Conversion of a Marples technical jack plane into a scrub plane

The longest journey

I have long been fascinated with scrub planes - the legendarily efficient way of reducing stock sizes. (old tool posts - first question, bedding angle?, blade radius on a #40)

Back in 2000 I dipped a toe into the deep waters of scrubbing by converting an old coffin smoother into a better form for scrubbing. But in use, my fears about handling were confirmed. In any extended session, the coffin body does not provides an effective grip. So I decided (having seen a marvellous Clarke & Williams closed handled razee smoother) that that was the way to go. I even bought a wooden jointer just so I could trace its handle in preparation.

Nothing got done - I could procrastinate for my country. Until last Friday, 26th August. The local market of the fleas offerred up a Marples technical jack plane for the princely sum of £4.00. It's 14" long and weighs 1280g (2 lbs, 13 oz), which is around the right size for a scrub, the low handle position is favourable, and it has the usual laminated blade. All in all it struck me as a good starting point for a superior scrub conversion.

Marples technical jack plane

The blade that came with it was thinner that I'd expected or hoped; 0.140" or 9/64". Still, we'll see. The montage shows the the results of the various stages of preparation. The blade wasn't in too bad a condition (picture 1).

First, the blade was cleaned and flattened on 50 grit AlZi abrasive paper. To hold and move the blade I use a small piece of scrap, with a shallow slot to fit the blade, and rounded corners for comfort. This continued until the blade showed reasonably uniform scratch marks for a good distance at the end (picture 2)

I then worked through grits 180, 240, 320 in AlZi, then 400, 600, 800, 1000, 1200 in SiC. Using such finely gradated grits mean that each stage is no more than 20 strokes. The sheets of abrasive are simply placed on 400 grit SiC which is glued to the glass. (picture 3, with reflection of camera :-)

I then proceded to do some maths. Both Lie-Nielsen and Lee Valley use a 3" radius. This gives a sagitta of 0.127". I wanted a similar sagitta, but on a wider blade. The maths for this tells me that I'll need a radius very close to 4". I scribbled on the blade with a permanent marker, and used dividers to mark the curve on the back of the blade. A hand cranked grinder was then used, with the blade "square on" to the wheel to grind the edge to the marked radius. To grind the bevel at approx 25°, I worked out that the back edge of the bevel should be (0.140)/tan(25°) back, which is around 0.26". I marked a second curve of 4" radius on the face of the blade, and again used the hand grinder to remove most of the waste metal.(picture 4, with the remains of the second marked curve showing in the blue)

My notorious sharpening jig was used to finalise the bevel, although in order to follow the camber, a single wheel pillar replaced the normal back axle. This system will not grind a mathematically perfect curve, but it's fairly close. Having ground the bevel with 50 grit AlZi, I improved the texture up until 320 grit, which happens to be the finest AlZi I have access to. Since a scrub plane is used for rather vigorous work, the actual sharpening was done with a 30° secondary bevel, up to 2500 grit Sic. This doesn't take long, since the area of metal being worked is very small. Due to the camber, this was done with an old Marples #7418 honing guide , which has a narrow and "crowned" wheel.(picture 5, with barely visible secondary bevel)

Blade preparation

Handle for flattening

Perfecting the grind

Secondary bevel

I didn't want to use a cap-iron in my scrub, so the original wedge would not fit. I created a new wedge. This was a fairly simple piece of stock preparation, planing to width so the wedge would enter the body of the plane, sawing a rough taper, and then some careful fitting work (with a Lee Valley low angle block plane ) until the wedge held the blade firmly. I took particular care that the wedge fitted well near the tip of the blade, following a bad experience long ago. I then used a pencil to mark where the wedge touched the abutments, and copied the position of the 'U' cutout from the original wedge. Some enjoyable carving and paring work followed to arrive at the traditional shape.

blade retaining wedge

The new curved blade could not work with the original mouth. Since the blade sits at a 45° bedding angle, the sagitta of the blade projected onto the body is 0.707 of the original. A little maths eventually showed that I need a radius of around 5½" for the mouth. This was marked (using a piece of scrap to protect the body from the rear divider point) and the mouth carved away using a large sweep scribing gouge. I made a mistake at this point - when the scribing gouge followed through the stroke, I damaged the lower end of the abutments, which worried me. However the resulting mouth looks O.K.

I now realise, for people who don't like maths, you could just trace around the blade. If the blade is sharp, you might want to cover the edge with masking tape during the tracing.

reshaped mouth

reshaped mouth

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