forum.cncdrive.com

I bought a 4th rotary axis, which I am not sure if I should be happy about on a "you get what you pay for" basis or should have received something better. Are my expectations too high from these 300$ things? This is what I bought (Option 2):

https://www.aliexpress.com/item/1005002123527118.html?spm=a2g0s.9042311.0.0.47104c4dI8LQr7

Everything looks nice, but...



So the question is, if you think this is "normal" or not. I did complain to the seller, he asked to make a video to show the problems, so I made one today. I am still waiting for his reaction, but basically what I asked for is:

1. A new large gear or a chuck or both, because I thing that's the biggest issue.
2. A new MT2 live center., or at least a new pin for my MT2 taper.

Unfortunately, I don't know how to replace the pin or the large gear, so I also asked for instructions in that case. I don't know if the center pin is glues or not, but it sits so tight in the bearing that I can't remove it. I removed the three screws at the back of the large gear, but then there is nothing else to remove, and the gear still is firmly attached to the axis, so even there I need instructions because I don't know what to do. The chuck is more straight forward, I din'd try to remove it, but probably held only by three screws.

What do you think? I have zero experience with rotational axis, but dry running seems fun and works well with UCCNC.

I am also making a new 48V 7A supply just for this purpose, and also made a "Lathe mode" box, which basically is giving pulses and direction signal so that I will be able to run as a "lathe". I know, I know, don't laugh... it's not a lathe, but I will play with it and it may work for soft material. Anyway, if not good enough but I like the idea then I'll change the 1.8Nm motor to something considerably stronger, or a servo. Anyway, this device is based on an Arduino Uno and it will give pulses matching a set rpm, which can be set (as it is designed) between 0-350 chuck rpm. Of course, it has a display and it will accelerate/decelerate as well.

As usual, if there is an interest for this box then I will share the Arduino code, but will first make a box and everything before I will show it. It's a simple thing, but will be fun to try.

Hey,
ad 1 You are measuring the run out of the belt guards , they are pressed after the gear is made so don't expect they will be running true. Imo You shouldn't worry about that.
ad 2. Measure the runout of the chuck backing plate, take the chuck off, chuck introduces it's own error when You insert the drill or brass rod and measure).If it's ok (0,01mm or something around that) at backing plate, then I would look for better quality chuck. Measure at the mounting surfaces of register.

Let us know what did You find

OK, thanks. I will do that as well. But the thing is that the belt is pressed to the side on part of the gear, and if I move the belt to the inside, after a one or two rotation it creeps back to the other side on the same part of the gear wheel, so I think the gear is skewed also. You are right, I planned to remove the chuck and measure the chuck plate as well. Anyway, I am also waiting for the sellers reaction. According to the description, it should be better than this, at least according to my understanding of the specifications.

Try to move the gear on the stepper motor , maybe it's pressed to far and it skews the belt to the side. That chuck k12-100 here's the specs of that chuck,
Lathe Chuck:
Diameter: 3.937"(100mm)
Thickness: 2.165"(55mm)
Center Hole: 0.866"(22mm)
Bolt Circle Diameter: 3.307"(84mm)
Runout: 0.003"

It's closer to 0,1mm than 0,05 stated by the seller of the 4 axis. You can always bump the stock to get less run out. I wouldn't count on the seller too much with new parts
Here are specs of Bison chuck , but it probably cost around the price of whole 4 axis unit. Normal units have 0,05 run out also but You can get precision ones with 0,01.

Ditto to everything said above.

Once you figure out if the 4th is OK, if you find you need better precision you can get a 4-jaw chuck and dial in the stock/part to zero runout before running your program. My 3-jaw is only good to about 0.003" or so when mounted on my 4th axis, but I have a small (100mm/4") 4-jaw I use when I need something to be perfect.

BTW, the best thing to use for a runout check is an old carbide end mill (shank) or ground dowel pin.

Dawid92 wrote:Try to move the gear on the stepper motor , maybe it's pressed to far and it skews the belt to the side. That chuck k12-100 here's the specs of that chuck,
Lathe Chuck:
Diameter: 3.937"(100mm)
Thickness: 2.165"(55mm)
Center Hole: 0.866"(22mm)
Bolt Circle Diameter: 3.307"(84mm)
Runout: 0.003"

Actually 0.003" is 0.0762mm, so rounded off the Chinese way it is 0.05mm, but of course, the way I learned rounding is that it should be rounded off to 0.1mm. Anyway, in my dreams I thought that 0.05mm is the worst case, but I guess that they prefer listing the best case. What I will do is that I will remove the chuck, clean the plate and the back of the chuck to see if that helps, and also rotate the chuck to see if that makes a difference. Maybe simple things like these can reduce the runout a bit, but if not, then maybe one day I'll bite the bullet (assuming the back plate is good enough, and buy a Bison or something similar. Right now I can only use it to make chess pieces or engraving in round things, or something which does not need high precision. Anyway, it is very good for the money and will be very useful as a learning tool to play with.

