Hi,
the minimum size of the ballscrew is that size required to preclude 'whipping' where the ballscrew spins that fast that it start getting out of line. There are some calculations floating around
that attempt to quantify it, but generally speaking the longer the ballscrew the bigger its diameter must be in order to avoid whipping. A related matter is the pitch of the screw. A fine pitch screw
will have to spin fast to meet your max velocity requirement and be more prone to whipping than a coarse pitch screw. The downside is that a coarse pitch screw requires more torque to achieve
the same acceleration and cutting force.
The maximum size ballscrew is that you can afford, and afford to buy a big enough servo to accelerate it. I've seen some guys thinking 'hey I'll get 36mm or 40mm ballscrews because they're so stiff,
only to find they need a 3 kW servo to spin them.
I have taken the approach that a fine pitch screw maybe a little slower but have greater cutting force authority......and so I have gone for 32mm diameter 5mm pitch screws. They are 648mm long
and will not whip at any realistic speed. In fact I spin them up to 5000rpm which give me a rapid traverse of 25m/min. The 32mm diameter means that they are stiff and accurate but require
decent servos to spin them, again I selected 750W.
Others favor course pitched screws, say 10mm or even 20mm. They argue that the screw need spin very much slower to get the required speed, and gearing could be used at need to increase
the torque of the servo to offset the greater torque demand for cutting forces and acceleration.
It is somewhat a matter of style.
The essential point is that the servo, any gearing, and the ballscrew must match the machine. If you think servos are expensive then you have not checked out the ground C5 and C3 grade screws.
One of the other contributors to this thread is of the opinion that some of the cheap Chinese made screws advertised as C3 and C5 grade while being less than a tenth of the price of the industry leaders like
THK, NSK, Bosch Rexroth etc. It is very seductive to believe that such high quality screws can be had at such bargain prices....but I disbelieve, I think is a case of the Chinese lying yet again.
Bear in mind that quality ballscrews are the absolute heart of any linear motion control system like a CNC. The better the ballscrews the better the result, and C5 and C3 screws (genuine) are worth way WAY more
than the servo that drives them. May I suggest you start doing your research on ballscrews now....they are commonly the most expensive piece of the whole puzzle.
C7 screws are rolled and have a linear accuracy of 50um/300mm. That sounds good but they can and do have cyclic error, and is uncontrolled but can easily exceed 35um/rev. If you are targeting 0.1mm
accuracy then these are ok. If you want 0.01mm accuracy they will not be up to it.
C5 and C3 are, and traditionally have been, made by grinding, and are mega expensive. A C5 screw is about 18um/300mm, which is better, but the real kicker is the cyclic which is 8um/rev.
C3's are better again, about 12um/300mm and 6um/rev. So both C5's and C3's are desired and required if you want accuracies of 0.01mm.
If a C7 screw, say 25mm diameter and 1000mm long is $100, a C5 25mm diameter and 1000mm long from THK or NSK will be over $1000, and C3's even more again.
If we as hobbyists could find C5 and C3 screws we could make much better machines, and I would advise caution before believing the advertising hype. Quality costs. BS is cheap.
I got my screws (ground C5's double nut, called BNFN by THK, 32mm diameter, 648mm long, 5mm pitch with two FK25 support bearings) secondhand from Korea. When I got them they still had the
original test certs....like they'd never been used, although they were advertised as second hand. I got three sets (screws/nuts/supports) for $1000USD including shipping to New Zealand.
Had I bought new they would have cost in excess of $2500 each. In short I could not have afforded to build my machine WITHOUT being able to get these absolutely superb screws at such a price.
I got new old stock THK linear rails and cars from the same company.
Once I secured these components (ballscrews and linear rails) I then designed my machine. I designed it to take advantage of what I could get second hand......I did not design my machine and then
try to find the really expensive and hard to find bits, I did the reverse.
https://www.ebay.com/str/industrialp....m47492.l74602
This place 'would put a horn on a jelly fish'.
Craig