In a perfect world we would have perfect fit up every time. However, as well all know, this isn’t the case. We mostly think of poor fit up as annoying and the cause of some delays. But the reality is it can have tremendous impact on our bottom line. Or, in many cases, compromises the integrity of the welded structure.
Poor fit up can be cause by poor workmanship, parts not cut to the right dimension, distortion during welding, volumetric changes during heating and cooling, improper or lack of fixturing, and a few other reasons. If you have bad fit up in your shop it is worth your time to investigate how to fix it. Or at least, minimize it. Here is why:
The strength of a weld is determined by the length and by the size of the throat. There is a difference between theoretical throat, effective throat and actual throat, but for now, we’ll call the throat the distance from the face of the weld to point of deeps root penetration.
When weld sizes are determined on the design phase it is done so that the weld’s strength is enough to support the load placed on it, whether it is in tension or shear. In the case of a fillet weld if you have a gap between both plates you will have to increase the size of your fillet weld by the size of the gap in order to get approximately the same throat size and thus the same strength. If you don’t know the size of your welds that are being produce get a weld gauge – using them is easy, but if you need instruction go to Proper Use of Weld Gages.
As you can see below, if a ¼” fillet is needed but there is a gap of 1/8” the new fillet size must be 3/8” in order to achieve the same strength.
The reason we need to overweld this way is because our throat is measured from the face of the weld to the point of deepest penetration at the root. The diagram below illustrates that this two welds, although significantly different in size, provide the same strength which is dictated by the throat size.
The double arrow indicates the throat. As you can see it’s about the same by overwelding.
OK, so what’s the big deal? The big deal is that a gap of just 1/8” can potentially cost you thousands of dollars. The leg size of our fillet weld went from ¼” to 3/8”. That’s just a 1/8” increment which does not seem like much. But do you know that is a 124% increase in weld metal volume? Yes, you read that correctly, that is a 124% increase in weld metal volume.
If you are not increasing your weld size as shown the throat of the weld would be reduced significantly and so would the strength of the weld. This may be enough to cause failure in the field and create a huge expense in warranty work, possible litigation and loss of contracts.
So the right this is to adjust your weld size. If your fit up is consistently bad, it can add up quickly. We like examples to illustrate points. So here we go…
Let’s say you have 120 inches of weld per part out of which 60 have this kind of bad fit up. Let’s see the difference in weld volume between these two:
Volume of 60 inches of weld on a 1/4” fillet = Cross Sectional Area x Length = 0.0313 x 60 = 1.878 cubic inches. Using the density of steel this volume equates to 0.53 pounds of weld metal
Volume of 60 inches of weld on a 3/8” fillet = Cross Sectional Area x Length = 0.0703 x 60 = 4.218 cubic inches. Using the density of steel this volume equates to 1.19 pounds of weld metal. As you can see, 124% more filler metal needed.
If you are building 100 parts per day, that bad fit up adds to (1.19 – 0.53) x 100 = 66 pound of additional weld metal needed per day. Over a year (250 working days) that’s 16,500 pounds! If you are paying $2 per pound of wire you are wasting $33,000. But don’t worry, it gets worse…
You are still welding at the same speed, since your deposition rate did not change. So to deposit more than double the amount of wire will cost you dearly in terms of labor.
Assuming a 6.5 pound per hour deposition rate, it would take you an additional 2,538 hours to weld those additional 16,500 pounds. And if your labor and overhead rate is $45/hr that comes to $114,210!!! OUCH!!!!
But finally some good news. If you have been profitable with this problem it means you can actually start saving a lot of money by controlling your fit up. Almost $150,000 in labor and material costs (and we did not include the additional shielding gas associated with the extra 2,538 hours of welding) can probably solve part or all of your fit up problem. You may need a better supplier that may cost you a bit more, but the parts will be cut to the right size. You may invest the savings in better fixtures. You may spend money in training your welders in the techniques necessary to control distortion. Or throw it at whatever the cause is.
If you are overwelding, not due to gaps and bad fit up, but simply because you don’t have weld sizes specified for your welders you may want to check our article on Cost of Overwelding.
Reference: Welding Innovation Journal, Vol. XV, No.1, 1998 – Dr. Duane Miller