George's 8x4 router-me-bob

[georgetheforge]

Thanks for those Jazz-

am looking into the pros and cons of using ball screw type motion for the x and y as well now- as it looks easier to set up than R&P-


so with that in mind does the panel agree that screws are more accurate/ easier to mount etc- ie less faffing to get the tension/ pressure on the rack/ pinion?

for the 'long' x axis (2500 ish) you'd just have the end block, the ball screw+ mounting block and the machined end bearing- plus the screw obviously-! (one set per side)

would you go straight on with the stepper or do a belt?

whose the best supplier for these- i imagine Zero backlash is what i'm after?

hope everybody's well

G

[JAZZCNC]

so with that in mind does the panel agree that screws are more accurate/ easier to mount etc- ie less faffing to get the tension/ pressure on the rack/ pinion?

YES

for the 'long' x axis (2500 ish) you'd just have the end block, the ball screw+ mounting block and the machined end bearing- plus the screw obviously-! (one set per side)

Your problem here with a ballscrew is the length, 2.5mtr is too long in my experience if unless you use a rotating ballnut.!! (Ask Jonathan regards rotating nuts)

would you go straight on with the stepper or do a belt?

I prefer using short timing belts to connect motors to help with resonance and allow easy gearing options.

[georgetheforge]

yeah i was looking at Jonathans post/ thread on this issue- looks like an interesting idea-!

is the problem that it can 'twist' in it's on length- or just it flapping about in the breeze getting its wobble on?!

my slight problem in this build will be the machining of the parts etc, as i'm yet to get a mill and lathe- but want.... hence my initial though of R&P.

i guess it's a toss up between the 2 as the ball screw method may be costly compared to a realtively cheap r & p...

keep going round and round and round with these decisions- sorry to keep asking pretty much the same questions over and over-!!

G

[JAZZCNC]

George please don't appologise and keeping asking the questions untill your satisfied you fully understand.

The problem with long screws is whip at higher speeds. The only way to avoid it is to use large diameter screws but these bring there own set of problems, like needing BIG motors, drives, PSU Etc or moving over to servo's and gearing etc !!!. . . .all expensive.

Personally I prefer ballscrews over R&P anyday and really it can't live with them in any department but they are not as simple to impliment with a long axis machine.

If it was me then it would be a no brainer and it would be getting rotating ballnuts without a doubt.

Regards parts machining then ask around several here will do them for you I'm sure. I would and will glady make them for you but unfortunatly I'm tied up for the next several weeks but if you can wait then I'd be happy to help.

[Jonathan]

Splitting the gantry into two box sections as you have done is a great way to do it. I'd put an aluminium plate on the back to stiffen it further, although it's probably fine as it is.

The gantry sides just look flimsy. Since you don't have an adjustable height bed most of the time the Z-axis will be operating near maximum travel, so the combined tool deflection from Z-axis and gantry bending will significantly limit the feed-rate and finish you can obtain. You should do what I put in the last post - eliminate the gantry sides and have the box sections for Y on the smallest plate, or box section, viable to link them to the X-axis bearings. Add to that an adjustable heigh bed and you'll have a really strong machine with as much clearance as you want for long tools/drill etc.

You need to consider how to accurately align the X-axis profile rails. Currently you can get them both parallel in the X/Z plane, but the spacing of the rails (i.e. parallelism in the X/Y plane) is fixed. No frame will be accurate enough to allow that unless you build in some adjustment, or fix the rails in the same plane as the bed (like on mine).

Correct me if I'm wrong but you've drawn the rotating ballnut mounts fixed to one end of the bed, not fixed to the gantry sides?

[georgetheforge]

i'll get there at some point-!!

came across these badgers- bet they're worth a pretty penny-! http://www.drives.co.uk/fullstory.asp?id=3388

good magazine if you get it? might also go to the trade show in april...

thing is i have the skills to machine them just not the machines-!! i'll have a play with the drawings etc and see where i get too

would you run 2, one at each side? or do you run into sync issues?

G

[JAZZCNC]

Ye you'll have to sell your house and the wife to buy those mate. . Lol

Yes absolutly run 2 screws and provided you don't over tune them they keep sync ok-ish.!! . . . I have my views on the sync issue and to be honest I won't go into it again in your thread but if you look around at some of the other post's on here I've gave them recently but I can't remember which thread.!!
(This one amongest others I think.?? http://www.mycncuk.com/forums/showth...estions-please!)

