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Possibly stupid question but does a corexy have to be square?

Posted by DjDemonD 
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DjDemonD
Possibly stupid question but does a corexy have to be square?
March 01, 2016 08:52AM
Can a corexy assembly be made rectangular? I am thinking about modestly scaling up a smartrapcore alu to accomodate a 30cm x 20cm bed instead of the usual 20cm2 one? I'd be looking to increase the width but not the depth to reduce the cost/complexity of doing this. I intend to beef up the smooth rods for the print head to slide on to 12mm to keep the deflection to a minimum.
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filipeCampos
Re: Possibly stupid question but does a corexy have to be square?
March 01, 2016 09:19AM
yes, it can be rectangular

30x20 is still pretty small, 8mm or 10mm rods will work fine. 12mm is overkill

Edited 1 time(s). Last edit at 03/01/2016 09:21AM by filipeCampos.
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Edvardas
Re: Possibly stupid question but does a corexy have to be square?
March 01, 2016 02:34PM
Check out a new XL300 smartrapcore [smartfriendz.com]

And to answer your question- no it does not have to be square. Mine is not.

12mm is overkill. If you care about deflection then forget smooth rods and go with rails or fully supported smooth rods.
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animoose
Re: Possibly stupid question but does a corexy have to be square?
March 01, 2016 03:37PM
Off-topic a little, but what speed can you get out of the XL300 smartrapcore? How does it compare to Eclips3d?

See my blog at [moosteria.blogspot.com].
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J-Max
Re: Possibly stupid question but does a corexy have to be square?
March 01, 2016 04:20PM
Hi guys,

I can't say about the XL300 but with Wooden Box SmartRapCore (upgraded to 8mm rods) we reach 150mm.s with a good print quality.

Sorry filipe campos, I desagree. 8mm rods starts to bend too much since 340mm long. That's approximatively what you need for a 200mm travel.
If you don't trust me, calculate with the Young's module with a dynamic load.
So I would recommend 10mm rods along the 200mm side and 12mm along the 300mm side.
Don't be affraid of the extra expense and load. It's not realy that much.

++JM

^ Things I said. My thingiverse here. My website there (in french).
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realthor
Re: Possibly stupid question but does a corexy have to be square?
March 01, 2016 05:16PM
You can have a quick read of one of my calculations some time back. I have found out that for 360mm of rod the max acceptable weight prior to deflection is around 270gr. If this would be for X carriage, if you have 2 rods, the max weight you can have for the whole X carriage is around 540gr before the deflection becomes an issue. This has been calculated at static conditions, so at high speeds on Y, when the carriage stops to return direction, there is additional load due to the inertia, which surely cuts that value back.

It all depends on your carriage weight really. If J-Max has the smartrapcore with bowden and he says at high speeds the deflection becomes an issue I would believe him better than my calculations at static load smiling smiley

Edited 2 time(s). Last edit at 03/01/2016 05:20PM by realthor.

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Edvardas
Re: Possibly stupid question but does a corexy have to be square?
March 01, 2016 06:50PM
Everyone seems to calculate rods for extruder's weight. But I have never seen any calculations to differ for X and Y axis. X axis has to carry the extruder only whereas Y axis carries the extruder and the rods of X axis and the bearings for Y axis. This can quickly become double of what extruder weights.

Y axis is also quite simple to have fully supported round rods mounted too. Their weight is not an issue of course.
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Edvardas
Re: Possibly stupid question but does a corexy have to be square?
March 01, 2016 07:03PM
Quote
animoose
Off-topic a little, but what speed can you get out of the XL300 smartrapcore? How does it compare to Eclips3d?

Infinite speed like on any printer (if extruder limitation is out of calculation)? It is all about acceleration and jerk speed.
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realthor
Re: Possibly stupid question but does a corexy have to be square?
March 01, 2016 11:17PM
I did a separate calculation for Y at the above linksmiling smiley .... Decided for 10mm on Y. I have different lengths for X and Y, it seemed logical to do the calculation separately.

Edited 1 time(s). Last edit at 03/01/2016 11:18PM by realthor.

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filipeCampos
Re: Possibly stupid question but does a corexy have to be square?
March 02, 2016 05:35AM
Builded my printer with 8mm Y rods 360mm. I agree that 10mm rods is better that 8mm, and 12mm will better that 10mm rods. BUT, in a small printer (30cm x 20cm) i pretty sure that using 8mm, 10mm or 12mm will produce the same print quality.

I not saying there is no bending at all, but the bending will be very small and most of the time always apply in the same direction.
In my opinion this is only a very small issue, that in practice can be negligented.

Most printer when designed was one of the main goals to be cheap to build without affect the print quality. Most of the printer in the market use 8mm rods and not 10mm, this is because 8mm will do the jobs with no visible problems on the print.

