German Equatorial Mount (GEM)
The Shafts
The next stage of construction was a pair of shafts and housings. I considered different types of bearings and in the end chose needle roller thrust and radial bearings, in 2 inch and 1 1/2 inch sizes. For shafting I chose 316 Stainless Steel and for everything else aluminium was the go.
Once I had the bearings in my hands I drew up the shaft assembly on my simple cad software. The drives will simply bolt on to the end of the shaft and there is plenty of room for future upgrades, meaning larger RA drive and digital setting circles.
I soon realised that the stainless steel shafts were the heaviest items. With a quick calculation I realised that if I drilled and bored the center out I would save a total of 3kg without sacrificing too much rigidity. I made up an extended 13mm drill and was able to drill 225mm deep into the stainless steel. This was followed up with different size drills up to 25mm as shown on the drawing and the final 120 mm was bored to 32mm diameter.
The rest was normal turning, boring and screwcutting, although I was surprised by how well the stainless behaved, sure it was hard and made the lathe grunt a bit but it finished beautifully. Normal high speed tool bits were used and a good chip breaker ensured that I wasn't plagued by long ribbons of swarf. After the stainless the aluminium was almost too easy.
Looking down through the 2 inch bearing and the housing to the 1 1/2 bearing. The square section has 7mm wall thickness and each end was bored to mount the bearing housings.
The whole assembly is lightly preloaded by tensioning the M38x1 nut.
The final photo shows the drives mounted on their shafts and the two assemblies offered up to each other.
The next up is a joining piece and telescope mount platform.