DFM / Waterjet Cutting

Design for Waterjet Cutting: How to Get the Best Results from Thick and Complex Parts

Why Design Specifically for Waterjet?

Waterjet is not a universal substitute for other cutting processes, and it’s not one-size-fits-all for the parts that do call for it. Designing a part well for waterjet is different from designing it for laser or plasma. The physics are different. The constraints are different. An engineer who understands those constraints can produce a drawing that runs cleanly on a waterjet table with minimal operator intervention and tight, predictable results.

The basic mechanics: a waterjet machine pumps water to pressures ranging from 60,000 to 90,000 psi and forces it through a small orifice to create a high-velocity stream. That stream picks up abrasive garnet in the mixing tube and exits at the nozzle. The abrasive stream erodes the material along the programmed path. The cut is cold: no arc, no flame, no laser optics, no phase change in the base material. The metal you get back from waterjet has the same hardness and metallurgical properties as what you put in.

That cold cut is the primary reason to choose waterjet for certain parts. Hardened tool steel, pre-heat-treated aluminum, titanium, laminated composites, and thick stainless all benefit from the absence of a heat-affected zone (HAZ). For parts that will see precise secondary operations after cutting, a clean HAZ-free edge is a meaningful advantage. The edge requires no scale removal, no rework of hardness changes, and no post-cut stress relief.

The tradeoff is that waterjet is generally slower than laser for thin sheet and has inherent geometry constraints that affect feature size and corner sharpness. Understanding these before you send your file to the shop saves everyone time. See our broader overview of what waterjet cutting is and how it works if you’re starting from scratch on the process.

Tolerances You Can Expect from Waterjet Cutting

Published tolerance ranges for waterjet are sometimes misleadingly optimistic. The tightest achievable number, often cited as plus or minus 0.003 to 0.005 inch, applies to thin material cut slowly on a rigid, well-calibrated machine with fresh abrasive at a controlled pressure. Production conditions involve tradeoffs. Cutting thick plate at competitive speeds widens the achievable tolerance band. Long cuts accumulate any table movement errors along the full path length. Parts near the edge of a large table may see slightly different cutting dynamics than parts cut near center.

Realistic expectations for production waterjet work on structural steel and aluminum: plus or minus 0.010 inch is achievable and common on parts up to about 1 inch thick. At 2 to 3 inches of thickness, plus or minus 0.015 to 0.020 inch is a more honest estimate for hole positions and profile dimensions. If your design has a feature that needs to hold plus or minus 0.005 inch in 2-inch-thick steel, waterjet is probably not the right primary process for that feature. Plan for a secondary machining operation to bring it to final size.

Taper is a related issue. The waterjet stream is not perfectly perpendicular to the material surface. At high cutting speeds or on very thick sections, the stream lags slightly at the bottom of the cut, creating a small draft angle on the edge. Most shops compensate by programming a taper correction angle, but some taper can remain. For precision assembly, check the edge geometry requirements on mating surfaces and discuss with the shop whether taper compensation is in their standard practice.

Our guide to sheet metal fabrication tolerances covers the broader tolerance picture across cutting and forming processes.

Minimum Feature Sizes and Internal Corners

Every internal corner in a waterjet part has a radius. That radius is non-negotiable: it’s the physical consequence of cutting with a round stream. The kerf width for most production abrasive waterjet work runs about 0.035 to 0.055 inch, so the minimum internal corner radius you’ll get is approximately half that, or roughly 0.018 to 0.028 inch. In practice, most shops describe this as a 0.03 to 0.06-inch radius minimum on internal corners.

If your part design requires a true sharp internal corner, waterjet cannot deliver it. Options include: accepting the small radius if your application tolerates it, designing a corner relief hole (a small drilled or pierced circle that prevents the radius from affecting the mating geometry), or planning a secondary milling operation to square up critical corners. For many parts, the waterjet radius is small enough that it doesn’t affect function at all. For press-fit locations or precision slots, it matters.

Minimum slot widths follow the same logic. A slot narrower than the kerf width can’t be cut at all. A slot only slightly wider than the kerf gives the stream no room to maneuver and produces poor edge quality on both walls. As a general rule, slots should be at least twice the kerf width, which means a minimum useful slot width of about 0.080 inch. Narrow slots in thick material are particularly challenging: the stream can deflect off the walls, the abrasive can bridge across, and the cut becomes unpredictable.

Web thickness between adjacent cutouts follows a similar minimum. A thin web flexes as the stream cuts alongside it. Once it gets thin enough, the cutting pressure can bow it, break it, or create a vibration that corrupts the edge finish on both cuts. The practical minimum web thickness for most waterjet work is 0.125 inch, and thicker material generally needs wider webs. If your design has webs narrower than that, discuss the geometry with your shop before sending the file.

