Hi all,
I've been planning on building a small benchtop mill-drill, about the same size of a RF45.
Yes I know I could spend only $3000 and get a chinese RF45 clone, but I prefer starting from scratch and make it exactly the way I want rather than spending a lot of time fixing and improving a chinese kit.
The main constraint of the build is weight (sadly). The floor the machine will rest on is less than ideal and has poor load capacity. So I want to stay below 500kg for the machine. Current initial design is already at 450kg.
This is a machine intended for prototyping, so speed and production features are not a concern. "Slow is smooth and smooth is fast".
Here's the current early design, a lot of details are still missing.
General
- Travels XYZ: 450 x 250 x 400 mm
- Servo motors on all axes. 750W with brake on Z, 400W on XY.
- Absolute linear scales on all axes (Mitutoyo AT715, easily interfaced with custom software).
- BT30 spindle with pneumatic tool changing.
- NC system based on LinuxCNC and EtherCAT. Custom software with emphasis on semi-manual machining and conversational programming.
Headstock
The headstock is built from bolted steel plates, between 15 and 20 mm thick, and weighs about 45 kg.
The spindle is a BT30 model from china, going up to 10k rpm. It is belt driven by a 3.7kW Kollmorgen AKM-54L servo motor. My plan is to start with a 1:1 pulley ratio for simplicity. This would give a max speed of 6000rpm and 9 Nm of continuous torque. If this turns out to be too low, I would switch to a 2 speed PolyV system, with some kind of quick change mechanism.
Motion system of the Z axis: 25 roller guides and 2505 ballscrew.
One of the major feature is to be able to use the machine as a drill press. The traditional way to do it is a quill and a spindle with a long splined shaft. This is complicated to make for a DIY build and an extra axis also hinders rigidity. The simplest way on a CNC machine to manually control an axis is a MPG, but you loose the feel of the machining forces.
My first idea was to design some kind of mechanism to disengage the ballscrew nut from the headstock and make the Z axis move on a rack/pinion drive with the feed lever. So far I haven't figured out any good way to do it.
My latest idea is to connect the feed lever to a small, independent servo motor mounted inside the headstock with a high 1:15 gear ratio. The servo encoder would act as a MPG to control the Z axis movement and the motor would be controlled in torque mode to provide force-feedback and mimic the forces seen by the Z servo-motor. Let me know how crazy this idea is
Column
The column is also built from bolted steel plates, 15mm thick. Current dimensions are 800x300x230 mm and about 150 kg.
It is hollow to allow mounting an air cylinder to counter-balance the headstock.
The best way to build the column is still to be defined (position/spacing/size of bolts).
The rails mounting surfaces are 2 flats of precision ground steel bolted to the column. They are called "precision ground" but are far from precise enough to mount linear guides as is. No shoulders are planned to allow spotting the whole column on a surface plate for scraping. An alternative to the steel flats for easier scraping would be using continuous cast iron but it's even more expensive and needs to be machined. Not sure it's worth it.
Base
Currently the base is a single piece made of E80 UHPC (Durcrete), assuming the 80 GPa modulus is realistic. I will have to make a few samples and check their stiffness.
Dimensions are 750x400x200 mm and weight about 120 kg.
What is still unclear to me is how the base should interact with the stand. Should the stand be part of the base, increasing its stiffness, or should it be independent from the base?
I assume in a C-frame structure the base is weakest in bending, so the thicker the better. If the stand could stiffen the base, maybe some weight could be gained...
There are still many things to discuss, but that's all for now.
Let me know what you think.