- Materials & Industrial Technology
Tips For Welding And Fabrication - Maintenance Cruz
To Weld Or Not To Weld, That Is The Question
It seams when technicians have to make a choice between welding part together, they most always choose welding. It is not hard to understand, much faster and a lot less work to join parts with a weld bead. Welding definitely has its place but, many project would benefit from the extra effort in bolted connections. A full welded frame installed in a high vibration application will usually fail at some point, bolting alleviates high stress in the welds. A bolted connection is stronger than a weld connection.
The other factor to consider is future maintenance, modifications, and removal of a fabricated form. If a frame is completely welded, modification becomes involved and time consuming. With a bolted frame, if you wish to modify you can fabricate a new section that can be bolted into the existing mounts. All the construction would take place in the shop and install is reduced to a short duration operation, unbolt, remove, install, bolt together. The other benefit is that welding equipment isn't required at the install site which reduces or eliminates any chance of fire.
There are definite benefits in choosing a bolted assembly but, it is hard to convince personnel to take the long road. Maintenance technicians want to take the path of least resistance and also just love to melt steel. Its funny how much emphasis is put on welding and welding skills in a maintenance department. Granted, it is preferred that your people have good to great welding skills but, I have always valued machining and electrical skills more. I can teach an electrician how to weld in a relatively short amount of time, the same isn't true in reverse.
An assembly that you can foresee being modified in the future is a good candidate for bolted assemble. Prototype equipment is also a good place to use bolted connections. Your prototype is ready for test at start up it becomes apparent that the main rails need to be shortened, pull the through bolts, trim the rails, drill, reassemble, try the test again. Same prototype, break out the cutting disks and cut out the rails, sand the connection points smooth, trim the rails, align and clamp the rails, tack, check level, adjust, re-tack, recheck level, weld, retest. Much more time is consumed trying to modify the welded frame and time is money. You can even use bolted connection during prototype and then choose to weld after successful testing.
I prefer assembly that are removable without having cut through welds. You add a feature in 2000, another in 2004, then another in 2009, now you need to remove the first unit but, it is now surrounded by permanently welded assemblies. You now can either cut out everything in the way or cut the unit to be removed into pieces for removal, okay, if you didn't want to save it. This is all just food for thought and just look around your site and you should find items that really should have been bolted in rather than welded. You may have had the exact experiences that I have had, running into situations where if bolting was chosen the whole operation would be easy, instead it is a major problem requiring more people and man hours.
Clean, Gap, And Jig
The quality of the setup before welding is key to the outcome. It is very common for a technician to cut some steel, throw it on a bench, butt the pieces together, tack, check square and weld. Well, a few steps are missing from that operation, first was cleaning and beveling. Cleaning the scale off of steel before applying weld is key to producing the strongest weld. Beveling also increases weld quality through increased penetration. An air gap between the joint is paramount to full penetration, mostly technicians butt pieces together and fire weld on the surface of the parts.
If you already have welding training this is no new news to you but, I wanted to start at the lowest level of shop mistakes I see. Many shops do not really have a "Welder" actually employed, a person who has actually completed welding certification courses. Many shops have maintenance technicians who have training in repair, but no official training in welding. Very common for technicians to have picked up basic welding skills in the shop through trial and error. This would be a great place to start for a maintenance manager, schedule a welding teacher to come to your site and give your people a crash course, well worth the money.
C clamps, right angle jigs, vice clamps, jigging table, levels, and squares are the primary tools of welders, not the weld machine. Top notch welders are spending the majority of their time cleaning/beveling material and jigging/clamping material square. This where training should begin for technicians, understanding that each joint needs to be cleaned/beveled before hand. That jigging and clamping is required to get square results. Tacking material together first and adjusting for "pull".
A technician is tacking a square tube to another square tube, he lays the parts on a table and tacks the tube at the corner. He takes a look after the tack and not only is material pulled way out of square it is also now twisted. Understanding how the weld is going to pull the material as it shrinks the steel is somewhat of an art, but is also pretty predictable.
Tacking two parts together with a 1/32th air gap between the parts allows for easy adjustment to square before the second tack. The air gap is essential and will give you those beautiful welds you have always wished you could run. The weld will have full penetration to the inside of the tubing and will leave a nice smooth bead. Un-beveled, un-gapped welds are the most common bad welding practices I see in maintenance. One welder I knew used copper pennies all the time for gapping weld joints evenly, works great and the copper won't weld to your work.
