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Pneumatic Rotary Actuator For CNC Milling


Apr 14, 2022
Hello All,

I was wondering if anyone has seen or heard of pneumatic rotary actuators being used for indexing in a CNC application? I've never seen it done, but I cant see why?

Here is what I am thinking...

Why not use a pneumatic rotary actuator as a 4th axis in a 3 axis mill? I have no doubt that an off the shelf pneumatic rotary actuator cannot hold up to the task, but why does no one make pneumatic indexing products for cnc applications that can handle the forces involved with cnc cutting? I am suggesting a simple device, one that can only index 180 degrees and then back, so a very dumbed down "4th" axis, but still useful nonetheless.

Take a look at the attached image, this is a cutaway of a classic approach to a pneumatic rotary indexer using rack and pinion style design. Using this design approach, could you not incorporate a shaft brake/lock and some adjustable hard stops and you could have a functioning dumbed down "4th axis" on a 3 axis mill? At the very least you could have a pallet system bolted to this and be able to access both sides.

I am thinking it could be very neat to have a unit like this and be able to machine 2 sides of a pallet or single part... As I mentioned I am only thinking about a 180 degree indexer, so machine one side, index 180 and machine the other and then index back "home".

Incorporating a brake would be required, so something like a belleville stack against a disc rotor, or even an air brake.

It could be programmed using an Mcodes? Trigger a relay that activates a solenoid to make the index? The brake could be deactivated using the same relay so that when an index occurs the brake is guaranteed to release.

The hard stops could be adjusted to dial in the proper indexing positions, and then after that it should remain accurate during indexing for a long time?

Wondering why nothing like this exists?

Looking forward to hearing the replies!


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No doubt a pneumatic indexer is old tech compared to a servo indexer and less capable... A new HAAS 5C indexer is $11,395 USD at the moment, not sure on the used market. A servo indexer is more capable in everyway, but couldn't a simple pneumatic indexer have a place?

You can buy pneumatic rotary actuators for robot applications for a few hundred bucks, I would think a CNC ready pneumatic indexer could be made and sold for around a thousand bucks? For 1/12 the price of a servo indexer I would be interested.
I used to run a Mori with a Dapra pneumatic indexer back in the mid 80s. According to the Dapra website they still make them. It uses a curvic coupler so very good rigidity with no brake required.

Pneumatic and manual indexers never gained great market acceptance in the CNC world. Motor driven indexers did well 25-40 years ago. Since then, full servo driven 4th axis has become the market leader IME.
Actually I couldn't get the m code to work on my machine, so I put a limit switch on a bracket and rapid over to trip the switch
Only job I used the haas for
Erickson made 50 gazillion model 400 and 600 pneumatic indexers. You can set the index increment and even cut a custom masking plate if needed. They index from 15 to 90 degrees per stroke stock. So to index 90 you'd double stroke it.

I have a couple working model 400's with the manual if you need one.