The Ballista

It seems that against all odds I stumbled into a winning combination of variables the other day when that six shot,  two inch  group emerged from the machine at 40 yards.  This is especially true considering that the bolt that was  used had been bound up with electrical tape to fix a  crack.   I’d expected a  long fuss and bother trying to coax that kind of accuracy out of the machine.  This is right up there with the best the old Gallwey could  do.  Unfortunately the luck did not hold when I changed bolts and tried firing a lighter Dura style bolt.   The first shot was fine, but the second developed an upward swoop and smashed into the back stop at such angle that it snapped the 7/8″ ash shaft like a cheap pencil.  It appears that one of the limbs tips started traveling a bit above where it should be relative to the bolt groove.   I suspect this causes the bowstring to lift the rear of the bolt as it is launched down the groove.  Any number of perturbations set in after that.   I believe that the  grip of the torsion spring on the starboard limb has become  compromised.  The 1/4″,  3 strand nylon rope that is in the  spring now,  seems to be breaking down under the  load.   Perhaps the anti-chafing gear is defective and allows parts of the spring to contact the side of the hole in the field frame.  I won’t know until I take out the current  springs and examine them.   It is  amazing that the machine  performed as well as it  did the other day,  given that the starboard bundle was  knotted together in four places.    Probably the only long lasting fix is to install new springs made from  3/8″  nylon double braid,  and improve all that anti-chafing clobber.

I still consider  the results of that first accuracy test to be a blessing from the Catapult Gods.    It tells me that if the machine can perform like this once, it can do so again.  It lights the way for what is to come.

………But seriously, this wouldn’t have been possible without a good backsight and frontsight.  Here are the photos of the expedient designs I came up with for this testing phase.  Eventually, they  (along with the electric winch etc.),   will be replaced with something more plausibly authentic.  The long sight radius of 80″,  makes this form of sighting particularly precise.

It took awhile to get here,  but we finally have some encouraging results from our first accuracy test of the Orsova reconstruction.  I spent most of the morning yesterday  balancing the rotational position of the washers so that both limbs were working in unison.  Initially, the best way to do this is by observing how closely the limb tips match up as they swing past the stock during the wind up process.  If one limb tip is drawn back further than the other, this indicates that the washers on that side need to be rotated to the next position to increase the torque in that bundle.  Ultimately the final adjustments are made by observing the bolt flight.   In the case of yesterday’s test,  the bolts kept shooting erratically and maybe a foot to the left,  relative to  where the bolt groove was aimed.     I tightened the washers on the left side to compensate,  and finally the bolt flew straight and true, shooting out of the machine directly in line with the bolt groove.    I decided to leave the sight alone and  just see if the machine would shoot some kind of a group on a new target.   The first shot hit about 4  inches  left of the bulls eye.  I took special care to sight the machine for the second shot, and then eased back the firing lever until the shot was released.   “Yes! Yes! Yes! Yes!”.   The second bolt had gone right through the hole made by the first shot.   When I finally got through dancing a jig and thanking the Catapult Gods for their mercy,   I pulled myself together and settled down to the serious work of sighting a third shot.   It went off without a hitch and again the bolt flew straight into the hole left by the previous two shots.   At this point I was beside myself with elation and Becky came out to see what all the fuss was about.  After a couple of minutes  of deep breathing I prepared for shot number four.  Now  my ever patient wife was looking on.   This time the bolt hit about 1 inch low and the hole in the target took on a more ragged appearance.  Shot five again followed the track of the first three shots and further validated that original group.  Shot six was a blow out because I screwed up the release.  In the photo below,  it is the lone hole close to the bulls eye.  Shot seven was a success and only slightly opened up the original group.   All seven shots were made with the same bolt to enhance consistency.  Here is what the target looked like as I wrapped it up for the day.

The range here is 50 yards.  The backstop is a 12″ deep wooden box, filled with sand.   The back wall of the box  is a 1/4″ steel plate covered with plywood.  The sand slows the bolt down to the point that when it hits the solid back wall,  no damage is  done to the projectile.  I elected not to set up the chronograph for these shots,  but the velocity appeared to be in the 350 fps range.

From my perspective,  this level of performance was essential for what comes next.   There are any number of improvements that need to be made yet,  but today’s performance vindicates all the work that has gone into this so far.   Right now,  more than anything,  I feel an overwhelming sense of relief.

Actually it began about a month ago when I made a start on a fourth pair of limbs.  They have now been installed and test fired 20 times at full power.  So far they seem to be holding up fine.  The spring steel braces on the limbs are a much better design and that seems to be why they are holding up so much better.  The convex impact block seen attached to the limb in the photo below, can be packed out with shims to adjust the stopping point of the limb.  As noted in an earlier posting, this adjustment controls how exactly the string is picked up in its center by the catch.  It is also another way to adjust how slack the bowstring will be when the limbs are at rest.  This is vital to make sure the string is protected from the massive shock the limbs generate at the  end of their power stroke.  When properly adjusted, the limbs are brought safely to a stop only by the bronze hardstops attached to the curved stanchion on the field frame.   So far,  all these parts are working very well.