Heat And Motion
There is a huge disparity with welding direction, motion, and settings. You can get five different answers from five different people. You start to wonder if anyone actually knows the true answer to these questions. There are exact specifics on how much heat, proper stringer motion, and which direction you should be welding in. These are the things they teach you in welding class, but with so many self taught welders out there, this information has become corrupt. "That is the way I was taught" is the standard phrase you here. It is really difficult to tell a person "Well that's great, but you were taught wrong." You can only overcome someones beliefs with credible data, in other words, they are not going to believe you until you put a welding manual in front of them.
It always surprises people when they actually look at a welding manual, as they are not prepared for how much there is to learn. It just isn't as simple as turning on the juice and commence burning. The American Welding Society has a complete set of books that cover just about everything under the sun that has to do with welding. I would recommend acquiring a set of these books for your shop and make them available to your technicians. You can, over time, present the chapters to your people in meetings or as monthly tech tips. This is a great way to get knowledge to your people without having to send them to welding school, although there is no substitute for schooling.
Annealing And Polish
Polishing your welds after they cool is a rust prevention method. When you weld steel at such high temperatures oxygen is drawn to the bead. Wire brushing your welds will remove the accelerated micro layer of corrosion that has begun. You can see rust immediately after you have finished a weld. If you do not remove this corrosion layer and just apply paint, the cancer will be working on the weld joint under the paint. Eventually the corrosion will make its way through the paint and the exposure to the air will accelerate the decay.
Stress relieving or annealing welds is a crucial ,but mostly neglected step in welding. When a weld is applied the end result is a highly stressed area in and around the weld. The high stress is subject to stress cracking, especially in either high vibration or high torque applications. If you are just building a stand or a table then stress relieving is not really necessary ,but if you are making a frame that is going to constantly absorb impact then stress relieving is an absolute must.
The simplest yet still effective method is hammering a weld as it is cooling. This basically shakes the molecules loose and relieves the contraction stress. Alternatively, you can reheat the weld with a torch to the point were the molecules are able flow and then slowly remove the heat allowing the area to cool very slowly. A pneumatic peen or scaler is a great quick tool to stress relive a weld adequately. This, also, is another area where you need to make a choice between a welded connection or a bolted one. If there is a high likelihood that a weld will crack at some point due to the nature of the stresses applied, maybe a bolted connection is a better choice. Your standard hand tools like end wrenches are heated and quenched at first to harden the steel then reheated and cooled slowly the regain flexibility reducing brittleness. This is the same thing with welding, naturally the area is going to harden from the extreme heat and the relatively quick cooling period. Annealing a weld area will return some of the steels ductile properties giving the weld the ability to have some degree of flexibility.
The Classic Over Build
Many shop fabricated jobs come out so over engineered it leaves you thinking a tank couldn't destroy it. This is where an engineer is very helpful because most technicians do not really know how much load a structure can actually handle. Most shop fabricated assemblies could be made with materials of half the thickness and much less structure. Its like making a bridge out of toothpicks, the strength is mostly in the structure and not the material.
A good place to make improvements is to understand what is under compression and what is under tension. Compression members generally carry the bulk of the load while tension members usually keep the structure from deflection. This isn't always true, like a suspension bridge where the cables are under tension carrying the load and transferring the load to the compression members or piles.
A good example is a square frame with diagonal cross braces. The frame rails are substantial, but the diagonal braces are just small round rod. The small rod round will have very low compression strength ,but they will have very good tension strength. When the frame tries to pull to one side or the other, one rod will control the deflection through the tension from one corner to the other while the other rod will be under compression and not really contribute. The whole structure is very sound and the materials are at a minimum to still give great strength.
There are also books out there that show good structural forms and assemblies to help give your technicians better ideas. Taking some time in the design phase to really assess the structure you intend to build can help reduce wasted materials and over building. Many over builds occur strictly do to the available materials. People get excited about building something and would rather start to build then wait for the proper materials to be delivered. You wind up having everything being built out of 1/4" thick angle iron, a frame that is holding up a 40 lb container.