Friday, November 13, 2009

A Spring Pole Lathe

Base pieces (left), rails (top-center) and side supports (right).

I have a new addition to the workshop. I've wanted to build a spring pole lathe for years after seeing one on the Woodwright's Shop on PBS. I found a simple plan here which I used. Unlike actual spring pole lathes which use a green pole as a spring to let-out and take-up the cord which spins the work, this design uses a piece of bungee cord. It takes less height and uses somewhat less floor-space.

The plan is pretty simple. You get an eight foot 2x6, 2 eight foot 2x4s, 3 six foot 1x3s, two 4 x 5/8 lag bolts, some wood screws, a 4' bungee and some thin cord (~ 10 feet). Cut the 2x6 into two 4' pieces to use as rails. I figured I wanted the work height at 41". I wanted the poppet centers to be 8" from the rails. This meant that the supports would be 41" - 8" = 33".

I cut a 2x4 to give two 33" pieces. I also cut the 2x4s to give me two 29" bases. I notched supports to allow each base to be recessed into the support. I then screwed the bases to the side supports.

Base pieces attached to side supports.

I put the rails on my workbench and flipped the support bases upside down. I then assembled the rails to the side supports using screws, making sure the rails were square to the side supports.

Rails being attached to the supports.

I had some leftover 1x2s from another project which I cut down to act as diagonal bracing for the side supports. I had to leave some room for the 1x3s to act as the uprights which hold the bungee. I cut the angles into the bracing and screwed them to the rail assembly.

The poppets are the working part of the lathe. They sit on the rails, and have sharp points on which the material to be worked spins. The trick is getting the poppets designed correctly so that the system is rigid, but adjustable.

The poppet height (the height from the rail to the point the material spins on) is to be 8" (i.e., the biggest piece I could work is an 8" diameter - I'll never work anything that big!). I have to leave 1" of material above the poppet point. As the rail is a 2x6, the height of the rail is then 5.5". The poppet has to extend down below the rail so I can mortice in a slot for the adjustment peg, so I left an additional 6". The length of the center part of the poppet is then 1" + 8" + 5.5" + 6" = 20.5". I cut two 20.5" pieces from 2x4 material, along with four 9" pieces from the 2x4s to act as the "shoulder's" of the poppets.

I drilled a 1/4" hole thru each poppet piece 1" from the end. I counter-bored the holes 5/8" thru most of the poppet pieces. I then screwed the lag screws through the poppet holes to act at the turning points (aka centers).

Center pieces for the poppets with lag bolts inserted.

I cut two 6" sections from a 1x3. I ripped these pieces at a slight diagonal along one edge and sanded them. They will act as the pegs to hold the poppets in place.

Pegs to be used to hold poppets fixed on the rails.

I measured the width of each peg at their midpoint, and used that distance (2.25" in my case) to establish the bottom of the mortise which accepts the pegs. Given the poppet is 9" above the rail, and the rail is 5.5", and the mid-point peg height is 2.25", the bottom of the mortise is then 9" + 5.5" + 2.25" = 16.75" from the top of the poppet. The top of the mortise has to be above the bottom of the rail (at 9" + 5.5" = 14.5"), so I put it at 14" from the top of the poppet. I mortised out a 1" slot in each poppet between 14" and 16.75".

A peg loosely fitted into a poppet mortise.

Nearly done. I took 2 of the 9" shoulder pieces and screwed them to each side of the poppet center. I then had a T-shaped piece which drops between the rails. One slides the poppets along the rails, and then lock them in place by hammering the pegs into each mortise.

Poppets with shoulder pieces attached and fitted to the rails with the pegs.

I attached the six foot 1x3 uprights on each side of rail assembly. They extend above the rails by ~ 4 feet. The bungee is stretched across the end of the uprights and act the the spring for the lathe. The cord is tied to the middle of the bungee, and then attached to a ~4' section of 1x3 (or 2x4) which acts as the foot treadle. I filed the lag screws down to remove the screw threads leaving them as simple points.

The final lathe (with support bracing, uprights, bungee, cord and foot treadle attached).

To use the lathe, the cord is wrapped around the piece to be worked. The piece is positioned between the poppets, and then the pegs are pounded into the poppet mortises.

Now you are ready. You step on the treadle board. It pulls on the cord, spinning the work, and pulling the bungee down. As you let up on the treadle, the bungee retracts, spinning the work the opposite direction. You only cut on the down stroke.

I still need to fine-tune the treadle. I also need to add a rest to the lathe (a rest is a rail which supports your tool when turning). I need to get some gouges and chisels to actually do anything. I'll start looking . . .

1 comment:

~ RM said...

WOW!!! I can't believe you built this! Great job! What are you going to make with it?? Can't wait to see~