Setting groove depth is one of the main goals when calibrating a lathe. For plastic cuts, we’re generally aiming for around 50–70 microns (0.05–0.07 mm). There's a few ways people do this, measuring groove depth directly with a microscope and reticle, or using cutterhead weight as a reference.
Most people we work with do use cutterhead weight in some form, so there’s nothing unusual about that. The problem is that weight only makes sense if you actually know what groove depth that weight produces on your machine. If you’re not measuring the grooves at some point, you don't know the actual cutting depth.
Once a machine is properly dialled in, there’s no issue with using weight as a reference. Where things go wrong is when people rely on weight alone during calibration or take the weight reading incorrectly. We always advise measuring groove depth rather than trusting a number on a scale.
The only weight that matters here is the effective weight of the diamond cutting into the disc, because that’s what creates the groove. Anything else isn’t relevant at this stage.
Spring scales
These are the long tube style spring scales with markings down the side. When used properly, they can give a useful reference, but they’re very easy to use wrong.
The correct way to use one is with the cutterhead lowered and the diamond touching a stationary blank disc. You hook the scale under the cutterhead and gently lift until the diamond just leaves the surface. The reading you take is the number the exact moment the diamond breaks contact with the blank, literally the very moment it raises.
What we often see instead is people lifting the head fully into the air, measuring with the head off to the side of the lathe, or not even touching a blank at all. Those readings don’t relate to cutting force and won’t tell you anything useful.
If measurements are taken right, we’ve found that on many lathes around 12g often ends up roughly in the 50 micron range, but that’s only a reference. Geometry, suspension, dashpot setup and the cutterhead itself all affect the result, so it’s not something you can blindly copy from someone else.
Digital scales
Digital scales sound like they should be the most accurate option, but we think they’re usually the hardest to use correctly.
As mentioned, any weight reading must be taken the moment the diamond breaks contact with the blank when it lifts, so to get a real reading, the scale surface needs to be at exactly the same height as the surface of your blank. That means the scale would need to be the same thickness as your discs, whether that’s 1mm, 1.5mm or 2mm. In reality, most digital scales are at least 4mm thick, even the 'stylus gauge' ones, which completely changes the diamond angle and geometry as it is not in the exact place of touching the disc and gives you a false reading.
There’s also the risk of damaging the diamond if the weighing surface is metal, and even small height differences can throw the measurement off by several grams. Because of that, we’ve not found digital scales to be a reliable way of setting cutting weight. If anyone has found digital cables the same thickness of a blank disc, we'd love to know!
Measuring groove depth
In our opinion, measuring the groove directly is the best way to do this. Using a microscope with a reticle ruler lets you actually see what you’re cutting and measure it properly.
A few microns either way can make a big difference. It affects surface noise, stylus life, and general sound. When we’re helping people troubleshoot cutting issues, incorrect groove depth comes up again and again, often because the cutterhead weight was measured inaccurately.
We already have a separate post that goes into groove depth measurement using a DIY method with a reticle (see HERE), so this isn’t meant to repeat that. This is more about addressing a common issue we see during support work, people relying on weight readings that don’t reflect what the diamond is actually doing.
Once you’ve measured your grooves and confirmed you’re in the right depth range, you can then note the corresponding cutterhead weight and use that as a reference. That works fine, as long as you know what it relates to on your own machine.