Small Gantry Router

**cropwell** · 06-01-2015

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It's one or both of these

**mike os** · 06-01-2015

thanks cropwell

**mike os** · 07-01-2015

more questions..... have I got this right?

ballscrew sizes... fixed & floating bearing for mounts (BK & BF?)
x axis approx. 1000mm between bearings, 20mm screw, 5mm pitch, critical speed about 2000rpm = 10m/min?
Y axis approx. 780mm between bearings, 16mm screw, 5mm pitch, critical speed in excess of 3000rpm=15+m/min
z axis approx. 400mm between bearings, 12mm screw.. critical speed<6000rpm

does this look ok?

**Clive S** · 07-01-2015

Originally Posted by mike os

more questions..... have I got this right?

ballscrew sizes... fixed & floating bearing for mounts (BK & BF?)
x axis approx. 1000mm between bearings, 20mm screw, 5mm pitch, critical speed about 2000rpm = 10m/min?
Y axis approx. 780mm between bearings, 16mm screw, 5mm pitch, critical speed in excess of 3000rpm=15+m/min
z axis approx. 400mm between bearings, 12mm screw.. critical speed<6000rpm

does this look ok?

It probably would be Ok with 16mm screws all round but in general for wood 10mm pitch would be used for x and y and 5mm pitch for Z ..Clive

**EddyCurrent** · 07-01-2015

If you order ball screws then refer to the 'standard machining' diagram and request dimension F = 30 mm, this will give you enough length to fit a pulley otherwise it's a bit short.

**mike os** · 07-01-2015

no where near ordering yet,,,, lol, I wish

I put 5mm pitch as it seems to be used a lot, any thoughts on 5 or 10mm pitch? not really worried about rapids as only a small machine

**EddyCurrent** · 08-01-2015

I was recommended 10 mm pitch for X and Y and 5 mm pitch for Z, I use a 1:1 ratio pulley/belt drive and it works very nicely.

**JAZZCNC** · 08-01-2015

Originally Posted by mike os

more questions..... have I got this right?

ballscrew sizes... fixed & floating bearing for mounts (BK & BF?)
x axis approx. 1000mm between bearings, 20mm screw, 5mm pitch, critical speed about 2000rpm = 10m/min?
Y axis approx. 780mm between bearings, 16mm screw, 5mm pitch, critical speed in excess of 3000rpm=15+m/min
z axis approx. 400mm between bearings, 12mm screw.. critical speed<6000rpm

does this look ok?

Nah at this size don't bother using differant size screws just go with 16mm all round and use 10mm pitch for X & Y 5mm on Z axis.
Critical speed isn't an issue if using steppers as you'll never get stepper to spin above 3000rpm and have any torque left.

One reason for 16mm is the intertia is less so you can have slightly higher acceleration. Connect the motors to screws with timing belts and pulleys to help with resonance but the other reason is that if you find you need higher resolution or more torque then you can apply a ratio to increase.

10mm pitch gives a good balance of speed and resolution. 5mm is ok if you need high resolution but not so much speed. This is why 5mm on Z axis because resolution is more important in this axis. Also the larger 10mm pitch means the motor/screw is spinning half the speed of 5mm pitch for the same feedrate. IE: 5mtr/min on 5mm pitch=1000rpm compared to 500rpm for same feed rate with 10mm pitch.
This puts the motor in better place on the motors speed curve giving more torque while cutting and still gives half decent feedrates. It also means the screw is spinning half the speed so less chance of whip for same feedrate as 5mm pitch.!
For cutting woods and plastics correctly you'll definately want 10mm pitch on X & Y for the faster feed rates.