Screw drive systems

Above, the Anatomy of a screw: threads, shank and head.

Words and photos: David Luckensmeyer

The anatomy of a screw comes down to variations on a theme: threads, shank and head. Traditional screw designs usually feature a single thread, shallow pitch, larger diameter shank, and often a slotted head for hand driving. More often than not, such screws are limited to countersink, raised and round head types.

Newer designs have single or twin threads often with a much higher pitch, a narrow shank and a drive solution that suits power tools. We can now choose from self-drilling tips and self-seating heads, to different thread and shank lengths. And specialty features abound, including asymmetric threads, reverse threads, shank ribbing, exotic coatings and materials, a plethora of head choices, and so forth.

What differentiates screws for me is primarily the drive system, or the connection between the screw head and the driver. Let’s run through some of the options.

Slot head drive

This is old-world tech reserved mostly for aesthetics reasons – decorative brass screws for example. Enough said.

Left to right: Phillips, Pozidriv and Square drives

Phillips drive

Named after Henry F. Phillips, Phillips drive is a crosshead or cruciform drive which has enjoyed universal use for nearly 100 years. Drivers come in many sizes (PH1, PH2, etc) but these do not fit screw heads particularly well. ‘Cam-out’ is a term used to describe drive slip when too much torque is applied.

There are numerous proprietary improvements on the Phillips drive, but the most well-known is probably Pozidriv (PZ1, PZ2, etc.). The modified geometry (marked by radial lines on each screw head) reduces cam-out and therefore increases torque transfer. A Phillips driver will fit loosely into a Pozidriv screw, but a Pozidriv driver will jam in a Phillips screw.

Lots of screw options are available these days. The Würth screw (foreground) features a self-drilling tip and self-seating head. The Spax Pacific screw (background) also sports a self-drilling tip, but the reverse thread up at the head is a highlight as this is designed to pull together components being screwed.

Square drive

Square drive (S1, S2, etc.) and Robertson drive (after P. L. Robertson; R1, R2, etc.) are often treated as identical, but the latter has a subtle taper to improve screw security and reduce slippage. Ironically, this improvement can marginally increase instances of jamming where screws jam on a driver, however having better screw security is worth the trade-off.

Less common, but increasingly more available is the Torx drive. The hexalobular design looks like a six- pointed star and uses a straight-sided driver. Drivers come in many different sizes and T20 is probably the sweet spot for furniture makers although some of my bigger screws take a T30 driver. Drive slip is virtually eliminated with this design.

Left to right: Comparing some of the drive systems available today – Phillips, Torx, AW (Würth) and T-starplus (Spax Pacific).

Torx drive

Hexalobular drive

My fave drive systems are refinements of the hexalobular drive. Torx plus drive is a big jump up in terms of a more efficient drive process: especially increases in screw security and torque transfer. But I haven’t found a convenient Australian supplier of Torx plus screws, although the security version is widely available (which has a small centre pin). Torx drivers will work for Torx plus screws, but not vice versa.

A closeup of two hexalobular drive systems by Würth (left) and Spax Pacific (right).

AW drive

For general cabinet work, my go-to screws are among the ASSY range from Würth Australia. The proprietary Würth AW drive is similar to Torx, but looks more like a round ‘gear’ than a ‘star’. The drivers are slightly tapered and in my experience the screw security is better again, with virtually no drive slip or jamming. I personally use the ASSY plus series which has a centring drilling tip. I didn’t believe such screws would work in ‘proper hardwood’ (for example blue gum) until the salesperson let me experiment with screws and drivers in my shop.

The difference between Phillips (left) and Pozidriv (right) is minor while the Square drive is easy to spot in the middle.

T-starplus drive

And finally, Spax Pacific is an Australian affiliate of Spax International, a German company with a long manufacturing history. The T-starplus drive is, in my opinion, the best system I’ve used in my shop. In particular, the screw security is outstanding, as there is an extra centre pin on the end of the driver which inserts into a small recess in the head of the screw. For applications where higher pull forces are needed, I prefer the extensive range of washer head screws. Spax screws also have a proprietary screw tip design to reduce splitting.

The hexalobular drive system has many variations based on the original star-shaped Torx drive (left). Two of my favourites are the AW drive from Würth (middle) and the T-starplus from Spax Pacific (right).

Overall, I prefer Würth screw tips and the Spax drive system – so I stock both in my shop. Würth Australia and Spax Pacific require an account, so it is not as easy as plonking down your money for a box of screws. Their products are priced on a sliding scale, with lower prices reserved for higher volume buyers. In my experience, both companies provide excellent sales support. I’d say AW and T-starplus drive screws cost approximately 20–25% more than screws with other drive systems. Give the screws a try, and then decide for yourself whether their advantages are enough to outweigh the price premium.

David Luckensmeyer is a Brisbane based woodworker and furniture maker, see www.luckensmeyer.com.au

Pictured here is a stainless steel decking screw with reverse thread, thick shank and small head designed to be buried (left); and a much smaller cabinet screw, zinc plated, with deep twin threads, and self- drilling and self-seating features (right).