Second CNC table build, here we go again....

[Robin Hewitt]

Or you could move the rails up closer to the gantry they support. The gantry only needs to be 2" above the bed, doesn't it?

[Davek0974]

In theory yes but my Z-axis needs a bit of clearance as its a pretty heavy built unit. I am trunk not to lift the rails - that was design idea one, but it means you do not have a clear open bed - you have rails either side which makes loading harder especially if juggling a bigger sheet on your own.

My high-rail option is sketched out earlier, it had no risers at all on the gantry - the beam was right down on the carriages.

[Davek0974]

Or you could move the rails up closer to the gantry they support. The gantry only needs to be 2" above the bed, doesn't it?

To add scale to my poor sketches, the bottom of the gantry beam is 140mm above the bed surface, not too bad??

[Davek0974]

Ok, stuff is on order.

The final result is 50x50x2 for the gantry beam and frame legs, 50x80x2 for the motion support/bed frame.

The final capacity is going to be the half sheet 1250x1250 / 4'x4' size.

Direct drive rack and pinion with a view to change to belt reduction drive or even ballscrews later on.

[JAZZCNC]

Good man thats what like to see no messing around.

[Davek0974]

Good man thats what like to see no messing around.

Oh I can mess around, to a point, then I just dive in and go for it.


The top box is the gantry beam, 50x50, the motor sits down the bottom on a plate that swivels on the point shown. It will be spring loaded but restricted so it cannot jump out of the rack and chew it up.

A couple of 75mm gusset plates under the beam on the inside and a rib or two on the outside should stiffen things up a fair bit.

Motors, controllers and rack arrived today :)

[Davek0974]

Steel came Friday, nice and straight and after a few stress tests (me standing in the middle of a 7m long beam) I realised just how rigid this stuff is

Construction question:

I was thinking of welding on some 100x50x10mm pads under each corner of the motion frame and similar to the top of each leg, then bolting the two parts (welded motion frame/bed and welded leg assembly) together, just to give the opportunity to shim out any variation in leg length etc. Good idea, bad or not worth it???

Next, the bed/motion frame - should I attempt to weld it up as one piece, or weld closing plates flush into the ends of the cross-members, drill/tap and then bolt together?? If yes then how to weld the closer plates in - only way i can think is to heavily bevel the plates then insert and fill bevel with weld tying into the tube sides??? Plates would be 46x76x10mm.

[Davek0974]

Got the metal cut today...

Ends squared up and deburred, also cut,drilled and tapped the mount plates for the motion frame and the insert plates for the cross-beams. All ready for a welding session now.

I'm fitting castors and screw-down feet so I can move it about and level it up when assembled.

While I'm gathering info, extraction....

On my little table I built an air-box that sits along the front of the table between the now unused water tray and the slats...


In the front i cut a slot with an area bigger than the pipe cross-section and also slightly tapered towards the middle in an effort to even out the air flow. It works well as long as the unused parts of the table are covered - i get no muck in the shop at all.

It's connected to this radial blower...



I know I can get more out of it by dumping the corrugated pipe and fitting solid duct but it's not going to do a table with an area four times as big.

I don't have any spare power capacity so simply sticking a big-ass blower in there won't do it, need to get smart here. I could maybe run two of these blowers, they are pretty cheap to buy and with external motors they are immune to metallic hot muck.

I had thought about a zoned system - a four zone would work ok with the same blower. Question is, how would I do it???

Any smart ideas on extract??

Don't want water - its only half as effective and makes the machinery in the shop rust.

[D.C.]

how to weld the closer plates in - only way i can think is to heavily bevel the plates then insert and fill bevel with weld tying into the tube sides?

If you are trying to weld thin steel and thick steel together you need to run the weld pool down the thick piece of metal and then quickly move across to the thin metal, back to the thick metal, down a bit across just enough, back again to the thick piece, down again etc etc

If you try running a bead equally down the joint, the thin metal will blow out or the thick metal will not get proper penetration.

A CNC machine is never going to generate enough force to cause a properly formed weld to fail, it will probably never get above 1% of the failure point. But if you decide to do nice 'stack of pennies' welds that would make an instructor happy you will be introducing a lot of heat into your frame that will cause stress and distortion over time.

Clamp it, tac it, clean it, back weld an inch at a time and drink lots tea so the heat doesn't build up.

This is an awesome video for demonstrating welding heat distortion, well worth 10 minutes.