Hi all.. continuing on with this very rambling topic..
Today we discussed motors. We're going servo as I said earlier, but the question comes up , how to connect the..ballscrews.. belts..ect..
The idea is to have as low a mass as possible to move, but with the rigidity required to get a good cut. Ive seen many designs, and we're not
close yet to deciding. Brian mentioned an interesting design where a timing belt is flipped inside out on a pulley and the pulley runs on a track with
a series of tapered slots on it. This woudl kill backlash problems as the rubber of the belt compresses into the track slots.. interesting.. might have to try something
like that. Ball screws are great and tight, but they do have a rotational inertia and that adds some acceleration time to the movement. Hi acceleration is the
ultimate target. Like all things in life, tradeoff's abound in the design of a router table. The stiffer the heavier, but the lighter the higher the acceleration,
the trick woudl appear to be getting just the right balance. Since we'll be using Servos that at fast speed are about 3000RPM, the perfect design is about 3000
rpm ratioed to give the highest speed we would want at the drive. The required acceleration then would simply be a function of the mass of the gantry. Y on the router tabel is
the long axis, so it will push the whoel gantry, its the worst one, the X is usually a cakewalk as it has less than 1/2 of the mass of the Y, and the Z is in a class of its own
as it has to lift or drop the weight of a carriage. Even with counterweighting, there is the mass to consider on that one. (Just because somethign weighs nothing, doesnt mean
you dont have to take its mass into consideration.. )
Id like about 12,700mm's per minute. Or about 500 inches per minute as my rapid. It may be overreaching, but we'll crunch some numbers and see. That means on a 4" pulley, Id need about
2*3.14*2 or about 1 foot per rotation on a 4" pulley with reversed belt, so an end speed of about 500/12 = 41rpm , so a reduction of about 75:1 woudl seem great under that scheme.
Gonna have to look around to see what I can find and make a compromise I suspect. More thought required. Just thought Id mention all this so you get an idea of what to consider in
some of the planning of driving the gantry about to get a better result. Ill keep you informed on the progress...
Art