Background

I've spent some time redesigning my turning target mechanism to create a more compact, easier to construct and less expensive version.  The new design is basically the same in concept, but the circuit has changed slightly as well as the construction.  Part of the new design was to remove the original tripod which now allows the user to choose their own means of mounting, whether it is still a tripod or perhaps attachment to an existing support.  The unit is still powered by a 12 volt, 4 (or 5) AH, sealed lead acid rechargeable battery which will last for several hours of use, however this new design has reversed the polarity of the system as will be seen in the new circuit diagram further down the page.  The system draws its short bursts of current (up to ~4A) only during the actual turning phases, a minimal amount (<100mA - depends on the relay used) while faced and none during edge time.

The heart of the turning system is a windshield wiper motor.  The particular one used for this project is described as a 2000-2001 Saturn L motor.  Other types may work, but this one started turning up at surplus places for around twenty dollars.  This type of motor has good torque and an integral switching system which can be used to limit the travel in one direction.  Therefore, only one more switch is needed to limit it in the other direction.  I found that for this alternate direction I also needed to provide a stop due to the inertia of the motor (and target).  The integral limiter stops the motor by reversing the polarity of the windings, but I didn't want to get that detailed in my construction of the other limit point.  In the present design there is a relay which controls the direction of travel, the integral limiter for edge position, and the added switch and bump stop for the face position.  The target does have a "bobble" at the end of its swing, but it turns in a relatively quick fashion and the "bobble" ends pretty soon.  The turning time is well less than a second, but is dependent on the weight of the target or system of targets.

This target system was designed to be faced by a short circuit contact device such as a switch or relay and edged when the short is removed.  Such a signal is provided by my Rangebox, a command playback and switching controller powered by a 9-volt battery.

I have built several of these, and decided to place this information into the public domain so others can build them.  If I can provide a means for someone to afford a turning system, then I'm happy to have published this page.  If help or more information is needed, contact me (15 Lakeview Ave, Tupper Lake, NY 12986).

A list of materials I use is at the end along with some source information.  Many of the parts are common type items found at many hardware stores, but some are from other places.  For some of the "others," I've also provided part numbers.

Of course, no printed instructions can be free of a safety reminder these days.  Due to the inability of me to be by your side through this, you will need to assume all risk involved with the construction and/or use of this system.  This includes, but is not limited to, the use of all tools involved.  Additionally, this system has moving parts!  Be aware of the pinch points and swing position of the frame when using it.  The battery should only be connected while you are not within the reach of the turning parts.  Disconnect the battery at any time you are within the swing radius.

Updates

Feel free to provide feedback on how this project has worked.  I will try to change it if better parts appear or techniques evolve.  I have reviewed this document countless times, but there is no guarantee of accuracy.  If an error is found, I will attempt to immediately get the correction out.  Please feel free to send me comments.

Construction

In an attempt to avoid confusion, I will refer to the motor and housing in the following manner:

Orient the motor assembly such that it is sitting in front of you with the shaft at the upper right.  This should place the motor horizontally to the lower left as in the picture.  Where the black motor cover meets the grey housing, two wires emerge and travel to the back of the connector on the bottom of the housing.  The mounting hole just above these wires will be referenced as #1.  Moving clockwise, the next mounting hole is #2 and the last one is #3.  The connections on the bottom of the unit will be referenced by the numbers given on the connector housing.  In case you get a motor which has different markings, this can be a key: A=53b, B=53a, C=53e, D=31b and E=53.  A-E will be from the left looking into the connector with it on the bottom and the shaft pointing upward.  The lever arm mounted on the shaft will be referred to as the arm.  This arm has a "hitch" type ball on the end.  Its orientation at this point will not be important.

Holes drilled in the washer and bracket will be numbered to match with the numbering of the motor housing holes to start out.  Subsequent holes made through the washer and bracket will be numbered as they are mentioned, as referenced in the pictures.

Steps to Construction - Working With the Motor and Washer

• Using a 13mm socket or wrench, remove the nut and lock washer from the shaft of the motor and tap the arm to loosen and remove it.
  
  
• The next step is to drill three 1/4" holes to coincide with the three mounting holes in the motor housing, with the washer centered about the shaft.  The easiest way I've found to initially place these holes is to place a small circle of paper (with a hole for the shaft) over the mounting posts and push a pencil into all three threaded mounting holes.  Then position the paper on the washer appropriately, mark the hole positions and drill pilot holes first.  Finally, the 1/4" holes can be drilled.
  
