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Enlarging the rostock mini

Posted by mrboost 
Enlarging the rostock mini
February 20, 2015 10:07AM
Morning everyone!

I've started building my second printer, a rostock mini! and I would like to make the print bed slightly larger then 6X6 inches. According to the rostock mini reprap page the design is good up to 12X12 print bed. I would like to make mine 8X8 as I have a ton of 8x8 heat beds and glass laying around. I know I need to make the frame (top and bottom plates larger) and the rods need to be longer also.

Now for the question: I need a little help modifying the SCAD files for the frame and some help doing the math for the longer robs.
I would really appreciate the help and accommodate if needed.

Thank you,
Christopher Koch
Re: Enlarging the rostock mini
February 20, 2015 01:02PM
I can't help you with the frame, but I can help with the maths for the rods. The critical factor is the ratio of rod length (measured between the centres of the bearings) to the delta radius. To work out the delta radius, take the points on the carriages midway between the pairs of bearings used to attach the diagonal rods, and work out the radius of the circle they are on. Call this the carriage radius. Then do the same for the pairs of bearings on the effector, and call this the effector radius. The difference between the two is the delta radius.

If you scale up the distance between the towers but keep the effector the same size, then the delta radius (and printable radius) will increase by more than that scaling factor. Increase the diagonal rod length so as to keep it the same multiple of the delta radius. On my Mini Kossel, the ratio of diagonal rod length to delta radius is 2.05. It may be a little different for the Rostock, but will probably still be close to 2.

Bear in mind that the print area of a delta is closer to a triangle or circle than a square, so it is usual to use a circular bed and not a square bed.



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: Enlarging the rostock mini
February 20, 2015 04:09PM
Hi dc42,

I've worked out (rounding down) to 250mm does that sound about right?

Thank you for the help!
Ck
Re: Enlarging the rostock mini
February 20, 2015 04:44PM
Quote
mrboost
Hi dc42,

I've worked out (rounding down) to 250mm does that sound about right?

Thank you for the help!
Ck

It's hard to say, because I don't know the Rostock design. However, my Mini Kossel has 215mm rods, delta radius of 105mm, a print radius of 85mm. If I were to scale it up without changing the effector or carriage design, a 250mm rod length would correspond to a delta radius of 122mm, which would give me a print radius of 102mm, so a printable diameter of about 8 inches. So perhaps 250mm is about right.



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: Enlarging the rostock mini
February 23, 2015 02:25AM
You might be interested in my fork of the Rostock Mini Pro repo:

[github.com]

When you load the main rostock.scad into openscad, it issues a bunch of echo statements containing the various delta measurements needed by the firmwares, build volume, belt lengths, etc.

I've rolled in some extra changes from other peoples repositories, but have made it more parameterized, particularly if you want to scale things up. If you look in the configuration.scad you can set mini = true to use the "standard" Rostock Mini Pro parts and dimensions, or mini = false to switch to the configuration I am personally building (of course you can set your own variables in the configuration scad to pick and choose how you want it). Mine has:

* 1m tall, 12mm smooth rod
* Extra bracing at the motor end and idler end for stiffness with such a tall model - including the ability to use threaded rod to help initially set the between tower spacing (see [minow.blogspot.co.nz] for the inspiration)
* Magnetic ball hinges on the diagonal rods
* Custom 623 roller bearing based carriages rather than LMnUU (hoping it'll be quieter, we'll see).
* Larger build platform (I'm using a standard 200mm square Mk2 pcb HBP)

My printer is currently in construction - currently the physical construction is done, now hooking up motors, belts, wiring etc.
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