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   Scarfing Jig - Part One

By David Kagan - Stillwater, Minnesota - USA

A Jig for Cutting Stringer Scarfs for the Seaclipper 20

To Part Two

My construction of the Seaclipper 20, a devilishly interesting trimaran designed by Jim Brown and John Marples, continues to progress at a steady pace. What's interesting about the build is that there are some tasks that are straightforward given my current skill and tool sets, and yet there are other tasks that have taken some thinking and planning in the noodle chair to devise a way to handle such tasks efficiently and safely.  One recent task in particular was totally new to me. I had now reached the stage in my building process where I needed to make the full length stringers for the two amas and main hull of the Seaclipper 20. I originally intended to tackle this building step at the outset, but I delayed making the stringers and instead focused on making other components. Frankly, I hadn’t yet figured out how I was going to make all the scarfs that were needed. And, I wanted to fabricate all the stringers at the same time for efficiency sake. With a stitch and glue background by and large, I had never scarfed stringer stock before. Embarrassing, yes, but true nonetheless. The one build where I needed long stringers, I laminated these up from three layers of thin stock. It is rumored that I have a natural propensity to avoid scarfing at any and all costs. Alas, the rumors had been true until the jig described in this article helped me overcome my scarfophobia.

For a few scarfs, hand tools would be the way to go. But I had too many scarfs to make for hand tools to be efficient. The three hulls of the Seaclipper 20 have a total of 16 full length stringers. By ripping stringer stock from 10 and 12 foot boards, I figured I could make each full length stringer from two lengths of stringer stock. That means I would need to glue up 16 scarf joints, which in turn means I needed to cut 32 scarfs. I would have to use power tool(s) to make the joints if I hoped to do this in a reasonable amount of time with a low scrap rate.

I almost used a table saw sled to make the scarfs until I researched this approach online. To my chagrin, I found out that many users of table saw scarfing sleds were getting badly injured by the wedge shaped offcuts being kicked back by the table saw.  Even people using sound looking jigs were getting hurt with alarming frequency.  So, no table saw sled.  Back to square one, or scarf one.  With the table saw out of play, I turned to my miter saw. I noodled this possibility for a bit. More than a bit, actually. As I continued to fabricate other boat components, I noodled the problem for several days. Then a few weeks passed with still yet more noodling as I made other boat parts. Then a rough idea for a scarf jig emerged from the noodling. The timing was fortuitous, as I was running out of other components to make. And surprisingly, the idea worked after only a handful of minor tweaks to the initial idea.

This article describes a jig and method that worked for me on the power miter saw to make a host of scarf joints in stringer stock with no scrap beyond the initial test pieces. The jig builds fast from scraps. You might take ten minutes to build the jig if you have all the scrap on hand. Best of all, my scarfophobia is cured, at least for stringers.

IMPORTANT: The jig is helpful, but safety equipment is important, too. To make scarfs using the jig, I also wore a full face shield to protect my eyes and face in the event that the saw were to throw an offcut at me. This didn’t happen, but it’s best to be prepared. Also, the chop saw is loud, so I wore hearing protection.

1 2

Figures 1 and 2 show the simple jig (Version 1.1; version 1.2 shown further below) sitting on the floor of my basement ready for use. It took under ten minutes to screw the parts together using Kreg pocket screws. Using these screws is great because you can take apart the jig after it’s served its purpose and the parts and screws can be used for other jigs. You don’t have to have pockets to benefit from the advantages these screws have to offer.

The jig has a 2 foot x 3 foot base. You can see the initial zero clearance saw slots highlighted in red. Ideally, you should have only one saw slot, but it took two tries for me to get a scarf angle I liked. More details on these angles are below. Each saw slot was made by making a cut through the base without any workpiece in play yet. That red zone is an important safety feature and is not merely added for purposes of this article. I need to keep clamps, screws, etc. out of the red zone to avoid conflict with the saw blade and guard. You can see that the base also has another slot that comes in from the right and ends short of the saw blade slot. This other slot is an artifact from an earlier jig using this same base board and serves no purpose in the present jig. Pretend it’s not there.

