CNC Y axis.

September 5th, 2016

This weekend I finished (mostly) the Y axis and base of the CNC machine. This is again made from L section aluminium extrusion. I really had to scrounge about at work to find enough bits to make it. I cut the angles on the drop saw again one lunchtime. I did the same as on the gantry and used angle pieces and small square plates in the corners to hold it all together.

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One thing that is really handy when working with aluminium like this is a very coarse file. I used the file I use when I do lead work on the car. The name of the file totally escapes me but it has a very coarse pattern, one side a curved cut. It works great on aluminium and doesn’t clog like a normal file.

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After bolting it together I made the base plate and the two lots of spacers that go between that and the linear bearings. The spacers are needed to lift the base plate above the edges of the extrusion used to make the base. After doing all that I drilled the holes for the rails and screw mounting again using a laser cut card template to mark where the holes went. It was only when I went to assemble things I found the template was wrong! I am not sure what happened but the spacing of the rails didn’t match the spacing of the moving bed.

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I suspect the issue happened when I drew the templates. I probably accidentally stretched something. Since the holes were only just off I couldn’t drill new holes and I didn’t want to drill or file the existing ones bigger so in the end I remade the base plate that mounted on the rails so I could use the slightly off rail holes. Instead of remaking the two spacers I just cut them up to make 3 sets of smaller spacers. I had been thinking about doing that anyway to help save weight so that worked out fine.

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The base plate is mounted with countersunk screws that sit just below the surface so that the top face of the bed is totally flat. I drilled six 8mm holes in it to be used as mounting points. The idea is to use MDF sheets with mounts that match the 8mm holes as changeable bases depending on if the machine is being used for laser cutting or 3D printing or milling.

The holes match the holes in the corners of the 3D printing heated base so my idea is to use 8mm bolts through a piece of MDF. The bolts go through the 8mm holes and are secured with nuts underneath. The tops of the bolts, where they come though the MDF, will be drilled and tapped for 3mm screws. The heated bed will then sit on springs sitting on top of the bolts with 3mm screws holding it down. That allows for height adjustment to level the printing base. It should become clearer when I make it I hope. I am rather making this up as I go along!

Since the base goes past the edges of the frame I needed to make cutouts to leave space for the nuts that hold my bases on. These were marked, then cut with a hacksaw and the bottom of the slot drilled so a piece could be broken out using pliers. The edges were then filed smooth.

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I also files grooves in the X axis to allow for the bolts heads there that hold the Z axis in place where they pass the edges of the frame. Having the axes go past the edges of the frame make the machines footprint smaller while still allowing full travel on the lead screws I have.

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It only occurred to me later that I could have used countersunk head screws there too and removed the need for the grooves!

I also bolted the gantry with the X and Z axes to the base. When I cut the base I deliberately made it fractionally wider than the width between the gantry uprights. This was to allow for any small variations in measurements. I wanted the base to be wider so there was a gap between the base and the gantry. If wider you can add in shims. But if the base had been narrower it would have been very hard to fix!

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I made the shims to fill the gap each side from some 1.2mm aluminium sheet.

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They filled the gaps perfectly. I still need to make another bracket that fits behind the vertical so that the gantry is held to the base in two planes and not just on the side. The extra mounting holes for that are seen above.

I also spent some time this week getting the electronics done. The thermistor for the heated bed arrived so I fitted that and I have had the bed and the extruder successfully heating. I even managed to squirt out some plastic but not very well as the bowden cable was all curled up and the filament was in a huge tangle on the desk. The feeder motor would skip sometimes. But I was able to extrude nice, even spurts of plastic. I also cut some glass to go over the heater. Really I should use borosilicate glass as it has the lowest coefficient of expansion. That’s hard to find here so I am just using ordinary thin glass. At least it is a flat surface to print on and the glass stops any damage to the heater itself.

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I need to get more of the metal clips to holds the glass to the heater.

I will need to measure the temperature of the actual glass I think. The bed thermistor is in the middle of the heater so the reading you get there isn’t necessarily the actual temperature of the bed. The temperature of the bed may not also be even, it is likely to be cooler in the corners. When everything is properly set up I will measure actual temperatures so I know how the shown value compares to the real world values. I plan to use a layer of cork under the heater to help insulate the base of it.

On both the bed and the extruder the electronics seem to maintain the shown temperature quite well, to within a degree or so. When it is all set up I can fiddle with the PID values (used by the software to maintain the temperature)  to try to make things as accurate as possible.

I still need to make a small power breakout board for running the various fans needed. The extruder has a fan that runs all the time. As do the electronics. You can also use a fan on the parts you are printing I gather but I am not certain of the details on that yet. It depends on what plastic you are extruding.

I have had all three axes moving but they need some adjustment, both mechanical and electrical (motor currents). I need to extend some of the motor wires too as the ones on them are too short. I bought some cable to do that today. I might see if we have the correct crimp pins at work for the motor connectors then I can make entirely new cables instead of splicing in new cable to the existing ones. It would be much neater. I also ordered some drag chain to help make the wiring neat. It is mainly needed on the Z axis as all the wires to that need to move.

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The above shot gives some idea of the size. The heated bed is a standard 200mm by 200mm one. I also drilled four 8mm holes in the base to work as feet and/or mounting points. Currently I have four 8mm bolts through the holes to act as feet but I am wondering if I shouldn’t bolt the whole base to some MDF to increase the stiffness of it. I don’t think that is necessary as there is movement in the bearings/rails way before the base ever twists.

Still to do is the limit/homing switches. I will probably mount them near the motors so I can keep all the wiring neat. This means the Z and X axes will have the limits on the positive end but the Y axis will be on the negative end. I am sure this is all configurable in software (I hope anyway!). I have an idea for making some off centre cams as adjustable microswitch triggers but I need to work out the mechanics of that first. Whatever I do they is plenty of room for mounting things.

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The other thing I did today was buy a new Dremel.

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These are very cheap at only $60NZ, I think because they only have two speeds rather than the multiple speeds of the other models. I am hoping I can use this as a spindle for light CNC work and PCB milling on my machine. I am not sure it’s going to be rigid enough for that (8mm rods actually flex a lot!) but as my old Dremel was starting to play up it was worth getting anyway. The first thing I 3D print, if this even works, might have to be mounting brackets so I can attach the Dremel to the machine!

It is mains operated but I think I can use a relay to switch the Dremel on and off based on the extruder signal.

All of this will become much clearer when I can make a small film of the machine hopefully working!

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