The belt layout is the very center of a CoreXY design, so we are spending some time exploring different layouts before we lock in on the layout we will use. This page starts with a quick documentation of the critical parameters of a CoreXY belt layout, then continues with several we looked at, with a critique of each.

CoreXY belt layout principals

"CoreXY" refers to a method of arranging two belts around a set of pulleys and connected to a single block so that by moving one or both belts, the motion of the block can be controlled. One useful thing about this design for 3D printers is that motors used to control the position of the belts do not move with the block so is easy to "bolt things down". a second useful thing about the coreXY design is that if the belts are ideal (no stretch) the position of the block is completely controlled by positions of the two belts. This help precision, but we must also worry about the fact that our belts _can _stretch and we'll need to add features to minimize the effects.

Generic Core XY belt layout

Most CoreXY belt layouts use two belt designs that are a mirror image of each other. This seems to keep things simple and we'll be following this practice on our printer. The picture below shows a generic belt layout where I've purposely located some of the pulleys in unusual locations. This is to make it easier to highlight the pulley positions that are required for a functional coreXY design.

The requirements for a functional belt layout are listed below. I'll refer to pulleys by the letter assigned to them. I'm following our self-imposed constraint that the two belts are mirror images of each other.

1. Pulleys D and E MUST BE fixed relative to each other and also in the Y-direction with the moving block. In practice, virtually all coreXY designs place these 4 pulleys on a single piece of material. We'll call this the "bridge". 2.The belt segments between pulleys D (and E) and the moving block MUST BE parallel to the X-axis direction of movement. Pulley D and E locations, pulley diameters, and the location of the belt attachment to the moving block are critical in maintaining this relationship.
2. The belt segment A-E and the belt segment C-D MUST BE parallel to the Y-axis direction of movement. Pulley A and C locations, pulley diameters, type of pulley (toothed vs toothless) and belt thickness are critical in maintaining this relationship.

That's it for hard requirements. There are additional considerations for the belt design to be considered. These are:

• The maximum Y-direction location of the block occurs when pulley C and D touch each other, and the minimum occurs when the pulleys A and E touch. The Y range of motion is the Y-direction spacing between pulleys A and C, minus the Y-direction spacing between pulleys D and E.
• The minimum X-direction location of the block occurs when the block hits pulley E (or D), and the maximum is when it hits the D or E pulley at the other side of the bridge. The total travel is the spacing between pulleys E minus the width of the block.
• The drawing shows pulleys D and E offset from each other in the Y-direction. This is not required, and many designs locate these pulleys to enable the designers objectives for attaching the belts to the moving block. Any placement is OK as long as the requirement #2 above is met. In practice, these two pulleys are almost always placed close together.
• CoreXY designs require that the two belts "cross" each other somewhere because of the topology of the two paths. This crossing is shown above the moving block in the diagram above and is the most common location for the crossing. Since two belts can't move intersect each other, most belt layouts shift one of the belts up (out of the screen in the diagram above).
• There is normally no good reason for pulley B to be placed way out in the boondocks as shown in the diagram above. Normal practice is to place it close to pulley C.
• Finally, one of the pulleys on each belt (usually the same letter on each, but it's not s requirement) is attached to a stepper motor. The amount of "wrap" around the pulley needs to be considered when selecting which pulley to use. In the diagram above, pulley A has the most wrap (180 degrees), and pulley C has the least, with pulley B in between. Some designs will add an extra pulley to allow more "wrap" on the motor pulley.
• Belt tension is also important in deciding how much wrap is needed on the motor pulley. More tension will make it harder for the belt to slip on the drive pulley (skip a tooth on the toothed drive pulleys we all use), but adds bending forces to all the parts trying to hold the pulleys in place.

OK, after all that, here is a picture of a more common belt layout. I've drawn a box around pulleys D and E to represent the bridge that holds them.