Steel Framed Router

[Jonathan]

We need to know the travel of each axis, and the pitch of the 16mm screws to work out if they are suitable. The main limiting factor is in theory the speed at which the screw starts whipping - critical speed.

The only 4Nm Nema23 motors I know of are these which are the same part number:

http://www.cnc4you.co.uk/index.php?r...&product_id=72

Their inductance is nice and low. The rotor inertia isn't stated...
I'm intending to get a few of them for my router so that I can put the ones from the router back on my mill. Not sure how much difference it'll make over the 3nm. I'll be interested to see what they're like.

[GTJim]

The X axis ball screw would need to be 1600mm long and the Y axis (gantry) 1500mm long. I was hoping to use 10mm pitch screws, so it seems your limited to either 16mm or 25mm. Whip is a concern.



The motors are the Nema 23’s on ebay from CNC4YOU and appear to have a low inductance and 4.0N.m hold torque, but I don’t know if that’s enough and what I’ll actually need.



Are there any simple rules of thumb to work out your requirements regarding motors and screws?

And one word of advice for anyone designing a machine, make sure you have lists and lists of standard size materials. I have gone through the pain of having to redesign this because I didn’t check on the sizes of steel or MDF. “D’oh”

[JAZZCNC]

Personally I wouldn't go with 16mm at these length's unless like Jonathan you used a rotating nut setup.
They are just too close to the edge for my liking and would need to be setup and aligned absolutly perfectly and kept this way, even the slightest miss alignment at this length would show up as vibration easily creating whip as the speed rise's.

I wouldn't go with twin 25mm screws either bit OTT and hard on the motors, esp little 23's.

I see you intend using belts and pulleys so you could always go with 20mm x 5mm pitch geared 1:2. If your wanting reassurence this works good then think I've mentioned it before.? . . I have a machine that use's this setup but with a single 6nm nema 34 motor driving 2*1500mm screws joined with belts. It works very very good and cutts every thing from paper to Ali and steel with super repeatabilty and accurecy.

I will guarantee you from experience with my original 2 x 3nm setup that 2 x 4nm 23's geared 1:2 on 2005 screws will handle every thing you can thro at it,. . then some.!

Seen as your using belts and for simplicity sake my first choice from experience would be 2005 geared 1:2 but if must use 16mm or need higher rapids than 10-11mtr/min then go for the more involved rotating nut setup. . . . Both will work good.

OR find 20mm 10mm pitch screws.?

[Jonathan]

Critical speeds with BK/BF bearings (approximate):

16mm dia, 10mm pitch, 1600mm long - 800rpm = 8m/min
16mm dia, 10mm pitch, 1500mm long - 910rpm = 9.1m/min
20mm dia, 5mm pitch, 1600mm long - 1000rpm = 5m/min
20mm dia, 5mm pitch, 1500mm long - 1150rpm = 5.75m/min
25mm dia, 10mm pitch, 1600mm long - 1220rpm = 12.2m/min
25mm dia, 10mm pitch, 1500mm long - 1400rpm = 14m/min

So given that in my opinion 20mm diameter 5mm pitch screws are the worst option regardless of the gearing since the critical speed is quite low. Also the moment of inertia of a 20mm screw is 2.44 times greater (since it's proportional to the radius to 4th power) than the 16mm screw. That means the motor has to impart that much more kinetic energy into the screw to accelerate it to a given feedrate. The pitch of the 20mm screw is half that of the 16mm screw it clearly has to spin twice as fast. As the motor torque is proportional to the angular velocity of the screw and the inertia you loose big time since the torque required is now a factor of (10/5)*2.44=4.88 greater between the two options!! This doesn't take into account cutting forces, which would make the figure look not quite as bad ... but still it's a massive difference.

Clearly I can use a similar argument as the above to say why the 25mm screws are too big for those motors...unless you rotate the ballnut. Rotating the ballnut is a win-win situation since screw whipping is no longer an issue and the moment of inertia of my rotating ballnut assembly on the RM2510 screw is about a third of the inertia of my 2m long screw. With that I get 15m/min (well over the critical speed) confortably and I'm limited by the 3nm motors, not the screw. For your size machine I reckon 16mm diameter with rotating ballnuts might be a good option...

If you can find 20mm diameter, 10mm pitch screws then that's potentially good option (though still the inertia is greater as above), but I bet it's going to cost you more than a rotating ballnut setup on the 16mm screw.

This does of course all depend on how fast you want to go. If you're not trying to get the land speed record then the RM1610 screws would be fine...

[JAZZCNC]

Don't want to get into an argument Jonathan but I know from real world experience all these calculations means nothing in DIY cnc.

All well and good if building cnc machine using high precision machinery built to exact specifications with top notch components all perfectly aligned.!. . . .but in the real world of DIY using a drill press and rudimentry tools trying to align screws etc using primative methods then it don't work like the nice calculations predict. . . . The extra beef of thicker screws takes up the slack of DIY building no matter what the calculations say should happen.!!!

My setup being a prime example that your figures dont work in the DIY world, . . . It happly spins 20mm screws at twice the critical speed you quote.!! . . Those figure say it shouldn't . . . but it does and has done for the last few years without any problems.!

[Jonathan]

The extra beef of thicker screws takes up the slack of DIY building no matter what the calculations say should happen.!!!

Perhaps, but you can't deny the simple calculations I did in the previous post demonstrate a significant torque disadvantage from using a bigger screw with lower pitch.

My setup being a prime example that your figures dont work in the DIY world, . . . It happly spins 20mm screws at twice the critical speed you quote.!! . . Those figure say it shouldn't . . . but it does and has done for the last few years without any problems.!

I know, my RM1610 ballscrew happily exceeds the critical speed by quite a lot. The thing is the critical speed formula doesn't take into account the position of the ballnut, which acts as another support hence leading to the calculation being an underestimate in a lot of cases. This is partly the reason I concluded that 16mm screws are optimal - from a torque point of view they are far superior to the others and in reality it seems the critical speed is higher than what I worked out, so ultimately you'll end up much better off than with the other options.

The calculations of critical speed may not mean as much as they imply, but I feel they mean more than nothing and give a good basis.

[JAZZCNC]

Perhaps, but you can't deny the simple calculations I did in the previous post demonstrate a significant torque disadvantage from using a bigger screw with lower pitch.

No regards torque It's undisputable obviously, but I know from experience with actually using them not calculating or theorising.! . . they have more than enough torque even with the disadvantage.

The calculations of critical speed may not mean as much as they imply, but I feel they mean more than nothing and give a good basis.

Yes to a small degree but simply too meny variables to explicitly say that 16mm are the best by far, to the point of dismissing the others completely.

I tend to work and put more value in experience than calculations and I know from my own experiences with helping others that long thin 16mm screws give more grief than the same length 20mm.
If you want to go looking around the zone etc then I think you'll find more unhappy owners of long 16mm screws than you will 20mm.

I know if asked to build a machine with 16mm screws of this length I would re-fuse and recommend 20mm with correct size pitch to suit the intend purpose of the machine.

My proposal to use geared 20-05 was work around to no cheaply available 20-10 and wasn't saying they are the ideal solution just an easy one that I know works thru practicle hands on experience. . . . The right pitch for the job wins every time but theres always other ways to skin a cat.

I'll take real world experinece over calculated theory any day . . . . IMO it's priceless and feel privilaged when someone shares there often hard learned experineces.