An angled fence block is mounted to the base board adjacent the red zone. The fence block has a 45 degree miter on one end. The angled fence block is securely attached to the base by three pocket screws.
On top of the angled fence block is a hardwood wedge serving as a clamping block. The wedge block is secured to the angled fence block by two pocket screws at the low end. The high end is used to clamp the jig to the miter saw. The wedge shape is not essential, but is nonetheless convenient as it allows short screws to be used at one end for mounting the wedge to the angled fence block, while leaving a high end on the other side for clamping. Who knew scrap like this would ever be useful? In the back you see a long thin piece of wood that serves as a zero clearance backstop. Hardboard, thin plywood, or the like could easily be used instead.

My shop boss in Fig. 3 says the jig is ready to use, so let’s go.

3 4

Amidst all the clutter in my shop, Fig. 4 shows the jig mounted to my miter saw with three clamps. Further down, additional figures will show close ups of some of these details. A first clamp secures the wedge shaped clamping block and the right side of the zero clearance backstop to the miter saw fence. On the left, a hold down clamp secures the base board to the saw table. Farther out on the left, another clamp secures the left side of the backstop to the saw fence.

Here are some additional observations about the saw itself and how it is set up. This is a Ridgid sliding miter saw with a 12 inch blade. The saw carriage is locked, so I am not using the sliding function. However, I think the 12 inch blade is desirable to make a scarf of an appropriate length. Maybe 10 inch blades will work, but I have no tool to test that.

I played with the angle setting to get a scarf of the right length. Recall the angled fence block was mitered at 45 degrees. I have the saw itself set at 37.5 degrees. This makes a 7.5 degree scarf (45 degrees minus 37.5 degrees). I initially used a 10 degree scarf, but that produced scarfs that were too short.

Here is a feature that is important but not so obvious from the photo. My miter saw stand is about 5 feet long, and the saw is positioned well to the right side of the stand. This allows me to operate the saw while standing directly to the right of the jig and to the right of the saw stand. Due to my paranoia that there is a risk that an offcut can be kicked by the blade, I did not want to stand in front of the blade or to the left of the blade. In an earlier version 1.0, I did not have a zero clearance backstop. I had an offcut get kicked backward and bind the blade and hit the saw’s fence loudly. With the zero clearance backstop, no pieces got kicked. Kicking bad. Backstop good.

5 6

Figs. 5 and 6 are close up views of the right side of the jig clamped to the saw.

Fig. 7 shows the clamps used on the right side of the saw.

You will observe that there is a ton of clutter on a workbench behind the saw. You are probably thinking, “Dude, clean that up, man. It’s awful.” I generally agree but I think it might take a full day to straighten things up. It’s a shop task I will have to face soon, as I will need the shop to be organized so I’ll have room to assemble the three hulls of this trimaran.

IMPORTANT: After you clamp the jig to your saw, test the path of your blade and make sure you are not going to hit any clamps and that you are not going to hit your saw’s metal fences or other components behind the back stop. Your saw path must be clear. DOUBLE CHECK THIS.

IMPORTANT: The stringers of the Seaclipper 20 are pre-beveled as they are ripped. Your own project also may use pre-beveled stringers. For pre-beveled stringer stock, the scarfs to be joined must be complementary. That is, you need a right handed scarf and a left handed scarf so your pre- bevels match up when you glue things together. For one piece of a complementary pair, the pre-bevel should face outward from the 45 degree fence block during a cut. For the complementary mating piece, the pre-bevel should face inward. Cut these in pairs so you can confirm that you are getting left and right handed pairs matched up. Keep the matched pairs together. I bundle each pair with masking tape and label them (1-1 and 1-2; then 2-1 and 2-2, etc.) in case the masking tape falls off, which it does from time to time.


To be continued...



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