China CNC routers - 3020 oldschool vs. 3018 GRBL control

[Kjelt]

How "long" is this ballscrew axis and does the linear guide not compensate ?
So if you move the Y axis do you notice the change in height of the table that is moved or does it get stuck move less easy etc. etc.

[TinkeringSteve]

How "long" is this ballscrew axis and does the linear guide not compensate ?
So if you move the Y axis do you notice the change in height of the table that is moved or does it get stuck move less easy etc. etc.

That axis is about 75cm long.
The "linear guide" are those 2 big round steel axes on the sides of the whole setup?
I have not seen the table move up/down, and it does not appear to be impaired in its movement throughout the whole length of the ballscrew.
The reason why I even found this at all is that I was moving back and forth along that axis and heard occasional clicking sounds. I didn't know what they were, thought "oh no, is it the bearings?" and looked under the thing...
But now I actually think the clicking comes from the plastic cable chain elements / hinges, making noises as the chain unfolds.

By "compensate", what do you mean exactly? If the rail guides on the sides force the whole thing to remain within the Z=0 plane, but the ballscrew wobbles, doesn't that mean it will crack the stepper motor's bearings at some point? Or the "sliding" one, when it's close to the wobbling source.

[TopLoser]

Google ‘oldham coupling’

Ideally the shaft would be better supported at the driven end, but you get what you pay for.

[Kjelt]

Yes toploser you're right i didn't see it before but there is no support behind the motor to axis coupling, 
That is insane. All designs I know off have the ballscrew axis supported at both sides and after the support the motor is connected and mounted.

[TopLoser]

Looking closer I'm not 100% sure it is a proper oldham coupling, probably just some flexible coupling pressed in between the two halves so removing the need for the support at the driven end.

It's shoddy and cheap but works, I can see why it's been done that way. I wouldn't lose any sleep about it in that application, even with the bit of wobble.

[Kjelt]

It is not devistating indeed but not as it is supposed to be. Probably the results are ok'ish, you get what you pay for.
Usually you have two sides supported with bearings where the side of the motor is supported with a fixed bearing (screw mounted) and the other side is loose (just stuck in a bearing). The motor side is usually fixed because that is the mechanically driven side of the axis having the most mechanical forces on it.
I can't see the entire construction so can't comment further.

[TopLoser]

I guess they saved a bearing and are using the motor bearing as the support. Shoddy, but there's not a lot of radial load (that flexible coupling absorbs most of it) so nothing should fall apart.

Can't see if it's possible but if the OP is concerned about the wobble then loosen the motor fixing bolts and move it about till the wobble is minimised.

But for the money....

[TinkeringSteve]

I just found a blog post by someone who also has a Chinese "CNC 6040" machine, visibly of the same basic design, alas the coupling isn't on any of the photos. He wrote that his machine was, after 8 years, still "going strong", and he did some aluminium with that...  (no mention of frequency of use, though)
I mailed him about whether his has an extra bearing there or not, out of curiosity

Looking closer I'm not 100% sure it is a proper oldham coupling, probably just some flexible coupling pressed in between the two halves so removing the need for the support at the driven end.

It's shoddy and cheap but works, I can see why it's been done that way. I wouldn't lose any sleep about it in that application, even with the bit of wobble

Hrm, and what about the sliding bearing that takes the wobble? Probably also some "plastic buffer" in there or what?
Looky there... that also looks interesting:


Reminds me of when I first unboxed a KitchenAid 550W blender and seeing the coupling: "why is there plastic???" (or even rubber, IIRC)

[TinkeringSteve]

Can't see if it's possible but if the OP is concerned about the wobble then loosen the motor fixing bolts and move it about till the wobble is minimised.

