[TheForge] Oxy/Propane reprise

[Paul]

Regarding preheat flames...they are just that. Regardless of what fuel 
you are burning the preheat flame is there only to bring the metal up to 
the combustion temperature.
Try an experiment on thin sheet metal using a welding tip. Heat the edge 
of a piece of sheet metal until it reaches a nice yellow heat and 
carefully turn off the gas flow and you will be able to cut the sheet 
metal just like thicker plate using the oxygen flow alone. Some 
commercial oxygen cutting systems use only an oxygen lance alone to cut. 
You can do the same thing with a cutting tip by turning off the fuel 
flow and continue the cut after the cut has started. The cutting process 
uses the heat of combustion of the steel burning in oxygen to cut the 
metal. The actual cutting is a balance of oxygen flow, metal thickness 
and cutting rate. If you move to slowly you run out of 'fuel' (the steel 
) and the 'flame' ( the burning steel ) goes out. If you move to fast 
the 'flame' does not generate enough heat to sustain the cut and the 
'flame' goes out. Many times if you move too slowly you can re-weld the 
metal after the cut. This your pre-heat flame in action. Without the 
oxygen flow through the cutting tip the cutting torch is just a small 
'rosebud tip'.
My experience in pipe welding regarding cutting torches indicates that 
the tip that most cutting/welding outfits come with are to large. They 
are designed to cut heavy plate rather than pipe (thin walls relatively 
speaking ~.2" -.35"). Just consult the cutting charts that come with you 
equipment or look them up online to balance the plate thickness and the 
cutting tip size.
See here as an example:

http://enet.smithequipment.com/public/docs/BDBinDoc.asp?ID={FCCFBF32-5B09-431E-B58E-1271BD10BD71}&DownLoad=0

for a pdf file of tip capacities

Paul