Projection welding:
Modified equipment to provide rapid response of loading system.
Large flat electrodes permit multiple projection welds being done simultaneously. Location of welds determined by that of projections.
However projections are an additional forming operation.
Spot welding requires specific size and shape electrodes to localize current for welding. Location must be assured by external means.
Cost comparison can be done on specific applications.
Even though I understand that Projection weld need an additional forming process on sheetmetal which directly increases cost. I would like to know your opinion in the below case.
If I take 2 sheetmetal brackets of 2.6mm thickness and need to weld them at 6 places on flat surface. which one will cost me more. spot weld or projection weld.
You did not specify the metal, you did not specify if there is room from both sides (the flat surface may be very large), and you did not specify the quantity for production.
If you ask for deciding on which equipment to purchase,
specify all the above, add a manufacturing drawing and send Requests for Quotations to a number of shops.
Different job shops will give you their offers.
Then you will have the answer you seek.