FE127E hole size, Butt joints and using PVA glue with MDF - diyAudio
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Old 30th May 2006, 06:53 PM   #1
bluegti is offline bluegti  United States
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Default FE127E hole size, Butt joints and using PVA glue with MDF

I'm making my second pair of speakers ever - the Folded Mono-pole's discussed in the
Fullrange Reference Project.

The information on-line and included with the FE127E drivers says the baffle hole should be 4 inches. I did a test cut in a scrap piece of wood and my drivers would not fit. I had to make a hole that was 4 1/8 inches. Is this just bad conversion from Japanese to English or have they changed the size of their frames?

The foreman at my local woodshop said that butt joints are bad and I should not use them because they will not hold. I suspect a LOT of people are using butt joints. Has anyone thought of a *better* way that is still easy to build?

He also said that using PVA glue (like Titebond) has a lot of water in it and that it would cause the MDF fibers to separate and make for a weak joint. He recommended using Gorilla Glue. I've used Gorilla glue before and it foams up and is nasty on the hands. Can anyone comment on the best type of glue to use with MDF?
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Old 30th May 2006, 07:13 PM   #2
poobah is offline poobah  United States
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A rabbit joint offers somewhat greater strength and aids in alignment during assembly... and not too hard to do. Do you have a table saw or router?

Glue choice starts a war here always. Gorilla (better) is good stuff... I use epoxy (best). Alot of damned fine speakers go together with PVA (good)... and I have used alot of PVA myself.

How do plan to finish these? That could aid you in choosing glue.



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Old 30th May 2006, 07:48 PM   #3
chrisb is offline chrisb  Canada
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It's not likely that Fostex has changed the frame stamping - rather that their specs for metric conversion are slightly off (i.e. when I get the driver holes machined by CNC at exactly 4", they are "a thick one" tight)

102.2mm = 4.023"

When this minor discrepancy is compounded by tooling variances, it's not unusual to have a slightly wrong size opening. If you've not put the box together yet, I'd strongly recommend you chamfer the rear side of the driver panel, at least in scallops around the mounting hardware. If it's still uncomfortably tight (and risks damaging the terminals, then a few twists of 60G sandpaper usually takes care of it.

There are at least as many thoughts on best type of enclosure joinery and adhesive as there are box designs. Personally, I go for the rabbeted edge, and yellow woodglue (Titebond, etc). Only if some panels are a bit slopply from repeated disassembly or plain cutting errors do I resort to moisture cured PU's (Gorilla) - they are definitely messy and need overnight cure.
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Old 30th May 2006, 08:29 PM   #4
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I use butt joints in MDF with Titebond I almost exclusively This type of joint is more than adequate for speaker building. Here is how the picture came to be:

I needed a jig for a project. I butt-jointed four pieces of MDF and clamped the assembly until set. Later, I decided this jig would not do and broke it up for scrap. I placed one arm on a board and struck the other arm with a hammer about two inches out from the joint. The MDF fibers broke in an arrow-head pattern from the center of the piece and tapering toward the hammer blow.

A PVA glued butt joint in MDF is stronger than the MDF itself. The glue that soaked into the MDF strengthened the material. The failure was away from the joint itself.

Rabbits, dadoes, furring strips and biscuits are all for alignment and ease of assembly. They add nothing to the joint itself.

Bob


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Old 30th May 2006, 08:53 PM   #5
dfdye is offline dfdye  United States
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Quote:
Originally posted by poobah
Glue choice starts a war here always. Gorilla (better) is good stuff... I use epoxy (best). Alot of damned fine speakers go together with PVA (good)... and I have used alot of PVA myself.
Anything past Titebond is OVERKILL. The only reason to use anything but PVA is make things harder for yourself. And Epoxy is terrible. (pow pow! )

Seriously, I use PVA almost exclusively, but I will throw in liquid nails on cross braces to further damp vibrations (though I have no idea if this actually does any good or not, it makes me feel warm and fuzzy)

I do like rabbets, but not for strength, just for alignment in the case of MDF. For hardwood joints the strength arguments are in full effect, but based on the composition of the MDF, I'm not convinced that you can add much strength by adding rabbets. The above picture is a little misleding, though since both of the pieces are edge glued. In a 90 degree rabbet joint there would be a face to edge joint that could benefit more from a rabbet. Remember, though MDF isn't directional within the plane of the sheet, there is definitely a directional difference between the FACE and the sheet. This being said, I guess I am waffling a little, but I usually rabbet just because it is so easy and it helps assembly so much, regardless of the strength rationalle. That would just be gravy for me.

Oh, and rabb-i-ts hop, rabb-e-ts are in wood. You should know better, Poobah!
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Old 30th May 2006, 09:04 PM   #6
poobah is offline poobah  United States
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dfdye,

You don't build boats with PVA... PVA ain't waterproof. PVA is brittle and becomes worse with age.

