PDX Zaph SR-71 Build Thread

StewLG

Member
2008-03-20 10:24 pm
I've been interested in building speakers for a while now, and although I'm most interested in more ambitious projects (namely an IB subwoofer), I figured since I'm an idiot it would be best to start with something small.

I ordered the Zaph SR-71 kit from Madisound, without the enclosures. (Remember, I want to learn how to work with MDF.)
I started work on it today.

IMG_0636.jpg


Based on some post somewhere - it would take me some surfing to find it again, sorry - I ordered these MDF screws from McFeeley's:

IMG_0650.jpg


I bought this kit:

http://tinyurl.com/23xdpk

Although I bought these because they were supposedly great at preventing cracking, it hasn't worked out that way:

IMG_0645.jpg


Did I countersink too far? Here's where my screws are ending up (after a lot of complaint from the clutch on my drill):

IMG_0642.jpg


Is this merely very poor workmanship on my part, or is this cabinet sonic junk now?

I'm going with just glue on the other cabinet for now, with no fasteners, just clamping it until it sets. I'm likely to re-do the cracked cabinet with screws you see above. At the very least it will be the cabinet I make all my mistakes on, before moving on to the other.

All suggestions and feedback welcome.
 

ShinOBIWAN

diyAudio Member
2004-02-25 9:13 pm
UK
Ah well you live and learn, don't feel bad about it though. MDF really is quite poor with screws put into the thickness of the material so I'd say forget them altogether. Just using PVA wood glue with clamps makes for a very strong joint, you'll rip the MDF apart before the glue gives way.

If you have to use screws then the smaller the better, imperial type 6 or 7 work well enough as long as you don't over tighten and pilot each hole correctly. I only use screws on very large cabinets though, say a sub enclosure and as few as possible at that.

Like you say, I think it would be wise to rebuild the cabinet with the split.
 

sploo

Member
2007-03-14 9:16 am
The advice already on this thread is spot on... but here's some more...

If you have access to a router, then it's a good idea to route rebates (slots and groves) around the edges of the faces of panels, so it glues together better (no need for screws). See http://en.wikipedia.org/wiki/Rabbet

If you can't do this, and feel the joint may be a bit weak, you can glue a corner together, let it dry, then drill 1/4" holes instead of the pilot holes for screws. You can then drop some 1/4" dowels in, with some glue, and trim flush.

If you really want to use screws, then I've found that clamping two pieces of scrap wood, one on each side of the wall that's going to receive the screw, will stop it splitting as you drive the screw in.
 
My process:

1. Spread glue
2. Clamp the pieces together
3. Predrill holes for screws
4. Screw in drywall screws - only until they are in, no need to tighten
5. Remove clamps, go back to step 1 for additional panels
6. Once the glue is cured, remove screws and fill the holes

This works for me. I learned early on that predrilling before gluing doesn't guarantee things line up. The glue is so much stronger than the screws that you don't need them once everything cures.

One day I hope to invest several hundred dollars in a full set of good clamps, and avoid using screws altogether.

Dan
 

sploo

Member
2007-03-14 9:16 am
Yea, clamps - especially the large ones - can be a bit pricey.

Whenever I buy some tools or consumables from my 'local' DIY store (actually an online place) I tend to add a clamp to the order. I've spent loads on clamps over the years, but spreading it lessens the pain!

Alternatively, you can buy some 1/4" by 1" metal bar, and make your own. I've got a load of the stuff, ready to make some (like this: http://www.woodshoptips.com/tips/050304/index.htm), but it's just another one of those jobs that never gets done :xeye:.
 
My technique (its slow but lines things up).

Glue only 2 panels at a time.
Line up via putting the join on a flat surface and pre-drill for the screws
Put greased lunch paper between your flat surface and the joint to be glued (stops your flat surface from becoming all covered with glue)
Glue along the joint, press together and screw. Just screw in enough so the joint "binds" (you'll see some glue being pressed out of the joint)
When glue sets, backout the screws. Saves money on screws & you don't have to accurately countersink. The glue is strong enough on a but joint
Repeat the above for the next join.

