Supporting X-Axis Expansion

A project log for Monoprice Select Mini Electro-Mechanical Upgrades

Endeavoring to build upon the existing fanbase work of this $200-ish printer.

Michael O'BrienMichael O'Brien 09/22/2016 at 11:410 Comments

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:

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.