experienced friends) is a big plus. As
shocking as the concept might be in FL, AC is negotiable, and would
likely not be repaired. The one in my car gets what it need$ =:0

Sorry for the digression.


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 _think_ I understand. On the sides backlash-safe sides, how do you
account for the endmill radius?



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.

Understood and agreed.



Don't be too discouraged if at first you
screw up a little. Most of us do.

It hasn't discouraged me so far



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).

Thanks for the warning.


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.

I suspect that most things I would do would end up being mounted with
holes inside the part - all the more likely if I buy a smallish table.


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.

My shop crane would do it, but the point is well taken. OTOH, I doubt
anything bigger would fit on my 31.


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.

Maybe the answer is to get a small horiz/vert table and then see if I
find need for anything bigger. There is certainly no hurry. For now, I
can do what I have in mind with an extra setup or two and a rounding
endmill.




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.

I am grateful for your thoughtful reply independent of whether or not I
agree with every part of it. As it happens, I find that I end up
agreeing with most of what you say.

If I were to look for an area to disagree with you, it would probably be
over the validity of mill-drill machines. You are correct to caution
people about the limitations, but the machine is serving me well. If,
as some here have predicted, I end up with a monster knee mill, the $900
for the mill drill itself will have been well spent.

Thanks!!!

Bill