Making a hole in a subwoofer cabinet smaller?

[uk_woodworker]

Hi, I have a problem to solve, and I thought I would bring it to the experts - hopefully someone can help.

I'm building a subwoofer, and have a cabinet made from 18 mm MDF. The front is double thickness, with a hole cut in each piece for the speaker. There is an overhanging lip on the inner part, to allow the speaker to sit recessed.

My problem is that the hole on the inner front piece is too large (by about 6 mm), and the difference means the screw holes to mount the subwoofer would be very close to the edge of the wood. I can't drill through without risking the wood breaking at the edge of the hole.

So I need to somehow shrink the hole so the screws (or bolts with T-nuts on the other side) have more support.

At the moment I've bent a piece of pine stripwood (4mm thick, 18mm wide) by soaking in hot water, to fit around the inner diameter. I was thinking of gluing it in, with small nails (avoiding the areas where the mounting screws will be) to hold it flush while the glue dries. Do you think that would be ok?

Also, would a wood screw or machine screw with T-nut on the back side work better in this situation? I have heard the T-nut would work better if repeatedly fitting/removing the speaker, as the wood screws would not hold up as well for this in MDF?

Picture attached showing the stripwood held in place within the hole while it dries. I used duct tape around the hole to protect the MDF from moisture.

Any advice is much appreciated.

 

[NazNomad]

The strip of pine will be fine just glued, no need for nails. You'll break the wood before you break the glue-joint.

T-nuts, perfect if you're going to be fitting/removing the speaker several times, especially in something as fluffy as MDF.

 

[SteveF]

have you got room in the cabinet to put 2 lengths of softwood\hardwood
you can then just screw through mdf into the softwood and even fit your t nuts if needed

Steve

 

[uk_woodworker]

The strip of pine will be fine just glued, no need for nails. You'll break the wood before you break the glue-joint.

T-nuts, perfect if you're going to be fitting/removing the speaker several times, especially in something as fluffy as MDF.

Thanks for your reply. That's reassuring.

The reason I wanted to use nails for the strip, is that I don't have clamps long enough to get pressure on the whole strip while gluing. There are some spots that aren't sitting flush without clamping force. Do you have any suggestions how I could apply pressure?

 

[NazNomad]

Even better... cut another ring of MDF bigger than the hole you have now, with the right sized hole for the speaker in it.

Then glue that ring behind the original hole and just use longer bolts for the speaker mounting.

That way, you only have to glue 2 thicknesses of MDF together and they're both flat surfaces.

 

[uk_woodworker]

have you got room in the cabinet to put 2 lengths of softwood\hardwood
you can then just screw through mdf into the softwood and even fit your t nuts if needed

Steve

Do you mean to stick a length within the hole, and a length behind the front? Something like in this (bad) sketch?

 

[uk_woodworker]

Even better... cut another ring of MDF bigger than the hole you have now, with the right sized hole for the speaker in it.

Then glue that ring behind the original hole and just use longer bolts for the speaker mounting.

That way, you only have to glue 2 thicknesses of MDF together and they're both flat surfaces.

I thought about doing it this way, but I was worried making the bolts too long might make the speaker more likely to move laterally? Especially if the bolts aren't surrounded by wood on all sides for some of their length.

I have the pine strip ready to go, so if that would hold then I might just go with that.

 

[SteveF]

like this

worse sketch than yours :lol:

 

[uk_woodworker]

like this

worse sketch than yours :lol:

Oh I see :lol:

I could do that, though there are 8 mounting screws so I would probably need to use 4 lengths.

That would be do-able, but as I said to NazNomad, I was worried the bolts might end up being too long, and lose some stability?

Also, I already have the bolts and T-nuts, so would prefer to keep the depth the same if possible, so I can work with what I have!

 

[uk_woodworker]

The strip of pine will be fine just glued, no need for nails. You'll break the wood before you break the glue-joint.

T-nuts, perfect if you're going to be fitting/removing the speaker several times, especially in something as fluffy as MDF.

Thanks for your reply. That's reassuring.

The reason I wanted to use nails for the strip, is that I don't have clamps long enough to get pressure on the whole strip while gluing. There are some spots that aren't sitting flush without clamping force. Do you have any suggestions how I could apply pressure?

I just realised a can use my bar clamps as spreaders by reversing the ends...

 

[Racers]

Rout the hole out oversized and make a stepped ring to drop in the hole.


Pete

 

[skipdiver]

Rout the hole out oversized and make a stepped ring to drop in the hole.


Pete

That would have been my suggestion.

I built a pair of LS5/3's once and i used t nuts to secure the speakers to the cabs.

 

[Mr T]

Rout the hole out oversized and make a stepped ring to drop in the hole.
Pete

I would have done it as Pete suggested. With an internal lipping as you are using if you form a steep scarf joint on the ends so the strip is just a tad too long to fit in the circle, by clamping across the joint the sloping surfaces can slide thus applying pressure internally round the hole. This works with hardwood lining but redwood may be a bit soft and just crush rather than slide.

Chris

 

[Racers]

Rout the hole out oversized and make a stepped ring to drop in the hole.


Pete

That would have been my suggestion.

I built a pair of LS5/3's once and i used t nuts to secure the speakers to the cabs.

Ls5/3 or Ls3/5?

I found a pair of Goodmans Ls3/5a at a car boot for £10 sold them for £765.

Pete

 

[uk_woodworker]

Rout the hole out oversized and make a stepped ring to drop in the hole.


Pete

This would be ideal, but I don't have a router or a bandsaw, so a low tech solution is preferred. I wouldn't be able to make a circle precise enough to sit flush with the hole.

 

[uk_woodworker]

Rout the hole out oversized and make a stepped ring to drop in the hole.
Pete

I would have done it as Pete suggested. With an internal lipping as you are using if you form a steep scarf joint on the ends so the strip is just a tad too long to fit in the circle, by clamping across the joint the sloping surfaces can slide thus applying pressure internally round the hole. This works with hardwood lining but redwood may be a bit soft and just crush rather than slide.

Chris

This is a really good idea. Unfortunately the only strip I could get hold of is pine, and I think it would just squash if I tried to do that.

 

[skipdiver]

Rout the hole out oversized and make a stepped ring to drop in the hole.


Pete

That would have been my suggestion.

I built a pair of LS5/3's once and i used t nuts to secure the speakers to the cabs.

Ls5/3 or Ls3/5?

I found a pair of Goodmans Ls3/5a at a car boot for £10 sold them for £765.

Pete

Yeah, brain f**t. They were Ls3/5a's. Bought a kit from Wilmslow Audio, many moons ago.

That was a great find for you. They seem to have reverential status, but in truth, the small active speakers i have now sound way better.

 

[Racers]

My Shahinian Arcs sounded much better than the Ls3/5a's, I didn't have a problem moving them on.

Pete

 

[skipdiver]

My Shahinian Arcs sounded much better than the Ls3/5a's, I didn't have a problem moving them on.

Pete

I should hope they did. They are about 5 grand aren't they?

 

[Racers]

They are now, but I paid less than a fifth of that!

Pete