George's 8x4 router-me-bob

[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.

[georgetheforge]

Thanks chaps-

What kinda clearance would you recommend?- obviously depends on what I'll be machining-!!

mainly 18/ 25mm ply and mdf, some poly/ foams maybe and maybe some ali if possible....

I wanted the side plates- rather than your solution- so i could keep the top surface clear for loading etc- i do have little triangular brackets which i forgot to make bigger when i was playing with the gantry height-

was tepted to 'split' the gantry so i had 2 sections with the ball screw between them fitted to the back of the front/ z plate. obviously that makes it heigher and was worried about twist/ aligning them correctly.... i did look at getting some 100 x 200 x 10mm RHS and getting the front lasered out to make a so a proper c section rather than an channel- almost like a chunky Unistrut....? then reverting to plan a having one of the rails on the front- bottom and one on the top.....?