Ok, now we are talking about the weight issue and moving the mass around and me still a complete novice at this. My question is would the steppers i already have be ok for the job. They are nema 23, 3nm (M60STH88-3008DF) and the drivers are MSD542 with a 48v power supply. I'm thinking that i would just have to change the drivers and get a 70v power supply
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.
06-08-2012 #63rsj: 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.
Last edited by deannos; 06-08-2012 at 01:29 PM.
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.!!
OK OK.. I should have qualified that thought process with 'theoretically'...
On a serious note, the laws of physics don't change, so if you're not getting within 70 - 80% of the theoretical numbers then something is very wrong. Now I don't profess, Jazz, to have your experience (or John's) of building these things, I'll admit I'm an amateur in the CNC world, but I've built, tuned and fettled enough race engines and other electro-mechanical systems to know that things generally, in my experience, behave the way I expect them to. For example a 10mm pitch ballscrew should generate 50N of force per Nm of torque and thats enough to accelerate a 50kg mass at 0.1m/s^2 i.e a velocity of 6m/min after 1 sec and 12m/min after 2sec. Now I'll accept that the error in the theoretical calculations above is an assumption of infinite travel. Of course, in reality on a 1m bed you'll need to start decelerating after 3secs in order not to run off the end (assuming your stopping torque is the same as your running torque), at which point you'll theoretically be doing 18m/min - assuming your motor is still producing sufficient torque at 1800rpm. So your overall transit time over the metre is around 6 secs minimum (probably closer to 7 or 8sec), which is an average speed of 7.5 - 8.5m/min, close enough to your 9m/min.
Last edited by irving2008; 06-08-2012 at 08:47 PM.
But none of the calculators allow for, neither can they really, all the other outside factors that DIY building create. They expect, Nah rely on every component being perfect and exact to spec with very little to no physical restrictions.
I don't disagree and can't or wouldn't due to the law of physics that the forces are correct but the forces are not the problem and they can only be realised from what they are connected too and this is where it all goes Pete tong.!!
Your theoretical 18m/min requires the stepper to give usable torque at 1800rpm.!! . . . At 1800rpm stepper motors couldn't pull the skin of rice pudding let alone a 40KG gantry and with only 48V they would struggle get near 1800rpm and have enough torque left to turn the screws with nothing connected.
Couple these things with other build and cheap component issues and the REAL figures are far from the theoretical figures. . . This is all I'm trying to say and get over but If my saying this upsets anyone who's into specs and calc's then I'm afraid it's "Tuff shit" it's true and realistic..!! . . . Ence . . KEEP It REAL.!!
IMHO the purpose of the theoretical calculations, recognising they can't model every real-world situation (but can get very close if you can identify all the 'gotchas') is to see if you're in the right ballpark... if the theoretical numbers come out below what you want to achieve then you know there is no point going down that route. On the other hand, if they come out way above then you know its possible, how close you get is then determined by how near to the ideal you can make it - and my experience is that its easy to get within 70%, 85-90% is doable with care, >90% is hard. If they are close you know you've got a marginal design.
Ok, here is my latest attempt, trying to incorporate every thing that has been suggested. So i will be using 20mm supported rails on every axis and 1610 ballscrews on every axis, along with belts and pulleys. The 2 rsj's will be 178mmx102mmx19mm and the gantry is 100mmx60mmx3mm steel box section. The gantry end plates and bearing plates are 20mm steel. The Z axis will be using 20mm ali plate. I have spread the gantry bearings out as been advised. Hopefully the drawings with dimensions will help.
The rails are 300mm,800mm and 1400mm. The screws are 350mm,800mm and 1450mm long
Also now confused after reading comments on the other post about the "effect of forces on gantry cross sections" and the following was mentioned-
I don't like box section for gantry's has it can produce resonance and vibration unless thick walled.? The bolted together extrusions are far denser therefore absorb vibrations far better.!! . . .Any resonance will transfer thru to the finish.
Last edited by deannos; 06-08-2012 at 11:25 PM.
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