Sweet dreams CNC - more than one step beyond...

[sweetdream]

Design spec;

Seems like a long time ago I asked the question "what do I want to cut with it?" "Don't know yet" was the unhelpful conclusion! But after much thought about what I needed, opposed to what I wanted. I came up with a basic spec; a very strong, fast CNC that could work anything I threw at it including metals and portable enough to fit through a standard door.

This CNC was not going to be an exercise in what was theoretically possible, just a no frills, very strong, quick and easy to build structure that a tank could drive over! It had to be built without recourse to another CNC or exotic measuring equipment, just the equipment I had, could borrow or was prepared to buy. I did have some milling done on the frame, (making the 26x 22mm holes for the conduits, we had a jig on the mill and just knocked them out but that could as easily have been done with a pillar drill but would have taken a lot longer. Apart from that milling job, it was built with power tools and a pillar drill.

A few additional criteria added during the build, the spec stayed the same and was met. It fits through a fairly standard door frame too!

For the frame, I went for a PVC thick wall tubing with copper plumbing fittings. Skate bearings and ACME thread and I found a couple of old plastic pallets to use as the bed too.. LOL, only joking, just checking you're still awake. I realised fairly early on that for the performance I was wanting, a hybrid of bolted and welded steel construction and profiled rail were the right option. So large quantities of 80mm thick wall box section steel were calculated. The Hiwin catalog and price lists took a beating too.

Other staples were the standard 2.4kw water cooled spindle, NEMA 34, 8amp drivers, Omcron switches, nothing unusual except the later addition of a 250 rpm single phase spindle to deal with any low speed spindle requirements, mounted parallel but alongside the main spindle.

Although I had a good idea of what I wanted to achieve and some ideas I'd not seen used before, when I started I did not know all the questions that I'd need to answer before the build was finished. For the benefit of anyone reading this before building their own CNC thats a small but important point. You may not know all the questions you'll need to answer. :-)

But of course, the devil is in the detail and how you put it together.


So first build a workshop! lol..




or convert an old shed. After repairing the roof [picture coming] putting in a concrete floor, [find pics] some storage arrives courtesy of hearing about them being thrown out!

The wooden carcasses are solid but the plastic trays look a little fragile. Might swap them out for wooden ones as and when they break.

I've ended up with the back wall of the shop, floor to ceiling with these bins and a bench cantilevered out between them at a suitable height for standing at the bench. I had enough left to put them on one wall from head high to the ceiling giving a total of about 200 bays, 17inches deep, with either 3 inch or 6 inch high trays. Some I put shelves in as seen in the Hammer store below.



The storage starts filing up...



All workshops should have a good selection of tools!



In case of working late, some refreshments may be required! :-) [update] the beer has long gone! There's half a bottle of gin, 5 litres of cider and some fruit based cordials in case of visitors.. :-)

[GEOFFREY]

Good way to start, prioritize refreshments. Well done sweettooth!!!- I mean sweetdream. G.

[njhussey]

Liking the cut of your jib...looking forward to seeing details of the build and the machine...

[Washout]

I think any man with a beer fridge in his workshop is going to fit in well around here

Although chilling Spitfire is not an approach I would advocate ;)

[sweetdream]

I think any man with a beer fridge in his workshop is going to fit in well around here

Although chilling Spitfire is not an approach I would advocate ;)

I thought the fridge was a requirement for joining this illustrious gathering??

There's got to be a joke about spitfire and cold in the spirit of the other ads.. just can't think of it..

[Jonathan]

I think any man with a beer fridge in his workshop is going to fit in well around here

Does chocolate in the 'fridge' at -70°C count?

[sweetdream]

Does chocolate in the 'fridge' at -70°C count?

..chocolate counts only if you can eat it!

[sweetdream]

As mentioned the frame was initially going to be 80mm box section, measuring the steppers showed that by careful arrangement, the frame could act as the stepper motor mounts, saving the costs of dedicated mounts [pic] this worked rather well on the X and Y axis but as there was no box section on the Z, it couldn't be done there. The Z axis had its own design spec, small, as compact as physically possible and strong, very strong.. The Z stepper mount needed to be adjustable in one axis (same as the X as it happened) but solid in every other. 4 pieces of 10mm alloy sorted that.

Getting the spindle as close to the gantry as possible, mainly to reduce the leverage effect from the spindle under load, was another area I spent some time on. It's ended up 140mm from the centre of the spindle to the centre of the Y axis and the Z axis is 144mm wide! This minimises the amount of table space lost to the spindle. The only parts on the table, profiled rail, and ball screws that have to be there but cannot be used by the Spindle when the Z axis is 1mm off the stop switches is 146mm on the Y Axis, 73mm each side.

It should also be remembered that smaller chunks of 20mm ECO plate are cheaper than Large chunks of 20mm ECO plate! :-)

[pic]

The height [?] is where the space was found to mount the cooling system pump / reservoir and the Z axis stepper pulley arrangements. A couple of junction boxes take care of the wiring and act as an anchor for the twin runs of echain.


Very happy with the result, the Z axis stays within the confines of the 4 Y axis trucks wherever it moves, the Horns do their job and the 5mm thick steel triangulated plates that were shown not to be necessary on the simulation, but for the sake of 30mins welding, "over engineering" was preferred to "always wondering". lol.

[JAZZCNC]

Haaaaaaaaaaaaaaaaaaaaaaa

[sweetdream]

The suicide test;

So the machine is running without smoke pouring forth, all the bells are ringing and the whistles whistling.. (why don't bells belling?) spindle has been powered up and a couple of small tests run. so still air cutting at this point. ...Commissioning continues.

Time to up the ante.

Bearing in mind that body parts may well be found in the same vicinity as gantries and other moving parts, cue the suicide test.

The suicide test
Set gantry at one end of the X axis.
Set speed in excess of 30 metres per minute.
Hold down the button and drive the gantry at the highest speed you can get into the stops at the other end of the X axis!
Wait for the very loud bang!

Repair as required



I hit the switch and there was a whirr and a blur as the gantry shot from one end to the other in what seemed like well under a second. There wasn't time to take my finger off the button before the limit switches did their job. I don't have an exact figure of the speed it reached but I can tell you that the gantry was still accelerating when it crashed into the the Omcron microswitches at the far end of the profiled rail. lol.

The switches have 3mm of movement, .1mm between open and closed. Assuming 4mm of total travel, the speed of the gantry when it hit the switch can be calculated from the time taken between the switch being activated and the distance traveled before stopping. The speed of light and the price of mars bars being unequal, the gantry was travelling in excess of a "greased lightning" when it got to where it was going!

Confidence went much higher after that test. The limit switches worked great, even in the worst case scenario of a runaway gantry, the worst that I'd expect to see was a crushed switch as it didn't get as far as hitting the metal stops.

If you are building a machine, fit the limits and estops before you wire the motors. Watching a gantry move at high speed is really not something you want to see from the wrong place. If your gantry moves as quick as this one did, you'll never make it to the power switch in time.