My CoreXY Build Plan
Posted by jnsbanman
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Re: My CoreXY Build Plan February 02, 2016 11:08AM |
If you look at the corexy structure, especially the XY mechanism and the way it typically fits into the machine's frame, it is very easy to use fully supported guide rails on the Y axis instead of end supported rails because the rails are typically parallel and adjacent to frame members. Attaching the fully supported rails to the frame member every 30 - 50 mm makes the rails as stiff as the much larger cross-section area frame members. There's nothing better for print quality and precision than rigid guide rails and a rigid frame.
For the X axis (on which the extruder carriage rides), I would also use a single linear guide instead of a pair of round rails. It will be lower mass, more compact, and the bearings are as near perfect as you can get. The rails are rectangular, typically wider than they are tall when the bearing block rides on the top surface, so if you have a rail that is 12 x 8 mm, turn it so the 12mm side is vertical to minimize flex, or mount the thing on an aluminum tube to increase rigidity.
I recently bought 3 linear guides >0.5m long on ebay, one new old stock and two used, and paid about $34 each including shipping. There are similar deals on ebay all the time- you just have to wade through many pages of linear guide listings to find them. At that price it seems almost crazy to use anything else.
This picture shows how linear guides can be positioned for coreXY. I have added a feature that may or may not be necessary, depending on the size of the printer and whether it will be enclosed and used to print ABS. If so, as the temperature rises the aluminum frame that holds the two Y axis guide rails will expand moving the two rails farther apart by about 1mm if the frame is 0.5 m square. If the X axis steel guide rail, which won't expand as much as the aluminum frame, is rigidly attached to the two Y axis bearing blocks, the thermal expansion will create a very big side load on the Y axis blocks which could cause the mechanism to bind. What I would do is rigidly attach one end of the X axis guide rail to the Y axis block with a spacer (gray in the picture) that is the same thickness as the X axis block, and then use a second bearing block on the X axis to attach the other end of that rail to the other side Y rail (on the left in the picture). That will allow the two Y rails to move apart due to thermal expansion of the machine's frame without causing any side loading on the Y axis blocks. This would also allow some error in the Y axis guide rail parallelism. I'm still thinking about what to do about the steel Y axis rails which won't expand as much as the aluminum frame members they are mounted on.
This all sounds complicated but the same problems exist when using end-supported round rails- thermal expansion will cause the supports to move (or flex) and the rails to flex. All these problems would disappear if the frame pieces holding the Y axis rails were made of steel instead of aluminum.
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
For the X axis (on which the extruder carriage rides), I would also use a single linear guide instead of a pair of round rails. It will be lower mass, more compact, and the bearings are as near perfect as you can get. The rails are rectangular, typically wider than they are tall when the bearing block rides on the top surface, so if you have a rail that is 12 x 8 mm, turn it so the 12mm side is vertical to minimize flex, or mount the thing on an aluminum tube to increase rigidity.
I recently bought 3 linear guides >0.5m long on ebay, one new old stock and two used, and paid about $34 each including shipping. There are similar deals on ebay all the time- you just have to wade through many pages of linear guide listings to find them. At that price it seems almost crazy to use anything else.
This picture shows how linear guides can be positioned for coreXY. I have added a feature that may or may not be necessary, depending on the size of the printer and whether it will be enclosed and used to print ABS. If so, as the temperature rises the aluminum frame that holds the two Y axis guide rails will expand moving the two rails farther apart by about 1mm if the frame is 0.5 m square. If the X axis steel guide rail, which won't expand as much as the aluminum frame, is rigidly attached to the two Y axis bearing blocks, the thermal expansion will create a very big side load on the Y axis blocks which could cause the mechanism to bind. What I would do is rigidly attach one end of the X axis guide rail to the Y axis block with a spacer (gray in the picture) that is the same thickness as the X axis block, and then use a second bearing block on the X axis to attach the other end of that rail to the other side Y rail (on the left in the picture). That will allow the two Y rails to move apart due to thermal expansion of the machine's frame without causing any side loading on the Y axis blocks. This would also allow some error in the Y axis guide rail parallelism. I'm still thinking about what to do about the steel Y axis rails which won't expand as much as the aluminum frame members they are mounted on.
This all sounds complicated but the same problems exist when using end-supported round rails- thermal expansion will cause the supports to move (or flex) and the rails to flex. All these problems would disappear if the frame pieces holding the Y axis rails were made of steel instead of aluminum.
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
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