How to Build a Custom Knob Mechanism for Your Hobby Machine

If you’ve ever stared at a plain, cheap knob on a 3‑D printer or a small CNC and thought “I could make this look cooler and work smoother,” you’re not alone. A good knob does more than look nice – it gives you better control, feels solid, and can even protect the machine from wear. In today’s post on Turn & Twist I’m going to walk you through a simple, step‑by‑step build for a custom knob mechanism that you can fit to almost any hobby machine. No fancy tools, no PhD in mechanical engineering – just a bit of patience and a love for tinkering.

What You’ll Need (and Why)

Before we jump into the build, let’s list the parts and tools you’ll need. I keep this list short on purpose; you can find most of these items at a local hardware store or online.

Item	Why it matters
Blank knob body (plastic or metal)	This is the part you’ll turn. Choose a size that fits your shaft.
Set screw or grub screw	Holds the knob on the shaft securely.
Small bearing (e.g., 608‑ZZ)	Gives the knob smooth rotation and reduces wear.
Spacer or washer (metal or nylon)	Keeps the bearing in place and separates the knob from the shaft.
Drill and bits (2 mm, 4 mm, 6 mm)	For making holes in the knob and shaft if needed.
Tap set (M4 or M5)	To cut threads for the set screw.
File or sandpaper	To clean up rough edges.
Loctite (optional)	Keeps the set screw from loosening over time.
Mini screwdriver	To tighten the set screw.

If you already have a knob that you like but it’s loose, you can skip the bearing and just add a set screw. The steps below cover both the simple and the “smooth‑as‑butter” versions.

Step 1 – Measure Your Shaft

First thing’s first: measure the diameter of the shaft you’ll be attaching the knob to. Use a caliper if you have one; a ruler works in a pinch but be as exact as you can. Write down the number – you’ll need it for the knob bore and the set screw size.

Turn & Twist tip: Most hobby machines use a 6 mm or 8 mm shaft. If you’re not sure, check the machine’s manual or look online for the model specs.

Step 2 – Choose or Make the Knob Body

At Turn & Twist I love re‑using things. I once turned an old coffee grinder handle into a knob for my laser cutter – it fit perfectly and looked great. You can buy a blank knob from a supplier, 3‑D print one, or even repurpose a kitchen knob.

If you’re buying, pick a knob with a bore (hole) a little larger than your shaft. You’ll later drill it down to the exact size. If you’re printing, design a simple cylinder with a flat side for the set screw.

Step 3 – Drill the Bore

Set your drill to a size a hair smaller than the shaft diameter. For a 6 mm shaft, a 5.5 mm drill works well. Drill straight down the center of the knob body. Take your time – a wobbling drill will make the knob sit crooked.

Once the hole is clean, test the fit by sliding the shaft through. It should be snug but not stuck. If it’s too tight, sand the inside a little with fine sandpaper.

Step 4 – Add the Bearing (Optional but Recommended)

If you want a smooth feel, drop a small bearing into the knob bore. Most bearings for hobby projects are 608‑type (8 mm outer, 22 mm inner, 7 mm width). The inner race fits around the shaft, the outer race sits inside the knob.

Insert the bearing – push it gently into the knob bore. If it’s a tight fit, tap it lightly with a rubber mallet.
Add a spacer – place a thin washer or spacer on the shaft, then the bearing, then another washer. This keeps the bearing centered and prevents metal‑on‑metal contact.

If you skip the bearing, just move to the next step.

Step 5 – Tap the Set‑Screw Hole

Most knobs have a small hole on the side for a set screw. If yours doesn’t, drill a 2 mm hole about 2 mm from the edge of the knob. This is where the screw will bite into the shaft.

Now you need to cut threads (tap) so the screw can tighten. Use a tap that matches the screw you have – M4 is common. Turn the tap clockwise with steady pressure, then back it out a turn every few rotations to clear chips. When you feel resistance, you’re done.

Step 6 – Assemble the Knob

Slide the knob onto the shaft – the bearing (if used) should sit on the shaft first, then the knob.
Position the set screw – line up the hole you tapped with the flat side of the shaft (if the shaft has a keyway) or just any spot if it’s round.
Insert the set screw – tighten it with a mini screwdriver. If you have Loctite, a dab on the threads will keep it from loosening later.
Test the rotation – turn the knob. It should feel firm, not wobbly, and spin smoothly if you added a bearing.

If the knob feels loose, tighten the set screw a bit more. If it’s too tight, back off a little. You’re looking for a “just right” feel – firm enough that it won’t slip, but not so tight that it grinds the shaft.

Step 7 – Finish Up

Give the knob a quick wipe with a clean cloth. If you used a plastic knob, a dab of silicone grease on the bearing can help it stay smooth over time. For metal knobs, a light oil works fine.

Now you have a custom knob that not only looks better but also works better. I installed my new knob on a small CNC last week, and the difference is night and day. The machine feels more responsive, and the knob’s design matches the rest of my workshop gear – a small win, but a satisfying one.

Common Pitfalls and How Turn & Twist Solves Them

Problem	Why it Happens	Fix
Knob wobbles after tightening	Hole too big or bearing not seated	Re‑drill a tighter bore or add a thin shim inside
Set screw strips the shaft	Using the wrong screw size or over‑tightening	Use a screw that matches the shaft material; don’t overtighten
Bearing rattles	Bearing not fully seated or spacer missing	Add a second washer or a thin spacer to lock it in place
Knob doesn’t fit the shaft	Wrong knob bore size	Sand the bore or choose a different knob body

A Little Story from Turn & Twist

When I first started messing with knobs, I tried to mount a cheap plastic knob on my 3‑D printer without a bearing. The first print I did after that was a disaster – the knob slipped mid‑print and the filament jammed. I learned the hard way that a smooth rotation matters, especially when you’re making fine adjustments. After that, I added a bearing to every knob I built. It’s a tiny change, but it saved me a lot of frustration.

Wrap‑Up

Building a custom knob mechanism is a perfect weekend project for anyone who loves hands‑on work. You get a functional part, a chance to practice basic machining skills, and a little boost to your machine’s look and feel. Keep the Turn & Twist spirit alive: take something ordinary, give it a twist, and make it yours.

Happy building!