Triangular Screws

I ordered some cast aluminium 'project box' enclosures from RS. Each one came with four screws and has four plain holes cast into each corner for attaching the base.

I couldn't find a matching tap, but though screws are tough to screw in, they could be forced to cut their own thread.

I noticed the end looked a bit odd and assumed they were slightly tapered. Just fiddling with a second box I looked more closely and the screws have they cross-section of a rounded triangle. This explained why they gave me a diameter to small for the M4 I had guessed them to be.

In effect, they are 'thread forming taps'. The sharp topped thread follows the rounded outline and looks machine cut rather than rolled. I wonder if they are cut round and 'squeezed' to profile, are cut with some sort of cam device, ground or some other method.

Has anyone else come across these fixings? I can see them having uses where a removable fixing and convenience is more important than great strength - like fitting lids on boxes!

Neil

Hi Neil,

Try looking up Taptite screws - first came across them in the70's

Keith

Tri-lobular or Taptite is what you have.

Yep, being using them for years, mainly to fit multi-pin automotive connectors. The main advantage is that the holes doesn't need to be pre-threaded but can be as moulded. Optimum number of lobes is five, but three will do.

Andrew

Apparently you can also get pentalobular screws. Wikipedia article which introduces the wonderful new (to me) word - lobulation.

John

Andrew beat me to it

Edited By Journeyman on 07/10/2019 15:56:34

Well, all new to me, but interesting stuff.

If I get lobulation, I shall probably visit the doctor for advice.

Neil

" Such thread forms are not a new innovation. A patent for machinery to roll-form the threads of the screws was granted in 1975. "

I expected that sentence to end with a date like 1875... I now feel old for my age...

Neil

I'm advised you can get tablets for that.

They are a cost effective method of screwing directly into holes which are in castings of zinc or aluminium alloys and which have a draft angle, not being drilled. Generally, they are not intended to ever be unscrewed.

I can recommend a good ointment!

When the proper GKN Taptite screws are removed they leave a thread amply good enough to receive a normal screw of the same nominal size. Some of the more economically priced brands don't. Good ones work just fine in drilled holes, albeit larger than normal tapping size. Aluminium alloys or ordinary mild steel. I have a box of M5 ones that call out 4.5 mm for drilled holes. I ought to have the full list of drill sizes for original GKN Taptite screws somewhere. But I've not seen it for 25 years or so!

If you simply screw a taptite in the resulting thread is a bit distorted and tight on the screw. Effective inherent self locking. Especially in plastic. Removing it cleans up the thread so the self locking is lost if you re-fit it. The thread is looser too and maximum torque reduced. Using a normal screw brings the torque / strength thing back up but the self locking effect is largely lost. Best to add a drop of screw lock.

Clive

Edited By Clive Foster on 07/10/2019 22:27:31

I suspect Neil that you have encountered them without noticing, they are widely used on vehicles.

Mike

Hi Neil,

Diecast box screws, including Taptites are normally M3.5. This odd size appears to be for compatability with 4BA which was used on the original Eddystone boxes (and electical "back" boxes)

This rather than the tri-lobular form is likely why ir seemed small for a M4.

Robert G8RPI.

Would make sense, I measured one at 3.2mm but it clearly wasn't a 1/8" thread and too coarse for 5BA. Of course I was measuring across one of the 'flats'.

Neil

Edited By Neil Wyatt on 08/10/2019 20:19:39

If the box is Hammond or of USA origin its most likely a UNC thread

Ian P

SPOILER ALERT

**LINK**

https://www.atf-inc.com/wp-content/uploads/2016/07/TAPTITE-II-and-Other-Trilobular-Products.pdf

MichaelG.

While most Hammod boxes are 6-32 UNC and tapped holes, not all are

https://www.hammfg.com/electronics/small-case/diecast/eddystone

6-32 is 3.5mm major diameter but coarser pitch than metric but a taptite will fit.......

Robert G8RPI

On a building project at work many years ago they were using self drilling screws to fix into RSJ’s. The threaded screws had a small “drill bit” on the end and were put in with a special electric driver.

Taptites were typically used for cost reduction ,to save the tapping and subsequent inspection of tapped holes,best results are when used in ductile materials,used on very high volume commercial work ie aluminium pressure die castings with minimal draft on the cored holes,saves drillng and tapping. roll thread taps have similar profile with minimal swarf produced and a stronger thread form again works better on wrought than cast material.