Adding a Fourth Axis or Indexer to your CNC

Adding a Fourth Axis or Indexer to your CNC

Category : New Stuff Useful Stuff

Fourth Axis or Indexer?

When we talk about a fourth axis it means that we have four simultaneous axis X Y Z plus one other.

That means you can move them all together (the moves are simultaneous) and they are all timed together.

It’s a bit like trying to pick your nose and scratch your arse at the same time. You can get mixed up and poke your eye out.

CNC Machines are much better at this kind of stuff than we are.

Below is what happens when you program an X Y Z and C (rotary axis) move in one block of code.Fourth Axis

The one below is on a lathe so the Z works  the same as the X above.

In this program it’s just a Z C move or on a CNC Milling Machine it would be X C

Fourth Axis


In the above example just the two axis (Z and rotary) are simultaneous..

If your machine has a simultaneous fourth axis, sometimes known as a full fourth axis.

On the position display you will see X Y Z but you will also see C (sometimes A or B I’ll explain later)

A full A B or C axis will be programmable in .001 degrees. That’s one thousandth of a degree (very small).

Typically G0 C185.326 rotate to 185.326 degrees

If a machine has an indexer and not a full fourth axis it normally won’t be shown on the position display.

Another thing to note about a full fourth axis is that it doesn’t stop at 360 degrees and start again at zero.

Angle wise 0 and 360 are the same but if you are talking simultaneous then it’s continuous.

Are you confused?

This is my pet goat his name is Beelzebub I named him after a famous politician.

Let me explain. In the example above (the fourth axis example, not my goat) you want the fourth axis to rotate more than once.

You programme C1440. which is 360 x 4 and it turns four times and the other axis works simultaneously.

So if the other axis (X) is moving 60mm. For each turn it moves 15mm. Just like a 15mm pitch thread.

Ok you get it now? Good. Very good. Just when I am starting to loose my fuckin patience with you.

The number of times I have to beat my students with sticks these days is getting more n more.

So What’s an Indexer

When we talk about an indexer on a CNC machine this means we have a rotating axis but we can only move it to a position. This can be 1 degree or .1 degree depending on the machine tool.

Whilst these are really useful you can’t do all that clever shit that you see above. So don’t get confused especially if you are footing the bill for a new or second hand machine.

Check the minimum angle you can index. If you have a part and it has holes at 10.5 degrees and your indexer is one degree increments then you’re knackered (or fucked but I have decided not to swear in these articles anymore).

You could be disappointed. “Sold a pup” as my mother used to say. I”l have to ask her what the fuck that means.

Don’t you just love them. I know they’re little portable shit machines but soo cute. Make lovely Christmas gifts.

Not on Linkedin though for fuck sake. Anyway enough gratuitous crap.

Retro Fourth Axis

Retro, a bit like sick and gay, this word has a totally different meaning to an old guy like me.

Political correctness to me means voting for the correct political party.

Now with regards to an indexer retro means it’s added on, like sticking a spoiler on your old Mini Metro.

What retro doesn’t mean, is wearing your dad’s old Parker Jacket and thinking you look cool when you actually look like a prick.

Anyway when you add an indexer to an existing machine you control it by a dedicated M code from the CNC Machine.

Your indexer will have it’s own little control system and programme. This programme will be a series of index moves probably followed by clamp and unclamp command.

The machine will send an M code to the indexer and won’t continue until it gets a signal back from your indexer saying it has completed whatever it is doing.

Now your CNC machine doesn’t give a flying shit what the indexer is doing, but it won’t carry on until it gets the signal back saying it’s complete. It’s like wall papering the hall through the letterbox, unless of course you are a gynaecologist.

You do have to be careful with all this stuff but it’s a relatively cheap way to get a programmable indexer.

Haas Fourth Axis

Haas machines come really well equipped and it’s usually relatively cheap to add a full forth axis years after you bought the machine.

After all they used make the bloody things, before they made CNC machines, I think.

They do a great range of add on bits n pieces.


This is all stuff to consider when you buy a CNC machine new. It can be really expensive to add at a later date as an extra. So bite the bullet, grasp the nettle and get one with your new machine. Give them my name so I get a fat commission.


Simple As ABC

A rotary axis can be called A B or C. This will depend on which axis it runs through.

On a vertical machining centre like this one the centre of the fourth axis runs along the X axis so it’s called an A axis

Fourth Axis

If it runs through Y it’s called a B axis. And Z well……. try n guess.

Full Fourth Axis Useful or Just Bullshit

In the real world there aren’t actually loads of jobs that need a full simultaneous fourth axis. Radial slots maybe radial engraving.

When you start talking about indexing to angles and just doing stuff, well there are loads and loads of uses. For example with a square part you can get all around it just by indexing so your part can come off complete.

Leave Your Fourth Axis Setup

Don’t forget to set your fourth axis as far to the right as you can so you don’t have to keep taking it off.

You should consider a base plate That way you can mount your indexer outside the machine stroke.

By mounting your indexer I don’t mean trying to shag it by the way.

Taking a fourth axis  off and on can be a real pain the arse.

Also don’t forget to use a G10 to store the work offset. If you don’t know how to use G10 then be ashamed of yourself and read this.

Add an Indexer or Fourth Axis

Adding an indexer or fourth axis to your small CNC Machine will increase capacity and allow you to rotate parts to get to areas you didn’t even know were there.

Here’s a free advert for someone

Surface AreaFourth Axis

If you consider the long cube on the axis above. You actually have four faces so it can be a lot of surface area when you add it up.

A lot of people make these cubes themselves and it can save money like baking your own cakes or performing your own keyhole surgery.


Congratulations you are the first person to get to the end of one of my articles without being bored shitless.

If you feel you have been affected by any of the issues raised in this post or need CNC Counselling then contact me.

If you want to learn to programme CNC Milling Machines

Siemens 840D Sinumerik training

Look no further Contact CNC Training Centre






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