Wednesday, 24 December 2014

Kossel Mini Variant - opening thoughts

I have decided that my next printer will be a delta printer, and although I have come up with numerous designs, the most recent one seems to be verging towards that of a Kossel Mini, so why not simply build a variant of one of these and have done with it.
Now I could buy a kit of printed parts for $39.99 (£25.76) + $11.81 (£7.61) P&P plus some additional import taxes, but decided instead that since I already have a 3D printer, lets stop being lazy and use that to print the parts and save a few more pennies.

The aim is to complete the build for under £200.00, ideally under £150.00, which I believe is very achievable compared to a purchase price of about $1125.00 (£721.15) for an off-the-shelf version.

Cost of PLA printed parts
FilamentCostTotal
Part(mm)(cm3)(£)Qty(£)
Effector5.139.812.40.17610.176
Frame Top (20x20)13704.133.00.46931.408
Frame Motor (20x20)34007.681.81.16533.495
Carriage5717.913.80.19630.588
Bowden Extruder13719.7330.47010.470
Total

6.137

Cost of Vitamins and Electronics

CostTotal
PartSupplier($)(£)Qty(£)
20x20 profile 600mmMotedis2.461.5834.73
20x20 profile 240mmMotedis0.890.5795.16
Slider for 20x20 profileMotedis2.711.7435.22
Carbon fibre Tube 5mm OD, 3mm ID, 180mm (6)Robotdigg5.003.2013.20
Tie Rods & Ball Joints (12)Robotdigg7.004.4914.49
GT2 Belt Loop 1350mm (675 tooth)Robotdigg2.401.5434.62
Geared Nema17 Stepper MotorRobotdigg28.0017.95117.95
Filament Gear DriveRobotdigg4.002.5612.56
Nema17 Stepper MotorsEbay12.558.04324.13
J Head Hot EndEbay24.9415.99115.99
SanguinololuIteadSutdio4.683.0013.00
ATmega1284PRS7.805.0015.00
Step Stick Stepper DriversEbay2.301.4845.92
F623ZZ BearingsEbay0.780.5063.00
Total104.97
I ordered some Sanguinololu boards from IteadSutdio a while back at $22.00 (£14.10) for 10 boards, the components, are made up from various bulk orders I have made in the past (thousands of resistors for $0.99, thousands of ceramic capacitors for $4.99, hundreds of radial electrolytic capacitors for $8.99, 10 packs of 40 pin headers for $2.99, etc), I boosted the total cost of board plus components to £3.00 to include some decent connectors, rather than just pin headers.

I don't bother with FDTI chips on the Sanguinololu boards, or the associated components, I either program them via the ICSP header if I need to burn a bootloader, or an external FTDI serial programmer if just updating firmware, I also use bluetooth modules for wireless communications.

M4 or M5 T nuts I buy from Robotdigg at $7.50 (£4.80) for 100, other nuts, bolts and washers I buy from Orbital Fastners in quantities of 100 or more. Something like M4x25mm Allen Socket Button Screws are £3.72 ($5.80) for 100, I just order multiples of 100 as I use them up pretty fast anyway.

As such I will allow a fairly generous figure of £10.00 for nuts, bolts and washers, bringing our total thus far to around £120.00.

I do not want to use a heated bed, so this will save huge amounts of power and I will be able to use one of my many surplus laptop power supplies as a power source.

The print bed will be an 8mm disc of acrylic that has been sanded on one side to give a frosted appearance, this should allow PLA prints to stick yet still be removable once the print completes - a 175mm disc of 8mm acrylic can be had on Ebay for £10.70 ($16.70).

I am intending to try the Acetal Slider from Motedis instead of linear rails, I am not aware of anyone trying this idea before, and I have no idea how well it will work - it may have way too much slop to be of any real use, but at only £1.74 ($2.71) per axis, they have to be worth a try.
If this does not work, then I can use their Acetal rollers instead, these are designed to run in the 6mm slot on the side of the 20x20mm profile, these are £2.67 ($4.17) each, and I would need 9 or more, so I think it is worth the shot on the sliders first, many people have made alternate carriages to run on similar wheels/rollers, so I should be able to adapt one of these fairly simply.
Then there is always the fallback option of Chinese Linear rails at $58.00 (£37.18) from Robotdigg for a set of 3 x 400mm rails and carriages.

Even if end up I trying all 3 options, the total cost should still be well below the target of £200.00.

