technique for measuring where to cut...

[imamartian]

How do you pros go on when it comes to cutting a bit of pipe to join to elbows etc, do you measure to the millimetre and include 1mm for the thickness of the elbow wall, plus another 2mm for the bit where the pipe stops...

do you see what i'm getting at? or do you guys just 'know'? or is there a rule to follow for elbows, and a different rule for straight couplers?

example - 2 runs of pipe, parallel, 300mm apart on centre. if i wanted to join the two pipes using two 90deg elbows, i wouldn't cut them in the same place, and the pipe to join them wouldn't be 300mm..

do plumbers need to be mathematicians too?

 

[ChrisR]

300mm apart, for 15mm pipe, would need 300 - 15 - about 2 to 3 mm, ie about 282-3mm.
The ends of the pipes nearly touch in the fitting, unless it's a "slow" bend for example.