Steel Framed Router

[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.

[Jonathan]

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.

It's almost impossible to have too much torque since it so directly affects machining and is so useful, and the 20mm screws enable a much greater factor of saftey than the 16mm ones, or higher feeds/acceleration if you choose that way.

When I did the motor tuning on Chip's router we got perfectly good feedrates (can't remember exactly what, long time ago). That's with similar length 16mm screws, but only 5mm pitch so nowhere near as good as 10mm pitch if you're after speed.

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.

Surely that is because the vast majority of people use 5mm pitch 16mm screws, which due to my reasons earlier are clearly going to cause grief.

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.

Yes, naturally so would I in an ideal world where it's a reasonable price to get 10mm pitch.

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

Gearing helps but you're still limited by the torque due to the high screw energy and potentially whipping. I will only ever use pulleys anyway as clearly it gets the most optimal torque and has the added bonus of reducing resonance.

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.

Yes experience is of course great if the situation is similar enough for it to apply. I probably lack in experience to you, but however valuable experience is, experience backed up with mechanics is indisputably a lot more useful.

[JAZZCNC]

experience backed up with mechanics is indisputably a lot more useful.

Ok Jonathan we'll have to agree to disagree on some things but I'll agree with you on the above.!
(Provided your experienced in mechanics.! . . I know I am got papers some where that tell me so.:whistling:)