[airbrush]

Hi

Wondering if anyone has tried out any of the cheapo chinese rotary off of ebay/aliexpress ??

[dharmic]

Yes.

I got one shipped with my equally cheapo router. It's an impressive demonstration of what a non-engineer equipped with a catalogue is capable of underachieving. Mine is a belt on toothed pulley, looks like a 3 or 4 to 1 ratio with a NEMA23 motor.

A piece of brass stock in the three jaw, tried to do a pass at 1mm DOC, not particularly hard or fast on feed or speed, managed to overcome the holding force on the stepper. Worse, it flexed the whole thing on the bed - its spine extrusion just twisted.

Utter crap.

Useable for softwood and plastic, so long as you use the tailstock as well, I guess.

But after playing with this a bit I've come to the personal opinion that a decent 4th axis needs a steel frame of some heft for stiffness, a much better reduction ratio and a NEMA34 motor at a minimum to be of any real use, even for light metal work.

[wizard]

Hi

Wondering if anyone has tried out any of the cheapo chinese rotary off of ebay/aliexpress ??

Nope haven't tried such, but on the other hand if you are going to buy Chinese put out a bit more money for something solid instead bottom of the line. Currently the exchange rates mean that you will get a lot of iron cheap. Shop around and you might find one of decent craftsmanship if not you can take the ""kit mentality"" that many small lathe and mill buyers have.

In a nut shell an indexer made out of real metal at the very least can be reworked into something decent. Go too cheap and all you will have is scraps. So before shelling out look at what you are buying to make sure it is robust enough to be useful and if needed reworkable.

[pippin88]

I haven't implemented it yet, but bought a harmonic gearbox. 100:1 I think. I would not use a stepper with simple belt reduction.

[airbrush]

no plans for doing metal with it..my machine isn't rigid enough to metals....wood only

This is for hobbyist use...

[aarggh]

Yep, got one thinking "that'd be cool", in reality not so much, googling to see if I was the only one to get a uselessly underpowered device that I could stop with my pinkie, I found pretty much all the el cheapo Chinese are variants of the same useless garbage.

Fine to look at, fine to do light engraving with, kinda, but completely useless for any real carving.

Don't waste your money, get a good one.

cheers, Ian

[airbrush]

If you're refering to the motor...i have my own nema 23 275oz stepper motor i would use with it instead of the one shipped with it ...same motors driving my machine.

There's really not alot to it...a chuck on a shaft with some pulley gears and a belt...doesnt seem like there should be much that could go wrong with that....if the motor is the down fall...like i say i would be replacing that one.

Again only for woodwork, hobbyist stuff.

[wizard]

no plans for doing metal with it..my machine isn't rigid enough to metals....wood only

This is for hobbyist use...

Your other options is to DIY something. You apparently have a router so the question is can it do aluminum? If so you likely can do far better than the chines crap out there. It wouldn't be an iron based machine with massive castings and so forth but for your usage it could be acceptable. Another option is to buy a rotary table/indexer and modify it to run from a stepper motor. This is doable but backlash is a problem on most of these indexers.

A DIY belt drive indexer can be very suitable for wood working but you can't build it out of baby components. You will want to use wide heavy duty belts and a good stiff stepper with possibly a bit of gear reduction. You can get around the spindle problem by buying lathe spindles as replacement parts or savage an old headstock. If you go the lathe spindle route buy something with a either cam lock or bolt on support, for chucks as you want to be able to go forward or reverse with not problems.

There are lots of ways to go about this so you might want to look for examples in the net. In the end it will still cost you more than a cheap Chinese solution but it will work. The biggest problem with belt driven indexers is holding torque, you will likely need a sizable stepper and a bit of reduction to maintain any stiffness at the spindle.

[handlewanker]

LOL.....a 4th axis belt drive.....didn't we discuss this at VERY great length some time ago.....seems the cheap 4th axis story gets repeated over and over again.

I would think that if you already own the cheap one you could beef it up with a larger stepper motor, but the resolution would need to be at least 1:10 reduction to get any real power and holding force from the stepper, also the belt needs to be one that has very little stretch etc etc.
Ian.

[dharmic]

Problem with forums is old threads go to the back of the list, a bit like that offcut of 37mm diameter 7 series aluminium that ends up in the back of the rack. If you were around when the offcut was created, you know it's there and can go dig for it. But if you started working at the shop after it went in the pile, you don't know it's there and wading through the rest of the stuff there's a good chance you'll never find it.

