That said PERSONALY I don't like slaving motors for several reasons.!! . . . Main one being that when it does go wrong, which more often than not is at high feeds, the damage thats done can be massive. Imagine the one of the motors stalling coming to a dead stop while the other keeps on going.!! . . .It's like a train wreck and scarey shit, esp on larger machines.!!
To be sure to avoid this you have to effectively de-tune and run slightly lower acceleration/velocity than you can using belts.
There are other reasons like keeping the gantry absolutely square is fiddly to setup and requires good quality accurate home switch's to be absolutely sure it's square every time you home.
Basicly What your doing with the switchs is setting the start point or zero position for each screw and by positioning the switchs you set the gantry square. . . .This is less than ideal to me.!! . . . Mainly because every time you home the machine the gantry gets twisted and co-jolled into square by touching one switch then the other potentially putting strain on the machine.
If you don't use home switch's to square then you can't be absolutly sure the gantry is square and if the motors start dropping the odd step here and there, which can and does happen if you push the machine too hard or over tune motors or it's not perfectly aliagned or setup causing binding.
Basicly it slowly loses position putting the screws out of sync and the gantry out of square.! . . . or if you stop the machine abruptly from high feed one of the motors can slightly run on more than the other due to inertia and position is instantly lost with no easy way to get back in position. . . . With belts this never happens.
Like I say it's a PERSONAL thing and there are lots that use slaved motors and are very happy with them. I've built machines that use slaved motors without any problems.!
It's just MY PERSONAL PREFERENCE to use belts on my own machine because I feel it's safer and more accurate and ounce setup it's pritty much done forgot and never changes.
Yes belts can snap but I've only snapped 1 in 4yrs and that was my fault not belt failure.!
(I'LL SAY IT AGAIN TO STOP ALL YOU SLAVED MOTOR USERS JUMPING ON ME.. . . . . IT'S MY PERSONAL PREFERENCE )
Good to see a fellow flyer on the forum. I'm sure you know this already but Chris Williams writes a good 'big glider' column in QEFI magazine. Once you cnc cut parts from CAD designs there's no going back to carbon paper and all those hand methods!
Good point you make about the rail itself deflecting - I was think only of the block itself which is of course open to allow the supported rail underneath. That makes things more complicated and either needs FE, or someone to do a quick test!
If you've never flown a model heli before then join a club and get trained because otherwise you will crash it in seconds! Lots of PC simulators out there which are worth a go because you can often connect your own Transmitter (using the buddy lead).
Thanks everyone for the very kind and useful advice. Here are the revised plans with most of the stuff hopefully taken into account:
1. Revised Z axis with bearing blocks fixed to the Z plate against the gantry
2. Gantry sides beefed up to 200mm width to reduce racking . 20mm thickness
3. More detail added in drawings
The endplates will be extended to allow for a future upgrade to twin ballscrews as everyone is suggesting! and belt driven to avoid synchronisation issues as discussed earlier.
Thanks for the guidance on sources of aluminium plate.
For the eagle-eyed viewers you may notice that the y-axis bearings are floating... this is not deliberate but something I realised that a 750mm ballscrew is the total length, not the threadlength and as I want to bolt the BF/BK blocks to the gantry sides then I will have to reduce the length of the y-axis round rail and support. Question here - is it best to dismantle the supported round rail then cut the rail and support seperately and re-assemble? or try to cut it all in one go?
I'm really getting into the 3D solid CAD in a big way - it really does help you work out how things need to fit together and how on a 300mm ball screw, the maximum travel you can get on the Z axis using supported round rails would be no more than 150mm at best!
Thanks again everyone!
The Following User Says Thank You to JAZZCNC For This Useful Post:
The distance measured parallel to the Y-axis between the spindle and the Y-axis ballnut/screw is currently quite large. This will cause the cutter to deflect parallel to Y as the rails offer little support in that direction. Between the Y-axis rails is a good place, hence why a lot of people mount them on separate pieces of extrusion.
The Following User Says Thank You to Jonathan For This Useful Post:
The vertical plate on the back of the Z axis (to which the Y axis ballscrew is attached) is trapped between the small top and bottom plates. This means it needs to be machined accurately to ensure the Y axis bearings sit correctly. It would be better to make this plate full height and have narrower top and bottom plates so that everything can be set before tightening the bolts.
The Following User Says Thank You to routercnc For This Useful Post:
Thanks Jazzcnc, Jonathan and routercnc for the comments on the Y and Z axis. Re-design in process to include using smaller seperate extrusions to reduce
alignment issues and enable the Y axis ballscrew and nut to be located under the Y axis rails and reduce the distance to the Z axis assembly.
Talking about the alignment of the axes for a CNC, I had the idea to buy a large thick piece of float glass (10mm x 1500 x 1000) to provide my "flat" surface from
which I could then make accurate measurements using a dial micrometer. By measuring the surface first, one could reasonably then account for the variations in the surface when measuring...
Been busy at work but have re-worked the Y-axis gantry design, taking into account all your comments. Have shifted the Y axis ballscrew closer to the Z axis gantry and used seperate extrusions to allow this and maintain torsional stiffness I think. I guess the main issue is the use of supported round rails - profiled rails would reduce the height and thickness. However given my other constraints of time, budget etc..lets see. The other approach is to use the extrusions in a L-shape as per Jazzcnc's photos. This has the advantage of mounting the BF and BK blocks on the Y-axis and not having to cut the rails. Will probably work up that design as well and make a decision. Now I just need some more money for the ali, steppers and electronics!
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