I will also move the stepper gear. I don't know why I didn't think about that. No the gear is not pushing the belt out, the belt is at the outer ring on that gear, but maybe if I move the gear a bit more in or out it might change things to better or worse and in that case I can try to find the best. I was also trying the remove the large gear, but don't know how. I have removed the three screws, but that didn't help, I could still no feel the slightest release and definitely could not feel that I can pull the gear off without causing damage. Do you, or somebody else, know how to remove it? Is the axis conical, like the MT2 taper, and it is also held by the pressure?

spumco wrote:Ditto to everything said above.

Once you figure out if the 4th is OK, if you find you need better precision you can get a 4-jaw chuck and dial in the stock/part to zero runout before running your program. My 3-jaw is only good to about 0.003" or so when mounted on my 4th axis, but I have a small (100mm/4") 4-jaw I use when I need something to be perfect.

BTW, the best thing to use for a runout check is an old carbide end mill (shank) or ground dowel pin.

Thanks. So your 4-jaw chuck is that good... Which brand is it? Mine is Sanou, but I guess that's just a common type, even if they call it high precision. How come that your 3-jaw chuck is worse than the 4-jaw you have? I am considering buying a 3-jaw as well, because obviously, a 4-jaw chuck can't be used on everything, a hexagon is definitely a problem for a 4-jaw chuck.

The drill I am using is as high quality drill, very straight and is normally used in my CNC spindle, which has a very low runout (0.02mm measured by myself).



I know that it is not causing reading errors. Anyway, next time I will also try something else, like a carbide end mill.

Maybe try different positions of the chuck on the back plate. One of the other positions might reduce the runout... or make it worse.

Some update:

Basically, the seller admitted that the MT2 live center is not normal and we agreed on my proposal refund which basically covers the purchase of a new live center. He could not ship one from EU, and I did not want him to ship from China, which is why we agreed on refund so I can buy from someone else. For the chuck play, he basically said that that's normal, "you get what you pay for", which I understand very well also, even though I was hoping for something better and it is actually outside his own specifications.Anyway, with the refund I paid 250 USD for the 4-jaw 100mm 4-th axis and the MT2 tailstock with the damaged center pin, which I think is still usable, unless I really need the very sharp pinpoint edge. I think that's a good price.

What I did so far to improve the situation is that I removed the chuck and the pulley and cleaned up everything. I reassembled the pulley and also moved the stepper pulley out a bit. After this it's a bit better, there is not as much play in the pulley as before, but of course, the chuck is the important part.

Before reassembling the chuck, I made some measurements on the chuck plate, and that shows no real problem as far as I can judge, so that's good. The thing I noticed was that it was very hard to remove the chuck, and the reason for that is because the chuck plate inner flange (I don't know if that's the proper name of it) is I think a bit too large in diameter, so the chuck is VERY tightly fitted on that flange, I think it is too tight because there is ZERO possibility to adjust the chuck. I will see if I will try to do something about that later, but for now I don't have time, so I just reassembled the chuck after cleaning the surfaces and made some new measurements. To my surprise, the play is now half of what it was before, so it is definitely within the specifications, which means that I am satisfied for now.

So for now, I am OK with this as it is, I will test it out for some real work also before I try to fix anything more. Perhaps one thing I will do is that I remove the chuck once again and try to clean that flange with some very fine sand paper, maybe some grit or something else will be removed by doing this which might improve it even more. I feel that before staring to grind anything I will need to learn some more about using the 4-th axis. I don't want to destroy it or make it worse, after all, a slow start with a less then perfect chuck is better than a destroyed and useless one.

Sorry, I missed your question a couple weeks ago. Sounds like you've got a resolution in progress.

Just to close the loop - so to speak - on the 4-jaw vs. 3-jaw question... My 4-jaw is also Asian cheapo. The difference is that a 4-jaw chuck with independent jaws can be adjusted to essentially zero runout, whereas a 3-jaw scroll chuck will have differences in runout because the scroll and jaw teeth are never perfect. And even if you adjust the 3-jaw to be perfect at one clamping diameter, it won't remain perfect at a different clamping diameter.

The solution to the 3-jaw is to relieve the "inner flange" on the back plate or open the counter-bore on the chuck. BTW, that 'inner flange' thing is usually called a backplate spigot (lathe term).

Once you have a few thousanths (inches) of clearance, the chuck is free to move around slightly on the back plate if the mounting bolts are loosened. The process goes like this:

1. Loosen chuck mounting screws slightly so they're snug and holding the chuck in place, but not fully tight
2. Put item/stock in the chuck jaws and tighten jaws
3. Put an indicator on the stock and rotate the spindle (4th axis).
4. Tap the chuck until the indicator shows low or no runout. I use a lump of lead or heavy copper/brass to nudge the chuck.
5. Tighten the chuck mounting bolts and re-check the indicator.

This will get the chuck perfect for that particular stock diameter, and is the same basic concept as "Set-Tru" chucks or similar lathe chucks. Those use radial grub screws to move the chuck on the back plate, but otherwise same idea. You shouldn't need grub screws for your 4th axis as the side/radial loads on the chuck should be minimal, or at least not enough to overcome the friction created by tight mounting screws.

Once you have the chuck set, you can insert & remove that diameter stock as many times as you'd like and the runout shouldn't change much. That's the advantage over a 4-jaw: a 4-jaw has to be indicated every time you take the stock out - even the same piece.