design choices when the budget will not stretch?

[m_c]

As Jonathan has said, it all depends on what you're trying to acheive.
You're basically asking how long's a bit a of string.

[wilfy]

so what decides how you choose the right size motor? i presume weight of what you are driving comes in to it.. does the size/weight of the screw come in to the decision? does the materials to be cut have any sway on it? i also dont understand how the word resolution is used in cnc.. my thoughts are it is similar to a picture where there are so many dot's making curves smoother, does the same thing happen when you lose resolution in cnc? you start to get blocky curves?

[irving2008]

so what decides how you choose the right size motor? i presume weight of what you are driving comes in to it.. does the size/weight of the screw come in to the decision? does the materials to be cut have any sway on it? i also dont understand how the word resolution is used in cnc.. my thoughts are it is similar to a picture where there are so many dot's making curves smoother, does the same thing happen when you lose resolution in cnc? you start to get blocky curves?

You might find this useful: What size stepper motor do I-need.

Yes the size and weight of the screw is significant. The materials to be cut do have some impact, in the sense that they dictate how much force is needed and therefore, to some extent, the force able to be transmitted by the screw, but generally the materials dictate the rigidity required for a given depth of cut (DOC). To machine steel at any useful rate needs big lumps of cast iron, not ally...

The screw pitch determines resolution, i.e. the smallest step that the machine can make. For instance a directly driven 10mm pitch screw, using a 200step motor, gives a minimum step of 10/200mm = 0.05mm. This is plenty good enough for wood working, but marginal for metal working, where 0.005mm is preferred. Also if you want to move at, say, 6m/min, then you need to spin the screw at 600rpm, 10rev/sec or 2000 steps/sec. At 600rpm theres going to be relatively little torque available so there is a danger of losing steps... its all a balancing act :)

[JAZZCNC]

so what decides how you choose the right size motor? i presume weight of what you are driving comes in to it.. does the size/weight of the screw come in to the decision? does the materials to be cut have any sway on it? i also dont understand how the word resolution is used in cnc.. my thoughts are it is similar to a picture where there are so many dot's making curves smoother, does the same thing happen when you lose resolution in cnc? you start to get blocky curves?

Can't really Add more to what Irving already said but I would like to comment on this portion of what Irving said.!

At 600rpm theres going to be relatively little torque available so there is a danger of losing steps... its all a balancing act :)

This comment I don't agree with and is not my experience.? Stepper motors using higher voltage giving best performance will easily work at 600RPM and have a usable amount of torque available, they certainly won't be in danger of losing steps unless cutting really hard and deep or moving/stopping heavy weight around fast.!

Irving's correct that it's a balancing act and very important you consider what you want the machine to do and the feedrates required, then use this information in conjunction with machine data like weight to Accelerate and de-accelerate to aid select correct screw/motor/drive/PSU choice.

All this said when it comes down to it selecting Screw/motor/PSU etc there's only so many options for a given size/style machine doing certain kind of work.? So don't rack your brains trying to figure what's needed just browse the forum's find a similar size and type of machine that looks like it works how you'd like and ask what they use.?

Think you'll find it boils down to only a few options for good quality accurate machine. Like these.!!

Small moving gantry woodworking/plastics only machine: 10mm pitch ballscrews, 1.85Nm, 50V drive, 40-45VPSU
Medium to large moving gantry woodworking/plastics only: 10mm pitch ballscrews 3Nm, 75V drive, 65-70VPSU

Small all round material moving gantry machine: 5mm pitch, 1.85Nm, 50V drive, 40-45VPSU (10mm pitch option costing resolution/torque which is usually preferred for Alu or higher detail work.!)
Medium to large all round moving gantry material: 5mm pitch, 3Nm, 75V drive, 65-70VPSU ( 10mm pitch is high consideration on this larger size machine and the trade off between loss resolution/torque and speed gain can be justified to a degree.?)

Fixed gantry machines can get away with using slightly smaller motors due to less weight being shifted and the fact they tend to be smaller machines anyway because of the room they take up (basicly double the cutting length or width).
Mostly you'll find fixed gantry machines are built for strength and resolution so don't need high feed rates so use high lead screws which give much higher torque.
so.!. . . 5mm pitch/1.85Nm/50vDrive/44-45v is common. With high detail and really low pitch screws then lower powered motors/drives etc can be used.

These examples presume ballscrews and decent supported linear rails if other methods are used then these options could change slightly with chance the motors/drives/psu may need upgrading to over come the higher friction with some methods.?

Then there's the Milling machine/lathe conversions which play to different tunes and very dependent on machine so won't go there.!

[MATT]

Here's a topical but tricky question for the illuminatti that keeps coming back in various guises:

Assuming the budget will not stretch to ALL of the following features, which one (or two) would you choose to compromise on?

Welded steel framed gantry
20mm alloy Z and Y axis
Water cooled 2.2kw spindle and VFD
M542 drivers
50v power supply
1610 ballscrews
20mm supported rail
variable height table

Unsure on how you budgeted for all the above items, but I need to ask how you intend to, or what you mean by variable height table.

I am also building one with a variable height table (large scale things don’t need as much rigidity, whereas steel does so tighten up the size and specs for cutting smaller steel parts).
Only now I am nearly finished it, I wish I had compromised on the space of the X axis and had a moving X table with a fixed YZ gantry that could be dismounted and adjusted up or down) instead of moving the whole fixed bed up and down. But sometimes you got to learn the hard way (I was told, but am always too stubborn, haha).

Have fun, and let us know how it goes, I will post my machine once the frame is welded.

[mocha]

Unsure on how you budgeted for all the above items, but I need to ask how you intend to, or what you mean by variable height table.

I am also building one with a variable height table (large scale things don’t need as much rigidity, whereas steel does so tighten up the size and specs for cutting smaller steel parts).
Only now I am nearly finished it, I wish I had compromised on the space of the X axis and had a moving X table with a fixed YZ gantry that could be dismounted and adjusted up or down) instead of moving the whole fixed bed up and down. But sometimes you got to learn the hard way (I was told, but am always too stubborn, haha).

Have fun, and let us know how it goes, I will post my machine once the frame is welded.

Hi Matt

In this context, it's a hypothetical question. It supposes that you are in the planning stage of a CNC build and that the feature list of the desired machine had those items in it. It also assumes that the budget can not be stretched, cajoled or persuaded to pay for all of them. So, which would be the areas that could be compromised with the minimum effect or to put it another way, for the benefit of those yet wanting to do a build but thinking they don't have the cash; what priority to give to the various elements. My initial design criteria were none of the above and my plans don't include any of those features. The width of my shoulders, the doorway is needs to go through and the price of jaffa cakes were higher on my list. :-)

For me the answer was simple, wait until I can afford to get the bits that I wanted. Simple but difficult if you see what I mean.

There are some great methods in other threads showing how the question of raising the bed can be accomplished, jazz posted a link for a good one as I recall. I went for an asymmetrical bed that I could just flip over and get different heights but if I was doing it again, I'd have a longer look at a packing system to lift the work to the optimum height.

Look forward to seeing your pics too.