The first batch of seven 1/4-20 heads. This set was made one at a time on the PS1 PCNC 1100 Tormach using the jig shown below.
Summary:
The first seven 4-40 heads. These have proven consistently more difficult to fabricate since small tolerance issues make a bigger difference in the relative geometry of the part.
For the last 6 or so years I have been happily using mcmaster thumb screws for my projects. They are good quality, come in a diverse range of shapes/sizes, and are often surprisingly inexpensive. Recently though, I’ve been looking to build a consistent visual language for my projects. I have also run into some cases where I needed lengths/shapes that MccMaster didn’t have. Which brings us to today: machining custom thumb screw heads for 1/4-20 and 4-40 socket head cap screws.
These heads are designed to thread onto existing socket head cap screws, where they can be secured with locktite, and provide a grip for hand-adjustment. They are aimed squarely at “medium polish” projects, and feature a narrow profile with unimpeded driver access to the original screw heads if needed. Perfect for adding some quick bling to a shop-made tool, or for when there isn’t much space to work with.
The manufacturing processes for these heads has two parts: First a “blank” is created consisting of an accurate OD, final bottom face, and threaded hole. That blank is then threaded onto the screw-locating jig where the head pocket, final length, grip features, and chamfers are cut. The bottom edge of each grip slot can then be de-burred with a triangular Arkansas Stone file. For my first 5 heads I made the blanks on the Tormach (for the not yet posted HSA stand project), but I have since settled on hand turning the blanks in batches before moving to the CNC. The process has generally been quite seamless. My only real regret with this project is that I didn’t design the fixture for batch-machining, because I find myself adding these to everything now!
The unit cost is very reasonable at around 0.1$ and 1$ each for the 4-40 and 1/4-20 variants respectively. Even accounting for tooling, that is still a nice savings over the commercial offerings, which run ~3$ a unit for both sizes, and come in a limited range of lengths.
Note: This article was written in 2023 and published in 2025 because I wanted to be sure I’d actually end up liking the design (I do!), before putting it out there. With that said, I’m currently iterating the design for a better hand feel, so I’m going to hold off on including design files for now.
References:
This project was heavily inspired by Clockspring, and the way that Chris’s projects all tie together with a consistent visual language. The screw-on-head design is borrowed from Inheritance machining, and I really like that extra versatility. Here are a few relevant videos if you decide to make your own custom thumb screws:
Clickspring Clamping Kit - https://www.clickspringprojects.com/clamping--workholding-kit.html
Inheritance Machining Clamping Kit - https://youtu.be/0oJDy3OVxu8?si=_LTCVdD8bQ8FikjG&t=791
Clickspring Cutter Sharpener - https://youtu.be/g7qq16ACArI?si=TFfxnHPt95NxqObR&t=407
Build Fix Create Brass Thumbscrews - https://youtu.be/OaBHStfGRrw?si=2rbOGDFoKn3MSu8x
Knobs on Projects:
Reflections:
So far I have used these thumb screw heads for three different projects (see gallery above) and have been very pleased with the results. They add some nice pop to each project, and are genuinely quite a bit easier to use than raw screws while being nice on the fingers than their 3D printed counterparts. I’ve also found they are great as temporary additions to projects, particularly for fasteners that are intended to be permanent on the finished build, but that might get turned a lot during testing (say to access an electronics compartment).
Using socket head cap screws with unmodified heads makes it much easier to manufacture batches, and provides maximum strength when used with a driver. The big trade-off is a slightly more industrial look, but for tooling projects I think that’s kind of nice, and appropriate to the style of my designs. One thing I have noticed is that there are a few cases where it would really be a lot more convenient to use set-screws (with their range of point options) to provide the thread. I don’t think I’ll end up solving that for these knobs, but something to think about for future builds.
Good Design When:
Space is tight, or a “tall and thin” look is desirable.
Precise or high torque adjustment via hex-driver is needed.
A smooth finger-friendly grip is called for.
When To Reconsider:
When special set-screw points are needed (nylon tip, cone tip, cup tip, etc) since those don’t typically come on socket head cap screws.
When very fine or high-torque adjustments must be made by hand, since the heads are still quite narrow.
Tips for Other Makers:
De-burring the vertical edges of each slot is key! I use a fillet (0.01 and 0.015), but sandpaper also works.
De-burring the bottom of each slot is less important, not everyone could tell in blind testing, but I use a triangular Akransas Stone “file” and that works well.
Thread starts are not consistent between screw manufacturers, so having a 1-2 thread deep clearance pocket is worthwhile.
When making blanks, facing to a flat surface for each blank is key. Otherwise the hole will move over time, even with repeated spotting. (At least on our makerspace lathe, I am a bit suspicious about the tailstock)
Future Work:
Look for ways to de-burr the very bottom edge of each circular plunge while still on the machine.
Design an analogous head for 1/2-13
Design 10-unit batch machining setup (perhaps 5 per for the 1/2in size).
Experiment with a curved key-seat cutter for putting a relief above the bottom edge.