ok, first attempt using sketchup 8

[JAZZCNC]

The problem with either of these options is mounting the rails with sufficient accuracy. I doubt either steel RHS or RSJ will we sufficently accurate to mount the two rails on opposite faces, since the variation in the 'height' of the section will change the spacing of the rails leading to binding. Seems a bit risky... could fix it with shims I guess but you don't want to go there.
Instead I'd use two pieces of the 100x60 box section with one rail on each as that will be far stronger than just one and solves the problem of maintaining rail spacing.

Rubbish.!! What makes you think both these will be flat surface or straight .? Single or 2 pieces make no difference the surface will still need some work and while 2 pieces will allow easy general positioning they won't be anywhere near parallel all along the length. If the section has any bend then it will still need grinding or shimming at several points and just because you have a rail on each piece doesn't make it any easier.
It's not difficult with a decent straight edge and files,grinder to the bring the main reference surface upto scratch then it's just a case of shimming or grinding the other rail surface parallel.

[deannos]

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.

Am i going round in circles

[JAZZCNC]

Ok thats good but bring the screw to the front, will be much stronger having the ball-nut close the Z Axis back plate.

Have you got the 20mm steel plate.? If not I think you'll maybe change your mind when you see the price.!! It's not really needed either and I'd use ali for easy working.

[deannos]

Ok thats good but bring the screw to the front, will be much stronger having the ball-nut close the Z Axis back plate.

Ok, thats no problem, i will do that. Is this to prevent racking etc

Have you got the 20mm steel plate.? If not I think you'll maybe change your mind when you see the price.!! It's not really needed either and I'd use ali for easy working.

No i haven't got any steel plate and a quick google has made me change my mind, but if i use ali how would I join this to the steel gantry?

[JAZZCNC]

Also now confused after reading comments on the other post about the "effect of forces on gantry cross sections" and the following was mentioned-

Am i going round in circles

Dean don't worry about it your only cutting wood the box section will easily handle it. The box can resonate but cutting wood it won't really matter or show.
There are also ways around this if it does by filling with sand. So long has the resonance doesn't affect the motors then you won't have problem and because your using belts then this won't happen and why it's good idea to use belts.

[wilfy]

Once you include torsional stiffness in the calculation the single box section is much better than RSJ:



The problem with either of these options is mounting the rails with sufficient accuracy. I doubt either steel RHS or RSJ will we sufficently accurate to mount the two rails on opposite faces, since the variation in the 'height' of the section will change the spacing of the rails leading to binding. Seems a bit risky... could fix it with shims I guess but you don't want to go there.
Instead I'd use two pieces of the 100x60 box section with one rail on each as that will be far stronger than just one and solves the problem of maintaining rail spacing.

Thank you for that, I thought that was what was said earlier, which is why I posted what I did, however I didn't realise rsj would be just as in accurate.

How do you join the 2 pieces of box out of interest, if its welding surely this can go just as wrong no???

[JAZZCNC]

Dean if your intention is still the same has before to only cut wood then ignore Jonathan he's talking over complicated bollocks again and it will easily do the job.

If you can get the Large box section for a tenner then obviously it's a no brainier and go for it.! . . Not disputing Box section won't be stronger but no where near has cheap or easy unless you can drop on something like this.

The RSJ will do the Job no problem the extra mass will help with cutting but does cost in acceleration and speeds.

I would how ever change the way you have the X axis bearings and use wider plate than the RSJ which bolts to it. I would then have gantry end plates which bolt to the RSJ and the bearings plates dropping down so the ballscrews can connect. This will help stiffen things up a bit.!! . . . Pretty much like I drew before.!

[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.!!