Knobs and shafts - Page 1
Forum section: electronics / repair

author: Specmaster
author_rank: Super Contributor
timestamp: July 16, 2017, 03:39:48 pm
content: Not sure if this is the right place to post this, if not feel free to move it into the correct section.I like to repair old test equipment, analogue scopes especially and a common problem I have come across and so far have not found a solution for is how do you remove control knobs when the little grub screws (allen key types) will not undo, the allen key just keeps slipping round? Its not a case of using cheap keys, or the wrong size as I have a brand new set of Draper keys in both metric and imperial sizes and I have selected the key that will fit the hex socket the best and snuggliest, so the correct size of key has been selected. So has anyone else come across this problem and resolved it with resorting to destroying the knob? Many of these knobs are extremely difficult to get replacements for as they may be 30 years old or more?The other problem I have come across is where the a knob has been removed successfully but will not go back on the shaft again because the shaft over the years has been so badly scored by loose knobs that you simply cannot get the grub screws to tighten up enough on the shaft to get a really good grip?Any ideas at all?

author: Armadillo
author_rank: Super Contributor
timestamp: July 16, 2017, 04:05:06 pm
content: For instruments, its unlikely that thread lock has been used, so most likely it is due to rust. Never there is a need to over tightened the grub screw on instrument.So before you damage the hex key slot, always apply WD40 to loosen up rust as much as possible [allow time], vibration by knocking, and expansion by heat as last resort.But when the grub screw is damaged, Man! the job became harder. you will need to use the "left hand" drill bits and screw extractor.

author: Specmaster
author_rank: Super Contributor
timestamp: July 16, 2017, 04:13:26 pm
content: Quote from: Armadillo on July 16, 2017, 04:05:06 pmI can see how the WD40 might work, but how do you apply heat without the possibility of distorting, discolouring the plastic knob, or even worse destroying it altogether?

author: Armadillo
author_rank: Super Contributor
timestamp: July 16, 2017, 04:26:27 pm
content: Not for plastic knob, but of course.Use vibration and serious knocking then.At the*LAST RESORT*,you can apply heat through the hex driver to the grub screw by conduction. WD40 loves some heat expansion to flow into the crevices.

author: Specmaster
author_rank: Super Contributor
timestamp: July 16, 2017, 05:59:39 pm
content: Surely though, aren't all knobs made of plastic?

author: WastelandTek
author_rank: Frequent Contributor
timestamp: July 16, 2017, 06:16:54 pm
content: While I am certainly NOT on the anti WD40 train, I really have to disagree in this case. In my opinion the absolute best penetrating lubricant on the market at the moment isPB Blaster, it has replaced both "liquid wrench", and "kroil" on my bench...really amazing stuff.Quote from: Armadillo on July 16, 2017, 04:05:06 pmAs far as the heat, yes, you insert the allen key, then carefully heat the key with a pen torch. Most old knobs are plastic with a metal core.

author: Specmaster
author_rank: Super Contributor
timestamp: July 16, 2017, 06:32:32 pm
content: Never heard of PB Blaster before but it is sold by Amazon UK, I checked. Yes, heating the key might work, I don't have a pen torch but a hot air gun might do it?

author: Armadillo
author_rank: Super Contributor
timestamp: July 16, 2017, 06:53:46 pm
content: Quote from: WastelandTek on July 16, 2017, 06:16:54 pmI have not heard of PB Blaster either....sound blaster, yes! Actually you didn't disagree the method, you are merely just offering another brand of the penetrant, that's about all. Not that we need to agree with the brand.

author: Armadillo
author_rank: Super Contributor
timestamp: July 16, 2017, 07:03:25 pm
content: Quote from: Specmaster on July 16, 2017, 05:59:39 pmNot that I know of, not sure. If plastic, probably a simple slotted head or philips screw is used.Just my opinion.

author: Armadillo
author_rank: Super Contributor
timestamp: July 16, 2017, 07:10:30 pm
content: The ultrasonic cleaner vibration head piece, sounds a bit crazy, but I reckon can deliver quite a loosening and non destructive power to that grub screw, just like a vibration torque wrench?Just an idea, have not tried before.Maybe someone could device a loosening gun out of it!

author: WastelandTek
author_rank: Frequent Contributor
timestamp: July 16, 2017, 07:21:46 pm
content: Quote from: Armadillo on July 16, 2017, 06:53:46 pmTrue, "disagree" was probably overly strong, it is all really just a lubricant carried in a solvent. The joke around here is that the PB has nanobots in it with tiny jackhammers and angle grinders. I have had really good luck with it.cheers

author: grumpydoc
author_rank: Super Contributor
timestamp: July 16, 2017, 08:18:14 pm
content: If not completely rounded out I have had some luck using a torx bit rather than a hex bit - after a squirt of WD40 though.

