Balance. Why is it needed? One end of the X-axis is unsupported. If you extend this out, there should be enough force to have thins begin to bend. I'm not an mechanical engineer so I cannot calculate how much the 3 Z-Axis rods will bend when you're mid way up the Z-height, and you're X is at 120-125 mm, but I cal calculate the torque with reasonable precision. If you're going to extend this axis, you need to worry about it a bit.
Here is a quick copy and paste with some distances and masses of components:
- Upper Rod: 46.9 grams
- Lower Rod: 62.0 grams
- Hotend: 115 grams weighed, upping to 130 to include wire et al.
- Motor: 223.5 grams
- End cap: 10.9 grams, 20 mm wide, 17.8 deep holes
- Z rod: 18.5 mm from Upper rod
- Motor: 23.9 mm from Upper rod
- X Carriage: 37.75 mm to 162.75 mm (stock)
- X Carriage: 37.75 mm to 257.75 mm (extended_
- End Cap: 219.7 mm (stock)
- End Cap: 305.7 mm )extended)
- Upper Torque: +576.87 gm*cm
- Lower +Torque: +1218.53 gm*cm
- Lower -Torque: -52.93 gm*cm
- Net Lower: +1165.6 gm*cm
- Motor:-120.69 gm*cm
- End Cap: +333.21 gm*cm
- X Carriage: +490.75 -> +1920.75 gm*cm -> +3350.75 gm*cm (extended)
I don't have the mass of the linear bearings, nor the Z-carriage so instead of guessing if it is balanced, I'm going to assume it is. Thus it will neither add or reduce the amount of torque on the arm/gantry.
Overall the gives me a base torque of 1954.99 grams * cm, to which I'll round to 1955, without a hotend attached. In case you don't have a visual memory for this, it means that there is nearly 2 kg of force pulling the arm down towards the bed due to the mass of the arm,, but it's pulling 1 cm from where the M4 rod is. Please note this is with the extended X-Axis.
Just having the X-carriage with the hotend on it midway on the bed, x=110 mm, this force is almost doubled. If I balance this perfectly, I still have a variation of +/- 1.43 kg from 0 mm to 220 mm to to imbalance. The stock setup only has a variation of about +/- 0.8 kg from 0 to 125 mm. So, by nature of extending out the X-Axis, I have 76% more torque that I'll be asking the Z-Axis rods to handle in a perfectly balanced world. Yeah, methinks that 8 mm rod for one of the 3 is a requirement now.
If I bolt some mass into the graphite composite in parallel to the farthest Z-Axis rod, about 33.9 mm from the M4 rod, I'll need 1 kg as a counter balance and even if that were lead, that is a hair over 88 cm^3 of volume of a cube with 4.45 cm sides. However, if I make a pulley to lift up the gantry at it's very end, look at the end cap distance of 305.7 mm, then I'm only looking at ~110.7 grams for the counter balance. That seems rather doable.