Half circle jig

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Blog entry by Bluepine38 posted 04-01-2012 08:31 PM 6449 reads 0 times favorited 1 comment Add to Favorites Watch

I have read about a lot of circle jigs, but I needed a half circle jig. I have made bowls from a board using a
compound slide rest with a parting tool, but a parting tool thin enough to not take out too much wood will
bend and not give an accurate cut. I tried SASmith’s scroll saw method, but my hands are a little too beat
up to make a good cut. WoodguyNC, Taidsturning and others were using halfcircles and a bandsaw to make
bowls from a board, but once again my hands and eyes were not able to produce accurate cuts. I needed a
jig and could not find one. I planned and thought for a while and was stumped, so I went to visit Duane,
a friend who is a retired carpenter and cabinetmaker. We brainstormed, and this is the result. Both my bandsaws have a mitre guage slot so I cut a piece of cherry the same size as the Shopsmith
slot and glued it to a piece of 20” by 12” plywood so the top of the plywood would be even with the
inside edge of the bandsaw table.

slot in the plywood stopping when I was in the middle of the plywood. I shut the saw off and backed
the plywood out, then used a wood bit to drill a hole with edge above and just touching the end of
the slot so I would be able to tilt the table without the saw blade binding in the jig.

I then placed the end of the wood track in the guage slot and starting up the bandsaw I cut a

in the plywood on my router table centered in the middle of the plywood. I stopped by the box store
and picked up a genuine made in USA plastic cutting board and used a hole saw to cut two circles
that would be centered over the t-track and each end of the track. I used a jig saw to connect the
two holes and remove the material in the middle. This took care of the body of the jig, now I needed
a pivot point. I had an old butterfly bolt used to draw countertop mitre corners together and the
oblong threaded nut from it just fit into the t-track. I needed a 1/4” ID by 1/2” OD bushing that would
fasten over the track and be held down by the nut and a flathead socket head capscrew. I only needed
one that was 1/4” thick, but had to buy one that was longer.

I messed up a couple of pictures, so this next picture shows the t-track placed in the slot I routed

This is the best I could do on this picture, I used a machinist chuck and a drill chuck in my tailstock to
drill a counter sink in one end of the bushing. This next picture show me turning the end of the
bushing back to 1/2” after the countersink had flared it out. I also used this compound slide rest to
trim the bushing to length.

I used a 1/4-20 by 7/16” flathead socket head capscrew to fasten the bushing to the track. The
1/4-20 bolt has a 5/32” hex the same size that the Shopsmith uses on its tools, so I only need one
hex key when using this jig.

This picture shows the bushing and the holding plate that will fit over the bushing. I cut the round
plates out of a scrap piece of 1/4” scrap I had available using a Lenox 3” bimetal hole saw, these hole
saws can be used in either metal or wood. I then drilled a 1/2” hole the plate and made sure the
bushing and the plate had a close fit, but could still move freely. I now need a way to hold my wood
to these plates.

This is the completed clamp. The right angle was a piece of 3/16” X1-1/2” X 2” angle iron I cut about
1” long and drilled a 5/16” hole in the center of the 2” part about 3/16” from the other leg. This acts
as the clamping pivot. I needed a knob to tighten the clamp so I used a 2” holesaw to cut a couple
of circles from some scrap, now you and Stumpy know why I can not throw anything out.

This shows the wood after I turned the knob and am drilling it for a 1/4-20 X 3/4” long E-Z Lok flanged
insert. To allow the clamp to hold the wood, I cut a 2” length of tube from my road bicycle, mountain
bike tubes are too big, and folded the tube back and forth over the end of the clamping arm. As you
can see from this next picture I drilled a hole too close to the edge of my disc.

I then had to drill one centered half the way between the edge and center and countersink it for the
clamp to work properly. The screw I used for the clamp is a 1/4-20 X 2-1/2” flat head socket head cap
screw. Before you drill the hole for the clamping screw, scribe a line through the middle of the hole
and center of disc, then scribe another at right angles to this line across the disc. This second line is
the one you will line up with the edge of the half circle board you will be cutting, the first line you will
line up with the center of the board. I placed a piece of double stick tape over part of the disc the wood would be fastened to before I centered the wood and clamped it down, between the tape and the
clamp the wood stays securely fastened.

Here is a picture of the jig in action, and here is a picture the completed cuts.

This shows the reason for needing two clamps and making the discs easy to remove and insert
with accuracy. You determine the width of the bowl rings you wish to cut and measure and mark
these distances out starting on the outside of the wood. Then you use a square and run the
lines down the face you will be starting your cut on.

As shown in this picture you then draw a line from the top of one line to the bottom of the second
line. This gives you the angle you need to cut. Place this piece of wood on your bandsaw table and
tilt the blade until the blade is aligned with the angle mark. Clamp the table in place, clamp your
two half pieces to the plates being sure they are lined up with your scribe marks, clamp the jig to
the band saw table after sliding the jig over the blade. Then line up your saw with the first angle
cut and cut both half circles before you adjust the jig for the second cut and proceed until you are
finished. If you do not want to use steel, I have found that corian countertop sections will also work,
but with the plastic cutting board being 5/16” thick, the steel plates with the double stick tape came
out flush with the top of the plastic. I have been fighting posting the pictures for a few weeks because
my old computer setup got wiped out by a virus and I had to learn a whole new system. I have not
even tried to master sketchup, and my plans are mostly in my head and a few pieces of scratch paper.
If anyone has questions I will try to answer them. I am in the process of making a better jig that will
include a ruler along the edge of the t-track that will make adjusting the pivot point easier, for now, I
just measure the distance I need to move the pivot in relation to the blade loosen the screw measure
and move until it is where it needs to be. Thank you for looking.

There are a couple of places where the first line in a paragraph came out below the paragraph, but when
I check the original it reads correctly, so chalk it up to age or April 1st.

-- As ever, Gus-the 79 yr young apprentice carpenter

1 comment so far

View SASmith               's profile


1850 posts in 2986 days

#1 posted 04-01-2012 09:09 PM

Clever solution you came up with.
I had to make a 1/2 circle jig when I built my wooden cyclone.
You can see my jig in pics 3 & 4. Your solution is much more elegant.
Thanks for sharing.

-- Scott Smith, Southern Illinois

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