Preserving self-tapping screw holes - Page 1
Forum section: electronics / testgear

author: GeekPriest
author_rank: Contributor
timestamp: August 28, 2015, 05:25:12 pm
content: For the handheld gear with self-tapping screws (going into plastic), like the majority of multimeters these days, what is the best way to preserve the unit if it is opened multiple times?

author: TheSteve
author_rank: Unknown
timestamp: August 28, 2015, 05:37:45 pm
content: Before screwing the screw back in turn it slowly backwards(counter-clockwise) while applying a very gentle amount of pressure until you hear a small click - you can often feel it as well. When done correctly it will align the screw with the previously cut threads in the plastic. The screw will generally require less force to screw in as well when aligned. Not cutting new threads each time is key to preserving the already tapped holes. Of course you never want to over-tighten the screws either as it also ruins the threads. Done properly you should be able to remove/reinstall the original screws into most plastic many times without any kind of problem.Further - always use a screwdriver with the correct bit on the end so you have a good feel of the torque you are applying.

author: nctnico
author_rank: Super Contributor
timestamp: August 28, 2015, 05:57:23 pm
content: Quote from: TheSteve on August 28, 2015, 05:37:45 pm+1

author: Fungus
author_rank: Super Contributor
timestamp: August 28, 2015, 06:33:13 pm
content: Don't use electric screwdrivers.Do it by hand. Feel what you're doing.

author: SeanB
author_rank: Super Contributor
timestamp: August 28, 2015, 06:52:58 pm
content: Quote from: Fungus on August 28, 2015, 06:33:13 pm+1 there. Electric drivers are for the assembly line when making that thread into the virgin plastic, or in something using metal castings with cut threads and a machine screw.

author: JackM
author_rank: Regular Contributor
timestamp: August 28, 2015, 06:53:29 pm
content: Quote from: TheSteve on August 28, 2015, 05:37:45 pmSame technique I use! I feel as though this is one of those things that everyone discovers on their own but hardly ever passes it on.

author: GeekPriest
author_rank: Contributor
timestamp: August 28, 2015, 06:55:51 pm
content: Which is why I'm glad I asked the question. It seems obvious now, but I'm glad I had the turn-it-backward-for-a-bit tip pointed out to me!

author: nctnico
author_rank: Super Contributor
timestamp: August 28, 2015, 07:14:14 pm
content: Quote from: JackM on August 28, 2015, 06:53:29 pmActually this trick was taught to me during an internship. It is probably also described in service manuals in the chapters nobody cares to read.

author: Ian.M
author_rank: Super Contributor
timestamp: August 28, 2015, 07:31:09 pm
content: If the pillar with the screw hole in is particularly fragile and prone to splitting or already split, if possible knot a few turns of strong cotton thread round the outside of the pillar and saturate the thread with superglue. N.B. superglue is usually a poor choice for repairing split/broken pillars as its too brittle. Glue the pillar back together with a more appropriate glue (I favour plastic pipe cement, which works well on ABS and rigid PVC) before supergluing the cotton thread reinforcement round it.Partially stripped out holes can often be improved by inserting a longer screw with exactly the same thread into the lower case half on its own and heating the screw head with a soldering iron till the plastic softens and reforms round the thread. Let cool fully before touching the screw again. Don't over-cook it!

author: Gyro
author_rank: Super Contributor
timestamp: August 28, 2015, 07:45:13 pm
content: QuoteIf all else fails, inserting a length of striped PVC wire insulation into the hole before the screw should restore some purchase - Obviously not if the screw is performing some structurally or safety critical function.

author: SeanB
author_rank: Super Contributor
timestamp: August 28, 2015, 07:55:10 pm
content: Large PVC drinking straws often are the right diameter to fit the inner plastic pillar, or you can use some heatshrink tube to hold the pieces together as well, using some PVC weld to get the bits to fit back into one more or less intact sleeve.Of course does not work pn those cheap plastic parts which seem to be designed to crumble to powder within a year.

author: DimitriP
author_rank: Super Contributor
timestamp: August 28, 2015, 08:11:34 pm
content: Quote from: JackM on August 28, 2015, 06:53:29 pmYup. I came up with it when I was nervous the first time i ever tried to replace spark plugs...It's dark, you can barely reach and don't want to screw it up by crossthreading it. The "click" works.Give it a few days and Apple with apply for a Patent..."Procedure and method for preserving existing threads and cross threading avoidance" (Unless HP already filed for it) !

author: DimitriP
author_rank: Super Contributor
timestamp: August 28, 2015, 08:15:02 pm
content: Quote from: Fungus on August 28, 2015, 06:33:13 pmUse the smallest handle screwdriver that will do the job . That way you are not able to "go hulk" when reassembling. Unless you ARE hulk

author: GeekPriest
author_rank: Contributor
timestamp: August 28, 2015, 08:15:15 pm
content: Quote from: DimitriP on August 28, 2015, 08:11:34 pmWhat's funny is that I use this technique all the time with car work. It just never occurred to me to do it with these self-tapping plastic screws!(Here's hoping that the front brakes are all functional on "Vato" -- my newly-acquired white beater pickup -- after this week's work on it.)

author: GreyWoolfe
author_rank: Unknown
timestamp: August 28, 2015, 10:34:36 pm
content: Quote from: DimitriP on August 28, 2015, 08:15:02 pmHulk not screw in puny human screws, Hulk smash them in!

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: August 29, 2015, 04:02:51 am
content: IanM-I've used the same technique, and spray or dip the scew in silicone or dry-lube first. This seems to reduce the 'grab' of the solidified plastic.Another bodge is to use 'BluTak' or plasticine to make a bund or dam around the pillar and flood with epoxy.Regards, BT