Okuma Air-Through Spindle Modification

In this trick we show how we modified our vertical machining center for "air through spindle" functionality.

Why put air down the spindle?...

The biggest reason was that we were too dumb to know better!

Technically, this all started as part of our engraving endeavor. We took it upon ourselves to find a way to engrave part numbers and other markings on our hydraulic manifolds, in the CNC, during machining. Our preliminary scan for commercially available solutions failed to uncover tools with suitable performance and cost.

At first, we played with air powered tools (Click here to visit our engraving page) but in the end found that a spring loaded carbide did the best job. Before we discovered the spring loaded carbide however, we were certain that we needed to get air down the spindle to make a reliable, "tool changer friendly", pneumatic tool. While the spring loaded carbide will handle most of our industrial engraving, we expect that our pneumatic high speed spindle will permit us to do ornamental engraving. Furthermore, our high speed spindle has already proven itself as a terrific tool for plastic work and it saves us from spinning the Okuma's spindle at full tilt on low cost jobs.

We bought our machining center used and, while it has many nice options, it lacks through spindle coolant. As we worked on the air tools, it became obvious that if we used through coolant tools, holders and retention studs, we could use air for chip control. At the moment, we are only using it with a few tools but we are planning on purchasing tools with coolant passages and modifying some of our existing ones.

While air will work for some applications, coolant certainly has advantages in others. We have not tried it yet, but we are planning on installing a pneumatic lubricator, or mist maker, and experimenting with misting type cutting oils. There are certainly applications for minimum coolant, or near-dry machining, and we hope that our air through the spindle will let us participate in those projects.

How to do it...

Air is already plumbed to the spindle of most machines so that the tapers can be blown clean during a tool change. The problem is that the air passage is only sealed when the draw bar is opened. When the draw bar is closed, the hydraulic actuator lifts away from the top of the spindle and the air escapes through the gap. Our solution (shown below) was to install a spigot in the top of the spindle that travels through a seal installed in the draw bar actuator.

8,000 RPM Air Seal

  7. SKT CAP #6-32 UNC X 1/2 LG
  8. ROLL PIN - 1/8 X 1/2 LG
  9. O-RING 5/8 ID X 3/32 CS

My assumption is that every manufacturer will be similar but dimensions for fit will, of course, be different. I think even from Cadet-Mate to Cadet-Mate there will be differences. Now that we have climbed and monkey'd around on ours, I can see the differences in other Cadet-Mates even from the same year.

The only Okuma part that we modified is Item 6. It is an aluminum fitting that originally had a BSP thread tapped in the hex end. We bored it out so that it cleared the tube (Item 1) by 0.010-0.020" diametral. The idea is to compensate for eccentricity between the draw bar actuator and the spindle, thus relieving the bronze bearing (Item 3) from excess load.

A critical detail to making this system work is the spindle spigot (Item 2). The top of the spindle is not really made for this type of modification and we found that the bore in which the spigot is installed in, runs eccentric. In our case, the runout is 0.005" TIR.

When we first discovered that the runout was in the spindle, our first thought was: "Game over." But with a little persistence, we came up with the idea to turn the spigot in a 4 jaw, purposely making the nose eccentric to the base. We had to fool around with the orientation but after a few install's and dialing's it came in at 0.0005" TIR. If you are going to attempt this, I would say make sure you have 0.001" TIR or less. If the spigot does not dial in, be patient, throw it away and try again.

The seal itself (Item 4) is made from Teflon and has a special exciter spring and profile that allows the seal to handle the high speed. We purchased it from Bal Seal in California (www.balseal.com). They have a $US500.00 dollar minimum order so I have quite a few spares. E-mail me here: info[at]qsine.ca (change [at] to @). if you would like to purchase a couple. I believe it will handle speeds greater than 8,000 RPM but e-mail me with your requirements and I will see if Bal Seal will approve it before you proceed.

The seal is nominally 5/8"OD x 1/2"ID x 3/32"CS. If you want to use the one we are using the bore in the tube must be 0.625" +0.001/-0.000. The spigot nose diameter must be 0.500" +0.000/-0.001. For best life Bal Seal recommends a surface finish of 4-8 micro-inches (polished) and that the shaft (spigot nose in our case) be made of steel and have a hardness of 52 Rc or harder; which means heat treatment.

We have not installed a hardened spigot yet. The test spigot shown is just 12L14 free machining steel, polished by hand in the lathe. We lack sufficient hours to make a statement on life but because we have so many seals left over, we decided to try the soft spigot and see what life we would get. We have also made up an aluminum spigot and had it hard anodized 0.001" deep. The hard anodizing makes a very hard surface and finishes very smooth. Once we fail the 12L14 spigot we will install this one for test. My guess is that it will take a while before we publish results on this.

