Steel Framed CNC router

[JAZZCNC]

Hi Joe,

I just stumbled across your thread by accident whilst Googling "budget cnc router" and I cannot believe nobody has commented yet.

Thanks for sharing your work. I am impressed and inspired.

Kind regards,

Kenneth

Kenneth Sorry but I've just got to say this and please don't think I'm just being negative, the only reason is to save you some pain and disappointment.!!

This design has some serious issues and use's just about every design you want to avoid for a good strong accurate CNC machine.!! . . . . I'll list them in bad news order.

Gantry driven from just one side with narrow bearing separation.
Flimsy Gantry using Unsupported round rails for strength.
Unsupported round rails on all axis.
Bad Z axis design and placement creating long lever affect promoting resonance.
Belt drive with no reduction direct onto motor giving poor resolution and causing premature wear of motor bearings.

Not lots of issue's but they are the worst things you could possibly build into a CNC machine.!! Let me explain some of them better.!

First the gantry driven from one side.? This should be obvious but it still amazes me I see it used or considered.!! Basicly driving from one side the twisting forces on the gantry increase greatly with every MM the Y axis moves away from the driven side creating an ever increasing lever. This results in the gantry racking and deflecting the tool massively, when cutting some hard materials like Aluminium it's very likely to cause sticking and stalling with even moderate depth cuts.?

Unsupported rails don't offer any strength regards combating twist etc and the gantry mainly uses them to do just that, they also act like spring boards. Just watch the bars when gymnasts use the parallel bars to see the same affect.? Even short length unsupported bars can suffer from resonance which will show at the cutter.

The Z axis besides using unsupported rails hangs too low down from the gantry basicly creating a long lever, which again will flex and resonate the tool when cutting any moderately hard material. Really it defeats the point of high sides, the goal should really be to minimise the distance from Y/Z axis mounting point to the cutting tool tip.
This can be down by either raising the material to the tool or lifting the bed plus matarial to the tool but in this case the low hanging Z axis restricts this.?

Belt drive works good and his very accurate but in this case it's badly implimented.?
It's direct drive with no reduction ratio means the pitch or distance traveled for 1 revolution is huge, this lowers the resolution greatly but it does allow high speeds which really arnt needed 95% of time.
The direct drive onto motor shaft stress and over heats the bearings and kills motors.!
To give you some idea the pulleys look around 20 Teeth so will be in the 30mm diameter region. Using Pi this gives a circumference of 94mm. So your distance traveled for 1 rev = 94mm So assuming the usual 200 steps per/rev and half stepping microsteps(400) the distance traveled for 1 step would be 0.24mm which is really not a good resolution.! . . BUT it gets worse.?
Stepper motors have a usable RPM limit of around 1000rpm with a 94mm pitch this gives 1000rpm*94mm=94,000mm/min (94mtr/min)which is great if you want to go from A to B very fast but with average cutting feeds being in the 2-2.5mtr/min region and for hard materials like Aluminium or some hard woods being more like 1000mm/mtr then this puts the motor in an rpm region it just can't work at and give smooth performance resulting in cogging and jerks.
IE: using average cutting feeds 2500mm/min the motor will need to spin at 2500/94=26RPM at 1000mm/mtr = 1000/94=10.6 RPM

So has you can see these are really low RPMs and just about useless for detailed or sub 1000mm/min feed rates which is common in every day cutting.

To use belt drive affectively and keep the motors working at RPMs they like best it needs minimum 3:1 ratio, the other thing being belt drive doesn't actually workout cheaper than ballscrews by the time you have have bought pulleys,belts,bearings and made plates etc and it's certainly much more work.!

Like I say I'm not being deliberetly negative just trying to save you some disappointment thru my experience of building belt drive machines.!! Thou the other issues are far worse IMO and will show in the work produced.!!

Hope this helps.!