a Steel Box Section Build with SBR20 & Ballscrews Plus a few questions

[grain_r]

Hey Jazz
Thanks for the design, so if the parts missing at the bottom it doesn't get any-more twist of the table when in use ? or is the sheeting around it structural and stops this. I love the slider system I have been thinking that it would be not the easiest to lift the bed and get the bolts in, I imagine its just watch your fingers time as you take the last bolt out. your design is very much more evolved most certainly. and I love that there is space still underneath, Would you mind if I use your example I feel bad not making the design myself although I will change it to suit.

I'm sorry to ask so many questions but on the bracing does this have to be specific dimensions e.g. a true triangle for the strength like 45 degrees or just has to look correct. i'm not sure how easy it would be to make it belt driven the only think I can think of is to put the motor on the bottom of the bed and then use a pretty long belt in a 'v' shaping. I see the gantry uses ali extrusion I imagine I would still be better to use the box section for my design.

Thanks very much for the help.

Thanks

Rob

[grain_r]

Hello Again everyone
I have been designing again and hopefully getting somewhere now thanks to the guidance from you guys. I'm not sure whether to go above the top rail and build up or if it is better to build downwards towards the table my thinking it that the strength of the side will help a little if it is below ? although I will have to shim with something to get a gap from the main frame. or would the gains (If any) really be worth it and just build on top?

[EddyCurrent]

If you are using round rail on the gantry I can't help thinking it's best to have one on the top surface rather than both on the front.
You need to draw the ball screws and motor mounts in place before thinking it's done.

[JAZZCNC]

Sorry to say this but that's a dreadful design. The gantry hanging down like that is acting as a lever and with no support at the ends it will resonate like crazy, this will show in the finish quality.
You need the Gantry supported at the ends connected directly to the sliding carriages.

[grain_r]

Hey Guys. Thanks for that I would rather know that it isn't correct that's why I though I would post it up just in case I had it wrong. I will have another run at it.

Is the frame about correct for the design ?

My thought are to add one rail at the back and one on the top this way I think i can get less overhang on the Z axis face plate ? although this might be another silly idea. I have even thought about speaking to chai and seeing how much the linear rails would be but because I have the rails already I think i should persist with what I have and then upgrade when I have more knowledge and know what I am using the machine for.

I'm struggling in my own mind how to make the Z a little and because most seem to use linear rails the designs don't fit as easily. if anyone can point me to a design of the Z on round rails it would be much appreciated as a guide

Thanks as always

Rob

[EddyCurrent]

Here's one, just search on Google for "cnc Z axis design" then choose 'images'

http://runawaybrainz.blogspot.co.uk/...thout-cnc.html

[grain_r]

Hey EddyCurrent Thanks very much for that that will certainly help me. didn't think of the Images part of google.

I was trying to incorporate your idea of one rail on the front/Back and one on the top or bottom but it seems to create quite a large gap on the back plate which I'm not sure if it will affect strength. I will get the pencil out I think.

Thanks
Rob

[JAZZCNC]

Is the frame about correct for the design ?

Yes the only thing to watch is the amount of adjustabilty on the bed. If you don't need all that height then raise the horizontal frame supports.
Also doen't hurt to put some 4mm triangle plates on outside to help support the upper legs like in this pic.

My thought are to add one rail at the back and one on the top this way I think i can get less overhang on the Z axis face plate ? although this might be another silly idea.

No not silly at all and machines are often done this way to allow ballscrews on front but like Eddy said before you need to include everything in the design ballscrews mounts the lot to see what clearences you have. Personally I don't like the Ballscrews on front and in firing line so put them at rear out the way or protected behind something with slot for access.

I have even thought about speaking to chai and seeing how much the linear rails would be but because I have the rails already I think i should persist with what I have and then upgrade when I have more knowledge and know what I am using the machine for.

Yes do it because there's far more work to upgrading than you realise so it will never get done and profiled linear rails are worlds apart from those round type rails so make the machine so much more accurate and reliable. With round type rails your constantly chasing the adjustment and they wear out and go sloppy in no time if not looked after. You don't get any of that with profile rails.!!
Chias prices for Linear rails are cheao and the quality is fine and you'll reap the bennifits 2 fold when you come to sell it.

I'm struggling in my own mind how to make the Z a little and because most seem to use linear rails the designs don't fit as easily. if anyone can point me to a design of the Z on round rails it would be much appreciated as a guide

Profiled or round there's really no difference other than clearence between front and rear plates and it's actually easier with round rail as they are deeper so you don't have to machine clearence for the ballscrew.
The design eddy posted is essentially how I make my Z axis which you can see here. I then put aluminium covers over them to protect every thing.

[grain_r]

Hey Jazcnc

Thanks very much for the great reply. I have looked through it and will add the corner bracing and I think I am going to go ahead with the build as original and then see what I actually use it for and then can build a higher accuracy machine with the initial machine later on down the line and I will have learned more about the subject.

I have had another look at the Gantry and I think I am much nearer on this design and I have tried to keep everything out of the way I still need to continue working on it but Just thought I would post it up to see if I'm on the correct lines this time.

I still need to work on the Motor and pulley details and the cable tray too.

I have been looking on aluminium warehouse and the pricing seems pretty good on there for 20mm there seems to be different qualities on there for not much difference in price.

[JAZZCNC]

Wouldn't have the bottom rail that way around personally due to cutting forces and way it doesn't support the front plate. Personally would have top & bottom.
Also I wouldn't make the Y axis wrap around the gantry like that. It doesn't add hardly any strength makes the whole Y axis heavier than needed lowering accelration and increasing inertia plus it makes building harder because unless the Ends are perfectly 90deg they twist and bind the bearings.

Also rather than having the gantry cross beam central to the bearing try offsetting it back a little to allow for Z axis and spindle balancing the centre of Gravity better.