Hey John, hey all,
It’s nice of you to go straight to the machine and take measurements. But what I meant was not how much it sags due to the weight at a standstill, but the deflection under load.
A milling bit does not go through the workpiece as it does through butter. Rather, the force with which each flute lifts the chip creates a counterforce that wants to bend the tool. And by tool I don’t just mean
- bending the milling bit,
but also - bending of the spindle axle and the play in its bearings,
- deflection of the spindle holder, caused by play in the linear bearings of the Z rails
and last - the deflection of the X gantry on the X rails (perpendicular to the X axis), which would be a compound of bending of the X rails and of play in the linear bearings).
Here are some strong leverage forces at work, which multiply the force that acts from the workpiece’s resistance to chip removal and feed, and must not be underestimated.
I.e. the suspension of the spindle holder on the Z rails is the weakest point of this machine. This is where the greatest leverage is at work because the spindle is a certain distance from the Z rails. The above-mentioned radial forces on the tool, which arise during milling and feeding, exert a great deal of force on the linear bearings of the Z axes. These bearings and the Z rails themselves are not particularly large on Onefinity CNC.
Furthermore, a large leverage force acts on the deflection of the X gantry on the X rails. It may be funny when Ben Myers steps on the X rails with his son and shows that these 35 mm hardened steel tubes with 6 mm wall thickness only bend a fraction of a millimeter. But at the end of the lever, here: at the end of the milling bit, these fractions are multiplied by the length of the lever = at the end of the milling bit(!). And it’s not primarily about the rigidity of the hardened steel tubes themselves, but rather that every bearing has play, here: the linear bearings that run on these tubes.
The Onefinity CNC differs from the many available portal milling machines in that the Z Slider is rather delicate, if you have in mind the heavy Z Sliders with large plates to put you custom spindle holder on which you find in professional portal milling machines.
If I want to judge how it looks with the rigidity of this portal milling machine, I would install a measuring device that measures this under load, while it is working. Right now I’m thinking of how I’ll do it.
Did someone measure the deflection at a standstill by placing a dial gauge at the end of the milling bit, placed towards rear, and then grasp the whole spindle with the two hands an trying to bend the whole Z slider towards oneself and forth? What did the dial gauge show?
Forces that occur on the bit when milling can be quite large too.