One ring to rule them all?

[routercnc]

Hi Rufe0

These are my thoughts for a machine like that -

1. Double gantry, as per your drawing, and the machine I linked to, and MechMate:



and Jonathans machine a while back-
http://www.mycncuk.com/threads/6484-...strong-machine


2. Box Z, as per the link machine and Jonathans using 4 rails on the Z

3. Twin ballscrews on the X,Y and Z axis to avoid racking and allow the bearings to be close to maximise travel. Z twin ballscrew is to help with lifting the weight of the 1m axis, plus keep the loading central to the spindle.

4. Hiwin rails all round, 20mm or bigger.

5. Height adjustable bed with vertical rails at each corner -
Manually
or hydraulic using 1 ton bottle jack in the centre
or ballscrews
Which ever option you must be able to 'lock it down' before cutting - like you would lock a quill or lathe saddle once in position. Maybe thick angle brackets around the edge to nip up and hold it fast.

The alternative to height adjustable bed is to use bedplate mounting blocks (scroll mid/bottom):
http://cnzbzg.en.alibaba.com/product...eers_cube.html


6. Spindle(s) capable of high speed medium torque [alum, wood], plus very low speed high torque [steel]. Maybe via a single spindle linked to a separate cartridge spindle with pulley drive to give different ratios.

7. Automatic Tool Changer (air powered), optionally with ATC carousel.


In terms of your calculations the bending stiffness calc is straightforward (load one beam then half the deflection for 2) but that only accounts for the Z axis weight and the plunge stiffness. For the sections you are talking about (200x100x8) etc this will no problem for simple bending. However, where you will come unstuck is having enough torsional stiffness when cutting in X (gantry moving forward or back), especially at 1m extension. I don't think hand calcs would be possible on a double beam in torsion so you need an FEA program if you want to work it out. I notice Fusion360 has FEA built in and this is 'free' so worth a look. This will give you deflections once you have learned how to use the CAD part of the software.

[Rufe0]

Hmmm well I've tried but I just cant get to grips with Fusion360. I tried one called LISA which was ok drawing wise but the actual FEA never worked.

As I see it the gantry and Z are like a T in cross section, which means its a simple bending calculation. I'm getting around 0.025mm deflection at 1 meter with 200x100x8 seperated 400mm center to center.

Think I'm going to have to scale down my plans though. I will have to find a cheaper alternative to what i'm looking at the moment, maybe just wait for stuff to come up on ebay. Currently I'm pricing it from Zapp Automation for ballscrew related stuff and cnc4you.co.uk for the rails/carriages.
£110 Ballscrew support fixed end
£36 Ballscrew support floating end
£88 Ballscrew
£132 Ballnut
£37 Hiwin 25 Carriage
£37 Hiwin 25 Carriage
£108 Hiwin 25 Rail
£40 Motor
£10 DIY motor driver
Thats about £600 per screw. I need 6 screws for the basic XYZ table thats £3600. +2 for the lathe tailstock £4800. +4 for the moveable bed £7200.

So now I have to decide whether it would be best to get an old bridgeport for around £2000.

[JAZZCNC]

Head East my friend.!! . . . .China is the place for ballscrews. Contact Fred Lee at BST automation on Ali express and get a quote. You'll be very pleasantly surprised. If you want higher spec ballscrews then just Ask because he sells higher spec than class 7 advertised.

He Also sells Hi-win rails much cheapper than you'll buy in UK.

Whats the £10 DIY Motor drive.? . . . . If your thinking to build your own then unless your really really really good with electronics and have full knowledge of whats required then I'd strongly urge you to forget it and buy some decent drives with the Money you'll save from Fred Lee.!!

[Rufe0]

Bloody internets been playing up for the last couple days but yeah I had a look and thats great, this will change everything. Thanks

[Rufe0]

Yeah I dunno about the controllers, I'm using an arduino and it seems promising so far, i've just got a rotary encoder in the mail today so will be testing that out soon as.

[irving2008]

Listen to Jazz on motor drivers.. nothing you can develop with an Arduino and an off the shelf motor shield will be useful. The typical Arduino motor shields are toys, good enough for a NEMA17 stepper for a 3D printer or a small robot, even the so-called high-current ones are limited in reality because they don't have big enough heat-sinks and are restricted to 24v. For the sort of machine you're contemplating to perform at any useful rate, those screws will require big NEMA23 or even 34 size steppers, and the driver will need to deliver phase currents of 4 - 6A @ 60-80V
.
Plus an off-the-shelf stepper driver has complex algorithms for anti-stall, anti-resonance, fault-alarms, etc. You won't get close to anything useable developing your own software or using open-source stuff for several months if not years of development...

[Rufe0]

Bah Fred can only do C5 ballscrews upto 1m long. Says he can do C7 longer but I think its probably worth paying abit extra.