My early attempts involved squishing a bare wire between the tabs with some pliers. However, some of my crimped joints were not well attached: sometimes the pins would deform, so they didn't fit into the housing properly, and often the wire would either fall out, or I'd end up breaking some of the wire strands. I usually ended up squishing and then trying to solder them, often also melting the insulation.
This caused a recent problem where my extruder stepper was not moving - I thought it was due to the firmware, but it was actually a failing stepper connection - two of the wires had bad, intermittent connections.
Using my obsession to understand every part of my 'rap, I did some digging. Mr Nophead had an excellent video which solved my immediate problems :-
Looking a little deeper, there are a multitude of articles on the subject : this article on pinball repair has an excellent overview and good advice. The *key* point seems to be using the proper tools. Using the proper crimping tool makes a good crimp joint easy. The Society of Robots tutorial also has a clear guide, and even our own RepRap forum had a useful discussion.
Time to bite the bullet and get a proper tool. Although you can spend over a hundred pounds on a crimping tool, every manufacturer has a slightly different tool, several different sizes and styles, and most are expensive.
After a bit more research, I found that the 'cheap' (<£15) muti-size crimping tools are only useful for automobile connectors (anything with three coloured dots on it - they are too large), 'bootlace ferrules' crimpers are also not suitable (wrong shape), and the telephone/RJ45 crimping tools are special. The 'molex' and other open crimping terminal styles we use are a lot smaller, typically described using wire gauges, e.g. 22-24AWG of 24-28AWG. However, I did find one reasonably cheap well-reviewed tool - the Multicomp HT225D - available from Rapid(£20) and Farnell(£37). The correct tools have a small 'B' shaped indent, which bends the tabs back on themselves to tightly hold the wire. A properly crimped joint does not need soldering and is more than strong enough for RepRapping connections.
Also worth noting is the huge variety of crimping pins, housings, gold/brass/tin platings, and huge number of options when you go looking to buy connectors. Note that certain pins fit certain housings - you need to make sure the pins, housings, sockets all come from the same range - there are a bewildering array - just see the molex site! The two Molex ranges I found most applicable to RepRapping were the KK range (cheap and cheerful, already used for the stepper and range sensors) and the SL range. Both are available in the 2.54mm (0.1 inch) pitch that the RepRap boards use for the endstops and I use for my steppers.
I personally prefer the SL crimping terminals (16-02-0088)- they look like they will connect better (holding from both sides) rather that the KK terminals that push one side against the housing. I also picked up some 3-way and 4-way housings for the terminals. According to the Molex site, they will use the 70058 range of pins, which include the 16-02-0088.
I went for the gold-plated terminals, as the header pins on my boards are already gold-plated, and the gold-plated terminals have a lifetime of 100 cycles (plugging and unplugging) - the tin ones only have 25 cycles. I also wanted some reasonably chunky wire for the steppers, so I got some 4-way 24AWG ribbon cable, which determined the size of the crimp terminals (22-24AWG) I needed.
I also picked up some male pins (16-02-0081) - so I can make some wire-to-wire connectors. Most of my wires have awful twisted and soldered bodges where they're connected together - covered with heatshrink when I remembered to put it on before soldering, and insulation tape when I didn't. I'd like to replace my bodges with proper pluggable connections, particularly with the stepper wires.
I hope this will fix my dodgy connection issues, and also will tidy my 'rap wiring up at the same time.