Limitations of cheap 4th axis components off eBay?

[Goemon]

Hi, I get the impression that with a slab of hardwood or aluminium 2" thick X 6" wide and 30" long you would be working on something that looks like a large flat panel.......that is, working on each of the wider flat sides top and bottom etc.

It must be realised that as soon as you get away from the centre line of the work piece the torque reaction that the cutter applies to the 4th axis head, bearings and drive are increased.

No matter how firm you hold the work piece between the tailstock and the drive there will be a deflection on each side of the work piece as at 6" wide it is 3" off centre.....this is something like stepping into a boat and having it dip down on one side from your weight.

Any deflection from the cutter forces will make any surface the cutter works on totally inaccurate.......especially in the Z axis area.

I think this would totally rule out a belt drive/timing pulley set-up mainly due to the low 1:6 reduction ratio, but also from spring in the belt drive, but the worm drive or harmonic drive types would be better on a broad faced work piece provided the backlash could be reduced to practically zero, and that was where the search for the Holy Grail was heading.

BTW.........if the work piece is a wide flat panel then I have to think that a 4th axis is not the ideal means to rotate it for surface machining as all you get is a curved surface......the Z depth will be constantly decreasing or increasing as the piece moves away from the flat plane......is that how it's meant to be?
Ian.

I am not worried about the feasibility of what I am looking to do. It's nothing new or unique. Plenty of people are already doing it. I'm just not sure if anyone is doing it on a cheap Chinese 4th axis, or what the minimum investment requirements are for a semi diy build.

I am back to being as confused as when I started which is a fairly common issue for me when I ask for advice on forums on topics with no definative answer. Conflicting advice would be fine if I knew all of you and could just choose to listen to the most knowledgeable but I don't.

To me it reads: you want a harmonic drive. No you don't, that's the last thing you want. A cheap pulley system will
do the job. No it won't...well... not if you want the end result to resemble the actual product. What you really need is a worm gear system. No! Definitely not that! My uncle Dave once built a 4th axis from toe-nail clippings but I have no idea if it actually worked. Wait... it may not have been uncle Dave, it was aunt Susan and she never even had toe nails....

I am back to attempting to use common sense, deductive reasoning, reading between the lines, trial and error and trying to identify common themes from the advice.

Starting with common sense and common themes, it sounds to me as though any type of motor and reduction system could work well as long as I buy "good ones" and use a motor / reduction combo that provides the right amount of holding torque and.... I mount the components on a suitably robust frame with a well supported grip. Similarly, I bet any type of drive could fail if I buy a low quality version.

I did a bit more research and found a good number of examples of 4th axis builds using rotary tables. I also found some impressive projects using harmonic drives. There is a good selection of those on eBay used at affordable prices. If I can figure out how to connect one to a chuck, I could see it working well.

[johncarpenter]

A way to attach a chuck to shaft. Make a chuck backing plate as a split collar. Shaft hole in split collar was slight interference. Wedged split to insert shaft. Tightened bolts to reduce spread induced by interference fit in split collar hole and hold collar on shaft. Use 4 jaw chuck to make up for any inaccuracies.

[ger21]

There's really very little to it. A motor, gear reduction, and a chuck, and some way to tie them all together.
Belt drive will work, but you need multiple stages to get enough reduction. I'd say you want between 20:1 and 50:1, depending on the motor you are using.
Planetary gearboxes are probably the easiest way to go, and there are tons of them on ebay. Just make sure to find one with low backlash.

I know two people that have purchased chinese rotary axis, and neither was happy with what they got.

[Goemon]

My "harmonic drive" was similar to item no 261858842965 on eBay. Not the same seller but looks almost identical.

Just make sure to buy it with 4-jaw chuck (much more convenient for woodworking) and 50:1 ratio (100:1 is a bit slow). For a few bucks less you can get it with 80mm chuck but bigger is better.

Here is a video of that rotary thing in action:

I have been looking at 5", 6" and 8" chucks / rotary tables. I assumed that bigger was better for what I am doing. I just not sure a 3 or 4 jaw chuck is right for my application though. I will never be holding round work pieces. Only wide flat long bars so the most likely scenario is that I will clamp the blank stock directly to the face plate using t-slot vise jaws, or buy on of those 4" round clamps which are specifically designed to hold flat pieces on 100mm rotary tables.

it looks like the gears on the rotary tables can be disengaged so a harmonic drive could be connected to it to give a different ratio.

