[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.