Friday, December 20, 2013

Hammerli 850 Cheek Rest

elevated cheek rest

Lately I've been busy improving my Hammerli 850 air rifle.  With the new trigger job, this rifle is quickly becoming one of my favorites.  The next problem to tackle is the height of the cheekpiece on the stock.  It is too low for my scope.  For best accuracy your cheek should be pressed firmly against the stock with your eye perfectly centered in the eyepiece.  To see through my scope, I must lift my cheek off of the stock.  Not comfortable and bad for accuracy.  Good target stocks will have an adjustable cheek piece.  If this were a wooden stock, I could just cut it and install a mechanism to raise it.  Not so easy with a hollow plastic stock; so I decided to make a new cheek rest.

kydex formed to stock

I purchased a sheet of kydex  from Amazon.  I went with a thick piece (0.118") for the extra strength.  Next I cut the kydex sheet in half (6" x 12") with a pair of sheet metal shears.  Then I preheated the oven to 350 degrees and placed the kydex in a Pyrex dish for three minutes.  I checked the sheet with a spatula to make sure it was very pliable.  Then I scooped it up and placed it on the stock folding it over the cheek piece with gloved hands.  I quickly got the alignment just right and held the kydex tight against the stock as it cooled.  In about 5 minutes the kydex had cooled and was rigid.  It formed perfectly to the shape of the stock and the pattern and color even matched the stock.

tracing the pattern

To make a pattern, I wrapped a piece of paper over the stock.  Then I traced the edges of the cheek swell with a scrap of aluminum sheet.  The aluminum worked really well, cleanly marking only the edge.  I carefully cut the pattern out with an x-acto knife.  I placed it back on the stock and drew some witness marks on the paper and stock so I could lines things up later.  Finally, I cut the pattern in half.

spacer taped to stock

The next step was finding the correct height for the new cheek rest.  I experimented by placing pieces of rod under the cheek rest until my eye was perfectly aligned with the scope.  Turns out a 1" rod and a Bic pen tube provided the perfect height (1.319").  I taped the spacer to the stock and put the kydex over it to test.  Perfect.  Now to make the bottom of the new cheek rest match the contour of the stock.  I coated the back of the pattern pieces with spray glue.  Then using the witness marks, I lined each pattern up on the stock and stuck them to the kydex.

hlaf of pattern glued to kydex

The bulk the excess kydex was trimmed away with the metal shears.  Then the kydex was formed exactly to the pattern on the belt sander.  Now to attach the new cheek rest to the stock.  I was going to make the cheek rest adjustable by cutting slots in the kydex but why bother?  I only needed to set it correctly one time.  And most adjustable cheek rests have large ugly knobs on one side to tighten the bolts.  I wanted a cleaner look.  So I carefully laid out the position of the screws and drilled two 3/32" pilot holes through the kydex and into the stock on each side.  I then opened up the holes on the kydex to 13/64" and countersunk them so the screws would sit flush.  I enlarged the holes in the stock to #25 and tapped them for 10-24 screws.  Although the stock is hollow, it is thick enough where the cheek rest is providing plenty of threads.

screw holes in kydex

The pattern peeled off the kydex rather easily and I cleaned up any residual glue with Goo Gone.  I cleaned up any uneven spots on the belt sander and dressed the edges with 220 grit sandpaper.  Finally I screwed the new cheek rest to the stock with stainless flat head socket cap screws.  I was going to back up the screws with nuts but the plastic held the screws tight.  I added a drop of thread lock to each screw just to be sure.

finished and installed

height relative to scope

The new cheek rest is great.  It looks like a factory accessory and is rock solid.  Now that I don't have to move my head around looking for the eyepiece, I can get on target much faster.

PS - If you are cutting kydex on a scroll saw, I found that a 15 tpi blade at 500 rpm gave a very clean cut.  No melting and minimal saw marks.

