Adding a Rotary Indexer to a CNC Router

      Detailed thoughts on installing a "fourth axis," similar to a lathe, onto a CNC, to hold and rotate blanks for machining complex pieces. June 29, 2007

Has anyone heard of putting a lathe or spinning indexer on their spoilboard (nemi board) and using it in synchronization with a CNC router? Applications would mainly include turnings.

Forum Responses
(CNC Forum)
From contributor C:
I think you are talking about adding a 4th axis/rotary positioner or C axis to your machine. Typically they will mount to your table, off the end, or however you choose to mount it, not to the spoilboard. Depending on your machine, you will need a driver card or another means to communicate with this new axis in conjunction with your XYZ. I have seen stand-alone controllers for Haas rotaries, but it seems scary to me to have to dual post and get them to synch at the machine. Programming is also fun; I ended up having to get pretty advanced software compared to my router. I know that some of the mid level packages claim to have 4th axis capabilities, but the first program I got that said it could, couldn't, except for very basic rotate, stop type movement. Ended up with MasterCAM X with the multi-axis module; couldn't be happier.

I don't do a lot of turnings, mostly carvings, but it is very useful for rope/twist turning. For one off's or small quantities, I still turn to the lathe with duplicator. Arguably faster as well.

There is a difference between a rotary 4th and a CNC lathe. Someone on the board may know of a high speed 4th out there, but I know I'm limited as far as how fast (rpm) I can spin. So your style of cutting changes, not so much turning as profiling along the axis, turn, repeat. I'm sure one of the guys with a large machine does things differently, but that's where I'm at.

From contributor L:
Check put Thermwood - they have this available.

From the original questioner:
We have a 4 axis machine. And now that I think about it, it seems like it would be smarter to use a duplicator for our turnings. That still leaves me with parts like a tapered clover leaf that is used on the corner of an island. I definitely intend on pursuing this venture. Just not a whole lot of people out there who have done it. I have seen some rotary tables online that could lay on the nemi grid. Then I have also talked with someone who said (correct me if I am wrong) that a rotary table could be coded with M-code and put on an A & B axis? Could someone explain this to me?

From contributor C:
I'm not sure I understand completely what you are trying to do, but it sounds like you want to lay your rotary on its back so that your axis of rotation is vertical, perpendicular to the table? Basically a lazy susan? If so, you might want to look at Techno's 4th. It's not the most robust thing on the planet, but it will lay flat as I'm describing. As far as mounting it, can you attach it to an oversize plate, aluminum or even decent ply, and clamp it to your table then? I've got T-track in my top, so that's how I would mount in this application.

What are these tapered clover leaves you need to make? I guess I can't picture them...

From the original questioner:
Here is the tapered clover leaf that I'm talking about.

Click here for full size image

From contributor C:
Thanks for the picture. Obviously the picture is just a representation, but looking at it, without dimensions, do you need to cut it on a 4th axis? From the little end view in the corner, the sides don't look like they undercut. Wouldn't it be easier to use long tooling and cut it on its back, as it sits in the picture? Even if you need to work the back/inside, I would think it easier to cut it on the table, back first while still square, then flip to do the other side, then a fourth.

Are they solid? I suppose there could be an advantage to cutting two at a time, back to back, on the 4th, then splitting on a bandsaw. But either way, if there's internal work, I see holding them as a pain. That was my biggest boondoggle when I started on my project with the fourth; both ends had machining that I had to devise a way to fixture to in order to mount to the 4th, but not get in the way of the carving. And then of course that fixturing had to mount precisely to the 4th, so you could zero it out properly, etc., because the piece was asymmetrical, and had to reference those end/internal cuts. Moral of the story there was a .008 drift from welding heat scrapped the first fixture, and doubled the cost as the 2nd one had to be machined out of solid stock.

In my opinion, the 4th is best left to cylindrical work unless absolutely necessary. It's fun to figure out how to use it to carve asymmetrical items, but there's a lot of nonsense involved with it. Looking at your picture, it doesn't seem too bad; it's not going to matter if your C zero is off a few thou or a little backlash in the 4th exists, or if you mount the blank a little crooked. If you have a steady, consistent flow behind it to warrant the fixturing, programming and time, then go for it. But I think if you can cut them conventionally 3-axis, you are better off.

From the original questioner:
I apologize about the error. This is the picture I meant for you to see. It does have the back cutting and the notch to accept the corner of a cabinet.

Click here for full size image

From contributor R:
If you have a 4 axis controller and proper design software, it will write all the g and m code. The 4th axis will be the "A" axis. This can also be done with a 3 axis controller if the design is done in the flat. You will fool the controller to rotate the part instead of working in the flat. As a comparison, my laser engraver is only 2 axis, but I can tell the controller that I am working in the round. It will ask for the diameter of the part to develop number of steps needed for the rotation needed to engrave 360 degrees around the part. It then sends the signal to the stepper on the rotary fixture to rotate the part. The graphic is set up as if the engraving field of the round part is unwrapped and laid out flat. It does not design in 3D. You would also need to make some changes in your post processor to conform to the gear ratio changes on the rotary fixture. Hope this makes a little sense. I probably just have enough experience to be dangerous to the general public.