LOL! So that's what the seller meant, after showing the factory my video, by saying "remove Y-axis board, adjust the position".
Ok I still don't get the "remove the board" part, but if they mean the X,Z motor position (he did not mention the motor...), that reply suddenly jumps from "0% sense" to "makes some sense"

[TinkeringSteve]

Btw., the basic setup of such a device can be seen here:
http://inventorartist.com/cnc-machine/

Now there are different variations of them around with some details different, resulting in different prices. But that's roughly what I have here.
Ah, my motor clamp is somewhat different:
http://i.ebayimg.com/images/i/131982947836-0-1/s-l1000.jpg
And see, it has "structural stability"! Lol.

[TopLoser]

I think you have to manage your expectations of what you’re going to get by paying a crazy low price to an unknown Chinese company. One that’s competing against countless other Chinese companies all trying to save pennies trying to be the lowest priced.

I’d be happy it arrived and worked at all.

Remind me never to sell you anything lol

[SparkyFX]

To calculate the actual influence of the wobble on the result, you would need to get the cosine error (the distance the leadscrew pushes or pulls the gantry for this amount of runout at this angle).
Of course you measure 35/100mm runout, but if your ball screw is at 0° angle, it might not move the gantry at all, depending on the bearing position and gantry position.

I assume based on the very shallow angle there is not much movement side to side or up/down, probably below the amount of play. So it does not directly influence the work. The gantry itself has more bearing surfaces holding it in place.

[TinkeringSteve]

I think you have to manage your expectations of what you’re going to get by paying a crazy low price to an unknown Chinese company. One that’s competing against countless other Chinese companies all trying to save pennies trying to be the lowest priced.

I’d be happy it arrived and worked at all.

Well, I chose a seller who had many ratings and almost 100% good on ebay, and the ratings came from devices such as that, not a lot of 1$ articles All the different varieties floating around have some differences by which they can be told apart. More or less. And I found a couple people who had "exactly that" and were content. But I guess they clone even that. Anyway, the buyers of such machines of the very seller seemed to be content. So "glad that it works at all" may be slightly exaggerated

I'm not sure about it being a "crazy low" price for a bunch of parts like that, with a minimalistic design. People who buy the "real machines", made e.g. in Europe, pay something like my car costs - new. I think my car is a bit more complicated than such a router, and also has some critical tolerances somewhere.
I was aware that that kind of thing would not be usable in the way heavier machines can manage. But they could at least pay attention to set up things as well as they can actually manage, not less

Remind me never to sell you anything lol

I very rarely buy stuff even remotely in that price range, let alone from China. There were no viable alternatives. I had to try, there was at least some good precedents to be found on the net

[Kjelt]

The cheap K40 china lasers are also usable but not good, there are whole lists of improvements from users you can make, doubling the total money spent.
However cnc routers depending on the material can get large forces on the machine parts. So I would stay away from ferro metals. Wood, plasticks are fine but even aluminium might be too much. You will know it when you are missing steps or the machine makes too much noise.

[langwadt]

I guess they saved a bearing and are using the motor bearing as the support. Shoddy, but there's not a lot of radial load (that flexible coupling absorbs most of it) so nothing should fall apart.

Can't see if it's possible but if the OP is concerned about the wobble then loosen the motor fixing bolts and move it about till the wobble is minimised.

But for the money....

stepper can take take quite a bit of radial load, they have to when used with a belt and they use normal ball bearings
they can't take axial load but that should taken up by the bearing at the other end of the screw

[TinkeringSteve]

Kjelt, btw., the seller himself clearly listed steel as a no no for this machine, so they're at least that honest

The guy with his machine that lasted 8 years replied to me. He said his does have support bearings on both sides, but he also bought an extra machine more recently, which is noticeably lighter, "but still ok", for his purposes. Interesting, so that's probably the "make things yet cheaper" of the past years. His aluminum profiles do look thicker, for instance.

[Kjelt]

I hope it will serve you well over the time you need

[spiritburner]

Tinkering Steve

I have just found this post and I am going through the same as you.....Did you actually get a definitive answer to your questions on which model to go through, i see the post now is quite old and the machines now have moved up a little,,, what did you do in the end.....tx for any help Neil