Thursday, December 19, 2013

Paracord Cobra Bracelet

cobra pattern paracord bracelet

After making my paracord pouch, I realized that my 9 year old daughter would really enjoy making things with paracord.  I figured a bracelet would be the perfect beginner project.  Easy to do and what young girl doesn't love bracelets?  So I picked out some colors of paracord that I thought she would like and some small buckles at Michaels.  I showed her the first weave and then she took over.  In no time she was weaving the paracord like a pro.  Here's how we did it.

First cut 4 feet of each color of cord.  Then fuse the two pieces of cord together by melting the ends.

Loop one end through one of the buckles.

String the cord through the other buckle and wrap the cords around your wrist to measure.  Tie the buckle to the cords with overhand knots.

Ready to weave.

She wanted purple down the middle with pink lines down the sides.  Whatever color you want down the middle is the color you start with each time.

The purple cord is laid across the bracelet.

The pink cord goes over the purple, behind the bracelet, and through the loop from the back.

Pull the cords tight.

after several passes
Continue, repeating the same sequence each time.  Purple over the front of the bracelet, pink under the bracelet.

Repeat until you reach the other buckle.

To finish just trim the ends leaving about 1/8".  Then melt the ends with a lighter and use the metal part of the lighter to push the melted cord against the bracelet.  This mushrooms the end of the cord and melts it to the surrounding cord so that it can't pull through.  That's it.  Enjoy your new bracelet.

Saturday, December 14, 2013

Paracord Pouch

finished pouch holding 3/8" steel shot

Now that I have made a paracord needle, it's time to put it to use.  These slingshot ammo pouches were what first inspired me to try some paracord projects.  A quick search on the web turned up this set of plans for a paracord pouch.  It is a slightly different design than the Simple Shot pouches but it should work.

Start with 30 feet of paracord.  Use a rubber band or tape to secure one end of paracord to can.  Also secure a sharpie to can to add slack to line.  (I used a pen but found it a bit tight afterwards.  The sharpie will give more slack.)  Wrap the cord around a soda can 25 times and remove the sharpie.

25 wraps of paracord

Add your needle to the end of the paracord and weave it up through the wraps by twos.  Start by going under the first two wraps.  Continue spacing each line about 1/2" apart.  Don't worry too much about spacing, just push the lines together, you can feel when they are close enough.

starting to weave

and more

Make sure to work any twists and kinks out of the paracord as you go.  Your last weave should go down and thread back under the bottom wrap.

last weave

Now it is time to start on the bottom.  Push everything down even with the bottom of the can.  The bottom is constructed by making half hitches around the bottom of the weave.  Alternate the half hitches through the horizontal wraps and through the base of the vertical weaves.

starting first half hitch

finishing first half hitch

first row of half hitches complete

Continue the half hitches around the base of the can.  Keep the hitches tight so you have a solid base.  As you get towards the center you have to skip some half hitches and maybe go every other one.  Otherwise you end up making the base cone-shaped rather than flat.  If there is an exact formula for this, I'm not aware of it.  The center gets a little tricky and I just went by feel, adding half hitches where it looked like I needed one.

completed base

Now remove the pouch from the can.  To finish the base, I passed the working end inside the pouch.  Then I secured it to the base with an overhand knot and cut off the excess.  I inverted the pouch and finished it off by melting the knot together, permanently securing it.

inside view of melted knot

The starting end of the cord is then woven back down the side of the pouch next to one of the other vertical lines.  Melt that end and press it to the adjacent line, fusing them together.  Take the extra cord (or cut a new piece) and weave it around the top to create a drawstring (see first picture).  Add a cord lock then knot, trim, and melt the ends of the drawstring.  All done!  Add your favorite junk and pull it closed.

