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Straight answers on choosing, fitting and looking after the torch consumables for a 45 amp class plasma cutter.
Some torches are designed to accept the consumables of a smaller model in the same family, and parts rated for a lower current are often used when cutting thin metal at lower settings. That only works where the maker says the parts are compatible, so check the manual or consumable chart before mixing.Parts rated below the current you are actually cutting at will burn out quickly, and the reverse is also poor practice. Set the machine to a current that suits the consumables you have fitted, and use one matching set rather than odd parts from different current ratings.
On a mechanised torch, a height controller needs to know where the plate surface is. An ohmic-sensing cap is a retaining cap designed with a contact point for contact-based height sensing, so the controller can detect when the torch touches the plate and then set the pierce and cutting height from that point.It is only for machine setups that use that method of height finding. A hand cutting job does not need one, and an ordinary cap is the right part there. Check the maker's chart and your height control documentation to confirm which cap suits your torch and system before ordering.
Many torches have a parts-in-place switch that stops the arc starting unless the retaining cap is fitted and seated. If you have just changed parts, check that the parts underneath are seated and in the right order, that the cap is on firmly by hand but not overtightened (a cap screwed on too tightly can also stop the torch firing), and that dirt or debris is not stopping the cap sitting down.If it still will not fire, check the manual for the fault indication on your machine. Never bypass or jumper the interlock, because it exists to stop the torch firing with the parts removed.
Fit the cap by hand until it is snug and seated, and do not use a tool or force it. Overtightening can damage the consumables or stop them working properly, and it can strain the threads and the parts inside the torch head. Cross-threading is the other common mistake, so start the thread gently and make sure it turns freely before it firms up.If the cap will not seat without force, take it off and check that every part is fitted correctly and that the threads are clean. Your manual describes the correct fitting for your torch.