Monday, February 13, 2017

Leather stitching clamp

I recently started leather working. It is a lot like wood working, both work with organic material, both texture feels great, both are relatively cheap to get started and both are more of an art than a labor work etc. One geeky reason is that is uses a lot tools that you can make yourself. When I was looking at a beginner set of tools, the stitching clamp caught my eye. It's purpose is very simple, hold leather together to be stitched. Most product on the market are pretty simple and does not look well made.

I'm always a firm believer that a well made, well thought out tool will inspire a better work. Let me explain a little bit about what a stitching clamp is, in case you don't know much about leather working. A leather stitching clamp, is usually a pair of wood jaw that holds a leather work together so it's easy to stitch.

The idea is pretty simple, but as any projects, the devil is in the details, how to make the jaw arm, how to attach the jaw, how the locking works, etc.

After some discussion with a leather worker friend, I settled on the following design:
  • make the jaw arm out of bent lamination, so it can be thin, large curvature and still very strong.
  • use a cam + threaded knob to lock it jaw, so it can be released quickly and fine tune the clamping pressure
  • the hinge of the movable jaw is cut directly on the wood like fox joint
  • the foot of the clamp is interchangeable, with a 1 inch tenon hold it in place

 First step and the most challenging part is obviously to make the bend lamination jaw arm. I start with gluing couple 2x4 together to make the form. I made mistake by not having enough material to make the form, the form break under clamping pressure. So the second form is made from multiple layers of MDF, I leave it thick and strong. The best way to make the curve form is to use a template to make the curve than a router bit with a template guide to cut out the exact thickness of the lamination. Then use a bandsaw to cut it open and trim route the rest flush. But since the form I'm making is about 5 inch thick, I don't have such long a router bit to reach the depth, I mark the line and free hand bandsaw it out, later hand sand it fair. This does not result in a perfect form, but close enough.

I've never done steam bending before, so this part of the project is new and most interesting to me. I made a steam box out of plywood, with simple screws, and  used couple dowels to elevate the wood strip off the floor. The caps are just friction fit. Steam is from a electric steam generator, very easy to use, and easy to top up water during the process. The tricky part if that after the steam is done, when the strip is out of the box, you only have about 10-15 seconds to clamp them before they become stiff again. Once they are clamped on the form, I gave it about 12 hours to dry and cool down. I steamed the strips for about 20 mins, time is depend on the thickness of the strips, the rule of thumb is about one hour to one inch of thickness. You don't want to over steam the strip, it will cause stain.

After the strips are formed into rough shape, it's time to glue them up. I used mini paint roller to spread the glue. Because my form is not perfect, there is some gaps after clamping. So on the second Jaw, I tried something different, I only used one side of the form and lots lots of clamps to keep the pressure uniform. And this worked better. The reason is that with two forms, you can't apply pressure where it needs, so unless your form is perfect, you will have gaps. But with one form, you can apply pressure wherever you need, so it's much easier to close the gaps.

Despite of some gaps, the jaw arm came out to be very nice, gentle curve, thin but very strong.

The hub of the jaw is made from a block of Ash. since I want to cut hinge directly on the hub like a box joint, I cut it into 3 pieces first. Then cut out the box join on one of them, the fixed jaw is joined like a large tenon, and pinned. After all the joinery is cut, I glue them all back together. The connection to the foot is a 1 inch diameter 2 inch long tenon. I drilled it out with a drill press than used a live center in the hole on the lathe to turn the outside round and put on a brass ferrule.While it's on the lathe I also shaped the hub a bit.

To thicken up the mouth of the jaw, I added a 3/4 Ash to it, it's pined in using a 1/4 dowel.

Next step is to housing of the clamp knobs. It's purpose is to strengthen up the jaw on the clamping spot and have a recess so the knob can sit in. The back of the housing needs to sit flush with the jaw. Since the jaw is curved, the back of the housing also needs to be curved. I used a hand plane to shape it, just test and fit, go slow. After glue them on, I pined them with 3/16 dowel. The bottom of the recess needs to be parallel to each other when the jaw is closed. I clamped the jaw in my vise so the bottom is horizontal, and hold the template over it, used a router with a bull nose bit and template bushing to route out the recess. It came out very clean.

After the jaw and hubs are done, it's time to put them together. The fixed jaw arm is glued and pinned, but the movable jaw can still take apart by remove the pin. To make it fancier, I made the fixed jaw side of the pin head square.

For the clamping system, one side is a screw on knob, the other side is a quick clamp lever like the one on the bike. I'll skip how I made the knob, since it's very standard, and focus on the lever. The lever needs to be made from Baltic birch plywood, because the pressure was from many different direction, a solid wood is along the grain, but relatively weak on the other direction. There's a hole cut in the lever, for a 3/8 rod with a 1/4 tapped hole for the threaded rod. in the center of the lever, there's cut out  to clear the threaded rod when the lever open and closes. You want to make the cut out as small as possible to leave the lever strong. After all is done, bandsaw and rasp to shape it to your like.

Finally finished with a oil/wax blend. I really love the result, it's simple, functional, and has a craftsman touch to it.

I also help my leather working friend made one. His clamp is made from a curly walnut, normally curly and bend lamination don't mix, but since we are trying things out, it's interesting to push the limit. It came out very very nice! the curly wood out of the steam box is like a droopy noodle.

Tape holder

Emma loves doing crafts, and tape if a tool she use a lot. I always want to make a cute tape holder for her. I prototyped with some ...