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Heated bed for Mini Kossel enlargement - advice needed

Posted by dc42 
Heated bed for Mini Kossel enlargement - advice needed
February 28, 2015 10:01AM
I am looking to enlarge my Mini Kossel to increase the build area. My plan is:

1. Increase the length of all the horizontal extrusions. I will stick with 15mm extrusion so I can use the same printed corner pieces.

2. Lengthen the diagonal rods in the same proportion as the increase in delta radius.

3. Larger heated bed.

4. Accept that the printable height will be less, because of the longer diagonal rods. I could increase the height of the towers instead, but then I would need longer belts and cables, and probably also 20mm extrusion instead of 15mm. I've never needed to print anything high anyway, my problem is with the XY dimensions.

For the heated bed, I intend to use a hexagonal aluminium plate about 330mm between parallel sides. It will be attached to the horizontal extrusions below it using six threaded steel pillars, one near each corner. That should ensure that it is level. I will place a 330mm diameter round glass plate on top of the aluminium, held in place using 3 or more foldback clips. The printable diameter will be 300mm.

Underneath the alu plate I will have a 300mm diameter heater, probably this silicone heater: [www.aliexpress.com]. I will use either mains voltage and an SSR or 24V, I haven't decided yet.

I have a few questions for the community:

1. What bed heater power do you think would be appropriate for this size heated bed? My estimate is 300W to 400W, but I don't have any experience in this area.

2. How does one attach a silicone heater to an aluminium bed? Is the supplied adhesive sufficient and capable of taking the heat, or does the heater need to be held in place by a clamp?

3. The heater doesn't have a hole in the middle, so the thermistor will have to go at the edge. Is this going to work well enough? What thickness of aluminium plate do I need to ensure the heat distribution is good enough to keep the temperature measurement error low?

Thanks in advance!

Edited 1 time(s). Last edit at 02/28/2015 10:03AM by dc42.

Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Heated bed for Mini Kossel enlargement - advice needed
February 28, 2015 10:29AM

If that Aliexprees list is the one I think it is then I tried to order from there and had the orders cancelled after 8 day's by Aliexpress themselves for not shipment so would be wary

I eventually got one from Keenovo of EBAY Heaterwas maybe a little more money but it has a built in Thermistor (In the centre of the heater) and the 3M PSA sticks like Sxxx to a blanket.

I have used a 400mm Diameter 5mm thick Ally blankblank also sourced from EBAY and he reckons he can cut custom shapes as well not the flattest tho good job I am using a 400 mm dia Candle Mirror on top of it.

Would try to make the heat spreader a little larger than the heater tho so that it doesn't interfere with mounting it level.

HTH Doug
Re: Heated bed for Mini Kossel enlargement - advice needed
February 28, 2015 11:25AM
Doug, thanks for that. I'll get a heater pad with built-in thermistor, now that I know such things are available.

I've done some sums and realised that if I increase the printable radius by 65mm (to get 150mm) while keeping the effector and everything else the same, the printable area will actually overlap the extrusions instead of being inside them. So I don't need a hexagonal bed to have 6-point fixing to the extrusions, a circular one will do. I want a 300mm printable diameter, with room for bed clips outside the print area and heater. So I am planning on a 330mm diameter bed and a 300mm diameter heater. This also leaves room for the bed support pillars outside the heater area.

Do you know a good source of compatible carbon fibre rods and Traxxas rod ends?

Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Heated bed for Mini Kossel enlargement - advice needed
February 28, 2015 01:41PM
What size are the Carbon Rod/Tube The Ebay link for Ally blank's will cut to whatever size you want.

Carbon Rod can be bought from Decathlon Sports shops (they sell it as Kite parts) or there is Carbon Fibre Products

Another source for Alluminium is Ally stuff There Ecocast Plate is superb for build plates.

and is excellent for Extrusion and bits for it (There Tee Nuts are very useful).

Traxas rod ends well there is a dilemma I know IGUS do some nice ones but Traxas need to come from a model shop you could try Traxas 5347 ends with balls looks like they come in packs of 12 for around £7.60

I am using Andy's Cherry Pi system on mine and that is 8mm balls with a 4mm hole blind drilled these are then mounted onto 6mm OD 4 mm ID tube (Carbon or Ally) and I have done a jig to get them all the same I use M4 x 16 Grubscrews and Epoxy to glue it all together
The balls sit in cups ont the carriages and effector and the whole lot is kept in place using Springs and Spectra line (Or similar I use stranded wire fishing trace line). Once you get the Hang of holding the effector in a certain position whilst feeding in the rods then it becomes easy and there is absolutely no backlash at all.

You could also look at andy Bigger slice of pi thread (This is what mine is based on) and this uses 20x40 extrusion for the towers and 20x20 everywhere else.

Also don't know if you have come across ESUN Filament but it is very good consistent and Very Cheap from ESUN Filament ABS is around £11 / Kg and PLA £12.30 / Kg (These prices vary daily for some reason ABS has been as low as £9).

Lot's of info there for you I am hoping that I can get mine fully operational next weekend (On Call this one) still waiting on some more extrusion to come from Motedis then I will be able to box it all in which will also help the rigidity of it and keep heat in for ABS prints.

p.s. sorry to be a pain but is there any news on a differential IR Sensor yet?????

anyway hope this little lot helps.

Oh some of the heater pads come with thermocouples installed rather than thermistors (my 400 x 400 is such a one and that is for the Core XY machine)

Re: Heated bed for Mini Kossel enlargement - advice needed
February 28, 2015 02:25PM
Doug, thanks for your input.

Regarding the differential IR sensor, I have given up on this, mostly because there is no standard size that will fit a wide range of deltas. I still have plans for a solenoid activated probe, which could be made to fit any delta just by extending the probe. I have a suitable lightweight solenoid already. There are a couple of options for the electronics:

1. Just a transistor to drive it, and an opto sensor to detect when the probe has been pushed up a little way. Electrically very simple, mechanically a little more complex in order to get repeatable accurate results from the opto sensor.

2. A microcontroller and a couple of transistors to drive it, and use the microcontroller to sense the change in inductance as the probe is pushed in. Mechanically very simple, but electrically more complex. Would be usable over a wider range of supply voltages, i.e. not just 12V.

One other nice-sounding idea I read about was a force sensitive resistor built into the print head mount, to sense when the head touches the bed. But the mechanics of that are probably beyond me.

I did wonder whether it was possible to detect head contact with the bed by sensing the difference in tension of the two parts of each belt, but I haven't worked out how to do this without introducing backlash.

Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Heated bed for Mini Kossel enlargement - advice needed
March 01, 2015 12:29AM
Hi Dave

I've just completed a core XY build and purchased a 400 x 400 silicone mat from a supplier on AliExpress. The mat has a built in 100K thermistor. It's mains (230V) powered and rated at 1000W. I control it with a 12V relay connected to the heated bed (D8) terminals on RAMPS. The mat is stuck to a 1.5mm aluminium sheet. The Z platform on my machine is built from 2020 extrusion to form a square of 440 x 440 which is covered by a 6mm MDF plate. On top of this are two 400 x 400 squares of 3mm cork mat, then the heated mat/aluminium sheet then a sheet of 3mm glass. This is the best set up I have ever used for a heated bed. It heats up in about 60 seconds and stays within 1 degree of my target temp over the entire print. My IR thermometer reads within a couple of degrees over the entire bed surface with the norm being that it is hottest in the middle and fades a little toward the edges. I've tried 24V systems in the past with 30A supplies. Mains is definitely the way to go but you need to make sure everything is cabled, fused and earthed properly to keep it safe.

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