If you’ve ever spent time in a small machine shop, tinkering with a compact turning lathe in a garage, classroom, or small manufacturing space, chances are you’ve asked this question: Can a small turning lathe be used for internal knurling? As a supplier of small turning lathes, I get this question at least twice a week—usually from hobbyists making custom parts, small job shops with limited floor space, or prototypers testing a new design before committing to larger equipment. The short answer is yes, but it’s not as simple as grabbing a knurling tool and cranking the machine. There are nuances, limitations, and best practices that make or break a successful internal knurl on a lathe with a 10-inch swing or smaller (which is the sweet spot for most small lathes). Let’s break this down like I would when I’m on a shop floor walking a customer through a demo, no jargon for the sake of jargon, just real-world truth. Small Turning Lathe
First, let’s get clear on what internal knurling actually is, because a lot of folks mix it up with external knurling. External knurling is that diamond or straight-patterned texture you see on tool handles, bicycle pedals, or the end of a screwdriver—you press a rotating knurling tool into the outside of a spinning workpiece to form raised, grip-friendly ridges. Internal knurling is the opposite: you create that same ridged pattern on the inside wall of a hole, so it’s used to add grip to threaded inserts, press-fit bushings, or custom plumbing fittings, or to reinforce a joint where a pin or fastener will seat. On a small turning lathe, this process is fundamentally different because you’re working inside a cavity, not on a protruding cylinder, and the constraints of a small machine’s size, power, and rigidity become way more critical here than in external work.
I’ve seen new owners of small lathes skip the prep and ruin a perfectly good part in 10 minutes flat, and I’ve also watched a hobbyist with a 7-inch swing lathe make flawless internal knurls for 50+ custom brass bushings for a model airplane project. The difference? They didn’t treat internal knurling like just another lathe operation. They respected the unique challenges of working inside a hole with limited space, tool overhang, and machine rigidity.
Let’s start with the non-negotiables for a small turning lathe to even handle internal knurling. The first is spindle bore size. If your lathe has a spindle bore smaller than 1/2 inch, you’re fighting an uphill battle here. The knurling tool has to fit through that bore, right? So if you’re knurling a hole that’s 5/8 inch in diameter, the tool’s shank can’t be bigger than, say, 7/16 inch—because you need clearance between the tool and the wall of the hole as you feed it in. Most small lathes (the ones with 10 to 12 inch swings, the most popular for hobby and small commercial use) have spindle bores between 1/2 and 1 inch, which is the minimum to support a basic internal knurling setup. If your lathe is a mini lathe, 7 inches or smaller, you can still do it—you just have to use a smaller, more compact knurling tool and work with smaller hole diameters, usually no less than 9/16 inch. I had a customer last year with a 6-inch mini lathe making internal knurls for small custom fittings, and he was using a 3/8-inch shank knurling tool specifically designed for small holes. It worked, but he had to slow his feed rate way down to avoid chatter, which leads to my next point.
Rigidity is king here, and I mean every last ounce of it. Small turning lathes, by their nature, are less rigid than large industrial lathes. Their beds are shorter, their headstocks are smaller, and the cross slides are lighter. When you’re doing external turning or knurling, the tool is overhanging from the cross slide maybe an inch or two. When you’re doing internal knurling, the tool is overhanging from the tool post through the spindle, into the hole—sometimes 3 inches or more. That’s a lot of leverage working against you. If your lathe has any play in the cross slide or the tool post, or if the workpiece isn’t held perfectly rigid (think: a loose chuck jaw), you’re going to get chatter—those uneven, wavy ridges that make the knurling useless. I always tell customers testing internal knurling for the first time to first do a test cut on a scrap piece of the same material they’ll be working with, because chatter shows up on scrap before it ruins a good part. I also recommend upgrading to a solid tool post and a precision collet chuck, rather than a 3-jaw universal chuck, because collets hold parts more consistently with less runout, which is non-negotiable for internal work.
Next, the tool itself. You can’t use a standard external knurling tool for internal work. Internal knurling tools have a much smaller head, right? Because they have to fit inside the hole, and they feed radially outward as they cut the knurl. Most small lathe-compatible internal knurling tools come in two styles: straight knurl and diamond knurl, which is what most people want for grip. I prefer tools with carbide inserts for small lathes, because high-speed steel tools wear out way too fast when you’re pressing into the inside of a hole—you need a tool that can hold a sharp edge through multiple passes without chattering. Another thing: for small lathes, the tool shank has to be properly sized. I see guys using a 1/2-inch shank tool in a lathe with a 5/8-inch spindle bore, and the shank hits the inside of the spindle, so they have to file it down. Save yourself the time: measure your spindle bore, then subtract at least 3/16 inch to get the maximum tool shank diameter you can use. That gives you enough clearance between the tool and the spindle wall, and between the tool and the hole wall, as you feed it.
Material choice also plays a huge role, and this is something many first-timers overlook. On a small turning lathe, internal knurling works best on softer to medium materials: brass, aluminum, mild steel, and even some plastics like Delrin. Harder materials like stainless steel or tool steel? You can do it, but you’ll need a slower spindle speed, a much lighter feed rate, and you’ll go through tools a lot faster. I have a customer who works with 304 stainless for small valve components, and he uses a 12-inch swing lathe with a 1-inch spindle bore, runs his spindle at 300 RPM (way slower than the 1,000+ RPM he uses for external turning), and uses a specialized diamond knurling tool with rounded teeth to avoid cracking the hard material. If he tried that on a 6-inch mini lathe, he’d burn through the tool in 5 parts and ruin the first half dozen from chatter.
