Metal cutting is one of the most basic metal fabrication functions. It can affect part costs when it impinges on all subsequent operations such as forming, welding, finishing and painting.
Cutting solutions principally fall into two categories: mechanical, also known as cold-cutting, or hot-cutting processes.
Mechanical processes are shearing, trimming, sawing, waterjet cutting, grinding, drilling and punching. These engineered processes are very effective with well-tested parameters and limits that manage tools or tooth design, grit size, tool rotation speed and feed rate for each specific metal.
Hot processes include flame, plasma and laser cutting. These are less definitive and can possibly allow a wider range of function. Their larger operating windows can be a benefit to the skilled operator or an obstacle to the neophyte.
Principally, these hot processes produce a narrow kerf of molten metal that is blown out by an invisible gas. The invisible gas, cutting oxygen, is the tool point of the process, producing quality cut parts or ones that need significant rework. This makes understanding of the process a bit more mysterious.
Their residual heat contributes to distortion, metal growth and shrinkage, making accurately cut parts a challenge for the machine operator and supervisor. This process is sometimes referred to as “black magic,” and it can be overcome with some simple attention to detail.
This article will look at only flame-cutting part accuracy. The range of the flame-cutting process in metal, primarily steel, varies in thickness from 30 gauge to 72 in., principally found within foundry and steel mill applications. Cutting accuracy depends on the table and torch, but it’s typically within 0.062 to 0.5 in. However, these tolerances are no longer acceptable to industry and can be improved.
Metal Tip Cutting Preheat Preheat Speed IPM Kerf Width
Thickness Size Oxygen Oxygen Fuel Gas
Pressure Pressure Pressure
0.25 00 85/95 10-25 5-15 25/30 0.050
0.38 00 85/95 10-25 5-15 23/29 0.050
0.50 0 85/95 10-25 5-15 16/25 0.060
0.75 0 85/95 10-25 5-15 17/24 0.060
1.00 1 85/95 10-25 5-15 15/22 0.070
1.50 1 85/95 10-25 5-20 12/16 0.070
2.00 2 85/95 10-25 5-20 11/15 0.090
2.50 2 85/95 10-25 5-20 10/13 0.090
3.00 2 85/95 10-25 5-20 8/11 0.090
4.00 3 85/95 10-25 5-20 7/10 0.110
6.00 3 85/95 10-25 5-20 5/7 0.110
8.00 4 85/95 10-25 5-20 4/6 0.140
10.00 5 85/95 10-25 5-20 3/5 0.180
The oxy-fuel torch, with addition of iron powder injected into the flame, can cut stainless steel and aluminum. It can cut through steel, stainless steel, aluminum, concrete or rock. It is a very effective tool for punching holes and then breaking the object along a hole-pierced line.
Cutting more accurate parts with the flame cutting process
Newer cutting machines have contributed a lot to more accurate cutting. The use of dual-drive systems for larger plate produces incredibly accurate movements for the cutting table. Smaller machines might still use a single drive on only one side while the other side uses an idler.
As speed and width of plate increase, so does the need for a dual-side drive machine. The new solid-state, fiber-optic controls are excellent at repeating the same movement with accuracies of less than 0.1 mm (0.0040 in.)
However, this mechanical performance can be upset when the rails, ways or gears are dirty or spatter lands on them. These parts are better protected in today’s machines, but nothing replaces quality maintenance when looking for accurate, high performance production.
A weak link can cause poor accuracy
Heat is one of the less visible enemies of accurate flame cutting. Plate coming in from an outside area at -10 degrees F and then being heated in one spot with a 5,000 degree F flame will move. Less movement is seen when the plate is allowed time to at least warm up to 50 degrees F.
The torch is the next issue. We’ve all seen an operator use a bubble level to determine the perpendicularity of the torch to the plate. However, it’s the oxygen that does the cutting, and it’s the invisible oxygen-gas stream that needs to be accurately positioned. To get the gas perpendicular to the work, the level is placed on the brass or stainless steel tube that has a smooth design to allow the torch to be raised and lowered depending on the thickness of the cut metal. However, usually this reading is totally inaccurate.
If you look at the isometric drawing of a common torch, you’ll see the head is independent with respect to its brass or stainless-steel barrel. The barrel might be perpendicular, but the head could be several degrees out. This very common error sacrifices flame-cutting accuracy, adds unnecessary expense and can lead to hours of grinding, trimming and re-measuring to make parts fit, because they won’t be cut accurately.
