I did write a reply but I think the mod's must have it ??
James
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I did write a reply but I think the mod's must have it ??
James
So what did it say.?????Quote:
Originally Posted by luke11cnc
Ok, still struggling a little with sketchup. This is my latest attempt and i have put the ballscrew on top of the i-beam. Would this work, i'm struggling with ideas on this.
Ignore this
Ok, i think i'm finally getting there. All plate is 20mm thick ali and also using I-beams. I'm not sure if the screw mounts are ok doing it this way. Would the bearing blocks need spreading out more. All the rails are 20mm with 1610 screws. Some pictures to show my progress and please tell me where i have gone wrong and where improvement can be madeAttachment 6471Attachment 6472Attachment 6473Attachment 6474Attachment 6475
I will have the ballscrew end machining increased from 15 to 30mm as i want to use belts and pulleys.
The RSJ's are 178x102x19mm, would it be better to use a smaller size on the gantry, 127x76x13mm.
Also just realised i haven't taken into account the thickness of the beams
Ok have redone the RSJ's and added steppers.Attachment 6482Attachment 6483Attachment 6484
\Quote:
Originally Posted by deannos
Although it's convenient to use RSJ on X and Y, you'd be better off using a different cross section. The torsional stiffness of RSJ is pathetic, hardly much better than a sheet, so especially on the gantry where the cutting forces will tend to twist the beam you would get a lot of deflection. Similarly RSJ is poor under shear stress, which is the case on the X-axis. To an extent you have compensated for that by using a large thickness, but with less material you could use box section and get much greater stiffness.
One reason not to use smaller RSJ is it reduces the vertical spacing of the Y-axis bearings, which is something you want to maximise.
What you've drawn is still fairly strong, so it would work - but that is purely because of the large thickness.
Try folding an RSJ cross section and a box section out of card then try and twist them and you will see how big the difference is. This spreadsheet lets you compare them.
Ok, would i be better using Ali extrusion, or are you talking about steel box section. What size would you suggestQuote:
Originally Posted by Jonathan
I'm still confused on the axis direction, by "Y" are you referring to the bearings the longer gantry is riding onQuote:
Originally Posted by Jonathan
I want to get it right the first time, or as best as can be
How do you use the multi quote option, i clicked it and nothing happens
Quote:
Originally Posted by deannos
I was thinking steel box section, but aluminium extrusion is pretty good too. If you're happy to do some welding then steel box section is the cheapest and strongest option for making a frame. The standard length is 7.5m, so it would be prudent to design the frame to use an integer multiple of 7.5m! I used 60x60mm, 3mm wall thickness box section for most of the frame and 100x60mm, 3.6mm wall thickness for the two lengths the X-axis rails mount on.
How long is the piece of RSJ you've drawn for the gantry?
Quote:
Originally Posted by deannos
I always call the axes on the gantry Y and Z and the other axis X. So by that definition in your case X is shorter than Y.
The what.. never seen that! I just copy and paste the [QUOTE] tags in the relevant locations. I've put them in on your post for you, so if you click 'Edit Post' you can see how...Quote:
Originally Posted by deannos
I can weld , it wouldn't win any contests though, i would like to keep the RSJ for the X axisQuote:
Originally Posted by Jonathan
It measures out at 1715mm, but could be slightly longer.Quote:
How long is the piece of RSJ you've drawn for the gantry?
Instead of using a rsj for the gantry, could i use 100mm x 60mm x 3mm steel box section. I can get my hands on a piece long enough for a tenner, and its very local.
what size rsj were you planning to use and how long?
Quote:
irving2008;33697]what size rsj were you planning to use and how long?
On the gantry the rsj is 178mm x102mm x19mm and is 1720mm long
I'd use the box section.
Reasoning:
a) 1.7m of RSJ @ 19kg/m = 32kg, whereas 1.7m box section @ 7kg/m will weigh 11.9kg, thats nearly 2/3 less to move around...
b) RSJ will deflect .004mm in centre under its own weight plus 0.0004mm per kg load (i.e. negligible), Box section will deflect 0.018mm under its own weight, plus .004mm per kg load. So a 10Kg z-axis will deflect the rsj gantry .008mm and the box section gantry .058mm. In the scheme of things this is negligible.