Tab and Bridge Techniques for Complex Cutouts

On a waterjet table, parts are submerged or floating in the cutting tank on a slatted support grate. When the machine cuts a closed contour, the finished part separates from the surrounding skeleton. If that part is small, it can tip into the gap between slats and catch the cutting stream on the next pass, ruining the edge and sometimes damaging the nozzle. Large plates can shift as internal material is removed, changing the actual cut position relative to the program.

Tabs solve this problem by leaving a small bridge of uncut material connecting the part to the surrounding stock. The width of a tab is typically 0.060 to 0.150 inch, wide enough to hold the part but narrow enough to snap cleanly with hand pressure or to grind flush in a few seconds. Where tabs fall affects part quality: a tab on a critical edge profile leaves a witness mark that needs dressing, while a tab on an interior cutout or an edge that will be secondary-machined is invisible in the finished part. Flag critical profile edges on your drawing if tab marks are unacceptable there.

On complex nested parts, the programming operator typically decides tab placement based on part geometry and nesting density. You can specify preferred or prohibited tab locations in your drawing notes or in a DFM conversation with the shop. For repeat production parts where consistency matters, specifying tab locations ensures the same finishing steps apply every run. For prototype or low-quantity work, leaving it to the operator’s judgment is usually fine.

Bridges between nested parts serve a related purpose: they’re wider spaces in the nest that keep parts from interfering with each other’s cut paths. The minimum bridge width is typically the kerf plus clearance for any taper at the edge, so roughly 0.060 to 0.100 inch at minimum. On thick material, wider bridges may be needed to prevent the stream from undercutting an adjacent part’s edge.

File Formats and Drawing Requirements

The file quality you send directly affects the time the shop spends programming the machine, and sometimes the quality of the finished part. A clean file programs in minutes. A file with geometry issues takes an hour to fix before programming even starts, and if issues are missed, they show up in cut parts. Sending a good file is the fastest and cheapest thing you can do to improve your waterjet results.

DXF is the universal exchange format for waterjet programming. Nearly every waterjet CAM software reads DXF. DWG works too in most shops, but DXF is safer for compatibility across software versions. STEP and IGES are for 3D solids: they’re useful for reference or for multi-axis waterjet operations, but for standard 2D flat cutting you want a 2D DXF of the cut profiles.

What makes a DXF clean for waterjet? Closed polylines with no gaps: every cut contour must close completely or the CAM software can’t generate a valid tool path. Correct units: if your design is in inches, save the DXF in inches and say so in the notes. A DXF that’s actually millimeters but opens in a CAD system defaulting to inches produces a part 25.4 times too small, and this mistake happens more often than it should. No overlapping or duplicate lines: a line drawn twice in the same place causes the machine to cut the path twice, leaving a wider kerf on the second pass. All geometry on one layer or a minimal organized set of layers: a DXF with 40 layers named in a system only the original CAD package understands slows down programming.

Include a dimensioned PDF or drawing alongside the DXF. The drawing is your official record of what the part should be. If there’s ever a discrepancy between the DXF and the drawing, the drawing governs. Put key dimensions, tolerances tighter than default, any inspection requirements, and material callout on the drawing. If you have features that require a specific orientation on the cut sheet (grain direction for visual finish on aluminum, for example), note that too.

When to Choose Waterjet Over Laser

The choice between waterjet and laser isn’t always obvious, and the right answer depends on your part’s material, thickness, geometry, and tolerance requirements. Here’s a practical breakdown for common decision points.

Thickness is the most common deciding factor. Laser cutting of structural steel in a production shop typically tops out around 3/4 to 1 inch for mild steel on high-power fiber systems. Waterjet handles 2, 3, 4-inch plate regularly, with some systems going considerably thicker at reduced speed. If your drawing calls out 1-1/2-inch A36 plate with close-tolerance profiles, that’s a waterjet job.

Material type is the second deciding factor. Copper and brass reflect CO2 laser energy and require fiber laser. Hardened tool steel can absorb enough heat from laser to create a hardened re-cast layer at the cut edge that complicates secondary operations. Composites and laminates (carbon fiber, fiberglass, Kevlar) can delaminate from laser heat. All of these are clean waterjet cuts. For reflective or heat-sensitive materials, waterjet is usually the right call.

Speed and cost favor laser for thin sheet. A fiber laser cuts 14-gauge mild steel at ten times the speed of waterjet, which means lower per-part cost when quantity is high. If you’re cutting thousands of brackets from 0.120-inch sheet, laser is almost certainly more economical. If you’re cutting a dozen mounting plates from 2-inch stainless, waterjet is the answer.

Detailed comparisons of these processes are available in our guide on laser cutting vs. waterjet cutting and our article on waterjet vs. plasma cutting. If you’re not sure which process fits your part, send us the drawing. We’ll tell you which process makes the most sense and why, before you commit to a quote.