How many words have been written on how many forums about the lack of rigidity in the Seig X3 vertical column? I have a Seig which was converted to CNC using a Promica kit back in 2011. Like everyone else, I was appalled at its inability to machine steel and although I have done some reasonable work in aluminium I thought it was about time I tackled the problem in a serious way.
I have read all the posts I can find on ways to combat the problem and I have considered lapping the column, scraping the gib, fitting a rigid frame around the machine and I have fitted a counterbalance. However, the real issue is the fact that the column must be open at the back so the motion of the ball nut can be transferred to the head via the side plates. Given the three sided nature of the column and the fact that there is not a lot of thickness in the casting, it is almost inevitable that there is going to be some bending and torsion as the cutter does its thing. In my view, the obvious solution is to completely fill the column and the base with epoxy granite. So what to do with the Z drive mechanism?
Before the Promica website and forum was taken down (damn! I had a really good post on there about the motor upgrade I did on my machine), there was a post by another user about how he had fitted the stepper motor and Z screw on the outside of the column. The poster claimed it worked just fine and unfortunately the information is now gone. So, I had to reinvent the wheel somewhat and this is the solution I have come up with.
The stepper motor (a 270oz Nema 23) is now moved to the top of the column complete with its 2:1 reduction belt drive.
Attachment 249722
The Z ball screw is inverted and the ball nut is fitted to the left hand side of the head using the original side plate as a mounting point. A new lower bearing mount is fitted to the bottom left hand side of the column and a 200n gas strut is fitted on the right hand side of the column. The limit switch is now fitted at the top right hand side of the column. While I am doing the upgrade I am also fitting double ball nuts to all the axes to try to eliminate as much of the backlash as possible.
Of course, this upgrade is a bit of a one way street. There is no going back once the column and base are full of epoxy and rock! To hedge my bets a bit I did a very crude lash up with some G clamps and some fabricated brackets as a sort of proof of concept. The photo here shows the first trial.
Attachment 249724
To my surprise the system worked as well as it did before (in the original Promica setup). At this point I did not have the spindle motor fitted so I reassembled the .75kw 3 phase induction motor and its bracket back on to the head which now had a mass of 35kg and made some slight improvements on the ball nut mount and the top motor bracket. Once again everything worked as it should and there was no sign of the Z motor losing steps or stalling. Both these trials were done with the counterweight removed.
Somewhat heartened by the trial I set about modelling the new hardware I would need on my CAD programme. The old Promica bearing mounts would be scrapped and new aluminium alloy castings would be used to replace them. The ball nut mount would be machined from stock alloy and a new channel would be machined to align the double ball nuts which are preloaded by fitting two wave washers in between them and then locking their location with M6 grub screws.
When I was fairly sure I had the design right I printed ABS prototypes of the new fittings and mocked up the drive in its correct location to check for alignment and interference.
Attachment 249726
It all looked good so I am now in the process of printing patterns for the upper and lower bearing mounts.
Of course, lifting the head from a point that is not symmetrical with the centre of the ways will be criticised but if you consider how just about every lathe carriage is driven along the bed using a lead screw and half nuts below and on the front of the bed it is pretty much the same scenario. Provided the gib strip is properly adjusted I cannot foresee any issues and in any case, it can't be much worse than the situation at present.
My machine was retrofitted with a large steel counterweight running via a cable because I was concerned about the added weight of the 3 phase motor I used to replace the original Seig motor. I intend to replace the counterweight system with a gas strut rated at around 20kg to provide some balancing force from the opposite side of the column. It is a fairly straightforward fit and I was always concerned about the counterweight fouling something on the rear of the machine.
The other concern that will be raised is the thought of the now exposed ball nut and screw picking up swarf, coolant and other contaminants. Not a good mix! I have purchased a bellows type leadscrew cover from a Chinese supplier on EBay which will need to be cut in half so that it can be fitted above and below the ball nut via some laser cut discs which can be clamped to both ends of the bellows.
Attachment 249716
For any Promica owners out there who purchased the single ball nut kit, retrofitting a second set of ball nuts is not as complex as I thought it was going to be. The biggest issue was sourcing the Thomson 7820827 ball nuts which are now listed as discontinued on the Thomson website. I managed to purchase three new ones from a seller on EBay and there may be an alternative from a different supplier that will do the job. I considered replacing all the screws and nuts but the cost was prohibitive. The original system used by Marc Huett to pre-load the double ball nuts seemed a bit complicated. My solution is based on some ideas I had seen on this forum which made use of a machined alloy channel which simply keeps the two ball nuts from rotating relative to each other. Since the Thomson nuts have a body 1" square it is a fairly simple solution, I will fit two wave washers in between the two ball nut bodies to establish the pre-load and then lock them in place with two grub screws. Here is the mock-up.
Attachment 249720

So, that is the plan. I will post some further updates and images as the build progresses.