[aarggh]

Problem with forums is old threads go to the back of the list, a bit like that offcut of 37mm diameter 7 series aluminium that ends up in the back of the rack. If you were around when the offcut was created, you know it's there and can go dig for it. But if you started working at the shop after it went in the pile, you don't know it's there and wading through the rest of the stuff there's a good chance you'll never find it.

That's probably one of the best usecase analogies I've seen!
Good one!

cheers, Ian

[wizard]

That's probably one of the best usecase analogies I've seen!
Good one!

cheers, Ian

I laughed hard, immediately thinking about the metal racks at work. These are small racks to be sure but even so things "disappear" into the dark recesses.

[wizard]

LOL.....a 4th axis belt drive.....didn't we discuss this at VERY great length some time ago.....seems the cheap 4th axis story gets repeated over and over again.

For wood working it certainly can be done but like everything else it needs to be engineered to be stiff enough for the planned work. For a home shop it is certainly doable. the problem is you nearly need a machine shop to build such an axis.

I would think that if you already own the cheap one you could beef it up with a larger stepper motor, but the resolution would need to be at least 1:10 reduction to get any real power and holding force from the stepper, also the belt needs to be one that has very little stretch etc etc.
Ian.

At some point it does become a bit cheaper, made easier, to dig up a harmonic drive. But too much of a speed reduction isn't good either as radial speed will be an issue.

[horbrarian]

Problem with forums is old threads go to the back of the list, a bit like that offcut of 37mm diameter 7 series aluminium that ends up in the back of the rack. If you were around when the offcut was created, you know it's there and can go dig for it. But if you started working at the shop after it went in the pile, you don't know it's there and wading through the rest of the stuff there's a good chance you'll never find it.

Being a newbie, I had a browse in the user settings and you can change the threads to display old posts first, rather than new first. Much easier to read.

[daniellyall]

if you must get a chines thing just note you will have to rebuild it, I had one lump of poop of a 4th axis the time spent rebuilding it was fun but it was hours and lots of new parts and still not finished, spending the money on a decent 4th or doing a scratch build would of been better, it's still not that good.

[airbrush]

https://www.youtube.com/watch?v=OxQu-VsQc-4

here's a simple one as well...probotix.
Doesn't look much different than chinese stuff..other than the bigger stepper..which is why i was hoping to just get chinese one and throw in my 275oz stepper on it.

[airbrush]

PDJ Pilot Pro CNC router parts and options

found this....who knows, looks like the same chinese stuff??

[handlewanker]

Hi, all the belt drive stuff is relatively cheap on EBAY and originates in China, but the spindle on all of them is free to move when a cutter is working as there is no brake, only the holding force of the stepper, to prevent the cutter from pushing the spindle of the 4th around........ even slightly is bad enough.

The 4th axis with harmonic drive at $400+ shipping is costly, but it does have the braking effect of the mechanism without the resilience that the belt drive has and the lower ratios available are good too.
Ian.

[airbrush]

I guess i'm a little confused when i hear that the holding power is whats important..as i would be installing the same nema 23 275oz stepper that is running all axis on ballscrews on my machine. So i'm assuming if it can drive my machine ..why wouldn't it be enough power to drive the 4th? Yes it would be on belt drive rather than ballscrews so i guess its not the same type of forces applied....

[handlewanker]

Hi....I'm puzzled too on that aspect as it appears to be a general view point.

On my mill when I try to manually move a stepper with power on the holding force is quite enormous.....the steppers are Nema 23's at 425 oz. (I think).

I think the EBAY 4th axis models have small steppers to keep the price down coupled with the fact that the reduction ratio with belt drive is only about 1:4 approx......there are models with harmonic drives that have lower ratios.

A cutter can generate a lot of side ways force to a job in a 4th axis and that would impact directly against the stepper motor's holding force when stationary and also cause missed steps when rotating.....it gets better if the reduction is large.......tight belts to prevent flexing under loads also put a load on a stepper motor.

When carving the rotating speed is not all that important unless you are watching the clock, so lower ratios in the 1:40 range can be advantageous.....a difference of opinions is sure to exist here, but for hobby work this is of no consequence.
Ian.