And meant a deep dive into a problem that I have not seen solved by 3D printing so far (and I can’t claim to have solved it yet, but things are looking promising 🙂
The question is: how to ‘cnc’ a manual Sieg X1 milling machine – the so called ‘micro mill’ that comes out in so many guises including the Clarke CMD10 featured here, available from Machine Mart, but in my case bought nth-hand and neglected.
While built down to a price, the X1 design is mostly ok – some parts in this CMD10 were made with great care, and some rushed.
There are proper shortcomings: the dovetail and gib that holds the z-axis straight are woefully short for the 10kg of overhung weight they support, for example. But ‘shortcomings’ identified by some folks are more from expecting a small cheap machine to do the work of a big expensive machine.
Anyway, back to this particular cnc project.
Its self-imposed rules are:
- 3d print where possible
- real thrust bearings
- no modifications to the mill parts
- no loss of travel
- strength sufficient for the duty expected of the manual original
- stiffness similar
- backlash similar (which is 40-50μm on x-y)
Disclosure: I have seen a set of 3d printed x-y motor mounts, like these from jimmit on thigiverse, but no 3d printed z (up-down) mechanism.
Heres is my prototype z-axis mount (right).
It is the grey bit on top of the black column, supporting the new z-stepper motor (right of the photo – the middle motor is the existing spindle drive). The z-leadscrew is below the stepper, and is not the new threadded rod (down the centre of the hollow column) added to resist the stepper motor’s cantilevered weight.
The leadscrew drive uses a 3mm pitch HTD belt and pullies bought (at great expense, it turns out) to match the thickness and keyway of the original hand wheel.
3d printed mock-up of the beld and pullies
Using a belt allows the original nylock nut to be used to pre-load the thrust bearing – some alternatives z-drives I have seen use an axial drive, with the motor-to-leadscrew coupler pushed against the bearing block to remove slack, which is not a very adjustable set-up, cannot generate predictable force, and is asking for trouble when trying to hold up the whole z-axis (I measured 22kg of force along the leadscrew to start the milling head moving upwards.
3mm pitch GT2 belts and pullies would have been a better choice, but getting these things as a hobbiest is not easy. Older HTD technology was more obtainable, and even then the available 47 tooth belt is not ideal, and it took a lot of hunting to find 15mm thick pullies with a 3mm keyway to fit the existing leadscrew – and (did I mention the expense before..?) cost more money the stepper motor…..
I am rather proud of this stiffening structure
Plastic can be used to replace the aluminium parts used by some other conversions because they tend to be built around a ~6-8mm plate.
While 3d printable PLA plastic is ~6x less stiff than aluminium (as far as I can calculate with no mechanical enginnering training – please put me straight below if you know better), the stiffness of a cantilevered beam is proportional to the cube of its vertical thickness (same ignorance caveat), and 3d printing allows thickness to be wrapped around an assembly in a way that is very difficult when making simple aluminium or steel structures.
Hence, even this prototype deflects less than 10μm when the motor is added – see before and after photos.
Left, no deflection with no motor
For 3d printer folk: the prototype only uses three preimiters and 20% infill (except five perimiters and 80% infill where the leadscrew passes through it to support the thrust bearings).
The idea is to print the final object in (less stiff but more temperature resitant) PETg, with five perimiters and 80% infill throughout to recover stiffness.
Right, 1.5kg motor added, dial gauge records <10μm movement
If this goes well, the final .stl files might be published on Printables.
Prototypes of the x and y motor mounts exist and, time permitting, the aim is to cover them in a later blog. Trying to navigate through the world of controller boards is proving more tricky….