Tuesday, 9 December 2014

Back up and running again

I decided the quickest way to have the printer back up and functional again would be to glue the broken Y belt holder back together again, then just hope it holds long enough to print a replacement.

I use Plastic Weld to glue PLA pieces together when necessary, it uses capillary action to reach all the tiny gaps to ensure a good join.
I paint it onto both sides of the joint, then paint over as much of the joint as I can after pressing the pieces back together and holding in place.

The Plastic Weld dries pretty fast, usually just a few seconds, but I decided for maximum strength, I would leave it overnight to fully cure.

Next morning I re-assembled the printer and tensioned the Y belt and all seemed to be holding ok.

I set off a new print of a replacement Y belt holder and cross my fingers that the print completes without disaster.

Well I needn't have worried, the print was a success and even after the print had completed, the Y belt had only loosened ever so slightly, so it is possible that the repaired part may have lasted for quite a while longer.

The new part has a much more dense infill pattern and 3 perimeter layers compared to 2 on the original, so should be much stronger.
The original part is on the left, the new part on the right.

I also decided to weigh the two parts for further comparison, the old part weighs in at 7g, the new part at 12g, so nearly twice as much plastic in the new part.

Additionally, the teeth have printed much better on the new part - they were somewhat "stepped" on the original, so hopefully the new teeth will grab the belt better as well.

To be fair, the first set of parts I should have concentrated on making is a replacement set of parts for the printer itself, that way if anything did break, I am not left high and dry.

I think I will print a few more of the smaller replacement parts - one of the top Z brackets broke before I even started to use the printer and has been mended several times already so this will be the next print candidate.

Tuesday, 2 December 2014

A Bridge too Far

I believe I sorted most of the X/Y drift by adjusting the MAX_TRAVEL_ACCELERATION_UNITS_PER_SQ_SECOND_X Y and Z values in the repetier firmware to be the same as my lowered MAX_ACCELERATION_UNITS_PER_SQ_SECOND_X Y and Z settings (1000, 1000 and 100).

The creep has not completely gone away but it is now down to one strands thickness, mainly in the X direction, every 10-20 layers, sometimes I can manage 40+ layers without any creep.

I have been using this new found lack of creep to print out the parts for the Revised Fully Printable Eggbot.
Here are the parts I have printed so far:
These have mostly avoided creep. The two idler ends needed opening out in order for the bearings to fit, this I did with a boring head on my mini mill.

Next I wanted to improve the X axis motor and idler ends as mine don't really hold onto the LM8UU bearings very well.
This thing looked to be a good basis for some new ends that would grip the outside of the linear bearings a bit better.
I modified the ends I had previously drawn in Sketchup.
I mount my motor on the opposite side to most people so I can have a sturdier frame with shorter X rods and still have the same available print area.
This Part was going to be an exercise in bridging no matter how I printed it, so I thought I had better do some bridging calibration using the Bridge torture test so I did not end up with filament hanging down in a mess.
The gap being bridged in these tests is 50mm, each test takes about 5 minutes to print (all test prints are shown upside down for comparison purposes).
The settings were as follows starting with the top left bridge and finishing bottom right, in two colums:

Test #TemperatureBridge Flow
12000.85
22000.65
32000.35
41950.85
51900.85
61881.00
71880.85
81860.85

The first bridge was using the settings I had recently been using for general printing, the best bridge is arguably the last one at 186C and 0.85 bridge flow, however, I feel that the temperature was a little low as it is also the most "stringy" of the various prints and had the worst bed adhesion.

Having settled for 188C for the print, I set it off and all was going well at first, then the printer seemed to be creeping in the Y axis which up until now had been pretty stable.

I halted the print after about 20 layers, as the print had moved too far and tightened the belt, then started again from scratch.

This next print was still creeping but nowhere near as bad as before, and then after about 65 layers, whilst I was having dinner, disaster struck, the Y belt holder sheared.
The printer just printed a mess from then until I stopped it wasting any more filament.
On closer inspection of the Y belt holder it looks like the person who made the part was using ridiculously low infill values as there is next to no material inside the part to give it any strength.
Needless to say, this piece or a suitable substitute are now up there at the top of the list of items to make once I can get the printer working again.

On the plus side, at least the bridging of the wide gap in the middle of the print went fairly well, with only a few mm of bowing to be seen in the middle of a 20mm wide gap.