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Davek0974
10-23-2014, 04:59 AM
I'm thinking of rebuilding my little cnc plasma table, it was a first-build and I have now seen various errors or flaws in my design but not sure of the best options to choose.

Here is an early picture before it got all mucky ;)
http://www.landyzone.co.uk/lz/members/davek0974-albums-problems-picture16323-cnc11.jpg

Some of the issues are...
The cross-shafts (A) bend with changes in belt forces caused by accelerating/decelerating etc.
The Side tracks are in direct line of the muck that is produced when cutting

My thoughts are.....

Raise the side tracks up by a similar amount as the gantry supports (B), this will get them into a cleaner zone and also allow side-indexing of larger sheets underneath.

Remove the gantry supports (B) and fix the cross-beam directly to the carriages (D), this should remove some twisting motion on the beam.

Remove the cross-shafts and belts and replace with ball-screws and twin slaved stepper motors, direct drive.

Maybe remove the x-axis belt drive and fit another ball-screw shaft there too??


Do these changes sound sensible and worthwhile, it's cutting well but i feel it could do better.
Would direct drive ball-screws be fast enough?

Don't worry about the flimsy appearance of the structure, in use it's secured to the walls of the garage where it runs so no wobble etc.

Or I could redesign the cross-shafts with sturdier end supports to prevent the shaft flexing maybe?

I'm fairly certain most of my issues are caused by the shafts flexing as it makes the torch wobble.

Any other ideas?

RussZHC
10-23-2014, 07:28 AM
Compared to others on this board my engineering skills are woeful but to me any twisting motion connected to the beam would mean an increase in cross sections somehow (either or both of the cross section dimensions larger) though your attaching the cross beam directly to the carriage should have some of the same effect.
My WAG (wild a** guess) for the cross shafts would be the same...increase the diameter...to me it looks like the end supports are about as close to the ends as you can make them.
I would never have even thought to look at the accel/decel being the cause of shaft flex, so take my thoughts with a grain of salt.

Related to the cross shaft, could there be any twist being caused by the shaft "torquing", an axial twist that causes a "delay" on the "longer" side (from the drive) so that gantry is actually moving a bit dog leg when direction is first changed? Or is that what you are referring to?
If so, to me, that is a diameter increase fix again.

Stu
10-23-2014, 07:38 AM
One thought on the cross shafts would be to add a bearing block in the center of each.

Stu

Davek0974
10-23-2014, 08:12 AM
I don't think there is much torque going on, the gantry is guided on the left by ball races on both inner and outer faces of the track this stops the gantry twisting on the right, there *might* be enough stability there to completely remove the right-hand belt and both cross-shafts but i would need to test that idea, probably not a good idea.

Thickening up the shafts is another idea but would add rotational mass which might affect acceleration negatively.

Another thought is to add outrigger bearings to both ends and spread these apart front-back with another side beam on each side so the belts are effectively boxed in - no chance of the shafts flexing then???

Yes, I did add a test block to centre of each shaft, printed it on my 3d printer and retro-fitted it. It makes a big improvement but really needs a ball-race or needle roller bearing in it to reduce friction. Big improvement though, that's why I leaning towards shaft flex as being an issue.


Any idea what the life expectancy of a needle roller bearing running on a mild-steel shaft would be?

RichR
10-23-2014, 10:25 AM
Hi Dave
First thing I would check is if the bearing blocks are moving relative to the frame when that shaft bends.

Davek0974
10-23-2014, 10:32 AM
Hi Dave
First thing I would check is if the bearing blocks are moving relative to the frame when that shaft bends.

I think I get your point, these pillow blocks are the self-aligning type - the bearing race can swivel inside the mount.

I guess in hindsight that's the wrong type of block?

Mtw fdu
10-23-2014, 10:34 AM
Dave

I have been keeping you up to date with my table I am building. It is now finished and I am now working through some technical issues. Here is a link of it with a very quick look over. If you want more details as mine is made from 40 x 40 rhs and 6 and 10 mm flat mild steel. Mine is very strong the way it is. The only thing I am disappointed with mine is the guy who makes them has made everything that mounts with screws or bolts are very small ie 3mm metric bolts have been used to fasten the belt to the frame. The mounts are ally and some of the threads have stripped because they have been over tighten.

https://www.youtube.com/watch?v=HdF_TeMXM1Y

I don't really like the ally framed tables as I think they are a bit weak structurally. I prefer to use mild steel tube etc which my table is a minimum of 3mm thick.

Mtw fdu.

Davek0974
10-23-2014, 10:47 AM
Dave

I have been keeping you up to date with my table I am building. It is now finished and I am now working through some technical issues. Here is a link of it with a very quick look over. If you want more details as mine is made from 40 x 40 rhs and 6 and 10 mm flat mild steel. Mine is very strong the way it is. The only thing I am disappointed with mine is the guy who makes them has made everything that mounts with screws or bolts are very small ie 3mm metric bolts have been used to fasten the belt to the frame. The mounts are ally and some of the threads have stripped because they have been over tighten.

https://www.youtube.com/watch?v=HdF_TeMXM1Y

I don't really like the ally framed tables as I think they are a bit weak structurally. I prefer to use mild steel tube etc which my table is a minimum of 3mm thick.

Mtw fdu.

Thanks for that, I'll check the link out tonight (no video at work :( )

I used alu as I had a lot of it lying around, yes steel would be far better.

3mm screws! That's a bit small, I think the smallest I have on mine is M6, maybe M4 holding switches on.

