CNC 3018 Pro - Z Axis Stability improvement idea..

[richardsenior]

Hello I'm Richard.

Like a lot of people during coronavirus lockdown.. I decided to buy something to make myself feel better.
So I bought a cheap chinese CNC router (it actually came from Czechia).
It's a CNC 3018 Pro. And it cost me 140 quid on EBay.
I don't know what I was expecting... I knew I wasn't going to be sending sheets of white hot steel 30 yards down the shop floor with a huge carbide face mill.
I knew it was probably a bit of a gimmick, but all I wanted to do was engrave a small aluminium box with a logo about an inch square.

The way I saw it.. it's a 3 axis cnc for 140 quid. I mean.. it's rude not to buy it!
Anyway... the machine has both amazed me and disappointed me in equal measure.

For example... I put a dremel 2 flute router bit in it, and it happily ran a pass 8" long 15mm deep and 2mm wide on a chunk of plywood without any complaint.
It's accuracy is flabbergasting. I can run the same grbl file from the same zero point over and over, and it's bang on.. Easily to 0.1mm

But when engraving aluminium, there is just enough play in the axes to cause the outline of the engraving to be fuzzy due to machine vibration.

If you get hold of the router bit and yank it about a bit, you can cause a good 2mm movement.
When you examine where that movement is coming from, at least 50% of it is due to the Y Axis rails actually bending. (they're about 300mm long and 10mm diameter).

Some people have cured the other 50% by fitting longer linear bearings in the z axis, and bushing the z axis lead screw.
But they're still going to get y axis torsion.

So I had an idea.
There's a gap of about 5mm between the back of the z axis and the two extruded aluminium struts (on which the control board mounts).
What if I just shim that gap with some PTFE?
That way, we're still using the guide rails for accuracy, but you can't bend the guide rails because the PTFE will touch on the extruded bars.

I note that those extruded aluminium bars are used by the openbuild machines.. And you can actually buy bearings that run in the slots.
That was another thought I had.. just to attach some nylon bearing wheels to the z axis, that would run in the openbuilds backing bars.

Has anyone tried any of this?

Before anyone goes down this route.. Yes.. I know I could buy a better machine.
I could also just send off my parts for professional engraving, and let them worry about things like z axis stability.
Or I could just lay on my bed all day and do nothing at all... You get the idea... Please don't evangelise, it's tedious.

Anyway..

Hello :)

EDIT :

I bothered to create this page on my modifications