A good technique I've read somewhere is:
- a little table salt on the join with the MDF helps stop it sliding all over the place
- (plunge router with flush trimming bit required). Panel cut the top, bottom, back with the one fence setting on your circular saw / table saw. These panels are then guaranteed to line up (all the same width). Then cut the side panels slightly oversized. Flush trim these against the top/bottom. Then same again for your baffle

I agree using long bar or other clamps would be ideal but decent ones do cost. If you are going to make this a lifetime hobby, well worth it, but for your first few builds, using screws is fine.

I'd say the splitting is due to two things:
- Your predrilled holes aren't big enough for your screws. I always use a drill bit the same size as the "core" screw diameter (minus thread) - so it is only the thread that bites into the MDF
- You are drilling too close to the end of the MDF (as above posted noted) - so at least 2" out from the end is required. Screws don't need to be right at the end... remember the glue does the job.

Cheers,
David.
 
The suggestions using only clamps and no screws are correct IMHO.

Box building has been difficult for me too. I think this is a common problem.

Buying beautiful finished cabinets is the best route (if possible). The money spent on tools, materials and failures will make the $200 spent on cabinets pale in comparison.

Then there's the MDF dust mess to clean up. the stress of murphy's law gone ballistic, the pools of white glue, the wife yelling and...etc. (pulling my hair out :bawling: )
 

camaudio

Member
2007-12-31 10:38 pm
i would never NOT use screws in any construction, and ive built many, many boxes

if you pre-drill well enough and keep the screws 4" away from the ends, you shouldnt ever get cracks

but if you even do get cracks, just fill them with glue and slap clamp on it for a couple hours, then proceed
example of screw placement
[IMGDEAD]http://www.lt1ls1.com/Speaker%20Build/tn_P1110056.JPG[/IMGDEAD]
[IMGDEAD]http://www.lt1ls1.com/Speaker%20Build/tn_P1110026.JPG[/IMGDEAD]
tons of screws...no splits
[IMGDEAD]http://www.lt1ls1.com/Speaker%20Build/tn_P1110027.JPG[/IMGDEAD]
 

Hezz

Member
2002-12-22 6:52 am
Utah
Stew,

The picture is a little fuzzy but it looks to me like you screw was too big for the hole. Also, if you screw down too tight it will crack. The drill should be a little larger than the inner diameter of the screw threads, and a little smaller than the outside threads. The more material that has to be displaced by the screw the more outward pressure the screw makes on the surrounding material. And the more likely it is too crack. MDF cannot take much so the drill size is critical.
 

camaudio

Member
2007-12-31 10:38 pm
Hezz said:
Stew,

The picture is a little fuzzy but it looks to me like you screw was too big for the hole. Also, if you screw down too tight it will crack. The drill should be a little larger than the inner diameter of the screw threads, and a little smaller than the outside threads. The more material that has to be displaced by the screw the more outward pressure the screw makes on the surrounding material. And the more likely it is too crack. MDF cannot take much so the drill size is critical.
well said!
 

MJL21193

Disabled Account
2007-03-10 1:20 am
Well, unless you're building speakers that will need to withstand a lot of physical abuse (ie road cases, and then you'd use plywood), there is no advantage in using screws with straight box construction. The glued joint will be strong enough.

Another problem with screws in MDF is that they will make a bulge. Depending on how smooth your final results needs to be, this can be very important. Even with the correct size pilot hole the screw will still split it, as MDF is so weak through the middle.

In the attached pic the arrows point to the bulges from driving screws to help clamp the curved baffle. Not a big deal for some, but if you expect to make this smooth for a gloss finish...
 

Attachments

  • im000887.jpg
    im000887.jpg
    47.5 KB · Views: 1,358