author: Specmaster
author_rank: Super Contributor
timestamp: July 16, 2017, 10:02:29 pm
content: Quote from: Armadillo on July 16, 2017, 07:03:25 pmBy plastic, I meant the part that we interact with, not the centre boss etc...

author: Specmaster
author_rank: Super Contributor
timestamp: July 16, 2017, 10:05:17 pm
content: Quote from: grumpydoc on July 16, 2017, 08:18:14 pmThere might some mileage in this method, I'll give it a go when I get the scope on the bench next.

author: WastelandTek
author_rank: Frequent Contributor
timestamp: July 16, 2017, 10:07:49 pm
content: be sure to support the knob from the other side (I usually curl a finger under it) while you are really "getting with it" with the torx driver

author: alm
author_rank: Super Contributor
timestamp: July 17, 2017, 01:47:14 am
content: Quote from: Armadillo on July 16, 2017, 04:05:06 pmRotary switches can require a fair amount of torque. If the shaft is not flattened, then I could see how over tightening the set screw might be tempting, although the better solution would be to grind a flat on the shaft.If you are going to heat the knob, then it might be worthwhile to try to remove the plastic knob from the metal core (assuming the set screw does not stick out too far). That would require some moderate heating, but might save the knob from being damaged by further heat.

author: texaspyro
author_rank: Super Contributor
timestamp: July 17, 2017, 04:24:10 am
content: Some knobs use Bristol spline grub screws... they look a lot like the bastard love child of a hex key and a Torx. Hex drivers can slip in a bristol spline.

author: tautech
author_rank: Super Contributor
timestamp: July 17, 2017, 10:25:43 am
content: Like any thread that has sat tightened for decades they've sweated and frozen some onto the threads.There's an arsenal of tricks that mechanical engineers use:HeatPenetrantsPercussion, two main types.* (FBH)Brute strength and ignorance.Or a combination of any or all of the above.Familiarity with the forces fasteners can handle (experience) and the correct tools for the job will loosen all but the most stubborn fasteners along with some brute strength.Sometimes one must resort to destructive measures to get things apart like wringing the ba***rds neck or the good ol gas axe.Personally I prefer the FBH (F***ing big hammer) or variants of it.Blind fasteners like grub/hex/Torx screws have their threads loaded on one face only and the small clearances in a thread from one face to the other can be used to some advantage. This is where the FBH come into it's own.You need get the percussion (shock) into the shaft of the fastener, just hard enough for the end of the fastener tobruisethe surface it's wound down onto and lighten the pressure on the thread faces, breaking the bond at the same time. On small delicate fixtures the offside need be supported with something that can cushion the impact and yet has a similar or greater mass than the FBH. (1)A few measured clouts with your FBH or FsmallH should shock the thread to and fro enough to break the decades old bond of the threads and loosen the tightness of the fastener, hopefully for it to be easily unwound.As far as TE, the tightest grub screws I've come across was an old HP 1740A.......so tight that I didn't dare attempt to try to undo until I had theperfect fithex keys. And they were tight, bloody tight but all came undone without further tricks.Gentle thermal cycling and penetrants can sometimes be useful for delicate patients and when you're stumped and not getting anywhere fast......sleep and worry about it later. You'd be surprised how a fresh mind looks at things differently.* 1. Percussion with a FBH and a dolly (1) on tight nuts will swell the nut and break the bonds.2. Use of some thin drift to get the impact onto the shank of the fastener so to not damage the hex, Torx etc.

author: BMack
author_rank: Regular Contributor
timestamp: July 17, 2017, 01:48:07 pm
content: Quote from: Specmaster on July 16, 2017, 03:39:48 pmI have not tried this as I have not come across this problem(at that point most customers have already lost the knob). You can use a thread checker and get the thread pattern, find a longer grub or get a normal screw of the same thread, cut the head off an cut a slot in the top for a screwdriver. If the original grub was flat, you can try a conical grub to grip a little better.With typical screw extraction I use a left handed drill bit but since you have a deepish hole, try the Grab-it.Ifit bites, it bites well. I had the other brand of these and they never worked, the Grabit version works a lot better, make sure to get the micro.https://www.amazon.com/Alden-4507P-Grabit-Broken-Extractor/dp/B000Q60UOO/