New Build Log ... 2440 X 1220 X 300

[Wobblybootie]

Thanks guys ...

[JAZZCNC]

I think Dean was suggesting R & P for the long axis only with ball screws for the other two. ..Clive

Yes I was but R&P on both isn't a problem it's just easier and better to use a Ballscrew and you can easily getaway with one at this width so best to do so.

Regards Motor sizing then this is where it's easy to go wrong for first time builders building this size machine. You will need Nema 34 motors for the X axis with R&P, roughly around 6Nm depending on Gantry weight etc. But these need the correct drives to get the performance you'll require and it's here most new builders often get it wrong. You will need AC drives to give the best performance not the usual DC drives you'll see around. These are more expensive or seem it until you factor in that they don't need a DC power supply and run direct from the mains. This offsets the cost makes them not so expensive but still more than DC drives but worth every penny and a must for good machine running Larger Nema34 motors with higher inductance.

Regards R&P you'll want Mod 1.5 and with a ratio between 3:1 & 5:1 depending on resolution and speed your aiming for and motors used etc. Resolution required for wood is not massive so not a big issue but speed can be more important so you'll go with ratio and pinion size that gives the best balance of both.
The pinion size used along with ratio will determine the pitch and resolution. The usable Motor RPM will have to be factored into the equation to determine the best balance of speed, resolution and torque.
For instance a Mod 1.5 pinion size of 20 teeth will give a pitch of (20x1.5)xPi= 30xPi= 94.25mm pitch which is massive but then we apply 3:1 ratio so this becomes 94.25/3=31.42mm pitch which is still quite large but better. Now factor in that typical Nema34 motor gives aprox 900Rpm of usable speed before torque drops away then you have Rapid speed of aprox 28,300mm/min with a resolution of aprox 0.2mm based on a micro stepping of 1600.!!
Now great Rapids but lowish resolution which will be higher than the calcs suggest as micro stepping isn't or shouldn't be relied upon for resolution.
So lets do the same for 5:1 ratio. So 94.25/5=18.85mm pitch 18.85 x 900 = 16965mm/min with resoultion closer to 0.01mm better and about right.

Hopefully this will give you an idea of how to determine the ratio and pully sizes etc to get the speed and resolution required.
The pinion size is important for both pitch and tooth engagement, too small and not enough teeth engaged in rack so can slip or shear teeth etc. Too large means larger pitch and more ratio.
More ratio means larger Pulleys on the driven side which makes building harder. Ie 20T @ 5:1 ratio means 100T on the driven pulley which is a large pulley.
Often when the ratio gets large then your better with a gearbox but comes with factors that need to be looked at, backlash being one and expense along with extra motor power for the extra friction compared to timing belts which are very efficient compared to gears in gearboxs.!

So again as you can see it's a little more complex than just saying I'll use R&P and if you don't size and factor these things like motors, drives and ratios etc then you'll 99% get it wrong and be disappointed.

With ballscrews it's easier in some ways except when it gets long like the X axis due to whip and interia etc. Even on the Y axis you will again still need to size the ballscrew size/pitch, motor, drive relationship correctly. Here it will go something like 20mm Dia 10mm pitch with nema34 6Nm motor on AC drive.
Wouldn't use 16mm dia as it's just a little on the long side and 20mm dia is a little large at this length for nema23 motors. Staying with 34 motors and AC drives for X & Y will give a better balanced machine electricly.
Use 16mm dia 5mm pitch for the Z axis with 3 or 4Nm nema 23 motor running 80Vdc drive with 68Vdc PSU. This will give you best performance with a heavy spindle setup and some overhead on the drive and power.

Hope this helps and shows why you need to go slow and design correctly.!!

[Wobblybootie]

Thanks everyone. BUT all change!! (for the better I think, read cheaper and Easier!!) I have spent the day on the floor with paper templates that I will be using. I have discovered that I can fit the the templates onto 2 half sheets IE 1220 X 1220 with very little waste.
So Ball screws all round and Hiwin 20mm blocks and rails.
I have sketched a frame for your perusal and comments that I will post shortly (when the kids have finished homework). Is there a preferred format?

[EddyCurrent]

Just upload jpeg snapshots of the drawings from whatever software you use.

[Wobblybootie]

First Draft.



Gantry to fit on 20mm rails atop the frame. Z travel needed 350mm

[EddyCurrent]

I think you would be better with something on these lines.
http://www.mycncuk.com/threads/6565-...4294#post54294

You don't need a back in it and your top beams are not suitable for carrying the X rails due to the legs coming up through.
If you notice, my top beams are bolted to the frame, this allows for shimming to get it within a gnats knacks, ready for the epoxy.

[JAZZCNC]

No very difficult to build that way and too weak in key areas. Not enough bracing at bottom and the top rails will be very hard to get on same plane.
Look at this and Work along the same lines as this with regards bracing etc but without the adjustable bed if not required with slightly lower sides and you won't go far wrong.

This machine is in progress and will have 4th axis on the end when it's finished.

Also Why 350mm Z axis travel.? That is quite a lot and if your planning on cutting anything hard you'll get lots of flex and resonance giving a poor finish.

Edit: Opp's Eddy beat me too it.!!. . . or like Eddy's done. . Lol

[Wobblybootie]

Why 350mm? Because the whole idea of building the machine is it to perform two tasks, firstly to cut forms from 18mm Birch Ply and then when the forms are assembled by stacking one on top of the other to mill to the final shape. If I cannot have 350 mm of cutting depth I am throwing money away.



I was hoping to be able to store ply sheets ( I buy 10 at a time) under the bed. However with all the triangular bracing everywhere that also appears to be a No No ... Oh well back to the drawing board.

[EddyCurrent]

For some reason, after reading you're other thread, I had it in my mind you were laminating at 90 deg. to that shown in the photo.
It's okay milling to 350mm depth but what length cutting tool do you envisage using ? you don't want the spindle crashing into the job.

[Wobblybootie]

I'm sure I said fore and aft ... nose to tail anyway now you see the size of the profiles that are cut.

Typical Forums ... lol ... Half say no need for Epoxy on a machine cutting wood ... the other half say it's absolutely necessary.

The Legs didn't come through the tops!! 'capped' they would have been flush ... I hadn't planned running the rails across them anyway. Thinking about it they could have been raised at the back to provide hard stops and mounting limit switches.