Kossel mini 170mm-200mm scale-up

Hello everyone!

I've been given the task of building a pair of delta style printers, and I believe I have a nice build list that ought to work rather well.

Anyway, it seems there isn't much in the line of PCB heatbeds around 170mm good for 24v or more than 100-120w. That's not going to work for me, I'd like 24v and rapid heat times! I believe I've got my bases covered as far as ramps, the arduino, and what it takes to run 24V safely or at all.

I've found a decently priced "MK3" 220mm/200mm heated area 12/24v, seems to be very similar to the aluminum bed I use on my i3. Instead of wasting all the extra area and letting it hang over the frame, I'd rather just scale it up a teeny bit. I found a google groups delta frame calculator and ran some numbers, and I think I have something that ought to work...

To get a 205mm diamater build area, I would need 285mm long frame sides with 228mm long rods. I will leave the same frame height +/-10mm, which means I will loose ~25mm of height unles I can shift the rails up or something. Either way would be fine, ~200mm tall is more than enough. The extrusions I order will come pre cut to 610mm, so it's there if I want it. I plan on using some nylon open ended carrigages I found, with built in belt tensioners.

It seems carbon rods for Kossel minis are ~180mm normally, PHSA4 rod ends are 24mm from the center to face. Together, that makes 228mm rods like I need. Three pieces of 20-2020-915 and 20-2020-610 each for the frame, I have a tormach mill so I can mill all my openbeam perfectly even and true after cutting it, plus I can mill or print a simple jig for accurate rod end assembly.

So, I guess my question is if my research has lead me in the right direction as far as the frame size. Assuming everything is cut square and true, will a Kossel mini really loose that much rigidity scaling up 30mm in diamater only?

Thank you for reading, and let me know if you would like to see my somewhat current build sheet!

I scaled a Kossel Mini up to 300mm print diameter. It is still rigid enough, although ideally I would have increased the parallel rod separation. So you should have no problem with 205mm build diameter.

In my delta build blog (see link in my signature) you will find a link to a Chinese company that will make a silicone bed heater to your size, voltage and power specification.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Ok cool, it's good to know it'll still be sturdy enough.

How did you calculate the lengths of all your rods, I would like to check the spreadsheet I found against a few other sources if I can. Do you think scaling up 30mm would necessirate spacing the rods?

I found a very nice 170mm/200W keenovo heated mat for around $38.00, not bad for 0.8w/sqcm. Unfortunatly I'd still need to buy an aluminum surface, another $16.00, and it would still be the small 170mm build size.

The MK3 200mm aluminum bed I've found is $28 and won't need an aluminum surface, so I can build to the bigger size, not have to custom order a 24V keenovo mat, and still come out a bit cheaper in the end. If I'm not mistaken it's designed to use around 120W on 12V (1.0-1.2ohm) or 240W at 2.4ohm 24V, so I should be able to get to 110c fast enough. I plan on using a 17A 400W supply, just because it's chinese and probably under-rated. I think this all ought to work well, we shall see!

Edited 1 time(s). Last edit at 11/01/2015 09:04PM by deezums.

It's still 120W at 24V. ( The resistance then is 4.8ish ohm )
At 12V you run two traces parallel, each has a resistance of 2.4 Ohm. Results in 1.2 Ohm.
At 24V you run the traces in series which results in 4.8 ohm.

-Olaf

Not to steal ypur thread.
But what determines the max build bed?

If you get longer rods does that mean you can print bigger then 170mm?

Is there a formual somewhere.

Thanks for any help

Quote
xile6

If you get longer rods does that mean you can print bigger then 170mm?

Yes, but mounting longer rods means less build height.

Quote
o_lampe
It's still 120W at 24V. ( The resistance then is 4.8ish ohm )
At 12V you run two traces parallel, each has a resistance of 2.4 Ohm. Results in 1.2 Ohm.
At 24V you run the traces in series which results in 4.8 ohm.

-Olaf

Well, that would make sense. I don't do ohms law often, but I do remember subwoofer coil wiring...

The description says this though, is it possible they are only using half of the avaialble traces in 24V? Could I possibly cut the double sided pad and run half the traces without melting them, I wonder? I wonder if each trace covers the whole bed?

Resistance between 1.5 and 1.8 ohm for 12V, series pin 2 n pin 3
Resistance between 2.8 and 3.0 ohm for 24V

I guess I could always upgrade to a silicone mat if this ends up being too low wattage, I've got an in at a local waterjet company if I want an aluminum bed cut to size.

Here is what I'm using to calcualte frame/build size. I have no idea how to calculate the length of linear rail you would need, not much a problem if you aren't using it...
[groups.google.com]

Edited 1 time(s). Last edit at 11/02/2015 10:27PM by deezums.