Glad you're getting there though.

RichR
10-23-2014, 11:36 AM
... the bearing race can swivel inside the mount.

And there's the problem. You could add a third pillow block midway between the two you already have, that should stiffen things up.

Paul Alciatore
10-23-2014, 03:40 PM
I would worry about the relatively light nature of the basic frame. That wall behind it is not a very sturdy thing to count on when you are looking for accuracy in thousandths.

If you are not going to have hundreds of pounds in CI, then you need to design the basic structure on very firm engineering principles. You should review the posts by Evan of his shop built milling machine.

peter76
10-23-2014, 03:54 PM
I use trapezium threaded rods on mine with evanuts ( search this forum ), with dual stepper motors for the gantry.
This is working great with no play for two years of intermittent use now
The threaded rods also help as an extra guidance.

Davek0974
10-23-2014, 04:40 PM
That wall is just plasterboard :) this is in my day job where I built it, it's now at home in the concrete built garage so things are a fair bit sturdier there ;)

Even before I moved it, it was cross-braced and had a side extension and water tray fitted to it which helped a lot to stiffen things up.

garyhlucas
10-23-2014, 06:47 PM
The bearing blocks you used area good choice, not a bad one. The self aligning is important if you are not mounting bearings down to a machined surface. The problem is clearly that you have excessive overhang on both ends. The forces add up too which causes the shaft to bend in the middle. So the third support you added would help a lot. Getting three bearing perfectly in line though is tough, so another spherical bear is a good idea. Hang it on the shaft and shim it out from the mounting surface.

Reducing the overhang on both ends would be a better approach. Get the timing pulleys as close to the bearing as you can. I'd move the motor pulley out board of the axis drive pulley, and add the third bearing on the outboard end of that to take the extra load from the motor.

Larger shafts would help, but you'd add lots of rotational inertia that would be detrimental too acceleration and deceleration. I've done wide machines with a pair of bearing on short shafts on each side, with shaft couplings and light weight tubular shafts connecting them from side to side.

Putting the linear rails up top on posts at the corners and removing the gantry towers would get the rails out of the plasma dust and allow you to cover them from the top easily. I'd consider using an aluminum beam above the rails with the rails on the bottom, so dust doesn't settle on them as easily.

Removing the gantry posts would both increase stiffness and lower moving mass, both a big improvement on small machines.

Overall it looks like you've done a pretty nice job. For your first one you didn't know what you didn't know, now you do.

kf2qd
10-23-2014, 08:06 PM
Stiffer shafts could be made by making the center part of the shaft out of tubing. The tubing would have larger diameter and without adding weight it would be stiffer.

You could also put motors on both end trucks. It would require 2 motors but you would be eliminating the flex of the shaft. You could then drive using rack and pinion. Plenty accurate for the process and strong. Cutting machines of all sizes still use rack and pinion for the stability and reliability.

Place your cutting surface lower, and mount your cross beam right to the end trucks. Your torch can hang below the rails without a problem.

Davek0974
10-24-2014, 02:22 AM
Thanks, an excellent couple of posts.

I like the tube idea and the outrigged bearing on the pulley end, also lifting the rails and using rack. I did contemplate using the timing belt in a fixed configuration with the motors on the trucks and letting them rack along it but dismissed it as my current idea only needed one motor and driver.

I see lots of R&P tables but the smaller ones seem to lean towards ball-screws or belts like mine.

There is definitely some improvement to be had by sorting out the shafts and bearings, I will look to that area first.

Ideally I would just build another table and transfer the electronics as this would give less downtime but I will have to think about that a bit as it would cost more but ultimately give a better unit I think. I could also use this table to cut parts for the new one.

Food for thought ;)

boslab
10-24-2014, 04:36 AM
Well you have a table now and as you say use it to build no2, I'm stuck on no1 after the steppers drivers computer etc went on holiday with the local burglars, I've replaced the plasma but the hypertherm was so expensive I bought a thermal arc 12 plus, I'm not even sure if it will work on a plasma table?
I think a steel framed machine would be a better choice for no 2 myself, extrusion is alright but expensive to use, let us know how you get on
Mark

Black Forest
10-24-2014, 04:56 AM
If it was me I would flip the Y axis motor. That would make room for the the X axis motor to be mounted on the gantry. I would have two X axis motors with one being slaved to the X axis. Do you have room in your control box to add another driver? Who built the control?

I would lose the continuous belt and fix the belt at each end and get rid of the shafts.

How wide is the base on the gantry with the bearings for the linear rails?

I would do the same for the Y axis as far as the continuous belt. Mount the Y axis motor on the Z axis carriage. Tried and true setup. There is a reason they do it that way!

Davek0974
10-24-2014, 06:36 AM
Thanks, good stuff.

The controls were all assembled by myself, plenty of room for a slave drive board, would possibly need a bigger PSU though.

The gantry trucks are 75mm wide.

Davek0974
11-04-2014, 03:00 PM
Stage 1 is under way, I've printed up four bearing mounts and am in the process of fitting two at each end spaced evenly on the cross shafts, the first one showed a marked improvement in stiffness and as I've fitted ball bearings this time, they also rotate a lot easier than the single, solid plastic support I had before.

One major fault I uncovered was a loose final drive pulley on the motor end of the front cross-shaft, this was forcing the torque to travel through the front shaft then back through the gantry on every move instead of driving the gantry from both ends. One screw was lost and the other was loose, that's been sorted out as well.

Should be a bit smoother.