From contributor C:
Do you want to do the back cut on the CNC as well? If so, that's going to be a royal pain, as it looks like the corner of that cut-out is pretty near center. How are you going to keep the tool from hitting for fixturing/drive/tailstock? You'll have to mount off center, which is where the software issue is brewing, based on my experience.

Looking at the new picture, I guess I would still be looking at trying to cut them 3 axis, on its side, then flip to do other side. It's tough to tell from the picture with it sitting in isometric, but it looks like two sides will get you all of it. Maybe not, hard to tell from picture.

Either way, I guess it does confuse me a bit more in regard to your original question of mounting the 4th the spoilboard. If you want to cut these on the 4th, I would think you would want the axis running parallel with X or Y, but when we first started talking, it sounded like you wanted to mount the 4th in the middle of the table so that the axis was up and down, parallel to Z. I don't see how that is of any help for this part unless you have a 5 axis or aggregate head, which even then I don't know why you would use it. So I'm back to my original point of mounting the drive unit at one end of the table. I have mine set up permanently under the far end in such a way that the gantry and spindle can never hit it, and is not on the working area of the table. Then I just mount the tail stock when I need it.

Contributor R, I hear what you are saying, but I don't know if a simple programming package is going to handle what he's doing too well. It might, but the software I started with would not deal with this part well at all because it is not symmetrical around the center, not a cylinder. It sounds like you are using a machine shop rotary indexer with a stepper on it. Neat. What are you using to program? Going back to the old software issue, I had to do something similar to what it sounds like you are doing, but the program would not project to curved surface in any kind of useful manner, just got all kinds of confused.

From the original questioner:
My original thoughts were to do exactly what you have done to your table. Is there a kit you bought or made? Can I see a picture of it?

From contributor C:
Be happy to take a picture, but not today - no power, no lights, hence why I'm responding in the middle of the day:) There was no real kit, the unit has some holes that align with our track, I just had to make a spacer to account for the difference in height from the top of the phenolic grid to the extrusions. I also added for extra spin diameter. I'll take a picture when I can.

From contributor R:
I'm a modelmaker at my day job and use Surfcam software and a Motionmaster 5 axis. My laser engraver has a rotary attachment which is almost a plug and play. I have been looking at the rotary table idea for carving ink pens that I turn at home and hope to do on a little benchtop mill. I don't have that set up yet.

I keep looking at your part. If you don't run those in production, how about mounting the blank between centers with an indexing plate on one end. Build several files that will machine the blank in stages. Start machining the blank based on a known angle that will cut the first back angle and first positive faces and stop there. Rotate the blank to another known angle and start cutting where you left off on the first program. Keep rotating and cutting to catch all the profile. I would do that back cut on the table saw before taking the blank to the CNC. With this process you might be able to cut two at a time and cut them apart after machining to help with production time a little.

Look at your file from the end and see if this is possible. I think you should be able to do it in 3 programs.

From the original questioner:
Contributor R, that is definitely an idea I have been playing around with. When I first started to pursue this plan, I was looking at a rotary spin indexer (manual and automatic). I think I am going to have to do exactly what you said concerning building several files and turning the part to known degrees. I just want to make an intelligent decision not only concerning the technique but also concerning finances. I would like to think that I can make this happen at an inexpensive price. But I am starting to think that that is only a pipe dream.

From contributor R:
The Grizzly H7527 is an indexing head and tailstock for $255. A lot of the hobby guys then put a stepper motor on the rotary table for the 4th axis on their mills. You could use a 3 or 4 jaw chuck on the head, or screw a piece of plywood on the end of your stock and then bolt that to the index head.

From contributor C:
Here's a picture of the fourth on the table with the gantry out of the way.

Click here for full size image

Here it is with the spindle up, but over the unit.

Click here for full size image

This is with the spindle down.

Click here for full size image

Spindle down from the side. The gantry is on its limits, except for Z, so you can see it can't be hit unless I have the fly cutter or another wide tool on. When in use, the plates that bolt to the platter with fixturing on it are long to get the work piece over the phenolic/work surface, but keeps the gantry/spindle away from the drive. Haven't had a flex issue yet, but the unit can be slid forward if necessary. The shot I didn't take would be of the underside of both the drive and tailstock, which I put dowels in that ride in the T-track groove to keep everything aligned.

Click here for full size image

From contributor I:
You don't need an indexer/4th axis to make that part. Simply flip indexing the part on 4 sides will do it. The cheapest and easiest way I have found to do this is using Cut3D ( It goes for about $300 and is definitely worth a look. You could set up an entire bed full of these and mill all of the one side, then flip all of them simultaneously to reduce cycle time.

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