Potential core z axis wiggle fix

I see a number of posts talking about wiggle and relatively little advice besides rebuild your core. I can tell you how I fixed my issues without reprinting and my findings as to the source of the issue… also how to make the core clamp tightening less of a process.

So the primo core is held by 4 clamps and each has three bolts, two pass through the core and one is on the outside and pinches each clamp. This is the one you’ll typically tighten to square the core. However, I was finding the clamps to not make great contact with the rail in places and tightening to make relatively little change or require what I considered far too much tightening. The result was a core I couldn’t perfectly square and too much wiggle.

The fix was to pass a drill bit matching the bolt through each of the 8 core holes that mount the clamps. It removed practically no plastic at all, but it did shave a small amount in spots and clean up the hole. This meant the bolts could be pushed (with some resistance) through the core now instead of needing to be twisted. This was not enough to be sloppy or wiggle the bolts around, just consistent and clean. I was finding that when these holes were too exact or tight, that the third bolt did very little to adjust the clamps as they worked too well at spacing two of the three bearings apart. My parts are printed in PETG-CF, so perhaps that extra rigidity is a factor there. On a well calibrated Bambulabs X1C with a 0.6 hardened head, if it matters.

Now as for tightening the clamps in a way that saved me some sanity… I inserted and tightened the third pinching bolt with the clamps fully removed from the core, laying on the table, snugged each down so that they didn’t freely spin with just your fingers, but didn’t put up much resistance to a hand tool and measured the opening of each clamp where the rail passes with a caliper. I tightened each (a very small amount) as needed so that the caliper measured the same on each clamp.

I now rebuilt the core hanging it on the rails with each clamp and each of the 8 core bolts only finger tight, not even past the nylon of each nut, completely loose. It was already feeling like an improvement. I snugged each bolt down starting with the top bolts on each clamp and then the lower. Snug being barely any tool resistance, but not spinning with my fingers. Tight enough each clamp could not be wiggled on the core. Once done, use your hand tool on each of the 8 heads without restraining the other side of the bolt and they should all feel about the same, turning pretty easily. Torque wrench if you have one with low enough range might be useful… but I did by feel.

At this point I had zero wiggle, the core moved easily on the rails and it was all square without any further adjustment (possibly luck, possibly consistency). My theory is that slight imperfections in the core holes are enough to angle or space the bolts such that the clamps struggle to do their jobs easily. Perhaps PLA is more forgiving, but maybe this write up will help others too, where rebuilds haven’t… I had personally rebuilt three times to find the exact same result before this fix.