Of loose screws and piezo glues...

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fender3x
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Of loose screws and piezo glues...

#1 Post by fender3x »

I told my wife that "I have a screw loose..." and she exclaimed "You sure do!" before I could say "...in one of my piezos" Turns out there are several of them. One that is loose enough to pull out, and about 7 more that loose enough that it is clear that they are stripped. So my first question is what should I do about that? I am thinking that I could put a little hot melt glue in the hole, of the wayward screws. I could also put a dot of hot melt on all the screws to hold them in place without making it impossible to remove the element. Alternatively, I could put a little ABS slurry in the hole. Or maybe a toothpick with a little CA glue in the hole? I can't be the first to encounter this problem... I am curious what others have done.

That raises another issue... I have been wondering what the best way to glue up a piezo array is? The last time I used modeling glue, it came in a tube marked "Testors" and fairly thick. I have read through posts, but it's hard to assess the pluses and minuses of different approaches. I am inclined to use an acetone slurry...and maybe some of the new thinner modeling glue. Like the Revell Contacta or Tamiya. Or MEK...although I am not crazy about having stuff that toxic in the house. Trying to do the glue up in a way that won't create too much of a mess, but will fill the gaps adequately. Don't really want to paint if I don't have to...

Thanks in advance for any advice!
One man's magic is another man's engineering. --Heinlein

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J_Dunavin
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Re: Of loose screws and piezo glues...

#2 Post by J_Dunavin »

I like the idea of hot glue in the hole, that would probably work.
Glueing the arrays.. follow the plans.
I made a jig to hold everything together, then layed down wax paper so the array wouldn’t stick to the jig. I carefully and lightly glued the horns together with pvc cement (just the cheap stuff) then I came back through and really globbed it in there in the back side. Only requires a little clean up from the front, including opening up some of the mounting holes. Once mounted I never had any issues.
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Seth
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Re: Of loose screws and piezo glues...

#3 Post by Seth »

First thought is use fatter screws. Second thought is drill the holes out, glue a dowel plug in the hole, re-drill pilot holes, and be a little more dainty putting the screws in next time.

I tried my best to put a really smooth cut on my piezo's using an abrasive blade in my table saw. Afterward, I peeled off the little melty chunks and gave the cut edges a light sanding with 1000 on a solid and flat sanding block to give a nice mating surface with essentially no gaps to fill. I used the Extra Thin Model Cement by Tamiya, brushing one surface, then the other, then the first again, then the second again, push them together firmly, and held them for a couple minutes. This produced seams with little to no pushout, although not as invisible as I'd hoped for. But, I'll probably be the only one to notice. I made a thick ABS slurry with the trimmings in acetone and glooped a thick layer over all the seams on the back that aren't part of the flange for added strength. It wasn't too brittle before, but it's pretty tough after that. Once I get the diffuser in place, I do intend on painting the entire assembly with Krylon Fusion Flat Black for an even consistent color. I'm also hoping it helps fill and hide the seams a little too. In testing, spraying a piece of the trimmed off bits showed it was indistinguishable from non painted trimmings once it dried and cured.
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Grant Bunter
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Re: Of loose screws and piezo glues...

#4 Post by Grant Bunter »

I think hot melt glue may push out when you try to re insert the screws.

The ABS slurry (errr you can get black ABS from Leland, or at least could) will integrate better with the housing because part of the process of it setting is melting the plastic at a local level.

That would get you basically a fresh start. I wouldn't try and pre drill, you should be able to screw in the right sized screws by hand easily enough.

As far as building the array, I didn't use model glue to start. I just used ABS all the way from go to woah.
I've built 8 arrays using the same method so far, and none of them have misbehaved.
I build my arrays on the array jig. It lets you check you've cut to the right length, and the correct angle, before you glue up...
Built:
DR 250: x 2 melded array, 2x CD horn, March 2012 plans.
T39's: 4 x 20" KL3010LF , 2 x 28" 3012LF.
WH8: x 6 with melded array wired series/parallel.
Bunter's Audio and Lighting "like"s would be most appreciated...

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Re: Of loose screws and piezo glues...

#5 Post by fender3x »

This is very helpful. Thanks! Interesting to see what others are doing. I was thinking of something along the lines of what Seth has done. My edges may not be as smooth, though, as I used a fine-tooth blade. Not bad, but it looks roughly like it's been hit with 60 grit sandpaper.

I have built a 45 degree array jig, and the pieces look like they fit reasonably well, but I am sure I will need some slurry for filling. I kept all the cutoffs so I have plenty of ABS in the right color for slurry.

Nice that Tamiya dries that quickly. Is it similar with the ABS cement or PVC cement? A lot of the youtube advice has people clamping or rubber-banding things together for the cement to set.... I have only used PVC cement with pipe. This will be my first go with ABS since I was a kid with a Revell airplane ;-)
One man's magic is another man's engineering. --Heinlein

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Grant Bunter
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Re: Of loose screws and piezo glues...

#6 Post by Grant Bunter »

ABS, well it's not instant, but quick enough that you can leave it on the jig to dry for a time to set, before trying to fill any gaps (if there is any) or reinforcing the joints.
BTW my piezo's are cut with the same blade I use for everything else on my compound miter saw.
Built:
DR 250: x 2 melded array, 2x CD horn, March 2012 plans.
T39's: 4 x 20" KL3010LF , 2 x 28" 3012LF.
WH8: x 6 with melded array wired series/parallel.
Bunter's Audio and Lighting "like"s would be most appreciated...

