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With the limbs now snug inside the irons,  it is time to cut the nocks for the bowstring.

The weight of these assembled units is now at ten pounds each.  By the time we add the tension strap this will probably climb to around 10 1/2 pounds.  The extra 1/2 pound seems like a bargain when we factor in the added strength the tension strap will bring.

In a nutshell, that is what his whole exercise is about.  Weight over strength,  multiplied by patience squared.

In the photo below, the roughed out limbs are laid on the table  in the same horizontal plane as they will be when called upon to perform.

This tangential grain orientation seems ideal for our purposes.  Tomorrow I will finish shaping these White Ash  limbs so they fit snugly into the tapered sockets of the limb irons.  Time to mix up some inletting black.

The Mk VIII limb irons have been heat treated to 40 RC using a double temper technique.  The basic procedure is as follows:  the irons were placed in the forge and heated to 1600 F and soaked at that temperature for 1/2 hour.  Upon being removed from the forge they were immediately quenched in my stinky old vat of used fryer oil.  After enduring ten minutes or so of olfactory  grumbles from the Rebecca,  the now room temperature irons were put back in an 800 F forge and carefully monitored to maintain that temperature for 1 hour.   This is the procedure known as tempering.  The next day this tempering procedure  was repeated and that it is why it is known as “double tempered”.  The idea here is to remove any last traces of brittleness.  These new irons  should have a tensile strength of 186,000 lbs per square inch, and at 40 Rockwell they should have mechanical properties similar to a soft spring.  That is to say, they have high strength but are still  somewhat ductile.

And so we are now ready to fit up some new Ash limbs to these souped-up irons.  There is a definite “been there done that” quality to this stage of Firefly’ s development.    The CG’s willing,  groundhog day is almost over.

The welding of the limb irons went well.  No porosity and plenty of fillet on the inside.   Everything has been well stress  relieved to prevent  cracks forming from any of that pesky old hydrogen embrittlement.

The weight on these irons is coming in at 6 1/2 lbs each.  That is 1 1/2 lbs heavier than the previous set, but much of that weight has been shifted back in towards the fulcrum.  The main thing is that they will be many times stronger than our previous attempt.  I have acquired a reliable little heat treat thermometer that will make all the difference when it comes to hardening them.  I had thought to send them out for heat treat but shied away when I heard the cost.

I am contemplating some lightening cuts that would further reduce the weight by almost a pound.  Because these cuts can be executed after the limb is completed,  it seems reasonable to first test the limbs in this slightly heavier configuration just to see how much velocity is gained when we shed a pound at the base of the limb, later on.    I’m guessing  no more than 30 or 40 feet per second.  We’ll see.

I have decided to hold off welding the limb irons until some E7018AC stick rod shows up on Wednesday.  The Tig welding will work best on the outside welds, but there is no way to get the torch head on the inside of the socket to make a decent fillet there.  Stick welding with my trusty old Lincoln AC arc welder seems to be indicated for that.    Not to worry though, there are other things to do.

In the bottom of the photo below, we see a thoroughly garroted winch drum that was made from Ash.  Above it is a new winch drum made from super hard Ipe.   This upgrade promises to do a much better job under the massive compression from the winch rope.

The winch rope that left these impressions in the Ash also caused the drum to emit enough snap, crackle and pop noises that I was forever fearing a limb was fixing to explode.  Not at all the thing when you are cocking this beastie and every errant  sound seems invitation enough to dive for cover.

These irons are made from 4130.  After a 1/2 hour preheat at 400 degrees F, the top strap will be Tig-welded to the limb socket with some nice relaxing ER70S-2 rod.    After that, they will be  stress relieved at 1200 degrees for a couple of hours,  and then hand worked into their final shape.

I’m still debating on heat treatment.    No doubt one of the voices in my head will win out.   I am making every effort to make sure it isn’t the same one as last time.  He was a right ponce.

The bending of the side straps went well.   At this moment,  I am profiling out the final dimensions on the Mk. VIII limb irons.  A glimpse inside the riveting world of CNC machining is provided in this next vid.  20110903140214(1)

If you feel like a snooze now, don’t mind me.  I’ll be back.

As promised.    Hot iron and a bit of thumping.   Click for vid:  20110902131552(1)

Chill winds have been fanning our little outpost in the woods.  Brrr…..goes my August complacency.   Still, one shouldn’t complain.

Art by John Kenney.

Below we see the new  limb socket in flat pattern , bolted to a bending jig.

Tomorrow I will hot bend the outer straps that form the majority of the limb socket.   I’ll video the proceedings.  That should be good for a laugh.

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