Second CNC table build, here we go again....

[F1transportationserv]

Vee rails with R&P drive - seems to be pretty common, reasonably easy to build, precision depends on rail mounting, backlash in rack, adjustment of rollers
Linear rails with R&P - very accurate transport, precision depends on backlash again
Linear rails with ballscrews - highest precision, highest cost, hardest build
Flat plate & skates with R&P - a very common method, precision depends on quality of plate used and how its mounted, easy to replace, lowest cost, needs to be kept clean from grit etc

What is backlash?
And when you say highest cost re ballscrews how much are we talking??

[Davek0974]

Backlash is the slop or lost motion you can get with rack and pinion drive, normally gears are run with a small clearance between the teeth, when used in motion control you take all the clearance out by spring-loading the pinion against the rack so there is no clearance at all, this works well until the pinion or rack wears and backlash comes back. This will cause steps in the cut finish where the direction changes or circles to come out not quite round etc, basically its a bad thing.

Ballscrews can have anti-backlash nuts fitted which are two nuts with tension between them.

Cost for a 1.4m 20mm screw with end bearings and nut in the UK is about £350-£400, length is also limited.

[m_c]

You do realise you can get anti-backlash gears?
Essentailly two gears mounted together and spring loaded out of alignment. You could do something similar using two gears mounted apart with a spring loaded belt between them, however you have to remember high acceleration forces could overcome the anti-backlash spring and allow backlash to develop briefly until the forces reduce again.
Then you don't get prefect motion with a straight cut rack, as you get a slight notchyness as each tooth engages/disengages with a resultant increase/ (there is a proper term for this, but it's essentially what makes straight cut gearboxes so noisy), but for a plasma cutter it's not really an issue. If you're really concerned, you could use a helical rack, but I wouldn't worry about it.

[Davek0974]

Thanks, i have read of some innovative ideas re backlash removal.

Without sounding rude to anyone here, I have just had a reply from a very well respected heavy-duty expert in plasma cutting process and machinery via email, his examples of work are what i would like to see and are well known.

Due to my budget constraints, it is felt that I should be looking at V-Rails and rack and pinion, or, ground, flat plate and skates with rack and pinion for my build.

This makes a lot of sense and is in line with 90% of the commercial production built machines available - no point in re-inventing the wheel.

I will price up and sketch out a v-rail build.

There is another option for drive - open-ended toothed belt with an omega loop drive - this is simply a length of timing belt stretched tightly from front to back with the motor on the gantry and the belt wrapped in a loop over the sprocket. It is supposed to be a very good system as long as steel corded belt is used and it's pulled tight.

My existing build uses a belt but its looped which doubles the stretch possibility and its too small at only 10mm wide, also due to the poor design at the shafts, i cannot stretch it very tightly.

Its too easy to forget things like a budget at times :(

Need to get back on track now I think ;)

[Davek0974]

In summary -

1 - Go for the belt option again this time, but get it right! I think belts, designed properly would outlast a rack, easy to replace too.
2 - Using Linear rails is a possibility, pretty much same cost as V-rail but easier.
3 - This time I will go for two motors on the Y-axis, this will remove having a long cross-shaft in the gantry and the resulting torque twist it suffers, at a risk of racking the gantry, but plenty of builds have two slaved motors and work ok.
4 - Keep the rails above the table, with a gap under for sliding in bigger sheets. Having the rails up high means a low gantry - good for stability etc
5 - Stick with the motors/drives I have, they are Nema23 size so easy to upgrade to Nema23 servos later on if needed.

[m_c]

Out of your list, personally I'd say go for two servos now for the twinned axis, rather than buying an extra stepper/drive.
Servos are what is going to make the biggest difference in achieving higher speeds while still maintaining accuracy.

[Davek0974]

Hmm, would be an idea but i already have the spare motor and drive :)

I bought an extra last time round.