Center assembly rocking...causing mayhem!

Mostly Printed CNC - MPCNC Troubleshooting
1

I have exhausted everything I can think of to the point of ultimate frustration so I am seeking you sage advice…

My Machine:
24x48" cut area
84oz Nema 17s all axis
400w Brushless spindle 12500 rpm
Interface is Raspberry Pi 3 with Protoneer Raspberry Pi CNC Hat and Pololu A4988 drivers
I use BCNC as my gcode sender
All Models and CAM generated in Autodesk Fusion 360
The machine is totally in square and the longer tube runs are center supported. I am using 23.5mm parts and 3/4" 23.5mm emt conduit.

Everything was working perfect. I ran numerous test jobs above the work surface without the spindle engaged. I even did a relief of my logo with only about a 1/16" depth running at 12-16mm in mdf.
When I cut plywood the first time set way conservative feeds at 5mm and 3mm plunge using the 1/8" diameter 2 flute Kyocera endmill from drillman. It plunged two holes and the third it seemed to grab the side wall, spun it self around, necessitating a mash of the emergency button. This has happened twice now and not in the same hole. The first time things were slightly loose on the rail carriages so i tightened them up…smooth as silk. I also noticed the center assembly was rocking slightly which seems to be why it is moving slightly off course on the plunge. The only solution that tightened it up was increasing tension on the long middle bolt in the center assembly. The problem is, if you crank the tension on this at all, it causes the leadscrew to bind!

Here is a vid of the movement:

Please help! Any insight you have would be welcome…

2

The only thing I can think of is take off the roller motor mounts and remove the whole center assembly. I know its a bummer but something wasn’t assembled right. Best to just re -do it.

Make sure the z axis is very straight, and the Stepper is mounted securely and square (I had one with a tilt and it messed with the threaded rod movement. Make sure the z rails are very parallel, put it together following the instructions, and check square and rail tension at every step. Make sure your nut lock is centered (it can move when tensioning the 5" bolt). I know its a bummer but it is worth it to get it right. I just made a new stainless steel machine and it is so rigid it has to be assembled very accurately because it will not flex to allow any corrections like emt will.

My suggestion is roll the tension (center bearings on the longer bolts) with each adjustment. It is a very clear indication of rail contact. Try and get all the bearing evenly tensioned.

The seems to be a common theme for some, putt it together what seems to be perfectly, use it a few times and it is all out of whack. We can only assume things have seated or however you want to word it. The second time around it seems to hold its square/tension better. Of the three I have in use I just checked and one needs to be readjusted, the other 2 are holding up very well.