Drilling screw holes

[object88]

I'm doing some mechanical planning for a stompbox, and I want to fix the PCB to the enclosure via some screws, nuts, and standoffs.  In particular, I plan having the screws come down from the top of the enclosure and "hanging" the PCB.

First off, any trouble with this idea?  The only thing I could think of maybe going wrong was socket'ed components coming loose if they were hanging upside down, but then I realized I could just drop the PCB further and flip it upright.  (Just need to make sure I have enough clearance...)

Second, I'd like to use flathead phillips screws, so the head is flush with the top of the enclosure... purely for aesthetic reasons.  What kind of tool or drill bit is available to make the right shape and size hole for the head?

I tried a quick search for various combinations of "flathead", "enclosure", "screw", and "Hammond", but didn't find anything of relevance.  Perhaps I'm using the wrong terms?  Thanks for any advice!

[Greybeard]

I don't know the correct term, but there is a countersinking bit avalable for flat head screws. That is what you are looking for. You could use a very large drill bit also, but this can get scarry and never creates a round counter sink because the drill bit has only 2 cutting edges. I have used both methods tho' and was satisfied with the results. in either case, pre drill for the bolt, then very slowly sink the larger bit just enough for the screw head to be flush with the surface. This is best done with a drill press at very VERY slow speed. On some boxes the material may not be thick enough to get perfectly flush tho'.

Greybeard

[object88]

Thanks!  I'll inquire at my friendly neighborhood hardware store for a countersink drillbit.  Good point about the depth issue; Hammond has a datasheet... oh.  The floor is only about 1/13ths of an inch thick.  The lid is about 2/9ths of an inch (maybe?).  I think that probably precludes flathead screws, but I'll investigate anyway.  Thanks again!

[eliktronik]

While this may not be the most mechanically secure way of doing this, I recently cut some 1" plastic standoffs in half, and epoxied them to the inside of a hammond box. Here's a picture:

[object88]

I appreciate the advice, but I'm looking to make something as physically rugged as I can... while still being relatively easy to disassemble and reassemble.  Because I'm sado-masochistic like that. :shock:

[eliktronik]

Hey, I understand completely. If you want it to look really slick, you could recess the holes a little bit extra, and after tightening very well and using a lock washer, you could bondo over them. That would have the added benefit of not requiring you to be painstakingly careful about drilling/countersinking the bolts.

EDIT: I'm (possibly incorrectly) assuming that you'd be mounting the pcb to the enclosure and not the lid. I personally think that is more reliable, as you don't stretch out all the wires every time you open the lid. I guess it might be more of a hassle though and not worth it.

[Paul Perry (Frostwave)]

If you have enough pots, and use vertically mounted PCB ones, then you can use the pots to hold the board to the front of the box.
Worked perfectly hundreds of times for me
Not that I'd build an AMPLIFIER like that, of course...

[jmusser]

The best screw for the job, is called an "undercut flathead screw". Instead of it being beveled clear down to the threads, it is only about half that thickness, and flat underneath the head. It leaves a lot more metal in the holes, and makes it a little less "hairy" to have to try to get the proper bevel deep enough for the top of the screw head to be flush with the enclosure. These are used all the time in the telecommunications industry. For the amount of weight you'll be supporting, epoxy or JB weld type stuff (also an epoxy) will be more than sufficient. I use it all the time for the same purpose. The main thing with that, is to rough up the area where your stanoff will sit in the lid, so the epoxy has something to get hold of.

[Paul Perry (Frostwave)]

Jmusser, is this what you mean? What we call a "cheese head" here?
http://www.uc-components.com/drawings/dwgcheeseheadslotted.html

Works OK if you can drill the large (blind) hole precisely, and the smaller hole exactly concentrically.
Personally, I have more luck with an angled countersunk hole.

[jmusser]

Lord no! I'd hate to have to bury that thing! Look at the picture on this page, under "Flat & Oval head (undercut)" http://www.midstatesbolt.com/headtypes.htm   The oval head would stick above the surface, and the flathead would be flush with the surface. You can tell in the picture with the undercut flathead, that you have a whole lot more surface to surface contact in thin metal. I believe this type of screw head is also used in some types of blind hinges.

[object88]

Thanks again for the advice folks!

My current plan is to mount everything to the enclosure and not the lid, exactly to avoid dealing with wire-stretch when opening and closing the box.

Regarding epoxies, my hands are not nearly steady enough to make a good placement.  With screwholes, I can make exact adjustments with a drillpress.  Gotta play down my weaknesses!  

I thought about mounting the PCB on the pots.  This particular mechanical design is for a bog-standard fuzzface clone, to keep things simple-yet-challenging-yet-fun, so there would be two knobs and a small PCB.  I initially wrote the idea off thinking that it would be too difficult to match the PCB and mechanical design, but I realize now that I'm doing that anyway. :oops: I will definately consider this as an easier alternative.

I will check out the undercut flathead screw, but I confess that I don't know how I would get the flatted sunken hole required... what kind of drill bit does that?

[dpresley58]

Regarding epoxies, my hands are not nearly steady enough to make a good placement.  With screwholes, I can make exact adjustments with a drillpress.  Gotta play down my weaknesses!  

The placement technique that works for me is to mount the standoffs to the board without tightening the screws too much. That way, they're already in place when epoxied to the enclosure. When I say "too much", just tighten them enough so they can be easily removed without applying too much torque to a fresh epoxy joint. After the epoxy has sufficiently set, the board can be released from the standoffs for further work if required.

I realize you're more or less sold on the screw-in approach, but be careful with the countersinking bit; you're not dealing with much thickness with these enclosures.

[object88]

Excuse me while I smack myself on the forehead.  That's a great idea, dpresley58.  I'll keep that in mind after I've screwed up (heh, I kill me) a few Hammonds.  

[Paul Perry (Frostwave)]

That's a great idea, dpresley58.

I agree! and a similar thing is this: if you are soldering jacks and/or pots to a PCB and then mounting them on or through a panel, it makes sense to mount them on the panel BEFORE soldering to the PCB. This will reduce strain in more than one way :wink: (dependign what has to be sone in what order, you can undo them from the panel & do more work before final assembly).