When a $90 carbide end mill chips or loses its edge, the temptation to re-grind it is strong. Carbide is expensive, lead times on specialty tools can stretch to weeks, and the damaged tool is sitting right there looking like it just needs a touch-up. But whether re-gashing is worthwhile depends entirely on the tool, the damage, and your equipment. We have re-ground hundreds of tools in-house over the years, and we have also sent hundreds out to professional services and scrapped hundreds more. Here is how we make that decision, honestly and without romanticizing the DIY approach.
Equipment for DIY Re-Gashing
At minimum, you need a diamond grinding wheel — we recommend a D64 resin bond wheel for carbide, which provides the right balance of cut rate and surface finish on the gash face. You need a tool-and-cutter grinder with angular adjustment capability, or at the very least a 5C collet fixture mounted on a surface grinder with a tilt mechanism. You need a loupe or digital microscope (40x minimum, 100x preferred) for inspecting the gash geometry before and after grinding. And you need steady hands, patience, and the willingness to sacrifice a few tools learning the technique before you trust yourself on expensive ones.
The upfront investment for a basic re-gashing setup runs $800-$1,500. A decent benchtop tool-and-cutter grinder accounts for most of that. The diamond wheel runs $80-$150 depending on diameter and bond type. A USB digital microscope with adequate magnification is $100-$200. If you are re-grinding fewer than 20 tools per month, the payback period stretches beyond a year, at which point the economics start favoring professional services.
Step-by-Step Re-Gashing Process
Here is the process we follow for a standard 4-flute carbide end mill with edge chipping:
Inspect the damage. Before touching the grinder, examine the tool under magnification. Identify exactly where the damage is — which flutes, how deep the chips extend, whether the damage reaches into the core web. Measure the remaining cutting edge with a micrometer. If the damage extends more than 0.3-0.5 mm back from the end face, you are likely looking at a significant regrind that changes the tool length and potentially the geometry enough to affect performance.
Mount in the collet. Secure the tool in a 5C or ER collet with minimal stickout to reduce vibration during grinding. The tool must be concentric — any runout in the fixture will produce an uneven gash. We check runout with a test indicator and reject any setup over 5 micrometers TIR.
Set the gash angle. Rotate the fixture to match the original gash angle. On most standard end mills, the primary gash angle is 45-55 degrees. If you have the manufacturer’s specification sheet, use it. If not, measure the existing gash on an undamaged flute with a protractor or optical comparator and match it.
Grind with light passes. Bring the diamond wheel into contact and take passes of 0.01-0.02 mm depth. This is not a roughing operation — aggressive cuts on a micro-geometry like a gash will crack the carbide or create subsurface damage that causes premature failure in service. We typically take 5-10 passes per flute, checking under the microscope after every 2-3 passes.
Check under magnification. After each flute is ground, inspect the gash face for uniform surface finish, the edge for sharpness (no visible radius at 100x), and the transition between the gash and the flute for any cracks or chips introduced by the grinding process.
Grind the relief angle. The primary relief behind the cutting edge must be restored after re-gashing. Typical primary relief angles are 6-10 degrees on carbide end mills. This step is critical — without adequate relief, the tool will rub rather than cut and will fail quickly from excessive heat.
Test cut. Run a test cut in scrap material at conservative parameters (50-60% of your normal feed rate) and inspect the result. Check surface finish, chip form, and listen for any abnormal vibration. If the tool performs well at conservative parameters, gradually increase to normal cutting data.
When DIY Makes Sense
Simple end mills with 2 or 4 flutes, 6 mm diameter and above, with only flank wear or minor edge chipping are good candidates for in-house re-gashing. At these sizes, the geometry is large enough to see and control without specialized CNC equipment. A practiced operator can re-grind the end face, re-cut the gash geometry, and apply relief angles in about 15-20 minutes. If you are running a job shop with a steady stream of worn standard end mills, the economics work out.
When to Use a Professional Service
Tools with complex gash geometries — such as variable-pitch or variable-helix designs, chip-breaker features, or proprietary rake angle profiles — should go to a professional regrinding service. Tools below 4 mm in diameter should also go out, because the geometry is too small to control reliably with manual equipment. Professional regrinding shops run multi-axis CNC grinders with sub-micron repeatability and use the original tool manufacturer’s geometry data to restore the exact profile within specification.
Professional regrinding typically costs 30-50% of a new tool’s price and delivers 85-95% of original tool life when done well. On an $85 end mill, that means paying $25-$42 for a tool that performs nearly as well as new. The turnaround time is usually 3-7 business days for standard tools, though rush services are available at a premium. We keep a rotation of tools going out and coming back so that we always have reground tools on the shelf when a fresh tool wears out.
When to Scrap the Tool
Some damage is beyond recovery. If the tool has cracked through the core web — visible as a line running perpendicular to the flute on the end face — no amount of regrinding will restore structural integrity. The tool will snap in service. If you see cobalt leaching, which appears as grey or dark discoloration along the flutes (the cobalt binder has been chemically attacked, usually by aggressive coolant chemistry), the substrate is permanently weakened. If the tool shows heat tint — that blue-purple discoloration from thermal overload — extending past the cutting edge and into the body of the tool, the carbide microstructure has been altered and hardness is compromised.
In these cases, put the tool in the carbide recycling bin. Scrap carbide has real value — typically $8-$12 per kilogram depending on market prices. We collect scrap tools in a dedicated container and sell to a recycler quarterly. It is not a profit center, but it offsets the sting of writing off a $90 tool.
The Cost Comparison
For a standard $85 4-flute carbide end mill: DIY re-gashing costs roughly $5-$8 in consumables (wheel wear, electricity, collet wear) plus 15-20 minutes of labor. Professional regrinding costs $25-$42 with higher quality assurance. A new tool costs $85. When the math favors DIY, do it. When the geometry is complex or the tool is small, send it out. When the tool is damaged beyond safe recovery, recycle it and move on. The worst outcome is a poorly reground tool that fails in the cut and crashes into a $15,000 workpiece. Knowing when not to re-gash is just as valuable as knowing how.