[awerby]

As others here have tried to tell you, it doesn't sound like you need a 4th axis at all. If you're cutting things out of 2" x 6" plates or boards 30" long, suspending it by the ends will give you a lot of unwanted vibration in the middle, which will mitigate against getting a clean cut, no matter how good the 4th axis is. Instead, all you need is a fixture that will let you flip the part over and re-secure it against the table without losing registration. This will not only save you money, but will ensure better results.

[johncarpenter]

Goemon, 4 jaw chucks excel in holding rectangular blocks.

[RCaffin]

Two different machining situations here. One is where you keep the cutter on the axis of the A drive and really use the rotation, and the other is where you use the A drive as in indexer, rotating 90 degrees each time, and move the cutter off the axis.
For the first case the programming is complex but the forces on the A axis is quite manageable.
For the second case the programmining is much simpler, but there are bigger forces (torque) on the A axis.

Is it worth buying a really cheap Chinese RT just for experimenting? To see what the wood can handle?

Cheers
Roger

[handlewanker]

Hi, one thing you must always understand.....if you get conflicting free advice only you can decide which of the two opposites are valid for your quest.

There are only so many practical variations for a 4th axis build and in the end it comes down to actually making one or throwing money at it if you lack the skill to make one.

You've heard all the evidence, now you must be your own jury, as you'll do your own thing in the end, but there are 4 things that will occur.....time....effort ......money......imagination.......if you lack any of those the project won't happen.......the culmination of those is that you only get out of it what you put into it, and a poor man always pays twice.....metaphorically speaking.
Ian.

[RCaffin]

I got a notification of a new posting about a 'harmonic drive' with lots of backlash, but I cannot find the posting.

Well, anyhow, looking at the video of the backlash, I will say that the gearbox does NOT (I repeat, NOT) look like a harmonic drive (HD). It looks like a cheap and nasty Chinese gearbox. The amount of backlash would be typical of a cheap gearbox. You would NEVER get that backlash from a functioning HD. I think the vendor has told a flat lie, possibly through ignorance. Dispute through eBay and demand your money back.

Yes, I do have a 4th axis I made myself with an HD.

Cheers
Roger

[handlewanker]

LOL, well Rog, never mind the quality, feel the width.

Maybe the term "harmonic" generated a thought that due to the noisy gearbox it was a harmonic drive.....it's all in the interpretation.

Just to digress for a moment...…..I see they are selling CBN diamond grinding wheels on Ebay……...for what purpose I might ask?...... T'is only one or t'other I thought...…………..diamond for carbide and CBN for HSS etc.
Ian.

[RCaffin]

CBN diamond grinding wheels on Ebay
Chinglish.
I have noticed that most of the Chinese on eBay do not know the word 'brass': to them it is all 'copper'.

Cheers
Roger

[handlewanker]

Yep....when I bought some CBN wheels for my Cuttermaster I found them on a Ukrain seller's listing.
Ian.

[rey8801]

I got a notification of a new posting about a 'harmonic drive' with lots of backlash, but I cannot find the posting.

Well, anyhow, looking at the video of the backlash, I will say that the gearbox does NOT (I repeat, NOT) look like a harmonic drive (HD). It looks like a cheap and nasty Chinese gearbox. The amount of backlash would be typical of a cheap gearbox. You would NEVER get that backlash from a functioning HD. I think the vendor has told a flat lie, possibly through ignorance. Dispute through eBay and demand your money back.

Yes, I do have a 4th axis I made myself with an HD.

Cheers
Roger

Hi! The post is mine! :-)
Here the link
https://www.cnczone.com/forums/linea...=1#post2321520

I made a video yesterday during the disassemble, just a random talk late at night, but it's useful to actually show how it is made. The backlash it's really from what I would call harmonic drive, but I am not an expert. What surprise me is that there are other users from this forum with the same drive and they claim almost ZERO backlash, so can be that I got a faulty one with maybe components out of tollerance. It seems the mesh inside the gearbox has a play before lock in place.



Thanks!