  
• Check for proper fit by trying to insert three M6 metric bolts through the holes in all three possible orientations and choose the one which best centers the washer.  If the holes are close but not quite positioned well enough, you can enlarge them with the 17/64" bit.  I like to slightly chamfer the holes with a countersink as well.  If there is a difference and you choose an orientation, mark hole #1 for reference.
  
  

  Note: All of the 1/4" holes in this project can be enlarged slightly with the 17/64" bit to allow easier fitting of the parts.  Depending on how precise your measurements and drilling techniques are, you may or may not wish to do this.  Additionally, I prefer to remove all the rough edges from the holes by using a countersink cutter.
  
  

• After finishing the three mounting holes, find a center point between holes #1 and #2 for another hole, located between 1/4" and 3/8" from the outside edge of the washer.  This point does not have to be exact.  Finish this hole (#4) with the 1/4" bit.
  
  
• There will be one more hole to drill in the washer (#5), but the location will be determined during the bracket work.
  
  

  Note: In my prototype I initially had a sixth hole which is now labeled "omit" in the pictures.  Additionally, hole #5 is shown in some pictures, as being close to the inside edge of the washer.  As explained later this hole should be placed more toward the outside edge of the washer, as shown in the bracket and washer images.
  
  

Moving to the Bracket

• Orient the angled metal bracket so that it is on one side with the other side facing upward but away from you.  This should look similar to laying a book sideways with its binding away from you and opening it up to have the cover upright on the side away from you.  Measure 1-3/4" from the left edge (or 2-1/4" from the right) and make a line parallel with the left edge on the flat horizontal surface.  Position the washer upside down over the line so that the line is centered through hole #4 and the center hole of the washer.  Next, move the washer so there is approximately 1/8" clearance between the edge of the washer and the inside back (vertical) portion of the bracket.  This will allow room for the motor housing.  Make any minor adjustment necessary to align the washer evenly on the bracket.  Mark and drill holes #1, #2 and #4 in the bracket and fix the washer to the bracket by bolts through these holes.  Find a center point within the washer's center hole to use for drilling the large center hole for the bracket.
  
  
• The next hole does not need to be exact but should be located as near the edge of the washer as possible.  With the washer attached to the bracket and everything oriented as before, mark a position for another hole such that it will be near the edge of the washer and leave approximately 1/8" of bracket at the front edge, where the left side of the washer meets the front edge of the bracket.  This will be hole #5 and will be located between holes #2 and #3 of the washer (which is upside down), but not at all centered.  Drill this one to 1/4" also.
  
  
• At this point remove the washer from the bracket and enlarge holes #1 and #2 in the bracket to approximately 1/2" so they will clear the mounting posts of the motor.  Also make the large hole for the center shaft post at this time.  I use a 1-1/8" bi-metal hole saw for this one.  This hole will quite probably be off the edge of the bracket, so be cautious as you cut it and be sure to smooth any sharp points.
  
  
• Next Mark and drill an appropriate hole for the mounting of the 1/4" jack in the upper right portion of the same flat as the holes previously made in the bracket.  See the picture for more information.
  
  
• Depending on how you plan to mount the unit, determine what holes will be needed in the back of the bracket:
  
  
  • If you want to mount this unit on a tripod, you may wish to drill and tap a hole for a 1/4"x20 bolt such as used by most tripod plates.  Simply position your hole and use the appropriate size drill for your tap.  As seen in the images, I also made appropriate holes for the spring pin found on most tripod bases.
    
    
  • Alternately, you may wish to mount the unit to a post type structure.  You can drill the back of the bracket nearly anywhere to facilitate mounting, but the areas near the center or near the jack/wires should be avoided due to lack of clearance to work with bolts and nuts.
    
    
• There will be two more holes to drill, but their positions will need to be calculated based on your switch and arm preparation.

Working with the Arm

• Loosely mount the washer to the housing with M6x12mm bolts through holes #2 and #3, and lightly place the arm on the shaft.  Move the arm to several points around its arc and check that it has clearance with the inside of the washer.  If not, remove any metal from the arm that will restrict its movement.  The points on the arm to check will be the two sharp corners and the two bottom edges of the first bend.
  
