Dumb question about screws - Page 1
Forum section: electronics / projects

author: gxti
author_rank: Frequent Contributor
timestamp: June 23, 2014, 09:48:18 pm
content: I need some pan-head screws for use with an extruded aluminum enclosure. Would this be suitable?http://www.mcmaster.com/#92525A311Thread-Forming Screw for Sheet Metal & Iron, 18-8 Stainless Steel, Pan Head Phillips, 6-20 Thread, 1/4" Length, packs of 100The enclosure is a Hammond 1455C802, and the datasheet specifies that the original screws (which are flathead and need countersinking, otherwise I'd just use those) are "#6 Taptight". Previously I tried using thread-cutting screws, but learned the hard way that "thread-cutting" and "thread-forming" are not the same thing -- they fit but it gets little metal flakes everywhere.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: June 23, 2014, 09:52:46 pm
content: "To continue browsing, please log in."

author: Psi
author_rank: Super Contributor
timestamp: June 23, 2014, 09:59:23 pm
content: as above, i got to see the screw once but after that it requires loginfyi, all Hammond extruded alu cases produce metal flakes even when you use the correct screwsFor one of my products i screw all 8 screws in, remove them, spray the case with compressed air before screwing them back in.

author: gxti
author_rank: Frequent Contributor
timestamp: June 23, 2014, 10:50:02 pm
content: Strange, the link works for me even though I'm not logged in at the moment. In any case, the part number is 92525A311.Removing the screws and blowing the flakes out is what I did the first time, but now I need to make 60 of these (with 4 screws each...). My wrists would never forgive me if I didn't go out and buy a power drill, but that was true anyway. Do these at least seem like a good choice?

author: SeanB
author_rank: Super Contributor
timestamp: June 23, 2014, 11:59:35 pm
content: Thread forming screw works well in aluminium, as it does not cut a thread creating chips, but deforms the alloy to leave a thread in the hole. That would work in your case, and when placing them in the first time I would recommend using a lubricant on the thread. Get a tub of cutting wax and simply dip the screw end in it before starting the screw. That way the force required to insert is a lot lower.

author: nanofrog
author_rank: Super Contributor
timestamp: June 24, 2014, 01:43:54 am
content: Quote from: AmmoJammo on June 23, 2014, 09:52:46 pmQuote from: Psi on June 23, 2014, 09:59:23 pmNon-US IP address perhaps, as it works for me without a login required.Here's a pic from the link.

author: jeremy
author_rank: Super Contributor
timestamp: June 24, 2014, 03:25:25 am
content: yes, mcmaster doesn't like anyone from outside of the US unfortunately. Although I hear some big universities can still put orders through them.

author: dexters_lab
author_rank: Unknown
timestamp: June 24, 2014, 08:50:41 am
content: link works fine here in the uk!personally i never self-tap into aluminium, i would always drill/tap and use a machine screw as it's so easy to tap aluminium.

author: mjkuwp
author_rank: Unknown
timestamp: June 24, 2014, 11:33:37 am
content: Do you care about corrosion? Stainless steel and aluminum... I am absolutely not an expert but it rings a bell with me that it will happen with this combination.

author: Gall
author_rank: Frequent Contributor
timestamp: June 24, 2014, 11:43:57 am
content: Quote from: mjkuwp on June 24, 2014, 11:33:37 amNothing bad if used indoors since both aluminum and stainless steel passivate. Not nice outdoors but it depends on the exact kind of alu alloy (it may be or may not be rated for such usage).

author: mrflibble
author_rank: Super Contributor
timestamp: June 24, 2014, 02:10:02 pm
content: Quote from: Psi on June 23, 2014, 09:59:23 pmSame here. I can see why too: website sucks bawls. Either the web developer or the business requirements dude needs to go back to school. Anyways, quick workaround: use (for example) incognito mode on chrome.

author: Towger
author_rank: Super Contributor
timestamp: June 24, 2014, 05:02:31 pm
content: Quote from: mjkuwp on June 24, 2014, 11:33:37 amHanse (yachts) embed aluminum plates in their fiberglass deck and attach all deck fittings by drilling and tapping the plates for stainless steel bolts.They have been doing it for years and there are discussions on their forum, but there are not many reports of actual problems with the method.

author: G7PSK
author_rank: Super Contributor
timestamp: June 24, 2014, 06:02:45 pm
content: No corrosion problems with stainless steel and aluminium but there is with plain steel.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: June 24, 2014, 09:35:21 pm
content: I hate to get off topic here but that's not true. Stainless steel and aluminum is a bad combination in environments where electrolytes are present. I speak from direct experience with both marine and food. If you look at the galvanic series you'll see the reason why. Unfortunately the aluminium sacrifices to the stainless forming aluminium oxide which is larger and puts pressure on the screw. Since aluminium oxide is impossible to remove without some rather nasty acids, penetrating oil doesn't work on it.Fortunately it's easy to prevent the problem by sealing the bolt. You can use gasket sealant, grease (silicone doesn't wash out as easily), food or marine grade antisieze, loctite, bonded rubber washers, paint, etc - they all work.The main problem with steel bolts is they're ugly and the heads waste away making removal difficult, but the galvanic corrosion problems are much less.