ok, first attempt using sketchup 8

[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. .

[irving2008]

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.

[JAZZCNC]

at which point you'll theoretically be doing 18m/min - assuming your motor is still producing sufficient torque at 1800rpm.

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.!!

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.!!

[irving2008]

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.!!

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.

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.

[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...

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.!

Hey, no offence taken Jazz,

Good now give us a hug.!!. .

[deannos]

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.

[deannos]

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.

[wilfy]

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?

[deannos]

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"

A couple more pics, it might help