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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
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. . :(
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
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.
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.
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.!Quote:
Originally Posted by wilfy
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.Quote:
Originally Posted by 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.Quote:
Originally Posted by wilfy
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.!
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.Quote:
Originally Posted by wilfy
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.
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?Quote:
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.!
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)Quote:
Originally Posted by wilfy
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.!!
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.Quote:
Originally Posted by wilfy
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 :)
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.Quote:
Originally Posted by mocha
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.
Can't really Add more to what Irving already said but I would like to comment on this portion of what Irving said.!Quote:
Originally Posted by wilfy
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.!Quote:
Originally Posted by irving2008
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.!
Hi MattQuote:
Originally Posted by 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.
Quote:
Originally Posted by JAZZCNC
This isn't always a bad thing though if I knew how much the router was going to cost when I started I probably wouldn't have done it BUT I'm so glad I did!
You don't count the cost on this one ;)
Why would you want a variable table height? Seems to me that's just more hassle than it's worth. I'd just move the gantry up and down instead with dual ballscrew and slides if rigidity was that essential.
I have toyed with the raising and lowering the gantry instead of just the spindle I should make a decision I guess lol since I want to start the ball rolling with making my mounts now.
The biggest hassle for me is because I'm using toy electronics (Arduino, cncshield, cheap ass steppers) is I would have to wire 2 steppers to 1 socket so I can have a dedicated stepper for each motor not rocket science just hassle. Or Buy better steppers and wire them to the Arduino.
Desertboy, is there any thread you've not pointlessly resurrected yet?
It's because it makes suggestions in the bottom right corner and I never notice the dates lol. Rather than I'm trying to resurrect 5 year old threadsQuote:
Originally Posted by m_c