Originally Posted by
joeavaerage
Hi,
You are correct, my calculations are correct for static thrust, but the accelaration calculation is an approximation and in effect ignores rotational inertia.
With a very heavy axis the inertia is dominated by the linear acceleration of that mass, with a light axis then the rotational inertia of both
the armature and ballscrew and whatever gearing is employed become significant.
My understanding is that its common in engineering circles to 'translate' linear momentum to effective rotational inertia and that gives a more accurate
picture. My own training is in electrical and electronic engineering and so my mechanical engineering calculations tend to be introductory or first
approach affairs.
This may be an appropriate time for me to extend my knowledge in this particular field.
Such an approach would not be acceptable in a professional environment it is (ususally) sufficient to draw conclusions about a system, and that is
good enough for me.
But that makes no allowance for rotational inertia per the above nor does it allow for cutting forces. Note that when was calculating for the axes in
my machine I allowed half the thrust be 'consumed' by cutting forces, whereas the 1/2g calculation did not. Note also that I said its not too shabby....
but also implies thats its not that great either, in fact I would call it borderline. If it turns out your (or my) estimate of those countervailing forces
are underestimated then once you've spent your money only to find that it performs sub-par.
I paid nearly $3000NZD for my three servos.......a big, I mean very big chunk of my budget. I absolutely cannot afford to make a mistake here.
Neither do I believe I have...but time will tell. What I've found over several years I've been fiddling with CNC is that if you get it right and chose
the right piece of equipment even if its expensive, because its right youll use it and use it and use it again over many years. I've also had
experience where I'll get something which is underspecified or poor quality because I was trying to save money, only to find that it in no way
meets my needs and is in effect discarded. Despite being cheap to buy it is in fact the most expensive on the basis that I get no use of it.
You have the time and a forum in which to conduct your research such that you'll have a very fair idea, even if not 'iron clad' idea about
how your machine will perform BEFORE you spend your money. Use it well.
Craig