K3MT
presents . . .

Ham Radio
Hints and Kinks

that I hope you find useful!


May 2, 1997
Reissued Nov. 2017
by K3MT

Tips on tapping Aluminum Holding a hex nut in a tight place
Placing washer or nut in a tight place Fastening nut to chassis
Testing bipolar transistor with ohmmeter

Tapping aluminum

Tapping aluminum can be difficult.  The tap often binds and may break off,
  both ruining the tap  and marring or ruining the piece being worked.

Here are some styles of tap I have used in the past:

  Flutes are the grooves between the rows of threads. 
  The common tap has straight flutes, and may have 
  two, three, four, or more, depending on its size.
 
  A Gunn tap has two straight flutes, and is ground 
  for production work. It cuts a curl, rather than chips, 
  and pushes it ahead of itself.  The curl 
  continues out the other side. 
  For hand work, these do well, but you need to remove 
  the curl from the bottom of a blind hole.
 
  A spiral fluted tap, readily available from machine shop 
  supply houses, has flutes that are not straight - they're spiraled! 
  Think of a common 1/4" drill.  It usually has two flutes, and it 
  is a twist drill - the flutes are spiraled like a coarse 
  screw thread. 

  A spiral fluted tap is very similar, except the high parts 
  of the drill have a fine thread cut into them. It cuts a curl that 
  enters the flutes and is pulled back toward the tap wrench.  
  I had a single 3/8-24 spiral fluted tap in a production operation 
  that tapped over 2500 holes through a 2" aluminum hex nut before 
  I ran out of work.  It was still in good shape!
 
  Gunn and spiral fluted taps are available from most any machine shop 
  supply house, and some larger hardware supply firms. Ask. 

Finger extender to hold nuts in a tight place

  Sometimes when working on electronic devices, you'll run into a situation 
  where you must put a nut on a screw, deep inside the apparatus.  All manner 
  of capacitors, PC boards, transformers, etc. are in the way: you need a 
  very long, skinny, bendable finger extension to do this.

  Here's a neat trick.  Just get some heavier gauge solder. Put the end 
  over the nut so it straddles the nut's hole.  Squeeze the solder with a plier 
  fairly tightly: some of it will extrude into the nut's hole and onto its threads.
 
  Here's a sketch of the idea.

solder holding nut

  Unroll some more solder, bend it around the way you have to, and snake it 
  through the guts of the apparatus until it is lined up over the screw hole.
 
  Now start the screw into the hole gently, and turn it gently so it starts 
  into the nut's threads.  It will push the solder back out.  You can now 
  use the end of the solder to spin the nut onto the screw until it begins 
  to snug up.
  
  Then, with a long screwdriver, wedge the nut against the chassis, and turn 
  the screw with another screwdriver until the nut is firmly seated.


How to slide a washer or nut onto a screw

  How can you put a washer and/or nut on a screw down inside a difficult spot?  
  While the screw may be easy to insert into a hole, the other side of the hole 
  may be impossible to reach with fingers or pliers.

  You can put the nut or washer in place by first turning the overall object 
  so that the screw is pointing "up."  Then slide the washer or nut, or both, 
  on some solder (or copper wire).  Pull the solder out far enough that you 
  can get it to contact the top of the screw.  

  Hold the washer and/or nut on the solder with your finger while you snake 
  the solder down into the apparatus, to touch the end of the screw.  
  Then let the nut/washer go:  they will slide down the solder and onto the screw.  

  This sketch demonstrates the idea.

sliding washer/nut onto screw

  Turn the nut with another piece of solder or a long screwdriver until it starts onto 
  the screw, then remove the solder.  Continue working the nut onto the screw until 
  it begins to snug up.  Then finish tightening the screw with a standard screwdriver.


Fastening a nut to chassis

  Sometimes when building electronic stuff your life would be easier if you could 
  thread an already drilled hole.  In metalwork, a pressed-in-metal nut - 
  a PIM - is often used at this time.

  You can roughly approximate this trick by using some Elmer's glue to hold a nut 
  onto a chassis or other sheet metal part.  

  This sketch shows the idea:

Nut fastened to chassis


Ohmmeter test for bipolar transistors

  A bipolar transistor has two internal junctions: a base-emitter and a 
  base-collector junction.
  
  In an NPN bipolar, the base-collector junction is reversed biased when the collector 
  is more positive than the base.  It behaves as if it were a diode.  Indeed, it can 
  be used as a diode!

  The base-emitter junction is normally operated forward biased.  It, too, 
  behaves as if it were a single diode.
 
  Both base-collector and base-emitter junctions can be considered as if they are diodes. 
  For NPN transistors, the base is the cathode.

  This sketch demonstrates the dual-diode nature of NPN and PNP bipolar transistors.

dual diodes of transistors

  Notice that a PNP transistor has the diodes turned around - the base is the anode 
  of both junctions.
 
  An ohmmeter connected between base and collector, or base and emitter, will show 
  if these two diodes are functional.  Just try the ohmmeter in the forward and 
  reversed biased connections on the two junctions.  If they do not behave 
  as diodes, the transistor is probably bad!

  If they do, note the polarity for low and high resistance - this will identify 
  whether the transistor is NPN or PNP.
 
  A common failure in bipolars - called punchthrough - happens when the 
  emitter contacts the collector with a short-circuit path through the thin base region.  
  An ohmmeter will detect this by showing a short from collector to emitter.
 
  Another trick for common, low-power transistors, is to connect the ohmmeter to the 
  collector and emitter, leaving the base hanging open.  Apply polarity so the collector 
  will be reverse biased (positive for NPN, negative for PNP.)

  Now wet a finger and thumb.  Hold the base and collector leads in this finger/thumb 
  clamp (don't let them touch).  This injects some current into the base.  

  If the ohmmeter resistance reading decreases, the transistor is working.  
  By holding the base and emitter leads between thumb and finger, the resistance should go 
  up.
 
  These simple tests often tell if you have a PNP or NPN transistor, and if it is good 
  or bad.


73

K3MT