How does a 4th Axis work

[Action-KAT]

Hi
I'm looking for some information on how a 4th axis works,
I have looked at a few previous posts here,
I'm still not sure how they work & why they need to run slow!
If any one can explain how they work in simple terms would be a great help.
I'm going to try to design make one for a build I'm starting as the price on some are very expensive and I can't see why.
cheers for your input

[ger21]

Not sure what you mean by how do they work? A motor turns them, usually through some type of reduction system. They don't have to run slow, unless you use a stepper with a high reduction. In that case, the steppers rpm limit may limit your top speed.

[BackyardWorkshop]

I made a home made 4th axis recently - if you ned a few ideas

http://www.backyard-workshop.com/pro...-4th-axis.html

There's a video of it working at the bottom and here's a page for the first piece off of it (and a video there too)

http://www.backyard-workshop.com/pro...xis-piece.html

For this one it's going "slow" because it's really just doing 3D carving around the axis - not really "lathe" type work...

other than that I love to watch the Hossmachine videos on you tube - he's built his own 4th axis and is light years ahead

[Action-KAT]

Many thank for the replies
Sorry I may not have made it clear what info I was after,
cheers GeeksGoneBad for the links,
I'm not clear on the differance with the gear ratios & head diameters.
the backyard-workshop link is more like what i am looking to do simple & straight forward.
I have seen some Indexing heads cheap, would it be better to go this route if getting one at a good price.
Would it be possiable to just remove the handle and indexer & mount a motor,
nice to here some input on this route too.
angain thanks for the info
cheers

[jsheerin]

The gear ratio would describe how torque and rotation from the input is translated to the output. If there's a high gear ratio, the input torque will get multiplied to a much larger output torque, but the output will rotate much slower than the input.

The other issue to consider is whether you can back drive the gears in the head. If you just have some timing pulleys and belts, you'll be able to back drive it. This means if the force on the output (from the cutter pushing on the material you're cutting) creates a torque larger than the multiplied holding torque of the stepper motor, your workpiece will start turning when you don't want it to. On the other hand, I believe a worm gear will lock up and not be back driven although you might want to do further research on that.

And yes, you could just mount a motor to an indexing head, although how well that would work would depend on the head. For instance, you could have backlash. If I remember correctly, the one I have has a lock to prevent the head from moving once you've positioned it (for manual usage), but that wouldn't help much for cnc.

[Action-KAT]

Cheers jsheerin
Looks like I need to do a lot more research before jumping into building my 4th axis.