The Table.. continued..
OK, so I haven't badgered Bob yet to keep the old faithfuls wooden table, but Ill go on the
assumption he's willing to go along with that. Most of this little followup will pertain to either
a wooden or a metal table. I cant really say this is the best , or the worst way to go, Ill leave that to you
to decide. Of course, you may have decided to purchase one outright. SO before we discuss building a table,
lets talk a bit about buying one.
It disturbs me when I get support letters from people who have purchased a system and feel let down. While
I'm in no way responsible for that, I'm empathetic enough that I truly feel bad for them, typically it can be
allot of money, or even if not allot, it can be all they had for their new hobby or business. And I hear it all.
From the people that send a check for $25,000.00 for a system which never arrived, and will likely never arrive, to those that spend $5000.00
on a system that performs very badly, I ve seen many bad purchases. I don't judge others systems, unless I'm buying them, and Id ask you not to send me ad's asking me
if I agree or disagree with the design of someones system they are selling, I CAN tell you what I look for, (and again, what the hell do I know?)..
When I look at a possible purchase of a table, and Ive considered and drooled over a few, I look at several thing. How well is it build, and weight tells you something there.
Not everything, but weight is a good guide. Its typically heavy to build strong. Does it have bearings on the table where the gantry hooks up to the table? How good are they?
Linear bearing, like Thompson linear rails cannot be beat. Two pipes with roller bearing on them is probably not what you'd want. The thing here is the gantry you WANT doesn't
easily swivel out of square. Some have two motors to keep it square, others just rely on the bearing to do it. Having two pipes with roller bearings are unlikely to do that.
Linear bearings have car's, or carriages that do not easily swivel out of position, they stay parallel to the rails they are on, that means stability and less vibration.
This is an important thing to look for, there are three weak links in any table, the connection from table to gantry( Y axis) , the gantry's side to side carriage (X axis),
and the Z axis which resides on the X axis carriage. They should be pretty beefy. The best Ive used or seen is either a single beam being the X axis or a dual beam one on top of the other.
Having a beam in front of a beam with a router carriage mounted between the two beams should be avoided. Not that it cant be done properly, but it interferes with tool changing
and can be a bit of a pain. Again, Ill post photos soon of what I mean , showing what we've done on ours, (trying to keep expenses down), and we've been very happy with it over time.
Dive systems on a purchase table are important as well. The ultimate of course is good Ball screws. Take not a Ball screw is not just a screw. Ball screws can be recognised typically
by a barrel looking nut with tubes on it. The tubes allow ball bearing to race around the screw very tightly allowing no backlash and very tight coupling. Thats the ultimate.
Prices on those have fallen a bit over the years and where you can use one, I more than highly recommend it.
Second to that is an ACME screw. AntiBacklash nuts can be made or added , and they can work very well, and are very inexpensive. They do wear out faster than ball screws,
but depending on the performance you want, even an Acme screw can work.
Often, the X axis ( since it runs 4 foot or less across a table) can easily use a ballscrew, but the long axis (Y) of many tables would require a 8 foot ball screw,
thats hard to come by, so I use a belt. Some use rack and pinion. I actually used belts on both X and Y axis, I like belts, very quiet, not any backlash to worry about,
can be hard to find though with timing gears to match. We stole ours from XRay machines so I didn't have to search hard, but personally, Id like to have ball screws this time, so I'll
be lobbying for those or at least one of them with a belt for the long axis perhaps. (Anyway, this is about purchasing at the moment..
)
Plywood tops are nice, they can be replaced or used sacrificially, but many of the units out there have aluminum tops, even a user with an aluminium top will often bold plywood to it
to use as a protector as it can easily be replaced after a couple years of mistakes and cutting. I guess my point here is that almost anything else can be modified or fixed pretty easily,
but basic construction can not. Look for well built tables that show the engineering skill used in its design, usually, if it looks tough, it is. An dont discount used tables, they often
are the greatest deals in the world, as long as they were good to begin with. Show a picture to a friend in engineering, ask him where he suspects any weakness, Even 1/4" steel
can flex, and you don't want flex, so its always good to ask, unless you know enough to judge. Its your money, but Id use caution. Ive seen pretty bad designs that end up
being just about impossible to get running. Take specs as a general guide, they are often inaccurate as to maximum speeds, and they dont mention accelerations which is more
important than speed. Ill take a slow table with fast acceleration anyday over a fast table with slow acceleration, we'll deal with accel and speed when building later one, but if buying,
check it out well, its usually a big purchase.
What should I pay? ---
Good question. Only you know the answer. Ive seen tables from 1000.00 to 100,000 dollars, and the quality range is usually not linear with price, so again, check carefully with
your requirments in mind. I can tell you building one isnt cheap unless you root out the bargains. Our large table cost about 2000.00 to build and does great work, but Ive seen
systems costing $20,000 that I wouldnt trade for my 2000.00 table. (On the other hand I sometimes drool over a $3000.00 one..)
EBay is great for parts, for almost anything on a router table, but like a flea market, junk abounds, be sure of what your buying, unless its a good risk. Good risks are there.
I got 3 yaskawa servos for soemthing like 600.00 , AC brushless with drivers.. thats a good deal for all the servo motors and drivers a router table will need, and the seller was great to talk to,
so I called it a good risk. They worked great. Linear slides also go for great prices some days, and are very nice to have. (I swear by linear bearings). Never discount Ebay just because
the parts may be old or used,many of them come from factories with tolerance much higher than youll ever have, and a set of linear bearings for a hundred or two is a great start to a
system that is extremenly tight and accurate. And even if you dont think your going to want "Extremely tight and accurate, trust me,..you will..)
Ill keep this forum to Wood routing, but those reading it and wanting to build plasma systems can consider the differences that they need. For example, for plasma work, rack and pinion (R&P) is great
, R&P can also be used for wood, but its not my preferance, been there, found it just abit crappy, belt is much better in my experience. There are different types of R&P though, and Im told the better
ones eliminate backlash as well as wearing very nicely, probably great for wood, I just havent used them.
(Boy the people disagreeing with me are starting to build up now., Again, this is all jusy my personal experience..
)
Anyway, when buying a system consider some of the above points, at worst youll be better informed, at best youll save time and money..
(to be continued..)
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