  
• Place the M6 x 40mm bolt through a plastic one-inch standoff and screw it into hole #1. Rotate the arm clockwise against this standoff.  The arm should now be pointing almost in line with and toward the motor.
  
  
  • Mark the point on the arm where contact is made with the standoff and remove material from the edge to allow the arm to move further clockwise against the standoff.  Remove enough so that the arm can swing slightly further than parallel with the back edge of the bracket.
    
    
  • Smooth all rough edges on the arm.  Recheck that the arm now has greater travel in a clockwise direction.  The relief cut will allow for less beating of the system and help provide for a full 90 degree rotation of the shaft between the face and edge positions while allowing for alignment with the mounting bracket.
    
    
• The arm will now need two holes drilled and tapped.  The first will be centered in the ball from the top and must NOT continue through the area which mounts the ball to the arm.  It should, at most, extend into the neck area just below the ball itself.  This should be drilled and tapped to 1/4"x20.  Take your time with this hole.  Drilling and tapping the ball needs to still leave the ball securely fastened to the arm and needs to be deep enough for a good grip by the frame bolt which will mount in it.  A bottoming tap will probably be necessary.
  
  
• The second hole is far more forgiving, but still be sure not to place it too near the edge of the arm.  In fact, my hole is close enough to the ball base on the bottom of the bracket that it includes removal of some of the ball base on the top of the bracket.  The placement will be somewhat imprecise but your placement of the switch will be based on the actuating screw in this hole.  Using the picture of the arm as a reference, drill and tap a 6-32 hole in the arm for a 5/8" screw to be mounted from the bottom.  This screw will be used to actuate the switch and the only restrictions are that it must be far enough out on the arm that it clears the edge of the washer, and the head of the screw must be capable of activating the switch without dragging on the bracket.
  
  

Switch Placement

• To begin this phase, mount the washer to the bracket by placing it on the top and using a bolt through hole #4 and another through hole #5.  Finger tighten M6x12mm bolts through holes #2 and #3 of the washer into their respective mounting posts of the motor.  Finger tighten the M6x40mm bolt through the one-inch nylon standoff and washer hole #1 into its respective motor mounting post.
  
  
• Place the arm loosely on the center shaft and adjust the 6-32 screw to the proper height to activate the switch when near its position against the nylon stop with plenty of clearance with the bracket.  At some point you will need to lock the screw in place with your choice of adhesive, but you may want to wait until final assembly.
  
  

  Note: This next step is very important to the operation of the motor at the face position.  It can be very time consuming to get this correct, but do take the time.  You must find the best mounting for the switch you've chosen to shut down the motor at the face position.  If this switch does not engage at all, the motor will try to keep running after hitting the stop.  If it does not engage far enough ahead of the stop, the motor will bounce against the stop, possibly several times.   Additionally, be careful that you don't mount the switch such that the actuating screw will overtravel the end of the lever.
  
  

• You should be able to move the arm through an arc of over 300 degrees, but you will limit its travel to 90.  At its extreme clockwise motion, the switch must be fully engaged.  Actually, well prior!  You must now calculate the positioning of the switch so that it is actuated at the earliest moment before the arm reaches its stop, yet such that it can't be overrun by the actuating screw.  Also ensure that the switch housing won't be struck by the screw during any of its travel.  Once you've determined the location, mark the holes, disassemble everything and drill the holes.  Use an appropriate bit size for the screws you'll be using.
  
  

Some of the Wiring

• Now let's do some wiring! You can use some of the speaker wire from the 100 foot spool for these connections.  You will only need about 80 feet to connect your target for 25 yard use.  These connections will use less than five feet.
  
  

Note: The shield of the jack must not be attached to ground in any way. Either use a plastic jack with isolated mounting like the one in the parts list (preferred) or use the ring and tip of a stereo type jack AND plug. Even with a stereo jack, if the shield is grounded and a monaural plug is inserted, it will cause a short of the shield to ground. This will cause an overcurrent!

• Bare the ends of a single lead about 3" long and crimp a 3/16" quick connect female terminal onto each end.  Push these connectors onto pins 53 and 53a.
  
  
• Measure and cut another single lead (~6" long) to connect between pin 53e and the relay.  Bare both ends and crimp a 3/16" quick connect female connector onto one end only.  Push the crimp connector onto pin 53e and solder the other end to the Normally Closed (NC) pin of the relay.
  
