New Build Log ... 2440 X 1220 X 300

[EddyCurrent]

The links I posted here might be of interest ?

http://www.mycncuk.com/threads/7960-...2243#post62243

[Wobblybootie]

Is Mild Steel box 100 X 50 X 3mm okay for frames or would I be better using 5mm

[EddyCurrent]

Well I used 50x50x3 and 100x50x3 albeit on a smaller machine. These sizes were ideal and provided the design is right there should be no flexing of the structure. You do though have to consider that the 3mm thickness is not much if you intend making threaded holes in it, and you will for the X rails at least. In my case I glued a piece of 30x6 flat strap inside to beef the threaded thickness to 9mm.
I think you should go with the 5mm thickness

[JAZZCNC]

First let me say for a first machine this will be challenge so don't go rushing into it without carefully researching and drawing up a detailed design.

Forget driving gantry from centre and really forget using ballscrews for the long axis unless your prepared to get into rotating ballnuts etc. Rack & pinion is the way to go for the long axis. Using R&P brings it's own challenges but they are much easier and cheaper to solve than ballscrews and mostly just means choosing the correct motors and ratio's.

Profile is fine and easy to work with but it's very expensive compared to steel. For a machine this size then you'll want minimum 80x80x5 steel.
Build it in such a manner that you don't really have gantry sides, make the sides of the machine high and sit the gantry directly on the bearings. There are several builds here that use this setup so just look around the build threads.
Hi-win style profiled linear rails are what you need so forget round rails and they easily handle the debris, infact much better than round rail does.


Regards a 4th Axis then if your wanting to use like a Lathe then forget using a stepper motor.! They just don't spin fast enough or have enough torque.
Even if you want just a rotary axis for 3D work then I'd look at using a Rotary Axis converted with a Stepper motor.? Reason being is the Resolution. Steppers connected to pulleys don't give enough ratio so the resolution is low and for better or more detailed work then much higher ratios are preffered.

Make the Axis rails longer than the bed and put the 4th axis on the end has messing around with removing portions of the bed will soon become a pain. Also if the 4th axis is below the bed this means the Z axis is over extended giving much greater chance of flex and poor finish. Having the 4th axis on the end higher up makes much stronger giving better finish quality.
Another reason for the top rails extending past the end other than 4th axis is that you can machine the edges or ends of panels or cut dovetails etc this is very useful in many ways.

Like I say this will be a very big challenge for a first machine but it is do-able but not if you rush into it so go carefull, be patient and ask questions.

Oh and forget Kress spindles they are toys. Like eddy recommends 2.2Kw or larger is the way to go.

[Wobblybootie]

Thanks for the pointers.

Okay, let me get this right. Minimum 80X80X5 steel box ... Build frame with bed set to give max Z needed directly off the top of the frame (I haven't worded it very well but I know what you mean).

R & P for X and Y with Lead/Ballscrew for Z. Hewin rails on all Axis. 15 or 20mm or what?

Rails mounted on face of Gantry or top and bottom, which would keep Z close to Gantry?

I thought thick Ply for the bed with replaceable MDF as a spoil board.

The machine will only be used to cut Ply, MDF(Ugh!!!) and natural Timber.

Good thinking re running the gantry past cutting area for edging etc.

I already know the maximum dimensions/diameter that the 4th axis work requires so maybe I can build it into the walls of the frame parallel with the Gantry(the work is not complex or detailed, imagine an egg timer but with a spheroid shape of smaller diameter instead of a narrow neck with a tenon and slot at each end They are the upright supports of the Rocking Horse frame).

Is there a way to work out what size motors/ gearing I will need to get good accuracy/repeatability/resolution?

Tim

[EddyCurrent]

20 mm rails minimum, forget 15 they are a pain to grease for one thing, I would never use that size again.
Gentry Y rails top and bottom keeps Z close to gantry
I use ply for the bed 2 x 18mm birch glued together and bolted down, plus spoil boards. It's strong but will flex under load if not well supported by frame cross members underneath.
The 4th axis is great but you could machine the supports in 2.5D as two halves you glue together lengthways.
I think Jonathan posted something about motor sizing, use the forum search box.

[Clive S]

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

[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.!!