Lets assume you have a small knee mill that you want to convert to CNC. Lets also assume you have made the decision to CNC the knee rather than the spindle for the Z axis. Based on these assumptions you end up with something like this ...
diagram1.jpg (Size: 13.17 KB / Downloads: 166)
Note that the ball screw does not run. The servo turns the ball nut on the top of the pedestal causing the knee to move up and down.
A process check at this time suggests this is a reasonable path to go down, but there is a challenge based on the weight of the knee, which would include the table and anything on it. This could easily go over 400 pounds with 600 or more being realistic numbers.
A possible solution is an "assist" being added to offset some of the weight. Though you can not eliminate the mass you can at least offset some of the effects of gravity.
If you chose to use an "air assist" rather than a "counter weight assist" you gain flexibility in the ability to adjust the amount of assist via an air pressure regulator and do not add any mass as a counter weight solution would.
diagram2.jpg (Size: 16.91 KB / Downloads: 160)
It should be noted that the air cylinder is single ended as there is only a need to push the piston up and never a need to push the piston down.
Another process check at this time suggests you are still on a reasonable path, but now the challenge is to make it all fit under the knee.
Looking at diagram 2 you notice that there are two shafts going to the bottom of the knee, they rod from the cylinder and ball screw. It then occurs to you that (a) neither of the turn, (b) both of them are held firmly at the top of their container ... i.e. the bushing in the cylinder holds the rod firmly in the center of the cylinder and the ball nut holds the ball screw firmly at the top of the pedestal ... and (c ) they both go up and down.
So what would happen if you replaces the pedestal with the cylinder and replace the cylinder rod with the ball screw. You use nor more space that was originally used by the manual screw and pedestal.
diagram3.jpg (Size: 13.72 KB / Downloads: 159)
There are no issues with sealing the ball screw at the top of the cylinder as it is a single ended cylinder and the rod side is never under pressure. In fact it is the exact opposite as it has to be open to the atmosphere for the cylinder to work in any configuration.
There are not issues with the ball screw and the piston as the ball screw does not rotate, the same as the existing rod does not rotate.
There are no issues with the ball screw moving laterally at the top of the cylinder causing some issue with piston alignment as the ball nut holds it firmly in place laterally.
Which brings you to the end of your thought experiment with a very uneasy feeling that you have missed something really important. The solution in diagram 3 seems to almost be a "free lunch" with the assist and no space penalty. Yes, there are some implementation details but they do not seem to be formidable.
So what was missed? What intuitively obvious gotcha was not thought of?
Arvid
diagram1.jpg (Size: 13.17 KB / Downloads: 166)
Note that the ball screw does not run. The servo turns the ball nut on the top of the pedestal causing the knee to move up and down.
A process check at this time suggests this is a reasonable path to go down, but there is a challenge based on the weight of the knee, which would include the table and anything on it. This could easily go over 400 pounds with 600 or more being realistic numbers.
A possible solution is an "assist" being added to offset some of the weight. Though you can not eliminate the mass you can at least offset some of the effects of gravity.
If you chose to use an "air assist" rather than a "counter weight assist" you gain flexibility in the ability to adjust the amount of assist via an air pressure regulator and do not add any mass as a counter weight solution would.
diagram2.jpg (Size: 16.91 KB / Downloads: 160)
It should be noted that the air cylinder is single ended as there is only a need to push the piston up and never a need to push the piston down.
Another process check at this time suggests you are still on a reasonable path, but now the challenge is to make it all fit under the knee.
Looking at diagram 2 you notice that there are two shafts going to the bottom of the knee, they rod from the cylinder and ball screw. It then occurs to you that (a) neither of the turn, (b) both of them are held firmly at the top of their container ... i.e. the bushing in the cylinder holds the rod firmly in the center of the cylinder and the ball nut holds the ball screw firmly at the top of the pedestal ... and (c ) they both go up and down.
So what would happen if you replaces the pedestal with the cylinder and replace the cylinder rod with the ball screw. You use nor more space that was originally used by the manual screw and pedestal.
diagram3.jpg (Size: 13.72 KB / Downloads: 159)
There are no issues with sealing the ball screw at the top of the cylinder as it is a single ended cylinder and the rod side is never under pressure. In fact it is the exact opposite as it has to be open to the atmosphere for the cylinder to work in any configuration.
There are not issues with the ball screw and the piston as the ball screw does not rotate, the same as the existing rod does not rotate.
There are no issues with the ball screw moving laterally at the top of the cylinder causing some issue with piston alignment as the ball nut holds it firmly in place laterally.
Which brings you to the end of your thought experiment with a very uneasy feeling that you have missed something really important. The solution in diagram 3 seems to almost be a "free lunch" with the assist and no space penalty. Yes, there are some implementation details but they do not seem to be formidable.
So what was missed? What intuitively obvious gotcha was not thought of?
Arvid