Again, look at the isometric view. The barrel is attached to the valve body using two 6-32 bolts 180 degrees apart. The head might free-float compared to the other end of the barrel, or it might, on some models, be attached using two more bolts. The operator uses two 12-in. crescent wrenches to replace the cutting tip. So, maintaining the perpendicularity of the head to the plate is quickly lost.
Another issue is that some lower cost tips are not drilled square to the plate (work) plane. A customer once told me that he was buying hand-selected tips for $25 each. These tips were factory tested for perpendicularity, and that he would reject at least half of the shipment. We switched the provider to an OEM torch company that had newer drilling equipment and the tips sold for $15 each. Tip reject dropped to less than one tip per 250 tips he used.
Part squareness improved to less than 1 degree, it had nearly zero angularity, and shape accuracy improved to better than two tenths of a degree when the cutting oxygen was truly perpendicular to the plate. Additional cutting speed was achieved in multi-torch cutting, because all the torches were cutting the same thickness. In fact, a 4.5-degree angle will add 10 percent more thickness to the metal when cutting.
The efficiency improvement benefited the bottom line while reducing the lead-time for the company’s products.
It helps to know your supplier. You should ask how tip accuracy is maintained. This might disappoint your purchasing department when they find a cheaper tip and you say no to its use, but the benefits are not in the cost of your products. It’s the quality you produce.
Improve your straight cutting torch performance
Warning – these steps should be preformed by trained torch repair technicians only.
The reason for detailing these steps is to explain their significance to accurate flame cutting.
· Obtain a pillow-block bearing with a 1.375 in. inside diameter.
· Mount the bearings to a flat surface
· Slide the torch through the bearings
· Install a flame-cutting tip
· Light the torch and turn on the cutting oxygen
· The bearings will allow you to rotate the torch while lit
· Observe the position of the oxygen-cutting stream, because it’s the cutting tool point.
· If it’s perpendicular to the plate’s position, then you’re golden.
Most likely this test will indicate that your gas-cutting stream is out of alignment—not on center.
Rotating the lit torch with the cutting oxygen on will quickly indicate the amount of angularity you’ll have in the cut parts.
To repair this deficiency, mark the high side and push it in the direction you need to go.
Then retest until the gas stream is true or perpendicular to where a cutting plate would be.
Shut off the gases and remove the fuel and oxygen-gas hoses. Then purge the torch with nitrogen at about 20 to 30 cfh. This is done to ensure that oxides don’t form inside the torch while you braze the head of the torch to the barrel.
Use a 56-percent, food-grade silver for brazing. It has a low melting point. Braze only three very small spots to attach the barrelhead. Do not overheat the torch, you could cause a problem with the silver brazing done at the factory that seals and secures the oxygen and fuel-gas supply tubes. If you are unsure or untrained in doing this, have your authorized torch repair center do it for you.
Once this is complete, you will need to leak-test the torch to confirm that you didn’t cause a leak in the internal tubes. Do not overlook this step. Some individuals will submerge the torch in water, making sure that the tube connections at the base of the head are under the water to check for leaks. This would provide assurance that the factory-made brazes weren’t damaged.
Next, you might have to sand the barrel on the torch to have it fit into the bearings. Use 80- to 120-grit sandpaper for this. Once it fits into the bearings, and you’ve leak-tested the inner tubes, you can relight your torch.
Adjust your flame and turn on the oxygen. Now rotate the torch to confirm that the oxygen-cutting stream is perpendicular to the base plate.
If it is, then you’re ready to cut parts that are square. If not, remember the brazed spots are small and silver is ductile. Apply downward pressure to the high side and adjust. Recheck the squareness of the oxygen-cutting stream, and you should be ready to go.
The last detail is to confirm that the cutting table is square with the bridge. A customer explained to me recently that his cutting table was square with the cutting bridge, but the cuts were not. The frame of the table was square to the bridge, but the cutting slates that supported the plate were not. This meant that the steel was not level with respect to the bridge. There was a five-degree slope that produced a parallelogram shape that contributed to more indirect labor to produce a proper part.
Lastly, remember to confirm that the plate at rest on the support slats is level with the bridge to produce a properly square cut.
If you have questions, please call me at 262 269 6117 or email Larry_Cherne@Praxair.com. Our productivity team would be happy to help you lower your costs and improve your profit.