  
• Measure and cut a 3 "foot" piece of twin lead wire, bare all ends.  Later you will need to solder the male coax cable connector onto one end such that it matches your battery connection, which in turn is made relative to your charging transformer's connector.
  
  
• Measure and cut a single lead (~6" long) to go between pin 53b and the ring (or shield of an isolated jack) pin of the 1/4" jack.  Bare the ends of the wire and join one end with the negative lead from the battery connection.  Solder this junction to the ring (or shield of an isolated jack) pin of the 1/4" jack.  Crimp the other end with a 3/16" quick connect female terminal and connect to 53b
  
  
• Measure and cut a single lead to go between the tip pin of the 1/4" jack and the relay.  Bare both ends and solder one end to the tip pin of the jack and the other end to one of the coil pins of the relay.
  
  
• Solder the positive battery wire to the opposite side of the relay coil and connect a jumper from this pin to the Common (C) connection of the relay.
  
  
• Measure and cut a single lead to go between the relay and the switch (~2.5" long).  Bare both ends and solder one end to the Normally Closed (NC) pin on the switch and solder the other end to the NO connection on the relay.
  
  
• Create a small lead to connect between the C terminal of the switch and one of its mounting screws.  Solder this wire to the C terminal and connect it to one of the screws in a manner of your choosing.  This connection is to ground the C pin to the bracket.  You will need a good electrical connection through this screw.  Don't use any insulating type washers.
  
  

Some Testing and Setting of the Stop Position

Note: The motor has a great deal of torque when it runs, especially when it starts.  Hold the motor solidly or clamp it down for this testing phase.  It may be easier to use clip leads for the following testing, but some short jumpers soldered to the remaining coax connector could be used.  Additionally, a clip lead (or similar), to short across the jack terminals will be needed.



• First, assemble the motor, bracket washer and circuit, but ensure that the arm is not on the shaft for these tests.
  
  
• Connect the negative lead to the battery.  Briefly tap the positive lead to the positive terminal of the battery.  The shaft should spin in a counterclockwise direction, but it should stop on its own at some point.  If it does not stop, recheck your wiring.  If it does spin briefly and then stop, connect the positive lead to the battery.
  
  
• Next, short the two terminals of the 1/4" jack.  This should start the motor in the clockwise direction and it should remain spinning as long as you leave the short.
  
  
• Test the switch by activating it several times.  Each time you activate it the motor should stop.
  
  
• Now remove the short on the 1/4" jack.  The shaft should turn in the opposite direction, but should stop on its own at some position.  Break and make this short several times to get an idea of what is going on.
  
  

Note: This last step is very important!

• Make and break the short on the 1/4" jack several times while watching how the shaft stops each time you remove the short.  Then with the short removed and the motor stopped in its own stop point, disconnect the battery.
  
  

Back to the Arm

Note: You must have the above stop point established when you mount the arm on the shaft the next time.  If you're sure this is done, move to the next step.  If you're not sure, hook everything up again and make sure.



• Clamp the 1/4"-20 to 3/8"-16 adapter into a vise and drill the 1/4" hole larger using the 17/64" bit.  To try to achieve the best straightness to the hole I drill it through the 3/8" end.  After you drill it out, tap it with the 8mm x 1.25 metric tap.
  
  
• Assemble everything with lock washers in all the appropriate places.  For the M6x12mm bolt in hole #3, check for clearance with the arm and either use an internal tooth lock washer or adhesive on this bolt as required.
  
  
• If you're sure the motor is in its counterclockwise stop point, place a small amount of red Loctite (#271) (or other adhesive) all around the contact area between the arm and the shaft and position the arm so that it is over the M6 x 12mm mounting bolt in hole #3.  If there is some movement to set the arm onto the shaft (alignment of the small grooves from previous mountings), favor being counterclockwise just a little.  Place the original star washer on top of the arm and tighten down into place with the modified adapter.  I do not use any Loctite on the threaded part of this shaft, but do ensure the adapter is tight, without stripping the threads, of course.  Use the arm for leverage against turning the shaft when you tighten the adapter.  Be sure to allow ample drying time for the adhesive before running the motor.
  
