Youtube reveals some totally heroic machines made in plastic and MDF with frames in off-the-shelf extruded aluminium beams. Cheap computers and software have brought CNC machining within range of the DIY brigade. I’d like to continue in this spirit but head towards something much bigger and heavier without increasing the cost to anywhere as much as that of a professionally made router.
Size: In order to machine round the outside of an 8’x4’ sheet we’ll need a long and strong X axis. A 14 foot long steel RSJ will make the heart of our CNC router. It’s long enough to overhang the ends of the support posts so that tension ties can counteract even the smallest tendency to sag under the weight of two 4.5kW router motors and two drills. The neat thing about an RSJ is that it is not only cheap but the ‘I’ section enables us to put drive gears and the drive rack neatly inside the web.
Torsional rigidity is not a strong point but is easily enhanced by welding a heavy tube or two inside one of the webs. Unlike cast iron, steel can pick up vibration so filling a tube with heavy chain and oil will make a cheap but effective damper. You only get really smooth finishes on a machine that is well damped.
Z axis: You could buy an off the shelf Z slide and hang that off the X axis for a conventional and simple layout but the logic of that is debatable. When machining flat boards etc there is very little Z movement so it would be better to put the Z under the Y table so that the two major movements are as light as possible. The Z frame is raised like a garage car lift with 4 chassis mounted screw posts rotating to drive ball nuts on the frame. Unlike the continuous chain of the garage hoist there would be a single timing belt for each screw sharing one of a pair of drive spindles which are both driven by a fairly short belt and the stepper motor. This way we get reduced belt stretch and a built in reduction drive. This layout enables a much bigger Z travel – 3D modellers note – and makes a fast and responsive machine with a tidy uncluttered look. Air balancing means there would be no heavy lifting for the Z stepper motor despite having to lift the entire Y table plus the job on top.
Y axis: This is the table going forward and back and a conventional 2 belt and shaft drive will be fine and neatly out of site at the rear of the machine.
As it’s a long machine it could be worth having a pair of tables rather than one big one. Production is greatly increased when one side can be reloaded while the other works and the 2 can be worked together to make one big table. As is common practice one makes the Y axis table tops out of Tufnol self machined to skim true and make the vacuum rubber channels.
Tooling mount: This the neatest twist and the basis is a wrap around box of thick aluminium plates enclosing a 1ft length of the beam.
Each of the 4 plates has a unique function. The top and bottom plates are for the slideways; Turcite blocks for the heavy downward loads and vibration damping, ball bearing rollers only for lateral location.
The drive sideplate supports the entire drive mechanism – see picture – the drive motor sits over the top of the aluminium box and drives the big pulley on the outside. All the rest is inside the web of the RSJ ending up with the toothed belt driving onto the rack with a sort of caterpillar to give more tooth contact. This plate is at the back of the machine and also supports a pair of Desoutter AFDK drills. These lovely drills just need a pulse from a 4mm air pipe to set them off on an automatic drilling excursion.
The front plate supports a swinging plate that holds a pair of angled router motors that are alternately selected with a push from an air cylinder; the cheapest alternative to a tool changer you’ll ever see.
The tool changer: The top pivot point is in line with each router motor axis so that they both swing into exactly the same position. The pneumatic pusher cylinder is mounted inside the box and pushes a peg that goes through a slot – neat huh? Between the motors a round steel plug is pressed into the plate and one of a pair of electromagnets pulls this to lock the plates together (better solutions in comments please).
So there we have a heavy duty industrial quality router with 2 tools and 2 drills; about £20,000 worth if you had to buy one. One could earn a comfortable living with one of these. Not a lightweight project but still DIYable especially if some of the complex parts are tested in MDF first and then self machined in aluminium or Tufnol to finish. Don’t get bogged down with cutting, welding and drilling heavy frames; the steel suppliers have all that kit and will make to your drawings in no time.
Levelling the beam: There must be a few ways to do this other than sending it away for precision grinding. Once fitted in place with some representative weights clamped on, the tension stays can be tightened to get close to straightness. A small steel block with a laser on it will show a dot moving up and down a target to reveal the bumps which will respond to a light touch of an engineers scraper.
When the beam is true the finally added hardened steel guide bars will provide the perfect flat finish.
Panels: Professional machines come with expensive steel panels. As the central box chassis needs to be stiff and straight use MDF panels bolted to the steel to make it neat and strong too. Hammerite paint on MDF is remarkably durable and will keep it all looking very tidy.
Bellows: With a rectangular box sliding on the RSJ there will be no problem in adding bellows to make the machine the ultimate in tidiness and this will be no bad thing for the unlubricated Turcite slides.
For further reading try following http://www.mycncuk.com -a fount of CNC knowledge- and please criticise or add ideas for improvements in comments.