There is another way.
Thank you Jon for starting this second Router Turning thread. There is a reason for "redesigning the wheel", if there is another wheel, and there is in this case.
I intend writing two posts with photos, firstly on the proprietry lathe that I have, sold round the world under different names and without any gears. Then on the small DIY lathe I am developing right now.
I'm told by others that the quality of this lathe is poor, the indexing is limited, and the spiral work is inconsistent.
In my experience, I have turned legs for a billiard table and I have turned billiard cues consistently and reliably. I have turned barley twist, right and left hand. I have turned 2, 3, 4 and 5 flute hollow helixes to outside diameters between 60mm to 140mm.
The chuck is 120mm I.D. but the router bridge permits a throw of 70mm, giving a max diameter for work of 150mm (6"). The working bed is 1100mm or 3'-6". I see Jon that you suggest working to 6'. With my lathe I can turn a very good (round) tennon on the end of one piece and I can drill a good hole dead centre in the next. So a 7ft rest (cowboy stick) for a billiard table set, made in three parts and glued, with two 3" deep mortise & tennon joints works fine. Similarly, a mortise & tennon the the middle of a 6' standatd lamp is invisible, all done on a 3'-6" lathe.
The spiral turning process relies on a wire cable passing round a pulley at the chuck. My lathe has three pulleys giving 200mm (8"), 150mm (6") and 130mm (5") pitch spirals. Hollow helixes need longer pitches than barley twist for the same O.D. but in practice, any of this work just looks great. I have thought long and hard about making this lathe chain drive, incorpotating a pushbike cluster to give 8 or 10 pitches, and managing a reduction in the rightangle drive. The importand thing is positive sprocketing or gearing on the chuck and no stretch along the track.
All that is a lot of engineering. When you see my DIY lathe in the next post you will see that it uses chain in this application. I'm now leaning toward a multi pulley wheel on this lathe driving a loop of small chain in lieu of the wire cable. I have made chain drives before (for clocks) and think that I can do it using kitchen cutting board material. Very much a thought in progress. The attraction of chain in this situation is that I can get positive grip on very small diameter pulleys, without relying on tension to deform a bowden cable wire and pull it tight onto a pulley. And small diameter pulleys give short pitches. A second attraction is the chance to play with variable pitch screws, but you will have to wait for next post for that.
Oh, and I have set it up as a copy lathe as well. A bit of a challenge but 30 turned posts like a balistrade. Needs work though. The shortfall in that case (and perhaps this lathe) is that the cutting occurs on the end of the router bit, the bit plunging into the work. On my DIY, the router bit does copy (& round turning) work by cutting on the side of the bit. This gives a much better finish from the lathe, requireing much less sanding to get a high gloss polish. But, that said, there are ways of getting round that problem also.
Check out the pictures.
And just a paragraph on DIY geared lathes. I have cut and lapped gears before. I have nothing but admiration for those that have built any wooden geared machines. As for me, if I cut wooden gears it will be for wooden clocks and I'll hang them on walls for people to marvel at. There are less gears in a wooden clock than there are in a lathe if you manage pitch with cog sets, and especially if you plan on reversing. There are years of work in some of the suggestions I have read in the other post. That's not to say that anyone shouldn't start building their dream. But, for me, there is another way. I've done it, I'm doing it. Before getting into that big a project, have a look at these pics and think carefully, do you want to make things or do you just want to build a lathe? A bit negative, I supose, but thats my opinion.
Tks 4 reading,
Don't teach me, for I may not learn, share with me, that we both may learn.