Machining a Turbine Disk in 304 Stainless

It wasn’t all that long ago that four- and five-axis machining was left to the big guys – but not anymore! The hobbyist and pro-sumer market for CNC machines has shown substantial growth in recent years, and inexpensive yet powerful CAM software such as Fusion 360 has brought this capability to the garage machinist, maker, or hobbyist.

Not only were we able to machine 304 Stainless Steel (a notoriously difficult material to machine) but we did so using a small form factor removable 4th axis, and a machine designed specifically to enable the garage machinist – all while nailing the tight tolerances needed for a jet engine turbine. We love machining, and we love that it is becoming more and more accessible at any price point.

The Setup

All of our Tormach mills in the shop are outfitted with a Saunders Machine Works fixture plate. These give hundreds of precision-bored reference locations making it simple to drop in and align any vise or other fixture.

Check out our line of Tormach fixture plates here!

Starting with Op1 we chose to use the SMW Modular Vise System alongside a pair of our aluminum Soft Jaws. Dropping the Mod Vise onto the fixture plate, we machined the circular profile of the 304SS stock.

We prefer this to setting up a lathe chuck or vise with V-blocks to hold round parts: the soft jaws have much more contact with the stock (as opposed to just 3 or 4 points), it is faster and easier to pull the first part out and drop another in place, and the 4-sided soft jaws mean you can even flip the jaws for Op2 (provided your part geometry allows it). 

Unfortunately, Op2 wasn’t quite as simple – here we needed to turn a custom fixture on the lathe and mount this in our microARC 4th axis. One of the main issues with a 4th axis like this can be the space it takes up on the table. It’s great when it is being used, but can often eat into crucial table travels when not in use.

We love to use our microARC 4th axis subplate because it allows for super simple setup of the 4th axis! Using two dowel pins for quick alignment, the microARC can be added or removed from the mill in seconds and the subplate even allows you to gain back some travel when the rotary is mounted by mounting slightly outside the range possible with the table t-slots alone.

The microARC 4th Axis is currently on backorder, but rest assured, Tormach will have more in stock soon.

Let’s Get Machining!

Being that this part is for a jet engine, we have some pretty tight tolerances we need to nail here which influences the machining strategies fairly heavily. For a full, in-depth video on how Jeffery made this part, be sure to check out the video!

Op1 is relatively simple with two things to pay attention to: the bore through the center of the turbine disk and the parallelism of the two circular faces on the part. For tackling the bore, we first needed to spot and then drill a small pilot hole. Drilling can be incredibly taxing on the machine, especially in harder materials.

The pilot hole alleviates some of the power needed to drill the full diameter and keeps the spindle from bogging down in the cut. Once the pilot hole is drilled we can drill to the final drill diameter and finally reaming the hole to 3/8” to hit our tolerance.

Jeffery ran into a bit of trouble with the parallelism of the part when flipping it for Op2, but with some careful measurement using micrometers alongside fixturing adjustments the part was finally decked within an acceptable tolerance. From here the same surfacing was done on the back that was done on the front, and the part was ready for the microARC!

There were two more tolerances to be mindful of in Op3: the final outer diameter of the part needed to be spot-on and the axis through that final diameter needed to be coaxial with the center bore we machined in Op1.

To ensure the turbine was mounted along the correct axis, Jeffery took plenty of time carefully adjusting, measuring, and readjusting the fixture post in the 4th axis chuck. Once it was aligned properly the partially-machined turbine disk stock could slide onto the fixture and be bolted down.

This process ensures that both axes through the center bore and outer diameter are coaxial (which will eliminate wobble in the end product when the engine spins up!) Step two is to machine the final outer diameter – so let’s do some turning (on the mill).

Using the Rotary toolpath in Fusion 360 we can use a standard endmill to effectively turn the part to the diameter we need while keeping this surface almost perfectly coaxial. Now for the fun part – machining the turbine fins! Using some slotting and some adaptive strategies Jeffery was able to successfully rough out the fins with a circular pattern turning the microARC from fin to fin.

Following that up with a 3D Contour and Trace to finish and we were done! It is AWESOME that we were able to machine this kind of complex part on a hobby machine while still hitting the necessary tolerances for a jet engine (and with some beautiful surface finishes)!

John Saunders

John Saunders is the author of the NYC CNC YouTube channel and runs Saunders Machine Works, a machine shop in Zanesville, OH.

John Saunders

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