Right Hand Left Hand thread ????

Ok here goes,
Can anyone tell me what is the difference between a left hand and right hand thread tap?
All I know is, if the thread on a bar is angled upwards to the right as you are looking at it, it’s a right hand thread, and left if it is angled upwards to the left.
Does it matter which (right or left tap) I buy, to make my own drive nuts.
I just can’t get my head around this.( if I tap a round bar say 40mm long and tap it with either hand tap, it shouldn’t matter , I just turn the nut round ??
The reason I am asking this is because I want to make my own anti backlash drive nuts out of derlin using trapizidol (12 x 3) lead screw, which I already have.
And if anyone can give me a few pointers on how to make these nuts I would be really grateful.
Sorry for sounding a bit thick, but hope you can help.

Quote:

---

if I tap a round bar say 40mm long and tap it with either hand tap, it shouldn’t matter , I just turn the nut round ??

---

Turning it around doesn't change it from RH to LH. You need a RH tap for a RH screw, and a LH tap for a LH screw.
Chances are about 99.9%, that you have a RH screw. RH means if you turn it clockwise into a nut, it'll go into the nut.

right hand threads tighten clockwise, left hand tighten counter clockwise.

Thanks for the info fellas. Just ordered a right hand tap so im going to have a go at making my first nut !!!! anyone know what is the correct size drill bit for a 12mm be 3mm pitch, trapizidol tap ??

good rule of thumb with metric is diameter minus pitch = tap drill size. 12-3=9mm

Here you go:Trapezoidal Coarse Thread DIN 103