Originally Posted by
joeavaerage
Hi,
while the calculation shows that 100W servos would work fine both Matec and I suggest 400W, for the extra few dollars why wouldn't you?...but that was determined
by doing the calculation with a particular ballscrew. If for instance you increase the diameter of the ballscrew to 20mm the calculation changes quite dramatically, it
less sensitive to pitch but none the less changes the inertia calculation also.
I would suggest you look very closely and make a decision about the ballscrews, you may have thought that 1000Euro for granite is steep, but just wait until you
price C5 and C3 ballscrews! Ballscrews are at the heart of the motion system, once you have those many of the other design decisions are simple and just follow along,
servo sizing for example.
When I built my first mini-mill I found 4 Koruda 20mm C5 screws second hand for $600USD, I got them and built around them. With my new mill I found
3 near new THK BNFN double nut 32mm C5 ballscrews and FK end supports for $1000USD delivered to New Zealand.....a steal. If I had paid for new ones I might have
expected $1500 to $2000USD EACH.
Given the price of ground (C5, C3) screws you're bound to ask about rolled C7 screws being 1/10th to 1/5th the price. You will point out, correctly, that they have an accuracy of
50um over 300mm, so with the small parts you are making the error is vanishingly small.....but its not quite so simple.
There are two pitch errors, the absolute error over 300mm but also cyclic error. A C7 screw can vary as much as 30um over just one revolution, and that's significant.
A C5 screw has an absolute error of 18um or less and a cyclic error of 8um or less, both specs are better, but they sure do cost!!! I regard C5 as the entry level
for an accurate machine. C7s would be fine for a plasma table or a wood working CNC router table, but for engineering parts C5 at least is required.
Craig