  

  Note: At some point, after you have determined the screw is adjusted properly for switch activation, you may wish to add some adhesive to ensure it doesn't move.
  
  

Connecting Up the Battery

Note: You will need to determine the polarity of the connection based on the polarity of your charging system's coax connector!



• Remove the plastic insulator from one of the 1/4" quick connect female terminals and crimp and solder it to the outside connection of the female coax connector.
  
  
• Cut the fuse holder leads to length and solder one lead to the center pin connection of the coax connector.
  
  
• Crimp a 1/4" quick connect female terminal onto the other lead of the fuse holder.
  
  
• Push the terminal mounted to the coax connector onto the appropriate terminal of the 12 volt battery.
  
  
• Install the 5 Amp fuse in the holder.
  
  
• Ensure the proper polarity of the coax connector on the 3 foot battery connection wire.
  
  

  Note: If the adhesive has had time to set, proceed with the following test and finish wiring the battery.  If it hasn't set long enough, you can move on to making the target frame and come back.
  
  

• One terminal of the battery should still be disconnected at this point.  Plug the turner cable into the coax jack.  Momentarily touch the loose connection of the fuse holder to the battery terminal.  The motor arm should either move to and stop or remain stopped over the bolt in hole #3.  If everything is good so far, disconnect the battery and provide a short to the 1/4" jack pins.  Reconnect the negative terminal on the battery and see if the motor turns in the opposite direction and the screw actuates the switch.  The motor should stop and the arm should be resting against or close to the nylon stop.  If this does not work correctly, remove the short to check the stop point over bolt #3 and then reshort the jack and try again.  If the arm does not position itself correctly over bolt #3, you may need to disassemble the arm and reperform the steps above under the heading, "Some Testing and Setting of the Stop Position."  If the stop point near/against the nylon stop isn't working correctly you will need to check the switch actuation and possibly the wiring.
  
  
• If everything seems to be working to this point, try connecting and disconnecting the short on the jack and watch to see if the arm moves to each position readily.  If it does, disconnect the coax connector and set the unit aside.
  
  
• Remove the fuse.
  
  
• Hot glue or epoxy the coax jack to the battery.
  
  
• Ensure the opposite terminal is fully installed.
  
  
• Replace the fuse.
  
  

Building the Target Frame

• Retrieve the 1/8" x 3/4" (or 1/8" x 1") by 2 foot bar aluminum and place two 90 degree twists in the bar about 4 inches out from the center.  This will allow the center to sit flat and the ends to turn up to receive the channel metal.
  
  
• Drill a 3/8" hole in the very center of the bar.
  
  
• Cut the 2 foot length of channel aluminum in half and place one piece over each end of the bar aluminum in a right angle orientation.  The bar should fit within the channel.  Drill a hole through the channel and bar aluminum at each end of the bar to allow mounting the channel pieces.  Use the 11/64" bit.  Mount each end using a 6-32 screw and nut.  When finished the target frame should be able to have a two foot wide piece of "backer" cardboard slipped into it between the pieces of channel aluminum.
  
  
• Measure and drill a 1/4" hole to correspond with the threaded hole in the top of the ball on the arm with a 3/8" bolt through the center hole into the adapter.  When the frame is completed, place a 3/8 lock washer on the 3/8 bolt and tighten the frame down onto the adapter.  Next, using the appropriate number of 1/4" washers to space the frame above the ball, place a 1/4" bolt with lock washer through the frame and spacers and tighten it into the ball.  If your threaded hole is too shallow for the bolt length, add a flat washer under the lock washer.
  
  

Making a Connecting Cable

• Strip both ends of the remainder of the 100 foot spool of speaker wire and install the 1/4" plugs connecting one lead to the tip and the other to the shield.  Note:  If the controller you are using has a different style connector, install the appropriate connector to that end of the cable.
  
  

Final Testing

• Connect the battery and whatever device will provide a signal to the 1/4" jack and test the system.  The target should now turn relatively quickly.  It may "bobble" a bit at each end of travel.  You may need to adjust the switch to get the best face condition.  Be sure the motor stops at both positions.  If a controller is not readily available, you should be able to activate the face position by shorting the connector at the controller end of the cable.
  
  

Suggested Tools and Materials List:

Note: Several drill bits will be determined by what your tap suggests as the starting hole.

