4th Axis Build

Recently I decided to build a 4th axis for my Shizuoka milling machine. I looked through some designs and decided on using a Harmonic Drive gear drive and found a good deal on a 50:1 gear reduction one. If you haven't seen a Harmonic Drive do a youtube search as they are quite interesting and work very well when the goal is to reduce lash to a minimum. I have purchased a second one and will be building the 5th axis that will attach to my 4th in the near future.

Now that I had the gear system figured out it was mostly a matter of designing the rest around it. I had a fixture acquired at auction a few years ago that was perfect for this application. It is 1" steel welded up and I added a 3/4" piece of steel to be the support for the rear bearing. For bearings I decided to use Timkens. I used the same that are in my trucks front axles which have an ID of 2.25" and the outer diameter of the race is near 4 inches. I would love to have had a through spindle design however the harmonic drive design would allow it. I did decide to have a threaded nose that matches my Tsugami turret lathe so the chucks would transfer over. I also decided to make the nose accept ER40 collets.

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I must say have CNC capabilities has changed everything about how I go about machining. Here I setup to machine the bore for the race. Since going through the ball screws and getting the lash down below a thousandth I was easily able to hit the desired bore size. I'm still learning my machine and what I can and cannot get away with. The end mill cost has been steep with my goofs.


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Here I have the races installed and the bolt pattern on the front for the seal plate.


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More to follow:

Now that the main casting was finished I needed to turn my attention to the spindle. I already had drawn up a workable model but needed some length dimensions from the main housing before I could finalize my numbers. I used my old Sidney lathe to rough turn the spindle down from some 3.5" stock I had.

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Here is the rough profile and spindle thread after I finished turning it. I left the body oversized so I could grind it in on the Brown and Sharpe #13 grinder.

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Coming along and starting to mock it up as I go. You can see the stepper motor (Nema 23 - 425 oz in) and how it will be tucked under the spindle.

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Boring out the inside of the spindle.

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Here is the bearing seal plate machined and installed as well as more mock up to check the fit in the main housing. The harmonic drive can be seen in last of the four photos.

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Now that the spindle is fit I focused on the rear plate that would mount the motor and part of the harmonic drive.

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For my homing switch I used a small micro switch and on the spindle I installed a dowel pin that I rounded the end on so the switch arm could roll over it to engage.

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Here is a picture of the completed spindle. One thing I failed to mention is that there is a rear idler shaft out the back of the spindle to support the pulley for the harmonic drive. Inside the spindle I installed a small bearing and part of the harmonic drive there is a bearing. Worked out really well to keep it compact.

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Here is a better view of the idler shaft on the spindle

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I used an Amphenol connector for the electrical connection.

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About ready to do some machining finally. I installed a 48v power supply for the motor along with the Geckodrive g201x which is the same as the other drivers for the mill. After fooling around with the setting for a bit I was able to get it to work. I'm looking forward to having some fun with the addition.