Professional Guide to Sharpening Metal Lathe Cutting Tools

A lathe is a machine tool that rotates a workpiece rapidly; the cutting tool is then slowly moved into the workpiece, removing material as the piece spins against it. Lathes are a common part of professional and amateur workshops, and are highly versatile tools.

The cutting bits for a lathe, in particular a metal lathe, require occasional sharpening. Lathe tool bits aren’t like the often fragile bits found on a normal drill; their blocky shape and the overall design of the lathe means that the tool itself does much of the work, not the bit. 

However, lathe cutting bits do require sharpening or dressing up, refinishing the edges that come into contact with the workpiece and preventing them from rounding off. Bits that have lost their edge can result in chattering or poorly-finished surfaces.

This guide will give you the basic steps and equipment needed to keep your lathe tools in top shape.

Tools Needed

• Grinding wheel – the main instrument for sharpening metal tools since time immemorial. You may have an old-fashioned stone wheel, or more likely you’ll use a modern bench grinder.
• Bits to be sharpened
• Personal Protective Equipment – face shield and ear protection are vital. Avoid loose-fitting clothes.
• Grinding jig – a grinding jig is a custom-made guide for your bits, which holds them in place at the proper angle to the grinding wheel. Setting up a jig requires measuring precise angles, but once set you’ll have a ready-to-use guide for quick sharpening.

Preparation

• Secure and dress the wheel

Make sure your bench grinder is fastened securely to the floor, with no wobble or give. If you’ve used the bench grinder recently, you’ll want to dress it – removing the old bits of debris caught in the stone. This is easy to do using a diamond sharpening tool; simply turn the wheel on and draw the tool across the surface of the stone.

• Know your angles

When you sharpen a lathe tool bit, you’re really re-drawing a number of critical angles. These angles on your bit determine how quickly or aggressively the tool cuts into the workpiece and even how large the chips and shavings are. Most of the angles are slight – between 5 and 30 degrees – but you’ll need to know how they interact with each other. 

Images from metalartspress.com

• Secure your jig/test contact

Make sure your jig is tight against the guard of the bench grinder. Feed the bit forward slowly to make sure that it moves smoothly against the bench and has good contact.

Grinding

The actual grinding process is fairly straightforward, but there are a few things you need to remember.

• Grind in ten-second increments

You’ll likely see sparks immediately. That’s fine, but there’s a real risk of overheating if you go too long. Every ten seconds, cool the bit in a bucket of water.

• Grind each face separately

Another straight-forward step. Cool the bit before you move to the next face, and be careful not to over-grind any one face.

Sharpening your metal lathe tools is easy to do, but there’s a bit of knowledge required when it comes to the correct angles. Do your research, know the best angles for your tool and the appropriate metal, and use an angle jig to prevent mistakes.