design choices when the budget will not stretch?

[mocha]

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

[JAZZCNC]

There's only one and that's the table.!! . . . Some times people need to accept they can't afford to build just yet and save up a bit more.!
99% More often than not if they do proceed before ready it does cost them much more in the long run. . :(

[wilfy]

i'll give my answer from a noobies perspective.

welded steel framed gantry... i dont know if i'm understanding this wrong, but i see plenty of alu profile gantries even on here there are people using profile frames

20mm alloy z and y axis, again not sure if i understand right, but if you mean the mounting plates then yes i'm looking this way myself and think it's needed

water cooled 2.2kw spindle and vfd, personally i've not dug deep enough yet to find the answer to the power needed for most operations, but my understanding is if you dont start water cooled then you will end up here anyway, but as i said the power at 2.2kw is something i feel might be something you can compromise on, personally i'd been looking at 1.5kw

m542 drivers and 50v power supply are both things i feel come down to your personal preference. your desire for speed will have alot to do with how much you want to spend on drivers and psu... me personally i keep telling myself i'm going to keep cost down by staying with cheap drivers.. the reality is that i will more than likely want to upgrade to better motors once i realise what i should have bought.. hey i might even get to the stage before i even buy my first motors..

1610 ballscrews, now personally with loads of reading i'd already decided ballscrews were going to replace that original thought of using threaded rod.. however i'm not sure what the difference is between 1605 and 1610 yet and how they will apply to myself

20mm supported rail is pretty much a given for the size of table i'm considering and maybe table size is what decides whether you go supported or not, the arguements here for supported rails really do make it hard for you to consider anything but them

variable height table is something i never even considered i suppose a trade off would be to build a low table with the intention of packing it out everything you did any work that needed it raising, but then i imagine alot of our work is like this therefore i'll personally not even consider this yet

[Jonathan]

I'd already consider M542 drivers, 50V PSU and 20mm round rails a compromise for most machines. Any of the things listed are a compromise for some machines.


Also most of this depends on the size of the machine and the application, so it's not really possible to answer anything other than what Jazz has just said.

[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.

[JAZZCNC]

i'll give my answer from a noobies perspective.

welded steel framed gantry... i dont know if i'm understanding this wrong, but i see plenty of alu profile gantries even on here there are people using profile frames

Big cost saving advantage IE: 50mm x 3mm box section 7.5mtr length priced today £28 VS 60x60 Profile £25 per/mtr = £187.50 per 7.5mtr length.!! Then you'll need about the same again for the correct fixings to make best use of the profile.!

water cooled 2.2kw spindle and vfd, personally i've not dug deep enough yet to find the answer to the power needed for most operations, but my understanding is if you dont start water cooled then you will end up here anyway, but as i said the power at 2.2kw is something i feel might be something you can compromise on, personally i'd been looking at 1.5kw

If small machine using small cutters then 1.5Kww will be fine, larger machine cutting harder materials or larger cutters desired then 2.2KW much better option.

m542 drivers and 50v power supply are both things i feel come down to your personal preference. your desire for speed will have alot to do with how much you want to spend on drivers and psu... me personally i keep telling myself i'm going to keep cost down by staying with cheap drivers.. the reality is that i will more than likely want to upgrade to better motors once i realise what i should have bought.. hey i might even get to the stage before i even buy my first motors..

Personal preference regards Drives/PSU doesn't really come into it regards speed.? . . Speed should not be thought about has been got from drives or motors but from the correct choice of screw pitch. Yes to get best performance from motors then voltage and therefore drives/PSU play a part but it's the screws and the relation ship with running motors in the right RPM range in conjunction with machine size and intended purpose or goals that make the difference between getting it right or wrong.
Cutting corners regards drive quality or PSU size is THE most common mistake new CNC'rs make.!! . .. . . unfortunately it's mostly after weeks or months head banging they find out.!

1610 ballscrews, now personally with loads of reading i'd already decided ballscrews were going to replace that original thought of using threaded rod.. however i'm not sure what the difference is between 1605 and 1610 yet and how they will apply to myself

Simple really 1605 16=Diameter 05= pitch or distance traveled per rev. The net affect 1610 can go twice has fast for the same RPM but with less resolution per rev.
You decide the best most suited for your intended purpose. With carefull screw choice to make best use of motor's Ideal working RPM range then you can tailor the machine to give the speeds and torque you require.

[wilfy]

Personal preference regards Drives/PSU doesn't really come into it regards speed.? . . Speed should not be thought about has been got from drives or motors but from the correct choice of screw pitch. Yes to get best performance from motors then voltage and therefore drives/PSU play a part but it's the screws and the relation ship with running motors in the right RPM range in conjunction with machine size and intended purpose or goals that make the difference between getting it right or wrong.
Cutting corners regards drive quality or PSU size is THE most common mistake new CNC'rs make.!! . .. . . unfortunately it's mostly after weeks or months head banging they find out.!

see there is me misunderstanding the reason behind different sized motors. however when it comes down to running the motors in the right RPM range, is it really a case of buying the right motor if you got it wrong or can you make the wrong motor work for you if you are using belts and pulleys?

[JAZZCNC]

see there is me misunderstanding the reason behind different sized motors. however when it comes down to running the motors in the right RPM range, is it really a case of buying the right motor if you got it wrong or can you make the wrong motor work for you if you are using belts and pulleys?

Yes you can recover to a point by using belts but something has to be lost in the process.? IE: (These examples assume motors run at correct voltage)
Too small motor regards torque will need to be geared say 2:1 but the screw speed is half'd so machine speeds are lower. Plus the motors are working harder and therefore the drives to achieve the same speed has correct motors would be, this shortens life expectancy.
Supplying Higher voltage to motors will increase rpm's slightly but also create excessive heat and eventually damage the motors.

It's possible to have too large motor.? Large motors like Nema34's run quite bit slower than smaller nema23's and RPM's can be too low for the ideal required machine speeds. Gearing say 1:2 will increase screw speed but torque and resolution is lost. Same applies regards voltage.

The way gearing can work is if you match the screw to the motor.? In this case a lower or to high spec'd motor can be made to work in a way you'd like and still have the speed and resolution you want or happy with. This is done by choosing either a higher or lower pitch screw than other wise would be chosen if the ideal motor/screw combo was available. . . BUT. . Still the same torque/resolution loses/gains applies just now your using them to your advantage.!!

Just know that selecting the right motor/drive/voltage/screw in the first place can't be beaten and gearing to make some short fall up always comes at a price.!!

[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 :)