But was the price of 8mm or 10mm is almost the same, for me i will go with 10mm rods on the Y and 8mm on the X to minimize the weight.

Edited 4 time(s). Last edit at 03/02/2016 05:49AM by filipeCampos.
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J-Max
Re: Possibly stupid question but does a corexy have to be square?
March 02, 2016 09:27AM
Hi guys,

Filipe, don't forget that even a "small" printer (to me 300x200 is not small) may print thin layers (0.1 ; 0.05) and a small flex may be noticeable on the print quality.
Actualy 340mm is the distance of the X 8mm rods on the regular Prusa i3 and every user experience their bed have a small hill at the centre.
Obviously, their beds are flat, it's a 8mm rod issue. Note recent versions of the Prusa i3 now uses 10mm rods (Rework 1.5, or so...), it's at purpose.
On the Z you can keep 8mm guides, there will be no issue. Along the 300mm side you're right, you probably want 10 or 12mm rods, as it don't affect weight at all.
On the X axis, well it realy depends of the overall lenght in use along the 200mm side. If it's under 300mm maybe you can keep 8mm rods,
but if it exceeds 300mm of overall lenght, you'd rather upscale the roads. The extra weight you'll get is not that important compares to a direct drive motor to carry .

++JM

^ Things I said. My thingiverse here. My website there (in french).
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filipeCampos
Re: Possibly stupid question but does a corexy have to be square?
March 02, 2016 10:30AM
i never noticed this problem (small hill in the center), but i never printed less of 0.15...
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realthor
Re: Possibly stupid question but does a corexy have to be square?
March 02, 2016 10:49AM
Can you tell a number for the weight of the X carriage you have on the 340mm 8mm rods? If you are in the 400gr (even more) range you should be safe. But I'd like to take some data from the fieldsmiling smiley so to speak.

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filipeCampos
Re: Possibly stupid question but does a corexy have to be square?
March 02, 2016 10:59AM
Already have several setups on the same printer; bowden mk8, direct mk8 and my actual is a dual mk8 direct extruder: +/- 800gr. You can check photos and setup changes on the g&c printer thread.
If you are plannig to go dual extruder like me then is better to go 10mm on both x an y axis. This was something i was considering to do, but so far i never see any problem on my prints.

ps: the material of the rods is important too: do not chose aluminium

Edited 2 time(s). Last edit at 03/02/2016 11:04AM by filipeCampos.
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realthor
Re: Possibly stupid question but does a corexy have to be square?
March 02, 2016 11:04AM
I was thinking dual extruder but one bowden and the other one some lightweight Nema14 geared (<300gr) so I shouldn't go too much above 400 (I hope so). But yeah, dual direct extruder definitely 10mm for X rods.

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filipeCampos
Re: Possibly stupid question but does a corexy have to be square?
March 02, 2016 11:14AM
But if you can (have $) go with linear rails, this days they are not so expensive, you buy 3 of them from chine (ebay). This will solve some problems rods have.
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realthor
Re: Possibly stupid question but does a corexy have to be square?
March 02, 2016 11:17AM
Already have the rods so stuck with them for now. I believe they can have a long life if you dimension correctly for the load they'll be subjected to.

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DjDemonD
Re: Possibly stupid question but does a corexy have to be square?
March 02, 2016 12:31PM
Great discussion and lots of good advice and ideas, hadn't thought to include the x rods and runners in my calculations, so thanks.

Rails sound very appealing and should be quite doable with the smartrap. I think if I do expand this machine I'll try with 8mm rods first as it's only 2 I need and they'll always get used somewhere else if I replace them with rails.

To pick up on the 300 x 200 not being that big, it isn't, but it'll make a big difference to me in that I'll be able to print 30cm in one dimension which is 50% improvement on what I can print now. Plus I'll feel less like I ought to have bought the 300xl instead.
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Anonymous User
Re: Possibly stupid question but does a corexy have to be square?
March 03, 2016 08:49AM
Quote
filipeCampos
But if you can (have $) go with linear rails, this days they are not so expensive, you buy 3 of them from chine (ebay). This will solve some problems rods have.

Linear rails don't solve anything because they are not designed to support but guide ! Just have a look at the mfg specs ! And a solid frame, it is steel, cast Al, cast iron, granite.

People who can't compute can just try with some known weights and a micrometer. You will be surprised.

Supported rods are Ok for the limited requirements of 3D printer. Yet, no guide will compensate for flimsy frame.

Extruded profiles with integrated V guides exists but I don't know about their availability for the general public.

