I've always wanted a 4 axis machine but previously dismissed the idea assuming it would be far too complicated. However, recently I've been giving it some thought and it doesnt seem so difficult at all. My idea goes somthing along the lines of getting a 4th gecko drive, hooking that up to a 4th servo motor/encoder and then connecting that to a home built rotatry table consisting of some precision worm gears and bearings.

What I'd love to know from people who have already built or attempted to build a machine with a 4th axis is if my idea has a chance of working, or have a missed out a lot/over simplified things too much ?

Also if anyone has got some photos of their 4th axis setups i'd like to see them.

I am also going to add a 4 axis to my machine at one end of the table.
I dont know if you are aware but the 4th axis will be a rotary axis for either your x,y or z axis. The most handy place to put your 4th axis is on the x or y axis.
This will alow you to machine long posts. You could do facny pattered table legs, gun stocks, baseball bats, anything that is long and narrow.
The least useful axis to put rotary movement on is the z axis.

Here is a diagram I borrowed from a well know 5 axis web site. I hope he doesnt mind.
I have also included a classic table leg the demonstrates 4 axis routing
http://www.tablelegs.com/lgimages/303-XLS.jpg
http://www.quantumcnc.co.uk/images/4%20axis%20dial.jpg

I was aware that the 4th axis is usually a rotary table and I did indeed plan to put it on the X or Y axis. No doubt you'll get round to building your 4th axis before I build mine, so any tips you'd be willing to share based on your experiences of construction would be much appreciated.

A 4th axis addon is probley the most simple thing you can do to your machine. Get a rotary table. Take the handle off it. But a timing belt gear inplace of the handle. Make a bracket for a motor to bolt next to it. Put a timming belt gear on the motor and connect the two gears by a belt. Hook it up to a drive and set your setting in software. Bolt it to the table. Square it and plumb it. Tail stock can take some thinking. I built a adjustable slide with a tig welder all thread 4" tube and 2" tube. It works but not great.
A cheap option is to go to harbor freight and buy a $69 wood lathe. Take the motor off and replace it with a stepper or servo and a timing belt. You would then probley need to do like me and take a angle plate from enco and mount you r spindle horizontal. Change the Z and Y around and then you can do much larger pieces..

Donny

www.whiterivermfg.com

That sculpture was done on a mill?

Donny, I have a 4" rotary table that I am planning to convert. It came with a tailstock, and has a #2 Morse taper in it, which is a requirement for my purpose. It is built like a tank, weights about 20 lbs. Cost about $200 with tailstock and a dividing plate set, brand new. www.littlemachineshop.com

HF has 3" and a 4" rotary tables for under $100 that look like good candidates for conversion, but no Morse taper in them.

The head and tailstock from a Taig lathe could be easily made into a lighter weight 4th axis too. The head is about $60, and a timing belt pulley can be easily fitted. The head has very convenient slots on top. It would be easy to fit a bracket for a motor.

All the stuff in the pics above were done on a 4'x8'x6" Router table with a rotary table and the router spindle mounted horizontal.
Fred, Sounds like your 4" table should be perfect for the job. Just need a moter and some way of connecting the two. You could alway pull your y axis drive and hook it to the rotary and still use your 3 axis controll. Just use Y as A and wrap your part around a cylinder in the software.

Donny

www.whiterivermfg.com

That one in the picture on that website, is HUGE.