HOW MUCH WIRE? HOW MUCH GAS?

I am frequently asked how to calculate the estimated welding costs on jobs being quoted. By using some simple math, we can make the calculations necessary to get a very accurate estimation on the amount of welding wire and shielding gas needed to complete the job.

For example, let’s assume a shop just got a new job to quote that includes welding 5,000 ft of 1/4 in fillet welds. How much welding wire do they need to buy and how many gas cylinders will they go through? Before answering these questions, notice that the way this question is phrased tells us that this shop has already figured out two important variables: the size of the weld and how many feet of weld are in the job. Also, they are considering the cost of shielding gas in the quote, which is very smart.

Weight of weld metal per foot can be calculated, or the values for fillet welds can be found, in Table 1 nearby. These values are taken from Table 12-1 in The Procedure Handbook of Arc Welding by the Lincoln Electric Company (Cleveland, OH). The calculation for the pounds of welding wire needed for a job is as follows:

Pounds of welding wire needed = (weight of weld metal per foot) x (feet of weld for job)

Sample Calculation #1

Pounds of welding wire needed for 5,000 ft of ¼ in fillet weld with flat weld profile (using values from Table 1).
Pounds of solid welding wire needed = (0.129 lb/ft) x (5000 ft) ÷ 0.85 = 645 lb of solid wire
Pounds of cored welding wire needed = (0.129 lb/ft) x (5000 ft) = 759 lb of solid wire

Knowing how many pounds of welding wire are needed for a welding job allows you to compare the cost of different sizes and types of welding wire. Welding wire is bought by the pound and the price per pound varies according to the type of welding wire (solid or cored), the diameter of the wire, and the type of packaging (size of spools, drums, boxes, etc.). So an accurate quote can be generated based on the shop’s desired welding process (e.g., wire type, wire diameter and the type of packaging.)

Shielding gas use can be calculated based on the flow rate (SCFH) used during production and the hours of welding needed to complete the job as follows:

Cubic feet of gas needed = (flow rate SCFH) x (hours of welding)

Sample Calculation #2

Cubic feet of shielding gas needed = (40 SCFH) x (100 hours of welding) = 4,000 CF

A typical “E” size cylinder of 75Ar/25CO₂ shielding gas contains a little over 300 cu ft of gas. Therefore, you need to purchase about 13 gas cylinders for the welding job. The trick is to figure out how many welding hours are in a job. This is best calculated by using deposition rate:

Deposition Rate (lb/hr) = 13.1 x (wire diameter)² x (wire feed speed) x (efficiency)
— Wire diameter in inches (in)
— Wire feed speed in inches per minute (in/min)
— Efficiency (1.0 for solid wire, 0.85 for cored wire
— This calculation is for steel only

Example: Wire diameter = 0.045 in (1.2 mm) solid wire, wire feed speed = 300 in/min
Deposition rate = 13.1 x (0.045)² x (300) x (1.0) = 7.96 lb/hr

Sample Calculation #3

Cubic feet of shielding gas needed for 5,000 ft of ¼ in fillet weld using 0.45 in solid wire, at a deposition rate = 7.96 lbs/hr and a typical shielding gas flow rate of 40 SCFH.

From the first calculation, we know that we need 645 lb of solid wire for 5,000 ft of 1/4 in weld, therefore:

Hours of welding for the job = 645 lb ÷ 7.96 lb/hr = 81 hours of welding

Cubic feet of shielding gas needed = (40 SCFH) x (81 hours of welding) = 3240 CF

You will need to plan for about eleven E-sized shielding gas cylinders for this welding job. Again, the type of shielding gas mix (75Ar/25CO₂, 100 percent CO₂ etc.) determines the cost of gas as well as the packaging. Knowing how many cubic feet of shielding gas will help you get the best price and service from your gas distributor.

The goal of this article is to help shops prepare accurate welding quotes using a few calculations. However going through the math and generating the numbers will also help you manage your operations for profitability. From these calculations, we know that welding at lower deposition rates and higher flow rates will cost you time and money. I’d like to thank Regis Geisler from Lincoln Electric for inspiring this article based on his passion for helping welding shops become more profitable.

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Kevin Beardsley, a registered professional engineer with 21 years of manufacturing experience, is an application engineer at Lincoln Electric , 22801 St. Clair Avenue, Cleveland, OH 44117-1199, www.lincolnelectric.com. For questions or comments on this column, contact Kevin at 216-383-2259 or kevin_beardsley@lincolnelectric.com.

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