Epoxy and urethane have elastic and gap filling qualities than can be a real assist to someone who might be building cabs with a hatchet and a file.

The real difference is in what I stated previously... what type of finish. A lotta guys complain around here about glue lines telegraphing through their finish. Epoxy used as a glue and sealer brings all this to a screeching halt.

Pow Pow!




Damn... bust ME on spelling???? How's 'bout we go Brit. and call them rebates?

BTW, I got just a nifty Indian cook book (from an Indian store no less), want some recipes?

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Old 30th May 2006, 09:50 PM   #7
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ok guys, beat this.

i build my sonosub with a 15" atlas (6.5 cu ft BR 20hz tune), and put the mdf ened caps on(roughly 1.2inches insertion) and glues it together with just....


STOLEN ELMERS SCHOOL GLUE!!!

thats right, and i havnt had any problems with a tight seal and DEFINATELY not strength, ive had this thing running FULL TILT! (which at my room happens to be ~120db at 40hz) and NO problems, still a perfect seal, still all holding together.

In fact ive ever tried pulling it apart, its not happening.

bottom line is, standard PVA glue is stronger than wood anyway, gorilla glue is just MORE stronger than wood(waaa waa waaa waa, here come the grammar police!)

OH and if you wanta REALLY easy contruction method, you should make all the angles at an AT OR NEAR PERFECT 45* angle, then lay the pannels out flat, edge to edge, put some masking tape on top, then flip it, stick some glue in the corners, and flop the top over. This method i read in an article by terry cain( of Cain & Cain loudspeakers) and he still uses it to throw together boxes to this day(not for production boxes i dont think though).

to make that easier though, your table saw should be able to cut with the good side DOWN.
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Old 30th May 2006, 09:55 PM   #8
dfdye is offline dfdye  United States
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Quote:
Originally posted by poobah
Epoxy and urethane have elastic and gap filling qualities than can be a real assist to someone who might be building cabs with a hatchet and a file.
You do bring up a very good point. I guess I don't really worry about that since I have been blessed with the ability to cut straight! (BANG!! BLAMO!! Adam West would be proud.)

Oh, and Titebond II and III are indeed water resistant and waterproof respecively, though the binder in MDF isn't. But we are planning on PAINTING our speakers and providing an additional "waterproof" layer there, aren't we? Need we revisit the waterproof argument and rehash how silly it is again? And while I'll agree PVA is indeed brittle, it isn't a problem with typical speaker or even sub vibrations.

Still, I love Bondo for gap filler if you have problems with glue lines, so I guess I really shouldn't talk about overkill. . . . .

(And rebates are those annoying mail-in thingies you get when you go to best buy and purchase ANYTHING. That's my story and I'm stickin' to it!!!)

Quote:
BTW, I got just a nifty Indian cook book (from an Indian store no less), want some recipes?

Normaly I wouldn't refuse, but I have just started on a diet for a few weeks to drop some winter pounds, and I can just see the butter/oil content of how I cook Indian food being a VERY bad thing for this . . .

Mind you, this is VERY tempting!!!!

If you find something that looks like it would pass my wife's review be sure and post it in the coonking thread! I would love to have something besides canned beans and chicken over the next few weeks!
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Old 30th May 2006, 09:56 PM   #9
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Quote:
Originally posted by dfdye
Oh, and rabb-i-ts hop, rabb-e-ts are in wood.
You mean of course, rebates.
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Old 30th May 2006, 09:57 PM   #10
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Just my .02$:

I think that with everything its about balancing out the trade offs. MDF vs. Veneer core (or even solid wood), or PVA vs. Ureathane. (Titebond II is touted as waterproof).

I've been a carpenter for 20 years, and I despise MDF. Yes, its more acostically inert, but its a pain to work with, and it has no strength. For me Veneer core is what I usually use.

For simple boxes, PVA does great. Just use plenty of it. It will hold MDF together fine for something like an FE127E. The speaker won't put that much stress on your cabinet. If you were building a PA it would be a different story (or even a sub). I just finished a Flamingo D-83 BLH and I use ureathane as the box is very complex and the gaps in the interior passageways needed some filling. The expansion of the glue is perfect for this, though my fingers are now going to be stained brown for a week But, it did its intended job. There is also a glue specially made for MDF (specifically mouldings) that is a 2 part super glue. I remeber using it years ago, but not sure if it would be good for boxes of not.

As for joinery, I'd say stick to butt joints for now. Other types are going to require more math to get the dimensions right, and could cause some headaches if you arn't used to this (my shop guy still can't build a door with out his cheat sheet). Once you have a good feel for building boxes, then start playing with other types of joints.

I'll relinquish the soap box now.

-D
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