A two-part CNC video hit the tubes recently. It documents some of our adventures learning to use this beast we have created. The truth is that when we were scavenging parts and building it (see the CNC machine design & make), we didn’t really know exactly how we would use it. It makes pro-CNC builders cringe. I mean we had some ideas: we know we wanted it to help us make more wacky furniture and other ‘functional-art’ from upcycled materials, for example. Apart from that, the goal was to make it ‘very capable’, perhaps even able to machine steel, while still big enough to fit a 4x8ft sheet on the bed. Versatility like this doesn’t come easy. As you get to larger format CNC’s, like the 4x8ft routers, it becomes much harder to maintain enough rigidity to make good cuts in metals.

Higher z axis clearance, which gives the ability to machine much thicker lumps of material, make the situation even more difficult in terms of rigidity. In the video we do some deflection tests on the machine to confirm this.

I always knew this was going to be a serious problem with this design, but had thought that the way the z-plate (which carries the spindle) rides up and down on its linear guides (which are THK SHS20’s BTW) in combination with cunning work holding, would allow me to get round this conundrum. By having raised up, structural sides which carry the gantry, this machine is comparatively very rigid near the top of the z travel, where the moment forces that attempt to exert leverage on the gantry with the length of the z plate are minimized (as in the pic below).

In this example, it’s the addition of the relatively tall mechanists vice and its swiveling base that really raises the workpiece up to where the spindle mount, the lower linear guide bearing and the long oversized SHS35 bearing blocks are in line. The massive concrete sides and the gantry’s strategic epoxy granite also help keep vibrations to a minimum.

One weakness of the current build is the baseboard; an 18mm sheet of ply under an 18mm sheet of MDF. It has certainly made a good learning platform. To my shame, more than once, I have accidentally plunged non-spinning endmills right up to the collet, through the relatively soft material, the servo drive only throwing an error as the collet nut tries to smash through! Anyway, for milling metals, the wood-based spoilboard has the twin downside of deflecting noticeably under higher loads, and not dealing well with any kind of moisture (think mist or flood coolant). As you see in the pic above a temporary semi-solution was using this sheet metal tray to contain cutting fluid.

To begin with, we were using a cheap amazon mist cooler. It did help: in the video for example, we use it to mill steel with decent results. It has a serious drawback. It was VERY inconsistent. Fiddling with the valves that control air and fluid flow would occasionally give you the perfect amount of cutting fluid and air-blast. More commonly it would spray coolant everywhere uncontrollably, or fill the shop with a dense fog, or stop putting out oil and let the cutter run dry – most often all three in the same cut!

It was changed for a DIY fog-buster type deal, which you can see in the video. It works much better to give minimal quantity lubrication (MQL), and blasts the chips out the way to prevent re-cutting.

In the second video in this little series, we have a go at milling slate (!) and steel and using the CNC as a surface grinder/ knife sharpener. Not a sensible operation, but one that involves some fun jig making, dust shoe creation, and us ultimately learning a lot.

Click any of the pics below to watch the video:

Nice blue chips, indicating a reasonable amount of heat being carried off by the swarf, and decent feed and speed.

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Posted: 12|30|22 at 12:16 am. Categorically squeezed into: Invent.
Tags: CNC
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