Quote:
Originally Posted by irving2008
so even though both are negligible as you put it.. you would still steer him clear of the easier/cheaper option of an rsj that needs little work over him welding box section and possibly leaving room for him to create errors...
or have i mis-understood... i'm watching this thread as the thought of rsj being an easy way compared to welding box section appeals to me
Not sure i understand your logic... given a lump of i-beam or a lump of box section, you're still going to have to drill holes to mount rails etc. He was suggesting the rsj for the y-axis element of the gantry... not sure what welding is needed???Quote:
Originally Posted by wilfy
Also he says he can get a section for a tenner... the cheapest 178x102x19 rsj on ebay is £19.50/m min order 4m.. plus carriage...
ok maybe i've mis-understood the whole thing... i was under the impression that if he used box section then he would have to do some welding or atleast joining 2-3 pieces where as the rsj was a solution that didnt need any work other than attaching other parts?
my thought are that he is using 2 pieces of box to make what would be the top and bottom of the beam... but i can now see you are talking about one big piece of box the same size of the rsj... sorry my bad
Once you include torsional stiffness in the calculation the single box section is much better than RSJ:
Attachment 6517
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.
Quote:
Originally Posted by Jonathan
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???
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.!
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.Quote:
Originally Posted by Jonathan
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.
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.
Quote:
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.
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.??Quote:
Originally Posted by irving2008
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.!!
Quote:
Originally Posted by 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. .:thumsup:Quote:
Originally Posted by John S
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.
Ok first I know people(some) think I have no respect or time for the Law of physics but they'd be wrong.!! . . . I'm realistic not stupid.!!Quote:
Originally Posted by irving2008
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.!!
Hey, no offence taken Jazz, I am a great fan of the try it and see school... My father was a toolmaker and a marine engineer and couldn't work the physics or maths to save his life... but he still made some great stuff and mostly it worked first time. And yes, I am pefectly aware that a stepper has bugger all torque at 1800rpm if it can get there at all, in fact the point I was making, clearly badly, is that while in theory you'd still be accelerating at the point you'd need to start decelerating in reality you'd be at some lower speed before that and therefore your average over the distance would be less than the theoretical.Quote:
Originally Posted by JAZZCNC
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.
Attachment 6528Attachment 6529Attachment 6530Attachment 6531Attachment 6532
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-
Am i going round in circles:dispirited:Quote:
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.
Yep I agree can be ok for a rough ballpark (Thou yours did seem a bit off base.!!) . . . You only have to look to the BIG BOYS in theoretical design and performance like F1 teams and see that even they can't predict and 100% confidently model every outcome.!Quote:
Originally Posted by irving2008
Good now give us a hug.!!. . :couple_inlove:Quote:
Originally Posted by irving2008
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.
Ok, thats no problem, i will do that. Is this to prevent racking etcQuote:
Ok thats good but bring the screw to the front, will be much stronger having the ball-nut close the Z Axis back plate.
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?Quote:
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.
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.Quote:
Originally Posted by deannos
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.
Bolt or weld Angle brackets to box then bolt thru Ali plate into brackets.Quote:
Originally Posted by deannos
Excellent, an easy solution i didn't even think of.Quote:
Bolt or weld Angle brackets to box then bolt thru Ali plate into brackets.
cheers Dean
I have moved the ballscrew to the front of the gantry and will be using 20mm ali instead of steel plate, too costly. Have added some steel angle to corners to connect gantry to end plates.Attachment 6533Attachment 6534
Would this design be ok, i thought i would use some 100x60 box section as bearing plates. I would have some left over from the gantry and it saves on the Ali plate. The extra height would help with the bed, bit more room to play with. This box section is only 3mm thick, is that ok. I have used 300mm, 2 x 800mm and 1400 SBR20mm rails. The ball screws are 350mm, 2 x 800mm and a 1450mm length.
Attachment 6582Attachment 6583
2 questions for me that dont mean much to your build sorry :D firstly is that still an RSJ you got across the Y if so your second picture doesnt make it look like that... petty i know..
EDIT: oh scrap that i just read it's box section... my misunderastanding eek... either way second question applies
secondly how do you plan on fixing the box section to what i think is the RSJ?
The gantry is box section and this will be bolted/welded to the bearing plates. I have put some steel angle holding the Gantry ends to the end plates as well, but the plates will also be bolted to the bearing plates. Rsj is only used on the base "X"Quote:
secondly how do you plan on fixing the box section to what i think is the RSJ?
A couple more pics, it might helpAttachment 6584Attachment 6585Attachment 6586