What's going on with shipping and the Z axis?!
What’s been going on with the lead-nuts?
The first issue was binding of the leadscrew nuts on the leadscrews. We sampled parts and vetted the manufacturer of the leadscrews and nuts before placing the final order, this was a manufacturer we had worked with in the past. However, when the parts arrived it became clear that the nuts manufactured for the large main order were not within specification. The nuts were significantly too tight on the leadscrews, and this would lead to either mild binding of the nut on the leadscrew or in bad cases the nut becoming entirely stuck on the leadscrew.
Obviously these parts were not suitable to be shipped. We hand sorted the entire stock of 800 leadscrews and nuts, we hand-picked pairs of nuts and lead screws which were within tolerance and ran freely and smoothly. We shipped our first orders using these individually tested pairs of leadscrews and nuts.
We complained bitterly to our manufacturer, and arranged for re-sampling, and re-manufacture of the entire order. We received our replacement order of parts, the nuts ran freely and smoothly on the leadscrews and we thought we were out of the woods and would be able to re-start shipping with minimal delay. Then we noticed that all 800 of the new nuts had been manufactured with M4 mounting holes on the flanges instead of M3 threads as ordered. Absolutely infuriating.
We did the maths, and figured that the it would take us longer to re-print and laser cut new parts to accommodate the M4 screws than it would to ship the nuts back to manufacture and have them modify the lead-nuts to add the correct M3 mounting holes. So once again we sent back the nuts and had the M3 mounting holes added.
What’s been going on with the Z-axis, why are some users seeing Z-wobble on their prints?
All the while we were shipping what we could, using hand-picked screws and nuts, along with some of the known-good sample parts we had received.
In the field we were seeing backers that were having issues with print quality due to Z-axis issues. This was confusing as the problems being seen in the field were not like anything we had seen before on all the printers we had built. Furthermore only some users were experiencing the problem, while others were able to achieve great prints.
Some users, like Olivia were able to eliminate their problems with Z by carefully tweaking and re-assembling their machines ensuring everything was aligned and tram. We took this as evidence that there wasn’t a hardware issue, and that the problem was simply due to things being not quite perfectly put together during assembly. We resolved that we needed to improve our documentation so that everyone assembling a machine would be able to align their Z-axis properly and eliminate wobble.
However our issues with the nuts had hidden something more subtle. We had been attributing Z-issues simply to the fact that the nuts weren’t quite right, and that new nuts with better documentation would resolve the problems.
We started assembling machines for those backers that opted for assembled machines and when we began printing burn-in and test prints we started to see similar issues to those reported by backers. This was despite using known-good nuts and leadscrews.
We adjusted, tweaked, swapped parts and printed more tests. Again and again. However we still saw the same issues.
BigBox relies on the bed, which is guided by 4 linear shafts on long LM8LUU bearings, to be free falling and smoothly lower as the lead screws rotate. We prototyped and built our early machines with standard silver-steel rod that we cut to size ourselves in house. Silver steel is a good option for prototypes as it is cheap and easy to cut to size, as well as being well toleranced for linear bearings. However it is not true linear shafting and has inferior precision and wear characteristics. So for the production machines we opted to use true linear shafting, hardened, ground, chrome plated, individually straightened and toleranced to a H7 fit, which is just about the best specification for linear shafting you can get.
Properly toleranced bearings and shafting have a slight pre-load. This means that the balls in the bearings are ever-so-slightly squeezed against the shaft to ensure precision of motion without play. This works brilliantly for the X and Y axes, the bearings have no play at all and run extremely accurately along the shafting. However on the Z axis this causes a very slight and subtle issue. The pre-loaded bearings have a slightly more snug and less free-running fit on the shaft, as the bed descends the bearings do not fall freely down the shaft, and require a small amount of force to begin moving.
On the X & Y axis this small amount of force is not a problem, as the motors on X and Y can easily push & pull the axis with a well-tensioned belt. However on the Z, this resistance to motion, though small, causes a slight deflection of the bed corners, resulting in a small amount of error as the bed moves down the Z axis. The leadscrews are extremely accurate in their motion, however the slight grip of the bearings to the shafts means that the corners of the bed do not always descend precisely in sync with the lead-nut. Because the print surface is attached to the corners of the bed this means that the motion of the print surface can be affected by the bearings not moving precisely in sync with the lead-nut.
The solution for this is twofold. The first aspect is that the rods must be extremely well aligned and tram (parallel). If the rods are not tram then there will be a radial (sideways) force applied to the bearings which results in the bearings moving less freely along the rods. To remedy this we have added the ability for the angle of rods to be adjusted as the machine is assembled. By running the bed up and down the rods as the machine is assembled the Z axis can be perfectly aligned and then locked into place with the new printed rod end caps.
Even with the rods perfectly aligned, the built-in preload of the bearings on the linear shafts still results in some grip between rods and bearings. In order to resolve this we have to reduce the preload of the bearings against the rods. If it proves necessary, we can send the rods back to our manufacturer and have them re-ground to a more free-running tolerance specification. We would be removing around 15-20 microns from the diameter of the Z-rods, and from our testing this results in a significantly more free running fit of the bearings onto the shafts. This means the bearings now fall freely with the motion of the lead-nut resulting in far more accurate bed motion, and results in improved print quality. Whilst this sounds like a daunting operation, our machinist is based in the UK and will turn around all of your rods in 3-4 days once we get them to him, resulting in no more than a 1 week delay.
We are in a position at the moment where we are fighting some very small issues and it is of course delaying shipment. We understand that people are really keen to get their hands on their machines, and delays are never a good thing. However we think it is worthwhile us taking just one more week to do these final tweaks as we feel we should be shipping the best possible machine we can rather than settling for ‘good enough’. We’ve come this far, and we’re at the final hurdle. It might take us a week to sort things out, but we’re very keen to resolve these final issues once and for all, and get the machines out to you as soon as we can.