finished pouch

Thursday, December 12, 2013

Homemade Paracord Needles

my homemade paracord needles

Listen.  I love Rainbow Loom as much as the next guy but I fear if I make too many more rubber band bracelets with my daughters, I may have to trade in my Man Card.  So I decided to try my hand at some paracord projects.  Seems like something the girls and I might like to do together.  For some of the trickier projects, people use Perma-Lok needles to weave the rope through tight spots.  It's just a tapered metal rod with a threaded hole in the base that you screw the paracord into.  The threads actually hold the cord tight as you pull it.  I'd never forgive myself if I purchased something that I could make so easily, so here we go.

paracord and cleaning rods

I could have made them from some metal rod but I figured if I looked hard enough in the abyss that is my basement, I would find something already threaded.  I remembered that I had a bunch of cheapo pistol cleaning rods that I had saved.  I have upgraded to more professional rods and have no need for these anymore.  They are 13/64" in diameter and have a 8-32 thread on the end that goes 7/16" deep.  They looked just about right so I screwed a length of paracord in and pulled on it.  It held tight.  Perfect.  All I would need to do is cut them to length and taper the end.  I decided to make two, one for me and one for my daughter.

8-32 thread

I cut the rods to 3" in length with a hacksaw.  Then I started to taper the ends on my disc sander.  The results were coming out too uneven for someone as obsessive as myself.  So I chucked them in my lathe and cut a 5 degree taper.

cut to length

Next, I blunted the tips with a hand file.  I finished them off with some fine emery paper followed by 0000 steel wool while still spinning in the lathe.

tapered and (top one) polished

That's it.  It probably took me longer to find the cleaning rods than to make the needles.  I have some more scrap cleaning rods in other materials and diameters.  I'll probably make a few more in different lengths and tapers just to see what works best for me.

Friday, December 6, 2013

Hammerli 850 Trigger Job

The Hammerli 850 is a great air rifle nearly ruined by a god-awful trigger.  Previously I posted about making a steel trigger for this rifle.  That was all about personal preference; it did nothing to improve the trigger pull.  The stock trigger pull isn't that heavy, it breaks at a reasonable 2.25 lbs.  But the pull is a very long and very mushy single stage that feels much heavier.  Mine was adjusted with no pre-travel, followed by 4 mm of movement that felt like you were pulling the trigger through chewing gum, and then another 11 mm of over-travel after the sear released.  There was absolutely no indication when the sear would release making the trigger very unpredictable and distracting.  And the over-travel essentially guaranteed that you jerked the trigger every time.  There is a useless adjustment screw but it only changes the amount of pre-travel.  It does nothing to change the pull weight or length of pull.  Time to work on the trigger.

Manual Safety

Before I worked on the trigger I had to do something about the safety.  It automatically engages whenever the gun is cocked.  This is not only a nuisance, but it breeds bad habits.  Engaging the safety should be a conscious decision.  Since 99.9% of the guns in the world have manual safeties, you do not want to get into the habit of assuming the safety will engage by itself.

Part that needs to be removed

Luckily this is an easy fix.  The safety block has a plastic tab that the bolt pushes on when cocking the gun.  Simply cut off the tab and you have a manual safety.

As an added bonus the bolt operates a bit more smoothly now without having to push on the safety.  Now to do something about that trigger.

Over-travel Adjustment

The worst aspect of the trigger is the over-travel.  Eleven millimeters may not sound like much, but it's huge!  To eliminate the over-travel I added a screw that limits the movement of the sear.  I drilled and tapped a hole for a 5-40 screw right where the sear spring is located.  I cut the head off the screw to make a threaded rod.  Then I carefully ground off a little bit of the screw and tested the trigger.  I repeated the grinding and testing until all of the over-travel was removed.  Then I used some Loctite to secure the screw in place.  I ended up with the screw protruding 5.5 mm above the sear.

screw added to sear

Adding a Second Stage

As I mentioned, there is 4 mm of movement needed to trip the sear.  Unfortunately, much of that movement is the sear pushing the hammer back against the hammer spring.  That accounts for most of the pull weight and trigger creep.  There is nothing that can be done about that unless you are willing to change the geometry of the sear and hammer faces.  Since they are die-cast in soft metal I didn't feel comfortable messing with that.  Luckily, someone came up with a way to add a second stage to the trigger.  What this does is, it allows you to put most of the pull weight and "mush" into the first stage, then just a little more movement is needed to trip the sear in the second stage.

