Spar lathe desgin & build

Discussion in 'Wooden Boat Building and Restoration' started by abosely, Jul 25, 2015.

  1. abosely
    Joined: Mar 2015
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    abosely Senior Member

    PAR, did you end up with a motor setup to spin a Spar lathe?

    I've been thing about how to build power one. Since deciding to Build a Narai Mk IV, I have two 36' x 5.5" Spars and two Gaffs, thought it would be nice to be able to spin them on a lathe.

    Thinking of using a 1/2" low speed drill, possibly a used Milwaukee 1/2" angle drill, their powerful and low rpm. On low setting it has 0-335 rpm.

    If used a 1/2" shaft and two 1/2" pillow blocks with pulley between them and shaft chucked in drill.
    That would eliminate side load on drill and give slow speed with high torque.

    Cheers, Allen
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I've used a couple of different setups, but both had a regular continuous duty AC induction motor. It doesn't need to be very big, because you'll gear it down to a reasonable RPM, which jacks their usefulness as a lathe.

    The first motor was a 1/4 HP 3,200 RPM with about a 20:1 reduction. The reduction was simply a 3" V belt pulley on the motor shaft, swinging a homemade 36" plywood pulley. The big plywood pulley had a 3" pulley centered on it, which in turn drove a 24" pulley. This spun the mast a little fast and whipping was an issue, so the next one lowered the end RPM by about 50% (around 100 RPM). The second motor was a torquey 1,500 RPM unit that required a much simpler (smaller) gear reduction. I now have a DC gear reduction motor that will replace the last assembly, but I haven't built it yet. This one will let me spin either way and vary speeds easily.

    With a spar lathe there's two basic issues, mast whip and speeds appropriate for what you're doing. For example, you'll want to spin it up pretty good for "tooling" the rough mast blank, but it'll need to be pretty slow for smoothing with paper. Mast whip can be controlled with roller skate truck assemblies and some ingenuity. I mounted trucks at two points around a U shaped bracket, made from 1" angle stock. This was clamped to the bench under the mast and the trucks adjusted to hold the mast straight. This bounced around a lot, so I placed a split length of PVC pipe around the mast, where the mast would ride, so the trucks rode on the PVC, not bouncing over the 8 sided mast blank. This works like a charm, but some movement is still possible, so the last improvement was to use bungee cords to secure the trucks, instead of hard fastening to the brackets. This lets the trucks absorb vibration and some movement without shocking the mast as it spins.

    Unless you have a lot of spars to build, this is a long way around to get some mast made. Your cordless drill might get it done, though whipping will still need to be addressed. There's a number of pictures and videos online with various lathe setups. One I saw uses a router to rough the mast round, but damn, any whip and you'll have a nicely machined piece of crap. I'd rather sneak up on it, knowing I'm not taking too much than try to really make a stiff spinning spar lathe that's safe enough to use a rotary cutter. I rough the mast with a regular tool rest, made from a length of 3/4" pipe and some adjustable brackets. I use it just like a regular lathe, with gouges. I only try to get it round with this technique (I use the magic marker technique), then move to paper.
     
  3. abosely
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    abosely Senior Member

    That's a good idea using PVC pipe for the trucks to run on.

    The drill I'm thinking of is 115V AC. Used to use them when was a Contractor & building decks. Man, if that thing caught the drill bit & you had your chin in the way, Wow! It about knocked ya silly! Lol

    I wonder if the spot where the trucks run was carefully sanded smooth & round in that spot a few inches wide they would have a smooth spot to ride on?

    I would enjoy building the lathe and having it work well partly for the satisfaction of doing it and I think if made well could get a really straight mast too.

    I have to admit, I like jigs & fixtures to make multiple cuts or components all exactly uniform. It seems to actually save me time. That is as long as I don't get carried away making them to complicated.

    Cheers, Allen
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I tried pre smoothing the areas where the trucks ran, but in use, you have to move the trucks as you shape the stick, so not as practical an idea. I treat the mast in two stages, the rough rounding from octagonal and the smoothing. I rough it round with a gouge, using a tool rest, after having covered the whole length of the mast with a magic marker crosshatch pattern. When I can't see any more magic marker, it's round enough to start smoothing. When smoothing I decrease the speed and use lengths of sand paper, with wooden blocks at each end. I bend these around the mast, about 1/2 way and just keep moving. I switch to pencil lines for this and work up through 80 grit. At this point it's hand sanding along the length to remove the smoothing sanding marks and prep for finish.
     
  5. messabout
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    messabout Senior Member

    You can make up a wooden spool to hold in the variable speed hand drill. Cover the drum part of the spool with some rubber sheeting for traction. . Turn a sanding belt inside out, loop the belt over the mast and drive the belt with the spool. That gimmick makes fast work of the sanding operation. Simple, cheap.
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I have a special attachment and oversize belt for my 4x21 belt sander, that does just that, but these types of tools only smooth and you can make a mess quick, if you're not paying attention (I've ruined more than one mast blank learning this). I find it easier to use the belt sand longitudinally on the lathe, without the oversize belt and roller thingie. This is only up to say 60 grit, maybe 80. Simply put, with the mast spinning, it's much harder to make flat spots and it's easier to see where you need to remove more stock (if the stick is marked up). Eventually, you just have to get out the paper to hand sand and this is where you find how careful you were, with the power tools.
     
  7. DGreenwood
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    DGreenwood Senior Member

    I met a spar builder back in the mid 80s in Bristol England.---R. Mason. His business was carried out on a Large Dutch barge that he had imported. He taught me to make birds-mouth spars.
    He had a lathe and had opinions about them. His claim was that if yo were producing many spars they were worthwhile, otherwise you spent more time building, learning and tuning the damn thing than it was worth.
    I went on to perfect my own more portable method. It begins with the birds-mouth method (usually) and then is followed by the old fashioned planing. Then, I have built a fitting that chucks up in a large drill motor and has a handle with a rubber wheel between. I then turn belt sander belts inside out and the wheel runs the belt over the spar. With a little time learning to control the belt you get a very fair spar very quickly with little risk of screwing the whole thing up like you can with a lathe.
     

  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    My origional setups where similar, though the later ones have refined all the issues, such as whipping and speed. The real key is speed, once you've got control of the spar. If you spin it fast, you can whittle away more than you want very quickly, but if you reduce the RPM's to 100 or less (I've found 60 - 80 ideal) you have more control, less heat build up, etc. I agree you have to build enough spars to justify a lathe.
     
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