The quarter scale Merlin's prop shaft is a scaled version of a design that's still in use today on full-size aircraft. It's externally splined, and its end is threaded for a castellated prop nut. In order to fit a propeller to this shaft I designed a splined hub that will become an integral part of a four blade prop. Its front also includes a threaded section for a spinner.
I've cut key slots before, but never splines - neither internal nor external. Cutting external splines looks fairly easy and is pretty similar to cutting gear teeth. Initially, the internal splines seemed more intimidating, and so I decided to start with the hub.
One of the options on the Merlin's prop shaft drawing is a variation of an SAE 16 straight tooth spline. I designed the hub for sixteen 3/4" long splines spaced 22-1/2 degrees apart. The splines in this hub can be cut using an ordinary 3/32" keyway broach. Since I had a .875" diameter broach bushing, this became the i.d. of the hub as well as the minor diameter of the shaft. Actually, I reduced the shaft diameter .010" to obtain a tooth end clearance of .005". I pulled this number out of the air with no experience or reference material to justify it. Broaching this i.d. creates sixteen teeth having a 22-1/2 degree included angle and a height of a .050". The prop splines must be cut using a form tool that matches the shape of these teeth.
My initial thought was to use a rotary to index the hub under a broach held in the locked spindle of my mill. I did a few experiments and realized the force required to drive the broach through a one inch piece of 12L14 was more abuse than I was willing to apply to the bearings in either my spindle or rotary. Instead, I manually indexed the hub, and I used a manual press to drive the broach. I CNC-engraved the top of the hub's blank with the 22-1/2 degree alignment marks and I scribed a corresponding reference mark on the broach bushing. Accurately aligning the broach for each cut was easy, and the whole splining operation took less than half an hour. After finishing the splines, the remaining machining on the hub was completed.
Instead of spending fifteen minutes grinding a single tooth form tool for the shaft splines, I spent most of an afternoon modifying a 3/16" HSS Woodruff cutter. The sides of the teeth were ground as close as possible to the theoretical 11-1/4 degree side angles, and the width of the flat end came out to .078". Normally, I would have turned a four tooth cutter from scratch as I did for the crankshaft gear for my two radial builds. In this case, however, I had a lot more material to remove, and I wasn't sure how well drill rod, even heat treated, would hold up against the Stressproof shaft especially if I had to make more than one shaft. The most difficult part of the Woodruff cutter modification was relieving the sides of the teeth. During testing I found this relief helped to reduce the height of some very tough burrs that tended to rise up on either side of the spline. These burrs interfere with the test fitting of the hub to the shaft while it's still fixture'd for splining.
I first tested the cutter by splining an aluminum test shaft. The splines looked great, and the shaft slipped into the hub with a close sliding fit. The clearances of the splines on the rear of the hub where the broach had entered looked nearly perfect, indicating that the spline cutter was accurate. However, the clearances between the shaft's teeth and the hub's slots on the front of the hub where the broach had exited were much greater. I checked the broach bushing and the perpendicularity of the press, and all was as it should be. I purchased a new duMONT broach since the one I had been using was part of an inexpensive import set. I made a new blank and reduced some clearances, but the results were essentially the same. Even with 3-4 teeth cutting at a time, the bottom of the broach drifted to the outside of the blank during every cut causing the splines at the bottom end of the hub to be deeper than those at the top end of the hub. Turning the set-up around on the table of the press had no effect. I examined several parts on other projects that I had key-broached, but none of them showed the drift I was seeing on these hubs. The only solution seems to be a rigid set-up to keep the bottom of the broach tight against its bushing, but since it wasn't obvious how to do that, I decided to save it for the next splining project.
Although not desirable, the tapered splines really don't create a significant problem in this particular application. Both hubs fit the test shaft snugly since the hub teeth fit into the shaft splines with their proper clearances along the entire length of the hub. It's only the shaft teeth that end up with excessive clearances to the hub slots. The two-piece propeller hub that I plan to make will sandwich the prop and should insure its perpendicularity to the prop shaft independent of the splines. I was concerned about possible run-out issues with the hub that would cause the spinner to wobble. But, with sixteen mating possibilities, it wasn't difficult to find one with near zero run-out.
I turned an 1144 prop shaft blank and adjusted the parameters of the splining program for the new material. When the part was completed it was immediately obvious that the splines weren't uniform. I had evidently over-tightened tailstock of the the fourth axis (again), and this caused the rotary to loose steps. My second attempt was more successful. Its fit inside either hub was smooth and very snug with no backlash.
The rest of the machining on the shaft was completed including a pair of shoulders for two ball bearings and a 7/8-20 thread for the prop nut. The bearing shoulders were turned concentric with the splines in a 4-jaw set-up. The center of the shaft was also drilled out in order to lighten it, and a mating position for the hub was found that produced a TIR of only .001". Punch marks were added to the hub and shaft for use during assembly. Finally, a bearing retainer was machined for the front of the gear case to limit the forward thrust of the shaft.