• countersink cutter (optional)
• small bit for pilot holes
• small bit for 2-56 size screw holes
• 1/8" drill bit
• 11/64" drill bit
• 13/64" drill bit
• 1/4" drill bit
• 17/64" drill bit
• 1-1/8" bi-metal hole saw
• 6-32 tap
• 1/4-20 bottoming tap
• 8mm x 1.25 metric tap
• tap handle(s)
• 7/16" socket or wrench
• 9/16" socket or wrench
• 10mm socket or wrench
• 12mm socket or wrench
• 13mm socket or wrench
• drill - a press would be a definite advantage but a hand drill may work
• hacksaw
• vise
• Flat file
• round file
• soldering iron
• terminal crimpers
• hot glue or epoxy
• Loctite red (#271) (or other adhesive)
• solder

Parts List: (Source and P/N of part)

Note: Some of the parts I used for this project came from All Electronics.  I am in no way affiliated with them other than being a customer.  There are several other sources for many of these parts or similar.  I provided this list with All Electronic part numbers solely for your information.  Parts availability changes frequently, so you may need to research a part to find a similar one.  For all the parts other than the isolated (plastic housing) 1/4" jack, many other styles of part can be used.  Feel free to use alternate vendors.  For items without part numbers, these should be readily available from hardware stores.

• 1,   Wiper Motor - the one I use is listed as a 2000-2001 Saturn L series motor (All Electronics P/N DCM-171).  Also available at many other sources.
• 1,   Sealed Rechargeable Lead Acid Battery - 12 volt, 5 AH (All Electronics P/N GC-125)
• 1,   Miniature SPDT Lever Switch with 5A contacts (All Electronics P/N SMS-204).  As long as the switch has >4A Normally Closed contacts, other lever types should work fine.
• 1,   SPDT Relay with 5A (or greater) contacts.  (All Electronics P/N RLY-163 has 15A contacts)
• 1,   Jack 1/4" panel mount with shield pin isolated from the panel when mounted.  (All Electronics P/N SPHJ-1 or SPHJ-2)
• 1,   Transformer 12V 200mA (All Electronics P/N DCTX-122 **[You will have to reverse the polarity for either this or the rest of the system!!])
• 1,   Coax jack This mates with the above transformer (All Electronics P/N DCJ-1)
• 1,   Coax power plug.  This also mates with the above jack.  These parts are all dependent on how you wish to connect to and charge the battery. (All Electronics P/N DCSID)
• 2,   Plugs 1/4" cable mount.  **Monaural is fine only with an isolated jack!  Note:  One of these connectors will need to mate with your controller.  Substitute as appropriate. (All Electronics P/N RAPS)
• 1,   ATC type fuse holder (All Electronics P/N FHP-22)
• 1,   ATC type 5A fuse (automotive blade type) (All Electronics P/N FSA-5)
• 1,   100 foot roll of 24 AWG speaker wire
• 4,   3/16" crimp type female quick disconnect terminals
• 2,   1/4" crimp type female quick disconnect terminals
• 1,   piece of angle aluminum 1/8" thick 2" by 2" by four inches long
• 1,   2 foot piece of Channel Aluminum 25/64" by 1/2" by 1/16"  Inside of channel is 1/4" to mount over the ends of the 1/4" bar aluminum.  This is sometimes referred to as trim for 1/4" plywood.
• 1,   2 foot piece of flat bar aluminum 1/4" by 3/4" if available (or 1/4" by 1")
• 1,   1" long by 1/4" center plastic standoff (1/2" outside diameter)
• 2/3,   1/4" flat washers
• 5,   1/4" lock washers (either split or internal tooth)
• 2,   1/4" internal tooth lock washer
• 1,   flat washer with just over 1" center and just over 2 1/2" outside diameter
• 1,   M6 by 40mm metric bolt
• 2,   M6 by 12mm metric bolts
• 3,   SAE 1/4"-20 by 5/8" bolts
• 2,   SAE 1/4"-20 nuts
• 1,   SAE 3/8" lock washer
• 1,   SAE 3/8"-16 to 1/4"-20 adapter
• 2,   SAE 2-56 by 1/4" long screws
• 2,   SAE 2-56 nuts
• 4,   SAE #2 lock washers
• 2,   6-32 x 1/2" screws (or possibly a little longer)
• 2,   6-32 self-locking nuts