The bearing (Item 3) is made from oil impregnated, sintered bronze. It is a press fit into the tube and we used about a FN1-FN2 fit. As well as being a bearing, it holds the seal in place. Be careful not to distort the seal when pressing the bushing in. The ID of the bushing should have 0.001"-0.002" diametral clearance about the spigot. I would recommend a break in period where the spindle runs from 300-3000 RPM for a minimum of an hour before cranking up to high speeds. If you get the clearance too small the bearing will run hot. For this reason, I would recommend that you stay with sintered bronze and avoid 660 bronze. If you get your clearance a bit tight, the sintered material will collapse and create clearance if thermal expansion demands it.

It is a bit difficult to tell from the drawing but the clamp (Item 5) is just a split, aluminum ring that uses a #6 socket capscrew (Item 7) to secure itself to the top of the tube. Once in place, the clamp fixes the tube in the draw bar actuator by holding the tube just snug to the o-ring (Item 9) under the fitting. The o-ring allows the tube to move but prevents it from rattling in the fitting.

The clamp is also drilled from the underside to sit, with clearance, over a 1/8 roll pin that is pressed into a hole we drilled into the fitting. The roll pin prevents the tube from rotating.

The tube is tapped with a female, 1/8 NPT on top to accept an air line fitting (see photo). We only connect to the air valve that blows the tool holders when a tool change occurs.

From there, we wired an external M-Code to the solenoid in the main wiring junction box. Thus, the solenoid is driven by both our M-Code and the control during tool changes. We added diodes to both control lines to prevent feedback from output to output. I am uncertain if this was necessary but better safe than sorry.

Unless your machine has an M-Code available, you will either have to purchase some from your dealer or wire an air control manually. I would strongly recommend using M-Code control. We were fortunate: an external M-Code was available on our Okuma because it has the tool breakage sensor option. The sensor was missing when we bought the machine and we have not replaced it yet, so the sensor air blast M-Code was available to us.

Piece of cake, right?! Well... maybe not. Keep reading if you want to find out about our trials and tribulations to get to this point. E-mail me here ???? if you want to ask any questions.

The Long Story...

The reason for engraving is that the hydraulic manifolds that we manufacture often require part numbers and port identification markings. We have used hand stamps but the numbers were never even nor straight and the results were simply less than professional. As I searched catalogs, the internet and just generally snooped around, I found people were using high speed spindles and engraving bits or dedicated engravers. Becuase there is so little engraving done on the parts, it seemed to make sense to make the CNC engrave. Furthermore, dedicated engravers are quite expensive and, for the volume of work we are doing, it is hard to come to terms with the price.

So then the decision was: Do we get a gear type speed increaser or a pneumatic one? As I was exploring the various options, the available equipment seemed quite expensive and both styles need a spigot attached to the spindle: the gear type for a reaction member, the pneumatic type to port air in. No big deal really but I did not want stuff bolted to the spindle - it just seemed sloppy.

While the investigation was going on, I stumbled across a Cat 40 tooling blank on eBay for $25.00. I then got the idea in my head that I could machine the blank to hold a die grinder cartridge and it worked(click here for details).

At first I thought that I could use the air blow used during tool changes but some playing around quickly showed that the air was not sealed when a tool was in the seat. Pressure could only be built when the draw bar was actuated.

So the tool blank got a 1/4" NPT air port machined in it's side for testing. Once we knew it would work, the next step led to the painful discovery that making a high speed spindle and making a high speed spindle that could be cycled through the machine's tool changer was not the same thing... not even close!

The supposedly "simple" air spigot that makes and breaks the air connection during tool changes turned out to be "not-so-simple" on the Okuma. I tried all kinds of combinations in 3D CAD to make it work. If I made the spigot on the tool clear everthing, then the receptacle on the spindle would hit the face mill holder: if I made the receptacle clear everything, then the spigot hit something - like the Z axis when the tool changer was in 2 of the 20 positions.

After giving up a couple of weekends with no success in CAD, out of frustration, I cut a big hole in the spindle cover to see if I could see where the air was leaking out. Unfortunantely, I could not see a thing as the workings of draw bar actuator were well concealed behing the many expensive looking parts. No further ahead and now I have to patch my machine... what kind of idiot would just cut a hole in the spindle cover!

The next week, I made a stainless cover and a nitrile gasket to patch my "handy-work" and the following weekend it was back to CAD. Another weekend yielded a spring loaded receptacle that would move out of the way if the face mill holder hit it. But what if we accidentally turned on the spindle with the air tool in? We say it will never happen but yeah, right! And what if this? What if that? Blah, blah, blah... spring loaded receptacle?! Good grief? I finally convinced myself that the air spigot is just a bad idea on this machine.

As it turns out, we hit a lull with paying work and had some time to do "internal projects". Kent and I got into our coveralls and started taking the machine's spindle apart. Neither of us have ever serviced a tool like this so we were a bit tense. Both of us though we may be only minutes away from destroying or prized machine but neither of us said a word about it until after we had it together again.

As it turns out, servicing the machine is quite simple. Once we had it all apart, we could see how they did things and nothing was intimidating at all. We even decided that when it is time for a belt change, we will do it ourselves. Once again, dad's saying: "The difference between complexity and simplicity is understanding" holds true again. I guess the one thing to add is that it may take a certain amount of courage to gain that understanding.