Re: Rack and pinion

> I need 98 inches of travel to cut wood. I have seen other people use
> the rack and pinion. I believe it would make traversing w/o cutting
> faster than a screw. I am also worried about a screw wobbleing at
> higher speeds.
>
> Dex

Well, Dex, It depends a lot on what your trying to make or cut, and
the accuracies you really 'need' to do it. You mention "wood" cutting,
and under the assumption on my end that you are a hobbiest, you likely
not need be overly worried that you will not be able to do what you
want with Rack and Pinion.

So, I think your on the right track. Rack and Pinion can do just fine.
I use a larger Router with a Rack, as well as a homebrew plasma cutter
with rack (cheapy Mcmaster stuff), and there are some things about
both that I admire over my smaller screw driven machine. Neither has
any fancy techniques to control preloads, just tight fitting teeth in
a rack, and I really have no problems with repeatability or accuracy
for what I do with them.

I never have to do much maintenance with the rack setup, whereas on
my smaller machine with Kerk leadscrews, there are always things that
I do, like clean the screws and nuts. Garbage gets in them, and then
you get stiction, and a dryness that leads to squeaks.

To turn a screw without wobble as long as you indicate usually means
you need to compromise resolution, IE, more distance traveled per
turn, or get a really big diameter. I don't think I would touch
anything less than 2" dia at that length, or anything less than a 1"
"lead". (off the top of my head of course). And, that method would not
be inexpensive. Depending on your width, you may need to run two screw
setups adding even more cost.

You could use a smaller diameter screw with the proper "lead"
(distance travel per turn) and turn the nut, but you must consider
that you still have the costs of that long screw (or two) and
antibacklash nut(s), Plus you need to build the mechanism to do so.
It's not all that hard if you have seen some examples of it, but is it
worth it for what you need ?

In the end, rack is comparably very inexpensive. Rack will work fine
and you can fine tune your resolutions. You can also get your machine
to run rapid rates quite fast, and I assure you, if your stuck with
100-200ipm on something that large, you will get quite bored watching
it run.

What if the rack gets sloppy over the next few years ? Replace it ! If
you do rack right, it can be totally replaced, inexpensively one might
add when it does wear out!

I have not spent much time comparing manufacturers specifications on
rack, but I am sure one thing you will want to watch for is how
accurate various companies make their rack so you have an idea what
you can expect for "come and go" over a measured distance.

Obviously spending more usually would indicate a higher quality, so in
that regard, The REAL issue is what you NEED your machine to do, and
how much it is worth to you. No sense at all building a machine so
accurate that you can grind space telescope lenses, but, well, never
really ever need to do it.

My next large format machine project WILL use rack.

Chris