Note that these thin rods have an advantage, their prevent seizing of the various bearings because of poor alignments. Stiff rods will not flex and something will have to give, the bearings or the frame. Oh yes, but the frame is flexible smiling smiley
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J-Max
Re: Possibly stupid question but does a corexy have to be square?
March 03, 2016 09:00AM
Hi guys,
Quote
MKSA
People who can't compute can just try with some known weights and a micrometer. You will be surprised.
It's damn right !
You can make Vgroove profiles like Openbuild's V-slot with regular 2020 profiles (any kind), a router table and a 90° V-shape Wood bit.
I worked great for us. Maybe I'll make a video on the full process.

++JM

Edited 1 time(s). Last edit at 03/03/2016 09:01AM by J-Max.

^ Things I said. My thingiverse here. My website there (in french).
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Edvardas
Re: Possibly stupid question but does a corexy have to be square?
March 03, 2016 09:04AM
How straight are 2020 profiles?
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J-Max
Re: Possibly stupid question but does a corexy have to be square?
March 03, 2016 09:10AM
Well, they are pretty straight. I can't see any bend or twist with my machinist straight edge.
You probably should not use too long parts. I used unsupported 500mm 2020 profiles and I did not noticed any flex on the horizontal rails.

++JM

^ Things I said. My thingiverse here. My website there (in french).
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filipeCampos
Re: Possibly stupid question but does a corexy have to be square?
March 03, 2016 11:52AM
Quote
MKSA
Quote
filipeCampos
But if you can (have $) go with linear rails, this days they are not so expensive, you buy 3 of them from chine (ebay). This will solve some problems rods have.

Linear rails don't solve anything because they are not designed to support but guide ! Just have a look at the mfg specs ! And a solid frame, it is steel, cast Al, cast iron, granite.

People who can't compute can just try with some known weights and a micrometer. You will be surprised.

Supported rods are Ok for the limited requirements of 3D printer. Yet, no guide will compensate for flimsy frame.

Extruded profiles with integrated V guides exists but I don't know about their availability for the general public.

Note that these thin rods have an advantage, their prevent seizing of the various bearings because of poor alignments. Stiff rods will not flex and something will have to give, the bearings or the frame. Oh yes, but the frame is flexible smiling smiley

This is a lots of unecessary comments... the linear rails is to be used on the printer model is building; a smartrapcore aluminium

[www.thingiverse.com]

I think linear rails are better for this printer because of the lack of rigidity of the frame.

Edited 3 time(s). Last edit at 03/03/2016 12:01PM by filipeCampos.
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DjDemonD
Re: Possibly stupid question but does a corexy have to be square?
March 03, 2016 06:57PM
Its interesting the relative nature of perception, my smartrapcore alu, IMO far from lacking rigidity is the most rigid printer I have by a big margin. But yes I still think there's a lot to be said for linear rails.
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J-Max
Re: Possibly stupid question but does a corexy have to be square?
March 04, 2016 03:17AM
Hi guys,

Well, I believe 2020 profile is pretty stiff for proper use.
For regular 3D printing applications, as long as you don't use too long rails, it will be fine.
But don't expect it to stand CNC mill applications, because it's not meant to be better than it is.
Some people make 2020 structures without any calculation, using 2020 when 2040 or greater is needed
Or they use weak connexions between profiles like plastic ones, light metal corners, or so.
IMHO the profile itself is not to blame most of the time.

++JM

Edited 1 time(s). Last edit at 03/04/2016 03:18AM by J-Max.

^ Things I said. My thingiverse here. My website there (in french).
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DjDemonD
Re: Possibly stupid question but does a corexy have to be square?
March 04, 2016 03:54AM
The latest version of smartrapcore alu, connects the extrusions to one another directly with the centre hole tapped to accept a bolt drilled through the other extrusion so its remarkably stiff. They did away with plastic corners as this method is much more simple and stronger.
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J-Max
Re: Possibly stupid question but does a corexy have to be square?
March 04, 2016 05:03AM
So true...

++JM

^ Things I said. My thingiverse here. My website there (in french).
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Edvardas
Re: Possibly stupid question but does a corexy have to be square?
March 04, 2016 04:12PM
Quote
DjDemonD
The latest version of smartrapcore alu, connects the extrusions to one another directly with the centre hole tapped to accept a bolt drilled through the other extrusion so its remarkably stiff. They did away with plastic corners as this method is much more simple and stronger.

Bolt does not go through anything that has been drilled. Drilled hole is there to access a bolt with a hex key.

And I must say that it is a great solution. I have connected a bunch of 2020 profiles this way today. Before this I was using cast metal corners and this is much cheaper and not as time consuming as I have thought. I used ISO 7380 m5 bolts that have 6mm of thread on a 2020 I-type slot 5 profile from Motedis here in Europe.
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