The sub rings are MDF - and as such I'm concerned about the screws vibrating loose and the MDF becoming "chewed out".

What could I do to sop this? Are there any special screws I can use?

T-nuts. http://www.lakesidebilliardsupply.com/tnut.jpg

Available from Mitre10.

Tnuts with a light thread lock compound would do the trick - although I've never actually heard of this happening. What thickness MDF are you using? Anything 12mm or greater should be fine as long as the screws are screwed down tight.

I've had screws pull straight out of 18mm MDF with the weight of my XXX hanging off it. Locking compound shouldnt be required with t-nuts, I can't imagine it vibrating loose if done up firmly.

I'm using 18mm mdf - the screws are quite long so the poke out about 10mm through the other side. I'll go to Mitre 10 or somewhere this weekend. I need to get some thin MDF to block up the service holes in my doors

I've had issues with the t-nuts not biting fully into MDF and falling out asily when unscrewing

hmmm - maybe I'll glue them in a bit.....

I have a question as to how they work. Do you hammer them into the wood and hope you've got them lined up properly?

thats what happens when you drill the holes too big.

Thanks for the advice mate, I'll give you a call next time I can't work out how to use a drill. I thought it was due to my ~30KG XXX hanging upside-down, inverted, without T-nuts in the back of a car with 650lb springs. But what would I know...

obviously not, seeing as my 25kg, inverted subs, in the back of my car, with 560 pound/inch springs in the rear never fell out.

Yep, T-Nuts are they go. I have used them with my 9515 and 3kwrms of power with great success and never gets loose.

I just coat my screws with a light coating of pva glue when i screw them in. Dont dip them in it or anything, just enough like loctite. Never had any real issues with them comming completely loose. Sometimes they need a nip-up or re-coating, but thats nothing major.

Ush

^^ Nail polish works well too.

+1, its happened to me as well




once again, you are the cancer killing MEA

T nuts are obviously the superior solution. If you are worried about them falling out of the MDF once undone, just use a bit of superglue, or liquid nails to hold them in place.

Phil

That's it USE T-NUTS, nobody wants to not be able to unscrew there sub. So scrap all the other ideas of coating the screws.

T-Nut it and if your really paranoid just check the screws every so often. But they wont come undone.

And using a threaded insert gives you some more bolt head style options too, I found a nice hex key heads in black that match my speaker grills for my door speakers rather nicely

but becarefull with the t nuts if you over tighter them and they lose grip of mdf then you will be chopping your box to get your sub out

I know that feeling too! Pain in the ass, but nothing a holesaw and some vice grips cant fix.

T nuts aren't the answer.

They are often too big in diameter, so encroach into the woofer hole. Then they can spin and just cause a big mess.

In 16 years, I've never had a woofer work loose when held in with the correct size chipboard screws. Even in cars with 25,000Wrms per speaker.

Phil

Thats a signature worthy quote, lol.

Seriously, drill the correct size pilot holes and use the correct screws, or use t-nuts with some thread lock on them if you don't want you sub coming out in a hurry (possibly ever without jigsaw etc).

DONE.

exactly, but somehow people have managed to have their subs FALL OUT when held in with screws?

most houses are only held together with nails, and they don't fall apart (much)

even with the sub inverted, the screws don't have that much weight on them, and its more vertical loading, pulling the screws sideways, than horizontal, pulling the screws out.

if you have a 30kg sub, with 8 screws, thats only 3.75kg/screw.

yes, there will be some forces acting on the subs mounting from cone movement, but not enough to rip a screw out of the correct sized hole.

I'm pretty sure a sub vibrating flat out would add to that...

I just use 10g screws with an agressive thread. We've mounted heavy subs on upside down baffles, rolled the car and they still haven't budged. I don't know why some people have a phobia of their subs 'jumping' loose.

Just predrill the first half of the hole and let the screw thread bite itself into the wood to give it a firm hold. It will never vibrate loose.

Yes it would, especially when you add in vibrations and bumps from the road, however it still mostly SHEAR forces (ie cutting across the screw) not axial forces that are trying to rip the screw out of the wood.
Inverted shouldn't make that much difference to regular mounted. Some, but not a great deal.

50mm 10g plasterboard screws ftw.

i chopped up the side of an old computer tower into small strips then nailed them to the inside of the box

I use M6 threaded inserts now for all my enclosures, they're good if you're repeatedly removing/installing you subs.
I've used t-nuts in the past and found they are much harder to drive into MDF and quite often come loose or spin.
Wood screws are good the first time, but after that they don't hold well or start spinning.

When I had my 9915 in the test box I kept taking the sub out to change things on the internal of the box...


Long story short, I kept spinning the sub slightly every time I put it back in so it had a fresh bite. That answers the questions if anyone wonders why sometimes subs are on an angle

+1 for multiple uses.

Otherwise a bit of blu-tac or sound-deadner in the pilot hole will work as a non-hardening glue on the screw, that wont come undone.

no, its still mostly a 'pulling out' force that act on the screws if its mounted so the cone is vertical

this is because the centre of gravity of the sub is towards the middle of the magnet. the magnet isn't mounted in the same plane as the screws so there is a lever arm. the sub rotates about the bottom screw just like a hinge.

SO if the sub is mounted normally (not inverted) the screws dont actually do alot because the force is trying to pull the 'mounting lip' of the sub through the timber.

if the sub is inverted, then the most load is at the topmost screw and the force is a 'pulling' force.

theres very little shear force if a sub has a large motor.

i use T-nuts because i frequently remove my sub from the box and the hole in the mdf is pretty much stuffed. after one or two removals. although it may not fall out, there is a significant reduction in the rigidity of the connection, and after all my hard work desiging a stiff box, i dont want to be let down because i was to tight to spend an extra $4 on some t-nuts

good thread guys!

i have made my own box today for my new sub, tomorrow im going to carpet it,
i dont know if i should mount the sub on top of the carpet (screw sub>carpet>mdf) or cut the carpet around the sub hole so the sub it attached to the MDF to keep it airtight.......

any recommendations???

Plasterboard are a bit fine in the thread.

Chip board screws are the only ones to use. As I said above, nearly two decades in this game, 1000s of cars built, never had a woofer come loose.

Phil

This is the threaded inserts the guys are refering to http://www.ezlok.com/InsertsWood/softWood.html I've been using these for a while now where I can, you can't use them all the time because you may not have the edge distance between them and your subs hole, but I don't just use them for that but for holding trim panels, amp rack etc all in place.

Bunnings sell them but have a pretty limited range, I've been searching for another supplier who can supply them in smaller sizes, I found a McMaster-Carr online hardware site but they wouldn't ship to AUS.

*Plasterboard

There the exact ones I was talking about

nice long meaty thread, not too large (8ga) I found 10ga needs alot more weight put on the driver unless you want burred heads/bits.

Longer screw > thicker. Means you need atleast a 3layer baffle too

These are plasterboard screws, they fail. Thread isn't agressive enough!