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CNC 4th Axis Basics: How They Work

Apr 13, 2013   //   by Bob Warfield   //   Blog, DIY CNC, Products, Techniques  //  6 Comments

This is the second installment of a series on 4th Axis Milling.  The first talked about why you’d use a 4th Axis.  In this post, I want to talk a little bit about the mechanics of how they work and what’s inside one.  We have a lot of readers who are interested in the details and even in building their own 4th axis.

To create a 4th Axis basically requires that the axis be well mounted so it can spin, and that there be some means of controlling that spin via g-code program, preferably with as little backlash as possible–backlash is the enemy of CNC.  The mounting process is not unlike mounting a spindle.  Typically there will be a shaft between bearings.  I suppose you could even contemplate sleeve bearings as opposed to precision angular contact bearings, although the latter are standard industry practice.  Tapered roller bearings can also work reasonably well for a 4th axis.  There are some cases I’ll get to where the 4th Axis is analagous to a lathe headstock, so that’s another way to think about it.

Let’s dig into some of design issues and mechanism types for a 4th Axis:

Geneva Mechanisms and Other Indexers

Perhaps the simplest low backlash approach would be a Geneva Mechanism.  They’re simple, reliable, and reasonably precise.  The drawback is that 4th Axis Continuous machining is impossible because the Geneva Mechanism has a fixed set of stopping places.  Hence can only be used as a pure indexer and not a true 4th Axis.

Here is a shopmade air cylinder driven indexer that’s pretty neat:

air driven indexer

A shopmade air-cylinder indexer…

Rotary Table Worm Gear

Hobbyists are often taken with the idea of using a Rotary Table.  They’re cheap, readily available (perhaps you have one left over from your manual machining days), already set up for workholding, and it’s pretty easy to stick a stepper motor on one and start using it as a 4th axis.  There’s just one little problem, only it isn’t so little–backlash.  The worm gear mechanism of most rotary tables has a fair amount of backlash.  They’re set up so you can adjust it out, but doing so makes the table prone to binding and wears out the bronze gears fairly quickly.  They require adjustment often to keep the backlash out.  Still, they’re not a bad way to get started with 4th axis machining, especially if you’re only planning on occasional low intensity use.

rotary table 4th axis

It’s not hard to convert a rotary table to a 4th axis, but they have a lot of backlash and will wear out fairly quickly…

Timing Belts

You could use a timing belt just like the kind that are used for the X, Y, and Z axes on a mill.  The problem is you’d really like to gear things down more than the timing belt allows.  Even a linear axis has the further reduction of the leadscrew.  This approach is doable, but you need to pay close attention to the resolution of your stepper or servo motor lest you wind up with not enough accuracy positioning the axis.  Again, you’re not going to be able to gear it down very much.  Because of this, I find servos seem to work better in design sketches I’ve done around this timing belt drives simply because they have way more resolution than the number of steps per revolution of a typical stepper motor.  There are other advantages too.

If you’d like to do some back of the envelope calculations for this style, I’ve got a spreadsheet for you to download that does that.  One neat thing about this style is it allows fairly high speed operation of the 4th axis spindle.  Enough so that with the proper timing belt ratios you could use it as a lathe headstock and attach the tooling to your mill spindle.  Pretty soon you’ve got a simple mill-turn setup that’s pretty potent:

See my write up on this neat combination 4th Axis and Mill-Turning gizmo

Harmonic Drive

The next step up is a gizmo called a “Harmonic Drive”.  I won’t try to explain how Harmonic Drives work, there is always Wikipedia for that.  Suffice it to say they offer nearly backlash-free gear reduction–perfect for this application.  Put one between your servo or stepper motor and the 4th Axis spindle and you’re done.

I got one off eBay relatively cheaply:

Harmonic Drive…

For a long time they were really cheap on eBay as they were Industrial Surplus and nobody knew what they were good for.  Unfortunately, the word is out so you have to look carefully to find a deal.  Nevertheless, one with an appropriate ratio would make a sweet 4th Axis.  You could either gut and remachine a rotary table or remachine a less expensive 5C Collet Indexer and you’ve have a real slick 4th Axis when you were done.