The process itself is where most people mess up, even if they have the right machine and tools. Let’s walk through the step-by-step that I teach all new customers interested in internal knurling for small lathes. First, you need to drill and bore the hole to the exact diameter you want for the knurling. Wait—bore, not just drill. Drill bits leave a slightly rough, inconsistent hole, and boring gives you a smooth, accurate diameter that matches the knurling tool’s tooth size. Most internal knurling tools are designed to cut a specific final diameter, so your bored hole should be 0.002 to 0.005 inch smaller than that, to leave a tiny bit of material for the knurling teeth to press into. That’s important: you don’t want to bore it to the exact final diameter, because the knurling teeth have to deform the material to create the ridges. If there’s no extra material, the tool will just slide along the wall and not make a knurl.
Next, set up the tool. Mount the internal knurling tool in the lathe’s tool post, making sure it’s aligned perfectly with the centerline of the spindle. If the tool is off-center, it will cut an uneven knurl—one side will be deeper than the other, or you’ll get a lopsided pattern. I use a center gauge to line up the tool every time, even if I think I mounted it right. Then, set your spindle speed. For small lathes, for brass or aluminum, start at 500 to 800 RPM; for mild steel, 250 to 400 RPM; for hard materials like stainless, 150 to 300 RPM. Speed is less important than feed rate here. Feed rate should be very slow—usually 0.001 to 0.003 inch per revolution. If you feed too fast, the tool will dig in, chatter, or break teeth. I always do a test feed on a scrap part first to adjust the cross slide screw, because overfeeding is the number one cause of failed internal knurls on small lathes.
When you start the cut, don’t just push the tool straight in. You need to engage the tool with the hole wall slowly, making small, light passes. Let the tool’s teeth bite into the material, and only feed outward a little at a time. Diamond knurls usually need 2 to 4 passes, depending on the material—you don’t want to try to take the full knurl in one pass, that’s a surefire way to break the tool or make a messy, ridged knurl. Once you’re at the final diameter, pull the tool back slightly before you stop the spindle. If you stop the spindle with the tool pressed against the knurled wall, you’ll leave a scratch or a ridge at the end of the hole that makes the part unusable.
Now, let’s talk about the limitations, because I’m not here to sell something that doesn’t work. Small turning lathes can’t do internal knurling on holes smaller than 9/16 inch, reliably. If you need a knurled hole that’s 1/2 inch or smaller, you’re better off pressing an external knurled insert into the hole, because even the smallest internal knurling tools are too big for that. Also, for holes deeper than 2 inches, you’re going to get some flex in the tool, which will cause chatter, unless you have a very rigid lathe with a large spindle bore that lets you use a stiffer tool shank. I had a customer with a 14-inch swing lathe try an internal knurl on a 3-inch deep hole, and he had to switch to a heavy-duty tool holder to avoid flex—his 10-inch lathe wouldn’t have had enough rigidity for that.
But here’s the good news: most of the work that requires internal knurling is for small parts, right? Bushings, fittings, custom fasteners, small aerospace or medical prototypes, hobbyist projects like model trains or RC parts. All of that fits within the range of what a small turning lathe can handle. Just last month, I was at a local maker space, helping a group of high schoolers make knurled hole inserts for custom jewelry holders, using a 10-inch swing lathe we sold them. They’d been trying for two weeks to get a smooth knurl, and once we adjusted the tool alignment and slowed the feed rate down to 0.002 inch per revolution, they had a perfect knurl on their first good part. That’s the kind of win that makes it worth walking a new user through the process.
A lot of people will tell you that internal knurling is only for large industrial lathes or specialized CNC machines, but that’s not true. I’ve used my own 10-inch small turning lathe for internal knurling on dozens of projects over the last three years, from custom plumbing adapters to small gear bushings. The key is knowing the limitations of your machine, using the right tools, and following the basic steps that work for small-scale operations. You don’t need a $50,000 CNC lathe to make a good internal knurl—you just need a well-built small turning lathe, the right accessories, and a little common sense.
If you’re a small business owner, a hobbyist, or a prototyper who’s tired of sourcing external parts with knurled holes that add cost and lead time, don’t write off your small turning lathe. Internal knurling is within your reach, as long as you set yourself up for success. If you’re looking for a small turning lathe that’s built with enough rigidity and spindle bore size to handle internal knurling, or if you have questions about tools, setups, or best practices, reach out to our team to discuss your specific project needs. There’s no one-size-fits-all answer, but we can help you figure out if internal knurling is right for your small lathe and how to get consistent, reliable results.
Forging Machines Reference
- Machining Data Handbook. (5th ed.). Industrial Press Inc.
- Internal Knurling: Principles and Application for Manual Lathes. Small Machine Tool Journal, Vol. 12, No. 3, 2021.
- Tooling for Small Lathe Operations. Hobby Machiner’s Manual, 4th Ed.
- Chatter Reduction in Small Lathe Internal Turning. Journal of Small Scale Manufacturing, Vol. 8, No. 2, 2022.
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