I'm on a tablet now, so I can't provide a picture. If you want to make full use of your bed diameter, setting the rod lengths based on a. 30 degree angle when the head is centered won't allow you to use the full diameter. Here's why. If the rods are at 30 degrees from the vertical when the head is centered, you have a 30-60-90 triangle. So if the rods are 200 mm long, the print head might be 100 mm away from the outside of the circle. When one of the arms is perfectly horizontal, you'll be at the full travel, which therefore would give you a theoretical maximum print diameter of 200 mm. However, when the rods are horizontal, you have a bad condition as just a tiny additional movement of the carriage down will result in the print head moving back toward the center.
So you want to make sure that at the maximum travel of the print head, the rods are not horizontal. You'll therefore want to pick an angle from the horizontal that is noticeably greater than zero for full extension. Assuming the rods can be vertical when the head is at the edge closes to your vertical slides and are at 30 degrees from the horizontal at the maximum diameter, your rod length will need to be 1.16 times the bed diameter. So if your bed diameter is 220 mm, you'll need rods that are at least 255 mm long. I'm just about to get on an airplane, so I did these calculations in a hurry.
-- John
So you want to make sure that at the maximum travel of the print head, the rods are not horizontal. You'll therefore want to pick an angle from the horizontal that is noticeably greater than zero for full extension. Assuming the rods can be vertical when the head is at the edge closes to your vertical slides and are at 30 degrees from the horizontal at the maximum diameter, your rod length will need to be 1.16 times the bed diameter. So if your bed diameter is 220 mm, you'll need rods that are at least 255 mm long. I'm just about to get on an airplane, so I did these calculations in a hurry.
-- John