[georgetheforge]

Hi y'all, spent a bit of the weekend messing with my drawings-

to see what i could come up with-! bit of a mash up of my lurking about here and there-!! to be fair mainly Jazz!

so the plan is as pics below- however.....unsure of the Z as I've allowed for 150mm movement/ gantry clearance, might be over the top, but i get asked for odd things and it might be good to allow that tolerance for poly etc- had half a though of making the 20mm side plate adaptable with another set of holes for the carriages @ say 70mm up so you could drop the gantry-

need to think about the rotating ball screw affair/ speak to Jonathan-(pm coming your way-!) current thinking is to have them below the 120x 80 edge frame connected to the 20 odd mm side plates- and whether the y ball screw is too far back off the spindle- seemed like a nice neat way at the time- feel free to comment-!

also work out belt lengths for the y and z to make sure i can get something to 'fit' but will probably get the bits and 'measure' them before any plates are machined (for which i need a friendly person to quote on as theres some trenching/ holes to accurately cnc..chicken and egg- 'oh if i had a cnc mill i could.....!'

the bed is currently long enough to have the gantry go to a dead space at the end which hopefully will help with loading etc- and is currently 25mm mdf which i hope to make into a vac bed of sorts when budget allows-!

rest of the table frame/ cable management / extraction to draw up but cant see any probs with that but will draw them before i start building to make sure the 'problems' are thought about-!

gantry will probs be a the same 120x 80 x 6mm RHS..... thoughts?Quite exciting really.... now to collate me list of parts/ electronics ( which scares me slightly!)









hope that all makes sense?George

[JAZZCNC]

hope that all makes sense?George

Makes perfect sense George.!

Some will complain the Ballscrew is too far back from the Y axis carriage, which it is slightly, but because you are using profiled rails they will help absorb some of the effects. Thou be aware this will apply twisting forces to the bearings which over time could cause premature wear.? You may want to consider moving the screw forward and higher sitting directly on top of the upper bearing plate.?

When you say 150mm of clearence between gantry is that from the Bed to the underside of gantry.? If so then again you may want to consider upping this if your wanting flexabilty.
The things that tend to cause the most problems regards clearence are drills and clamping devices.!! Often drills have long length's so by the time you have material, sacrificial base material then clamps etc you don't much left for clearence.
Also if you ever want to work with really small items you may want to add a vice to hold them and with only 150mm of clearence then again it's very tight even with normal endmills.

150mm is a good Z axis travel amount but really I would look to more clearence if your wanting flexabilty. It's very easy to pack the work material up towards the cutter but not so easy to move the gantry up.!!!

[Jonathan]

Some will complain the Ballscrew is too far back from the Y axis carriage, which it is slightly, but because you are using profiled rails they will help absorb some of the effects.

I was about to complain... it's true that you'll probably get away with it because you're using profile rails, but its unwise to rely on one very strong component to compensate for suboptimal design elsewhere.

20mm for the side plates for a machine this size sounds a bit small. To compensate I would make the 4 triangular brackets extend all the way down the sides as that will really stiffen things up.

The alternative is to eliminate the gantry sides completely, like on mine (or Jazz's) machine:


(Ignore the rotary table madness and yes my Y-ballscrew is badly positioned)

It's close to the strongest format.

Consider carefully what the profile rails on the X-axis are mounted to. Since you have orientated them with the mounting faces pointing outwards (i.e. in the YZ plane) the distance between the rails is critical, so you must have minimum one of the box sections bolted down, not welded, so that it can be adjusted. If you go for the gantry format in the picture above the rails are both facing up which would make this no longer significant.
Same is true for the Y-axis. 120*80*6 box section is a good size, but by putting the rails on opposite faces you're relying on those faces of the box section being parallel. To the tolerances required for profile rails they wont be, so you'll probably end up needing to use shims and stuff to get it to run smoothly. With supported round rails the tolerances are lower, so you get away with it, and you want them in that configuration to even up the loading on each block. However for profile rails (with 4 rows of balls which are the only ones worth getting) the force rating is equal in all directions, so it wont make much difference if you mount them facing or in the same plane. Mounting them in the same plane will make life easier ... lends itself well to getting the Y-axis ballscrew in the correct position - right between the rails.