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Training Articles

Solving PAC Cut Quality Problems - Plasma Cutting Small Holes and Intricate Shapes
By Dave Cook, Centricut Technical Services Director

Article originally appeared in "Welding Design & Fabrication" - September 1999

Plasma Cutting Small Holes and Intricate Shapes
Many fabrication shops spend a lot of time and money reworking parts that were cut on the plasma machine to remove dross or correct dimensional inaccuracies. Some cut quality problems are caused by mechanical and electrical problems of an old or poorly maintained cutting machine; others are related to the plasma process itself. Here we discuss the critical process variables that affect dimensional accuracy of plasma cut pieces. By carefully controlling a few variables, the operator can minimize or eliminate dimensional problems and the associated costs of secondary operations and scrap parts.

Small holes and intricate shapes: Small holes and intricate shapes such as slots, sharp corners, tight radiuses, etc. present special problems for the PAC machine operator. (For our purposes we will define a small hole as any hole less than 1.5 X the material thickness.) Not only is it more difficult to cut these shapes cleanly with conventional systems, but reworking out-of-tolerance parts is more difficult as well- reaming out undersize or non-cylindrical holes, and grinding into tight corners to remove dross isn't fun or cost effective. Many shops solve these problems by buying expensive high-tolerance cutting machines or even more expensive laser systems. But it is possible, with a well-maintained cutting machine and conventional plasma torch, to achieve near-high tolerance cuts with careful programming and a good understanding of cut quality variables.

Bolt- holes: Bolt-holes should to be cylindrical-that is the diameters of the top and bottom should be nearly equal-in order to ensure a good fit with the bolt. One critical parameter that affects cylindricity of the hole is cutting speed or velocity. Cutting speeds are entered as a lineal speed in inches per minute (ipm) or millimeters per minute (mm/min), but in a circle the torch must slow down to compensate for the natural lag of the plasma arc as it cuts. Most CNC controls automatically compensate for this phenomenon with an algorithm that factors down the velocity for holes. (Called centripetal limiting, this calculation takes into account the length of the radius, the torch acceleration, and the minimum corner speed, to adjust down the actual cutting speed around a circle.) The CNC programmer or operator may be able to adjust the lineal speed up or down to optimize actual circular cutting speed for improved cylindricity. In other words, this would mean programming different (lower) speeds for bolt-holes than for straight cuts on the same part.

Cut Height: Cut height (voltage setting) is another parameter that affects cut quality on bolt-holes. For small holes the cut height should remain constant throughout the cut. With voltage regulated torch height control (THC), the cut height is determined by an arc voltage setting, usually 100-180 volts. Depending on the responsiveness of the system, using THC for small holes may worsen rather than improve cut quality. It may be necessary to lock out the THC during cutting of small parts to prevent the torch from cutting too high or low, and to prevent diving at the end of the cut. The THC can be "locked out" by switching into manual mode after the pierce is complete, or reprogramming the part to specify corner slow down (no THC) during hole cuts. Newer, more responsive torch height controls may help with defects in cut caused by improper cut height.

Programming:
Lead-ins and Lead-outs: The type and size of lead-in and lead-out can have a significant effect on the quality of a part, particularly so with bolt-holes and slots. Two defects are common-we'll call them "divots" and "bumps." A "divot" occurs when the arc removes too much material at the end of the cut. As the plasma arc crosses the lead-in kerf (the removed material from the beginning of the cut) it transfers to the saved part, causing a small indentation or sometimes, a larger scooped out region. This makes the hole out-of-round. A "bump" occurs if the lead-in and lead-out do not overlap adequately. In this case, some of the material in the hole is not completely removed leaving a "bump" of uncut metal that prevents the hole from accepting a bolt. Finding the appropriate lead-in and lead-out to minimize divots and bumps at start and end points can be challenging. Operators can use a trial and error process to find the appropriate combination. Generally, a radiused lead-in with a very small or negative lead-out (negative overburn) to the saved part will produce the most circular hole. Sometimes a short straight lead-in works better with a small lead-out (positive overburn).

The Outward Spiral Lead-In: The outward spiral lead-in (see below) is a special type that can be very effective for hole cutting. (Note: This is different from the old "locking lead-in" used in oxyfuel cutting, which is generally not useful in plasma cutting.) The outward spiral lead-in allows the machine to reach full speed and the arc to stabilize before cutting the perimeter of a hole; it provides the smoothest machine motion throughout the cut.

Nozzle Size and Amperage: Nozzle Size and Amperage: In general a small nozzle with lower amperage and slower speed will produce a smaller kerf and a finer cut. For example with a 200 amp plasma system, the highest power (200A .086" orifice, .130 kerf) may not be suitable for cutting small bolt-holes and intricate details. Let's say you want to cut a precise ½" hole in ½" mild steel. A 100-amp nozzle with a smaller orifice (.059") and kerf width (.089"), cutting at a slower speed, will produce a much finer cut. (To get the best cut from a given nozzle always set the amperage at 95%-100% of the nozzle's rating.) The downside is reduced consumable life and slower cutting speeds; the upside is a near finished part with minimal rework.

When to use High-tolerance Plasma : There are some cutting tasks that require high-tolerance plasma. High-tolerance plasma uses a small nozzle orifice and intense gas swirl to super-constrict the arc. The result is an energy dense arc with a very narrow kerf that can be used to cut intricate details and very small holes. Conventional plasma systems can cut within .030" accuracy and produce cuts with usually 3-5 degrees of bevel angle (sometimes as low as 1 degree). By comparison, high tolerance systems on precision cutting machines can cut with accuracy of +-. 010" and 0-3 degrees of bevel angle consistently. High tolerance PAC systems can accurately cut holes as small as 3/16".

Six Rules for Cutting Bolt-Holes:

1. Use the smallest nozzle size that is rated to pierce and cut the material
2. Make sure the pierce delay allows full arc penetration before machine motion starts
3. Lock out voltage regulated THC
4. Use a radiused or spiraled lead-in a Program
5. slower cutting speed
6. Use a short or negative lead-out to the saved part