I've been wanting to make wooden jack and try planes for quite a while.  A few months ago I bought the irons for them and finally I'm getting around to the build.  But because I don't want to mess it up, prototypes were in order.
Before I begin, I'd like to credit 
Richard McGuire, whose jack plane build videos really helped me, and 
Caleb James, whose published free plans for a wooden jack also helped a great deal.  I don't have plane floats, so like Richard's plane build (and unlike Caleb's), I'm using a three-piece glue-up for my design.  And like Caleb's plane (and unlike Richard's) I'm using abutments to wedge the iron rather than using a cross-bar - I much prefer abutments.  I built a wooden smoother this way last year and it turned out pretty good.  Those blog entries are 
here and 
here.
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| The coffin smoother in red and white oak | 
The first prototype jack plane is in pine.  I started with two 15 1/4" pieces of 2x lumber glued together.  After squaring it up to about 2 5/8" tall (thick) and about 3 1/4" wide, I sawed off 1/2" thick pieces from each side, then planed the sawn surfaces to get me sides that were 1/2" thick and the main body that was 1 15/16" thick.
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| Two sides and main body | 
The iron for this plane is just shy of 2 1/4" wide.  When I add 3/16" recesses to the sides (they form the abutments) to the 1 15/16" main body, I get an area that is a little more than 1/16" wider than the iron.
On the sole of the main body blank, I measured 5 1/2" from the toe to mark the back of the mouth.  Then on the sides of the blank I drew a 45° bed angle and a 60° breast angle from that spot.
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| Main body lines laid out | 
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| Then cut out the triangle and planed perfectly square the bed and breast | 
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| It made sense here to create a recess for the cap-iron screw. I used chisels and a router plane
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As it turns out, the slot in the iron (and hence the cap-iron screw) was not perfectly centered, but was slightly off and I failed to take this into account.  I later had to widen the recess to accommodate this.
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| On the fore part of the main body I added a small wear angle (bottom left) | 
To start creating the abutment recess on the sides, I marked a line directly from the main body bed.  I did this rather than simply marking a 45° line on the side with a combination square in case there was any deviation from the 45° bed angle after having planed it.
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| Marking the rear abutment recess line | 
Next was to figure out where to put the front abutment recess lines.  I started on paper and found that this double iron forms an angle of about 1-2° (in the opposite direction as the wedge angle).
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| Worked out some angles on paper with the actual iron and cap-iron | 
I wanted a wedge with about a 10° angle, so the iron/cap-iron and wedge combined would be about 8-9°.  I created a temporary 1 15/16" wide, 10° wedge, placed the iron/cap-iron and wedge in the plane and marked a line on the side.
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| Marking the forward aspect of the wedge abutment recess | 
Then I used a bevel gauge to make a knife line at the penciled line, sawed the shoulders of the recesses to a depth of 3/16" and removed the waste with chisels and router plane.
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| Far side is being held away from main body, near side is in proper placement | 
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| And here are the three parts placed together | 
 Then while holding the plane together, I inserted the iron and looked at the sole.
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| When I moved the fore part of the plane to a location that would give a reasonably sized mouth I got the funny offsets that you see here.  I dealt with that later.
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Then it was time to glue up.  I lined up the parts as accurately as I could, turned the plane upside down and placed packing tape across the bottom to hold things in the right position while gluing up.
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| Then turned right-side up and added glue to the sides | 
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| After clamping and allowing to dry, I cleaned up the surfaces | 
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| As for that discrepancy on the sole, ... | 
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| ... I chiseled the wear back to line up with the sides | 
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| But this created a mouth quite a bit larger than I wanted | 
I didn't get any good pics of this, but I made a wedge out of poplar.  After sharpening the iron I took some shavings and the prototype worked nicely.
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| First shavings from a scrap of pine | 
This prototype works, but is not optimal.  Next time I'll write about what I changed to tighten the mouth.