Comments sought on new build: A CNC Router for RC Gliders and Planes

[Lee Roberts]

That's a smaller nema34 of the bunch, colossal aren't they !

.Me

[njhussey]

OK so I'm going to order a new 600 x 600 x 300 cabinet and parts for my electrical panel, can someone check to see if I need anything else...if I've missed something:

600 x 600 x 300 - 1 off
20A door isolator (presume NC contacts so contact is broken when door's opened?) - 1 off
Emergency stop panel mount button with NC contacts - 1 off
Green push button panel mount, NO contacts - 1 off
Red push button panel mount, NC contacts - 1 off
Green LED panel mount light - 1 off
Top hat slotted DIN rail - 2m length
2.5mm grey DIN terminals (gettting 50 off)
2.5mm Earth terminals (getting 25)
bootlace ferrules 0.75mm, 1mm, 1.5mm (100 of each)
Crimping tool - 1 off
25x40 slotted cable trunking - 2m

I've got 2 x 120mm 24V cooling fans which I'll get filters for which I'll mount on the sides
I've got 1.5mm^2 4 core shielded cable (about 25m) for connecting the steppers to the drivers
Was thinking of getting 3 core 0.75mm^2 shielded for the limit switches, is this the correct cable to get?
What other cable will I need?

I'll be ordering parts for my PSU, so far looking atthe below components. Can someone please comment on them please to make sure I'm going in the right direction?

Calcs:
Toroidal voltage V(DC)=1.4xV(AC), so 70/1.4=50V
Question, I've read that if yout supply is greater than 230V then the voltage output is higher which would be a bit too near the max 80V the AM882's can take if there's back EMF from the steppers on deceleration? So would it be better to get a 2x24V transformer as linked below?

Current: I've 3x4.2A steppers (Z axis, Y axis and a future 4th axix) plus a 6A Nema34 making 18.6A but 60% is 11.16 so call it 12A? So transformer needs to be 12Ax50V=600VA sdo nearest is 625VA.

Capacitors: C(uF)= 100000xCurrent/Voltage = 17,143 so a 22,000uF 100V capacitor should do....any ideas where to get an economical one or are multiple smaller ones better?

Bleed resistor, I'm either going to use one switched through a relay or is it not worth bothering with? I'm more inclined to use one as I'd rather discharge safely than just leave the steppers to drain the caps. My calcs are: R=5000000/2200=227Ohm, rated at V^2/(2xR)=10.8 Watts. Any suggestions on what to get here?



Toroidal Transformer - http://www.airlinktransformers.com/c...nge/CM0625224/

Rectifier - http://www.rapidonline.com/electroni...-mb352-47-3226

I'm sure there's more that I've forgotten but I'm sure that someone will suggest it (hope so...)

[Wobblycogs]

The advice I was given was to get a 2x24V transformer and having measured the output and input voltage I'd definitely say that's the best advice. The power to my house at least tends to be a few volts over 230V so a 2x25V transformer + back EMF would probably push it over the magic 80V.

I went for a 1000VA transformer in case I ever added a fifth axis, to be honest though I reckon I could probably add a 9th axis it's so over kill. At 625VA you should be fine with just three steppers, you may or may not need to put together an inrush limiter, you'll probably get away with a Type C or D breaker though - a type D breaker would stay in occasionally for me.

For capacitors I used 4 of these: http://www.ebay.co.uk/itm/1-NEW-NIPP...-/280934713637 Multiple small caps were a lot cheaper than one big one.

I have a bleed resistor which will bleed the capacitors in a few minutes if steppers aren't attached, if steppers are attached they bleed in seconds. I forget what rating it is but it's a lot less than 10.8W, 0.5W rings a bell. I know the recommendation is to bleed the caps in so many seconds but it was a play off between safety and heat produced in the case. To improve safety I fitted a UV style meter to the front of the case to tell me cap voltage and fitted a cover over the capacitors, total cost about £5.

I think you'll want more bootlace ferrules than that and I used a mile of single core cable (can't remember if it was 0.75mm or 1.0). You can get jumper bars for the DIN terminals to make them one large block which came in handy.

[Clive S]

Neil Have I missed the rectifier? The cable for the limit switches can be as small as you like as there is practically no current to speak of.
I have used 25 -25 with no problem and my personal opinion is that the bleed resistor is not needed. ..Clve

[njhussey]

Neil Have I missed the rectifier? The cable for the limit switches can be as small as you like as there is practically no current to speak of.
I have used 25 -25 with no problem and my personal opinion is that the bleed resistor is not needed. ..Clve

Yes you've missed it.....I'm going for a 35A 200V one from Rapidonline....

I'll get some 0.75 tri core shielded cable then for the limit switches.

I think I'll measure the supply voltage here and see what it is out of interest.

[njhussey]

Done some machining today. Also had the rest of my order from Gary @ Zapp this morning.



Machined the Y axis ballnut bracket that connects the Z axis to the Y axis ballscrew...



Seeing as I've got my MGD16 ballnut housings for the X axis I machined the right gantry side.



More machining tomorrow

[njhussey]

Both ballscrews are fixed to the frame now. Machined the second gantry side for the ballnut holder. I bought some bigger pulleys for the X axis (40 tooth ones instead of 20 tooth ones) as the standard belt I've got is 2525mm and there's a lot of spare belt with the 20 tooth ones. I'm using an 8.7Nm Nema 34 run with a 240V drive so I'm hoping the extra inertia from the pulleys shouldn't cause too much of a problem?









I've bought the transformer, rectifier, 4 4700uf caps and holders, a 600x600x300 cabinet, loads or bootlace ferrules, wire, din rail and terminals, door interlock....tonnes of stuff but I'm sure not everything i need.

I'll be picking people's brains about the safety relay/s for the safety features...

[njhussey]

Did a bit of machining today. Machined 2 x countersunk holes to suit M8 bolts on the Z axis back plate to take the Y axis ballnut bracket.





The Y axis ballscrew will fix straight onto the back of the gantry so won't need shimming.





Scratched my head a bit today trying to work out how to mount the Y axis stepper motor so I can adjust it to tension the belt. In the end I've decided to put the pulley on the rear of the stepper and mount the stepper on the front via a bracket...





I'm hoping that as the only tension on the stepper is from the belt that this arrangement will do?

[toomast]

Nice build sir!

I don't have first hand experience mounting stepper motors this way but it looks trouble. Maybe too much strain on motor?
I guess as long as you fabricated the mount you might as well run it and find out.

Do you have special machining on ballscrew? I notice longer area for the pulley? Or is it just my eyes...

[JAZZCNC]

Scratched my head a bit today trying to work out how to mount the Y axis stepper motor so I can adjust it to tension the belt. In the end I've decided to put the pulley on the rear of the stepper and mount the stepper on the front via a bracket...

I'm hoping that as the only tension on the stepper is from the belt that this arrangement will do?

Bad idea that dude.!! Bearings will suffer and could cause resonance which will affect motor. Just pocket the Mount deeper so pulley goes thru it.
Or have idler arm tensioner tension belt from side of gantry.