[peteeng]

Hi Ben - Construction extrusions are convenient (anything convenient has a downside) but they are expensive and very poor structural elements when compared to std extrusions or std SHS/RHS steel. See attached. You need to stay in the one universe I think. So stay in steel or aluminium world... Std Aluminium extrusions have square edges if that's what you want. There's lots of ways to solve various issues, you need to develop the design a bit more... eg if you use 3mm for the structural elements this is too thin for threads. So use a 6mm doubler (or backing plate as some say) that is threaded to match the bolt pattern. In this way you get local stiffness at hard points plus the thin bits are good for the global parts as this keeps the weight down. eg if you used a 100x100x3mm al square extrusion for the gantry then use a 20x6mm thick strip (AL or steel) and thread it to match the rail holes. Slide it thru the section and get a couple of bolts in and off you go. Al has the advantage that its easy to make things from, a cheap drill press, a jig saw some files and your good to go and it does not rust (my pet hate)

One thing to think about is what are you going to use the machine for? If it's for flat product, plastic and timber then the machine can be very light. If it's for aluminium then it needs to be very rigid...Peter

edit - regarding mounting rails on construction extrusion. Put bluntly it's not very stiff. Look at those thin edges your bolting too. I know its done but if you do the math you'll see that they flex quite a bit. Do it with a backing plate or thick tube and its much much stiffer and betterer....

[bensaad]

Hi Ben - Construction extrusions are convenient (anything convenient has a downside) but they are expensive and very poor structural elements when compared to std extrusions or std SHS/RHS steel. See attached. You need to stay in the one universe I think. So stay in steel or aluminium world... Std Aluminium extrusions have square edges if that's what you want. There's lots of ways to solve various issues, you need to develop the design a bit more... eg if you use 3mm for the structural elements this is too thin for threads. So use a 6mm doubler (or backing plate as some say) that is threaded to match the bolt pattern. In this way you get local stiffness at hard points plus the thin bits are good for the global parts as this keeps the weight down. eg if you used a 100x100x3mm al square extrusion for the gantry then use a 20x6mm thick strip (AL or steel) and thread it to match the rail holes. Slide it thru the section and get a couple of bolts in and off you go. Al has the advantage that its easy to make things from, a cheap drill press, a jig saw some files and your good to go and it does not rust (my pet hate)

One thing to think about is what are you going to use the machine for? If it's for flat product, plastic and timber then the machine can be very light. If it's for aluminium then it needs to be very rigid...Peter

edit - regarding mounting rails on construction extrusion. Put bluntly it's not very stiff. Look at those thin edges your bolting too. I know its done but if you do the math you'll see that they flex quite a bit. Do it with a backing plate or thick tube and its much much stiffer and betterer....

ok thanks
now if i go for the steel tube 100X6MM should i use the same profile 100X6 for all machine part table frame ,legs...or 6mm for the machine axis and thinerprofile say 3mm for the other machine parts

[joeavaerage]

Hi,
it depends on what sort of stiffness you want. If you want to cut aluminum then you want as stiff as you can get.....and that means 100 x 100 x 9, not 100 x 100 x 6.

By all means use lighter material for things like legs and stuff like that, but anything you want to bear cutting forces should be a stiff as you can possibly make it.

There is one certainty about CNC machines, there is no such thing as too stiff. Were you to go overboard someone might comment 'you could still do a good job
but with a lighter machine', but no CNC'er ever tires of an overbuilt machine, but tire immediately of an underbuilt machine.

Craig

[bensaad]

Hi,
it depends on what sort of stiffness you want. If you want to cut aluminum then you want as stiff as you can get.....and that means 100 x 100 x 9, not 100 x 100 x 6.

By all means use lighter material for things like legs and stuff like that, but anything you want to bear cutting forces should be a stiff as you can possibly make it.

There is one certainty about CNC machines, there is no such thing as too stiff. Were you to go overboard someone might comment 'you could still do a good job
but with a lighter machine', but no CNC'er ever tires of an overbuilt machine, but tire immediately of an underbuilt machine.

Craig

hi peter
why i planed for lighter machine because i have already the electronics part with nema 23 425oz stepper motor,But now I am confused if i can use these motors with such a rigid machine as you described or should i upgrade for nema 34 too

i pursached these motor because first i planed to build printnc cnc from threedesign but i found ballscrew not suited for large cnc 8X4 and high quality ballscrew are very expensive
but now i found this plasma cutter in alibaba and i liked its simple design and inspired me to build a cnc router like that

[joeavaerage]

Hi,

why i planed for lighter machine because i have already the electronics part with nema 23 425oz stepper motor,But now I am confused if i can use these motors with such a rigid machine as you described or should i upgrade for nema 34 too

To be honest I would be prepared to bet that if your 23 size 425oz.in steppers are low inductance, and I strongly suspect they are, then they are likely better than any 34 size stepper you could get anyway.

You are right to be concerned that a heavier machine might require more powerful motors, however the truth is 425oz.in steppers can and do a sparkling job even with heavier machines. You can always
gear them appropriately, which may count against your top speed, but they should have no trouble accelerating a heavy machine. If you are cutting wood and plastics then you might well miss the top speed,
but if you want to cut aluminum your cutting speed will necessarily be fairly slow anyway, and thus loss of top speed is a minor inconvenience.

Its time to decide what sort of machine you want, a fast lightweight machine for wood and plastics or a heavy albeit slower machine for aluminum. Just remember that if you choose a lightweight and fast machine
that it'll be fine for wood and plastics....but you'll be very disappointed should you try the occasional aluminum job, it will just not be stiff enough. Alternately if you make a machine stiff enough for aluminum
it will still do wood and plastics with ease, even if a little slower.

Its the capacity to do aluminum (or worse steel) that adds very much to the cost and complexity of the design, making a stiff machine is costly.

Craig

[bensaad]

Hi,



To be honest I would be prepared to bet that if your 23 size 425oz.in steppers are low inductance, and I strongly suspect they are, then they are likely better than any 34 size stepper you could get anyway.

You are right to be concerned that a heavier machine might require more powerful motors, however the truth is 425oz.in steppers can and do a sparkling job even with heavier machines. You can always
gear them appropriately, which may count against your top speed, but they should have no trouble accelerating a heavy machine. If you are cutting wood and plastics then you might well miss the top speed,
but if you want to cut aluminum your cutting speed will necessarily be fairly slow anyway, and thus loss of top speed is a minor inconvenience.

Its time to decide what sort of machine you want, a fast lightweight machine for wood and plastics or a heavy albeit slower machine for aluminum. Just remember that if you choose a lightweight and fast machine
that it'll be fine for wood and plastics....but you'll be very disappointed should you try the occasional aluminum job, it will just not be stiff enough. Alternately if you make a machine stiff enough for aluminum
it will still do wood and plastics with ease, even if a little slower.

Its the capacity to do aluminum (or worse steel) that adds very much to the cost and complexity of the design, making a stiff machine is costly.

Craig

hi craig
the motors are from stepperonline
Electrical Specification
Manufacturer Part Number: 23HE45-4204S
Number of phase: 2
Step Angle: 1.8deg
Holding Torque: 3.0Nm(425oz.in)
Rated Current/phase: 4.2A
Phase Resistance: 0.9 ohms± 10%
Inductance: 3.8mH±20%(1KHz)
Physical Specification
Frame Size: 57 x 57mm
Body Length: 113mm
Shaft Diameter: ?10mm
Shaft Length: 24mm
D-cut Shaft Length: 20mm
Number of Leads: 4
Lead Length: 500mm
Weight: 1.8kg