Globoidal Cam

There are many more odd mechanisms that eliminate backlash and perform the reduction function.  I wanted to mention one more since Haas just launched a new high-speed 4th Axis that uses this approach.  It’s called a “Globoidal Cam”.  Here’s a couple styles of what they look like:



The “pins” are bearings so the motion is very low friction and precise.  There’s a pin on either side of the “worm cam” (my made up term) and the thing is made with enough precision that there’s no backlash or freeplay available to the pins.  They hold that cam pretty tightly on both sides.  It would be interesting to try to machine a Globoidal Cam with enough precision to work in such an application.


Enough about the rotary motion, we also want to be able to stop our 4th Axis.  In addition, we want significant holding force when stopped.  If we’re using the 4th as an Indexing tool, and stopping to machine, note that the further off-axis the part is, the more leverage it has to try to turn the axis.  If you’re face milling or something equally aggressive, you may be very glad you’ve got a brake of some kind to lock the axis.  Here’s an air-activated disk brake on a shopmade 4th Axis:

4th axis disc brake

A 4th Axis Lathe?

One of the things I didn’t mention in the prior article on what a 4th Axis is used for is the possibility of using one to turn your mill into a lathe.  If you’ve got CNC lathes, there’s probably little point in doing so.  But if all you’ve got is a mill, and indeed, for certain operations even if you have a lathe, it can save you a lot of time and effort to be able to turn parts on your mill.  Traditionally, this can be done either by chucking up the part in the mill spindle and putting the lathe tooling on the table, or by sticking a lathe headstock on the mill table and mounting the turning tools on the spindle.  Both methods work, but with a 4th Axis that is capable of spinning fast enough to act as a lathe headstock as well as capable of indexing as a 4th Axis, there are a world of possibilities.  I’m not going to delve into them too deeply here, but check out some of my posts about CUBEstudio’s 4th Axis that can Mill Turn for more.


That covers the intro to how 4th Axes work.  Next up will be a post on 4th Axis Workholding.


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CNC 4th Axis Basics: How They Work
5 (100%) 1 vote


  • The setting/scale ring on some torque wrenches have a plastic harmonic drive. The ratio is about 30:1 so turning the setting control 30 times rotates the scale ring once. This is possibly an example of a harmonic drive that a lot of people already have.

  • Just fell upon the term cnc 4th axis and even 5th axis machining. May I ask why? Coming from research within game engines, virtual reality and 360 deg. screens I do not see any additional axis, only XYZ. Adding a rotary table does not add an axis but creates the possibility to rotate around one of the three axes. In the video above, seen from the cameras point of view, it seems that while we exploit rotation of the mill around the x-axis we gain benefit of no movement along the z-axis. It is not until the machine shows indexing and the mill stops rotation that it does a little z-axis movement. So again we do not add an axis, in that case we actually stop using one of the axes while rotating on another.

  • Steffen, while it’s true there are no more axes from the perspective of a game engine or computer graphics, in the real world there are more axes. There are more axes being programmed and driven, and these additional axes provide access that is not otherwise possible with a pure 3D CNC machine. That matters in these applications, hence it needs to be called out.

    • Hi Bob

      Thanks for the answer. I see now from your perspective, that a 4th axis is present when you focus on the physical axes within the four motors driving the cnc robot. I can see why the designers of these machines focus on the inner workings of what facilitates the movement, but a user don’t I guess. They focus on the possibilities. A game engine is a match to the real world though regarding the three axes of the real world three dimensions. To avoid confusion between what type of axis is important, I still think the best naming scheme for this is to be found within other parts of robotics and engineering. The naming scheme I am thinking of is degrees of freedom (DOF) and would be an exact match to axis numbering wise. A 4th axis cnc would be a 4 DOF cnc and so on. So from 3 DOF and up all of them would be 3D capable machines. The future most able cnc would be the 6 DOF cnc which would be able to do heaving, swaying, and surging like 3 DOF cnc but also full 360 pitching, yawing, and rolling. Additional DOFs would be redundant.

  • Stefen check out 8 axis lathe on youtube. Axis is a term used to describe a motor moving a part of the machine, not a physical dimension. Two lathe turrets facing each other are programmed to move towards or away from each other using letters that designate different axis, even though all motion is along the same line.

  • […] thousands of holes in large tubing that was too long for the machine’s travels.  A sliding 4th axis fixture was created for the […]

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