Manually rotating the shaft of the completed gear reduction assembly smoothly spun the crankshaft at nearly twice speed with no binding or rough-felt areas. Now there's even a flywheel effect due to the large prop gear. The next step, while I'm working at the front if the engine, will be to finish up the rest of the prop components. - Terry
I've cut key slots before, but never splines - neither internal nor external. Cutting external splines looks fairly easy and is pretty similar to cutting gear teeth. Initially, the internal splines seemed more intimidating, and so I decided to start with the hub.
One of the options on the Merlin's prop shaft drawing is a variation of an SAE 16 straight tooth spline. I designed the hub for sixteen 3/4" long splines spaced 22-1/2 degrees apart. The splines in this hub can be cut using an ordinary 3/32" keyway broach. Since I had a .875" diameter broach bushing, this became the i.d. of the hub as well as the minor diameter of the shaft. Actually, I reduced the shaft diameter .010" to obtain a tooth end clearance of .005". I pulled this number out of the air with no experience or reference material to justify it. Broaching this i.d. creates sixteen teeth having a 22-1/2 degree included angle and a height of a .050". The prop splines must be cut using a form tool that matches the shape of these teeth.
My initial thought was to use a rotary to index the hub under a broach held in the locked spindle of my mill. I did a few experiments and realized the force required to drive the broach through a one inch piece of 12L14 was more abuse than I was willing to apply to the bearings in either my spindle or rotary. Instead, I manually indexed the hub, and I used a manual press to drive the broach. I CNC-engraved the top of the hub's blank with the 22-1/2 degree alignment marks and I scribed a corresponding reference mark on the broach bushing. Accurately aligning the broach for each cut was easy, and the whole splining operation took less than half an hour. After finishing the splines, the remaining machining on the hub was completed.
Instead of spending fifteen minutes grinding a single tooth form tool for the shaft splines, I spent most of an afternoon modifying a 3/16" HSS Woodruff cutter. The sides of the teeth were ground as close as possible to the theoretical 11-1/4 degree side angles, and the width of the flat end came out to .078". Normally, I would have turned a four tooth cutter from scratch as I did for the crankshaft gear for my two radial builds. In this case, however, I had a lot more material to remove, and I wasn't sure how well drill rod, even heat treated, would hold up against the Stressproof shaft especially if I had to make more than one shaft. The most difficult part of the Woodruff cutter modification was relieving the sides of the teeth. During testing I found this relief helped to reduce the height of some very tough burrs that tended to rise up on either side of the spline. These burrs interfere with the test fitting of the hub to the shaft while it's still fixture'd for splining.
I first tested the cutter by splining an aluminum test shaft. The splines looked great, and the shaft slipped into the hub with a close sliding fit. The clearances of the splines on the rear of the hub where the broach had entered looked nearly perfect, indicating that the spline cutter was accurate. However, the clearances between the shaft's teeth and the hub's slots on the front of the hub where the broach had exited were much greater. I checked the broach bushing and the perpendicularity of the press, and all was as it should be. I purchased a new duMONT broach since the one I had been using was part of an inexpensive import set. I made a new blank and reduced some clearances, but the results were essentially the same. Even with 3-4 teeth cutting at a time, the bottom of the broach drifted to the outside of the blank during every cut causing the splines at the bottom end of the hub to be deeper than those at the top end of the hub. Turning the set-up around on the table of the press had no effect. I examined several parts on other projects that I had key-broached, but none of them showed the drift I was seeing on these hubs. The only solution seems to be a rigid set-up to keep the bottom of the broach tight against its bushing, but since it wasn't obvious how to do that, I decided to save it for the next splining project.
Although not desirable, the tapered splines really don't create a significant problem in this particular application. Both hubs fit the test shaft snugly since the hub teeth fit into the shaft splines with their proper clearances along the entire length of the hub. It's only the shaft teeth that end up with excessive clearances to the hub slots. The two-piece propeller hub that I plan to make will sandwich the prop and should insure its perpendicularity to the prop shaft independent of the splines. I was concerned about possible run-out issues with the hub that would cause the spinner to wobble. But, with sixteen mating possibilities, it wasn't difficult to find one with near zero run-out.
I turned an 1144 prop shaft blank and adjusted the parameters of the splining program for the new material. When the part was completed it was immediately obvious that the splines weren't uniform. I had evidently over-tightened tailstock of the the fourth axis (again), and this caused the rotary to loose steps. My second attempt was more successful. Its fit inside either hub was smooth and very snug with no backlash.
The rest of the machining on the shaft was completed including a pair of shoulders for two ball bearings and a 7/8-20 thread for the prop nut. The bearing shoulders were turned concentric with the splines in a 4-jaw set-up. The center of the shaft was also drilled out in order to lighten it, and a mating position for the hub was found that produced a TIR of only .001". Punch marks were added to the hub and shaft for use during assembly. Finally, a bearing retainer was machined for the front of the gear case to limit the forward thrust of the shaft.
Manually rotating the shaft of the completed gear reduction assembly smoothly spun the crankshaft at nearly twice speed with no binding or rough-felt areas. Now there's even a flywheel effect due to the large prop gear. The next step, while I'm working at the front if the engine, will be to finish up the rest of the prop components. - Terry