ok, first attempt using sketchup 8

[irving2008]

Now I've read the whole thread, looks like we repeated the discussion on box v rsj from last January lol. JazzCNC is right in that either will do and arguably the rsj is the simpler to construct with, but lets examine the 'weightier' issue of a 40kg gantry (32kg rsj + other parts) vs a 20kg gantry (12kg box + other parts). You say, from earlier, that you plan on using 16mm screws x 2, each 800mm long driven by 2 slaved motors @ 3Nm each on 48v.

A rough calculation suggests that would be good for:

rsj: 9m/min cutting and 16m/min rapids
box: 11m/min cutting and 20m/min rapids

given the same acceleration. If the former speeds are good enough (and I think they are IMHO) then the choice is down to personal preference; either will work.

There is no need to increase the driver voltage, upping it from 48v to 60v has a marginal effect (increases the rapids on the rsj version to 18m/min, doesn't justify the extra expense IMHO).

One other consideration. the lower mass of the box version would allow one motor to drive both X screws at approx 6.4m/min cutting and 11.5m/min rapids @ 48v. Again upping the voltage has relatively little effect, increases rapids to 13.3m.min. If these speeds are acceptable, then this could mean buying one less driver and stepper motor and a smaller PSU (but adding pulleys/belts to the cost). So you need to factor this as well.

[deannos]

rsj: 9m/min cutting and 16m/min rapids
box: 11m/min cutting and 20m/min rapids

Well these speeds are plenty enough for me, a hell of a lot faster than what i'm getting now. My plan is to use 1 motor and belts and pulleys, so the box section it is at the moment. Plus i would use less ali plate in the Z axis. Also the price is right as well, but i'm waiting on some quotes for rsj,also local so could change, but would then have to factor in the extra stepper on top. Also i guessing i can use just 1 piece of box section and not have to double it up.

[JAZZCNC]

You say, from earlier, that you plan on using 16mm screws x 2, each 800mm long driven by 2 slaved motors @ 3Nm each on 48v.

A rough calculation suggests that would be good for:

rsj: 9m/min cutting and 16m/min rapids
box: 11m/min cutting and 20m/min rapids

Whattt.??? . . . Irving you been sniffing solvents or something.? No way will he get 9m/min cut and 16M/min rapids with 48V and 10mm pitch dragging an RSJ that weighs 40Kg and would be lucky if would get that if it was Box section and weighed half the weight.? So certainly won't get get 20M/min rapids and 11M/min cut.??

This is what I mean about calculations and reality not being the same.! The real figures will be much less at this 48V voltage. The fact the motors are slaved dictates they need to be slightly detuned to avoid one stalling. The real figures to avoid stalling and keep accuracy by not losing steps under Acceleration/De-acceleration and take into account the DIY build factor will be around 8-9M/min rapids and 6-7M/min cutting with 48Vdc if your lucky.!

But it actually gets a bit worse because at 48V on 50V drives then he's too close to the drives voltage limit so for the drives safety, voltage around 44-45 will be best. This will drop speeds ever so slightly.!
At 48V the 50V drives will probably handle the odd spike back EMF will produce but they are still running at there upper limit so there life expectancy will drop.
Now the 75V drives and around 65V will give better speeds and still not be pushing the drives to the max so IMO they are very much worth the extra.

Dean don't expect much above 11-12M/min rapids and 8-9 M/min cutting and expect to keep good stability and accuracy even with the 75V drives and running 65-70V voltage. Yes with lighter gantry you'll get a bit more speed but nothing to shout about and bare in mind with real conservative figures of around 9-10M/min Rapids and 7-8M/min cutting you'll still have plenty of speed for cutting woods and know the machine will be reliable.!

Keep it real.!!

[John S]

this is what i mean about calculations and reality not being the same.!

Keep it real.!!

rotflmao.......................................

[JAZZCNC]

rotflmao.......................................

Ye does sound a bit ALI-G. . . . But know I'm correct that perfect world calculations often don't match real world DIY reality.??? . . . Thou so long has it's made your Ugly mug smile John it's made my day. .