I checked my supplies and found a length of 2.36 mm stainless steel drill rod.  I cut it to length and just placed in under the trigger to test it out.  It looked like it would work so I cleaned the parts in acetone and epoxied the rod to the trigger with PC7.

rod epoxied to trigger

During reassembly I burnished some molybdenum into the trigger and sear contact points and oiled the pivot pins.  The results were amazing.  Now the trigger pull is as follows.  There is no pre-travel, a very smooth 5 mm pull through the first stage, followed by a definitive stop before the second stage, then < 1 mm of movement to trip the sear with absolutely no over-travel.  Pull weight is now just 1 pound.

the finished trigger and sear

The original adjustment screw now determines how much of the trigger movement is in the second stage.  Turn the screw all the way in and you end up with a long second stage pull.  Turn the screw out and you transfer the trigger movement to the first stage.  Turn it out far enough and the sear trips in the first stage, giving you a single stage trigger again.  I have mine adjusted with the screw about one and a quarter turns out.

The trigger is so much better it makes the rifle much more enjoyable to shoot.  Now I can concentrate on the sights and not lose focus wondering when the gun is going to fire.  Don't get me wrong.  This is no match trigger, but I certainly can't blame any bad shots on the trigger anymore.

Tuesday, November 26, 2013

QUADframe Battery Tray

battery tray on QUADframe Pro Six

One of the goals of our hexacopter is long flight times.  Our hex is large and relatively heavy (3.3 kg without batteries) so that requires large batteries.  We planned to fly with either three 6S 8,000 mAh batteries or two 6S 10,000 mAh batteries.  These batteries are large and heavy.  QUADframe sells a battery tray that mounts between the arms.  Unfortunately those mounts are designed for much smaller batteries.  Time to design a new battery mount.

QUADframe battery tray

After some playing around with the frame and batteries, I determined the best place to mount them was over the center plates of the frame.  I removed the protective dome and decided to mount the plate on the four standoffs that supported the dome.  I wanted to make the tray out of honeycomb carbon fiber.  Unfortunately I didn't have any left and I didn't want to wait to order it.  So I made my own carbon fiber laminate.  I epoxied some thin carbon fiber sheet I had to both sides of some good quality 1/8" plywood.  I clamped it in a vise between some boards as it dried.  The next day I cut it to size on a wet tile saw with a diamond blade.  Sorry, I forgot to take pictures as I made the laminate.

over-sized plate attached

Next I marked and drilled the holes to mount the tray.  The GPS mount that I made was in the way of the tray.  So I cut a groove in the tray for the GPS support.  Had I known the batteries were going there, I would have mounted the GPS somewhere else.  With that done I mounted the tray and tested the fit of the batteries.  The tray was longer than needed, so I cut it to the same width as the center plates of the frame.

screwed to standoff

To hold the batteries in place I used some heavy duty Velcro tape and Velcro straps.  I cut a slot in the tray to pass the straps through.  The standoffs were higher than needed so I cut about 1/2" off of each to bring the center of gravity down.

finished tray on frame

The tray is very strong; doesn't even budge with over 6 kg of batteries on it.  As a bonus, it also protects the flight control systems from crashes.  The 10,000 mAh batteries mount front to back, while the longer 8,000 mAh batteries mount lengthwise.

10,000 mAh batteries

8,000 mAh batteries
Even with all of that weight on top, the hex flies pretty well.  The 8,000 mAh setup balances slightly better because the third battery mounts below the hex on the landing gear (see first photo).  At some point in the future I may move the GPS mount so I can move the 10,000 mAh batteries closer to the center of the hex.  But for now, it works.