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Seth
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Re: Of loose screws and piezo glues...

#7 Post by Seth »

Had another thought about the stripped screw holes... maybe put a little super glue on some toothpicks, push them in the holes, and slice them off flush with a razor blade.
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Two 2x6 shorty SLA Pro's
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Re: Of loose screws and piezo glues...

#8 Post by fender3x »

I may try the toothpick method as potentially the simplest and least messy. I have a busted tweeter so I can try up to three approaches ;-) Thanks for the tips!
One man's magic is another man's engineering. --Heinlein

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Seth
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Re: Of loose screws and piezo glues...

#9 Post by Seth »

I think that'll work well. With the toothpick deal, Grant's right, no need to predrill. I'd only recommend predrilling pilot holes if you drilled them out and plugged them with a hardwood dowel.
Build in process - 2 WH6, one Alpha 6a loaded, one PRV Audio 6MB250-NDY loaded

Two 2x6 shorty SLA Pro's
One T39, 16", 3012LF loaded
Tall AutoTuba, 20" wide, 2x 8" MCM 55-2421
TruckTuba, 8½" wide, 2x 8" MCM 55-2421

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Re: Of loose screws and piezo glues...

#10 Post by fender3x »

Just a quick note on some findings from searching the internet.

It seems that ABS glue is about half MEK, most of the rest being split between Acetone and ABS resins, with a tiny bit of coloring agent.

PVC glue has about 50% of a different solvent called tetrahydrofuran, and around 30% is MEK and Acetone, with most of the rest being PVC.

Tamiya extra thin is a simple formula, of half acetone and half butyl acetate. No resins, so not very viscous.

People make slurry by dissolving ABS in either acetone or MEK

I am guessing that the difference in formulas is the use of more ABS for viscosity, and varying the amount of acetone to decrease set time. MEK seems to have the longest set time, acetone the least, and bytyl acetate about in the middle. If I am right about that, the Tamiya and the ABS glue should have about the same set time. A bit quicker than MEK, but not as quick as acetone alone. Since ABS will dissolve completely in either acetone or MEK, I am guessing they are pretty similar in terms of bonding strength.

Of course this is all just from reading and not from actual experimentation, so offered FWIW. YMMV ;-)
Last edited by fender3x on Thu Sep 01, 2022 1:12 pm, edited 1 time in total.
One man's magic is another man's engineering. --Heinlein

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Re: Of loose screws and piezo glues...

#11 Post by fender3x »

Just built my first two 16-element arrays, and wanted to report on my experience. I used a glue/slurry of about 50ml acetone plus 10g of ABS cut-off material from the tweeters. I was pretty impressed by how well this worked. I was afraid that acetone would harden too quickly, but it was actually just about right IMO. I did two at a time, and there was time to paint each of the sides to be glued. Once I had the tweeters positioned, they were pretty well stuck in a minute or two. After about 2-3 minutes I could pick the array up off the jig to paint the side getting the next pair. This seems to be about ideal for gluing parts without using any sort of clamping other than a few seconds of fingers.

The "glue" becomes more viscous as you use it. By the end of the glue up it is slurry-like enough to thicken joints. At the end of the build I was able to fill any remaining gaps and strengthen all the joints. It dries quickly, but I let it cure overnight just in case. By morning the arrays were really strong. I did a break test with a couple of pieces of cut-off I glued and reinforced together and the break was more around the seam than on it.

A couple more things worth mentioning. The color matches perfectly, although the acetone makes it shiny rather than matte. A quart of Acetone is roughly the price of the smallest can of ABS cement. And you already have more ABS than you need in cut-off material. It's rare for me to find something that appeals to me quality-wise, aesthetically, and to my inner cheapskate.

Edit:
Forgot to mention that I "cured" the loose screw problem poking a toothpick up from the bottom of the screw lug, and then tightening down the screw. Once the screw was tightly in place, I packed the bottom of the lug with acetone-ABS slurry. What I hope is that this holds the screw and toothpick in place, but still allows me to remove the element if that ever becomes necessary.
One man's magic is another man's engineering. --Heinlein

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CarterKraft
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Re: Of loose screws and piezo glues...

#12 Post by CarterKraft »

thanks for the details.
We like pictures :)
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Seth
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Re: Of loose screws and piezo glues...

#13 Post by Seth »

CarterKraft wrote: Fri Sep 02, 2022 9:07 am thanks for the details.
We like pictures :)
Indeed. Let's see how they came out... show me your tweeties! 🙂
Build in process - 2 WH6, one Alpha 6a loaded, one PRV Audio 6MB250-NDY loaded

Two 2x6 shorty SLA Pro's
One T39, 16", 3012LF loaded
Tall AutoTuba, 20" wide, 2x 8" MCM 55-2421
TruckTuba, 8½" wide, 2x 8" MCM 55-2421

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Re: Of loose screws and piezo glues...

#14 Post by fender3x »

This is how they look as of today. Fresh off the jig with no sanding or painting.
CT1.png
CT4_back.jpg
CT3_center_joint.png
CT2_side_joint.png
One man's magic is another man's engineering. --Heinlein

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J_Dunavin
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Re: Of loose screws and piezo glues...

#15 Post by J_Dunavin »

ok, ok see how the screw holes are out of round? I tried to use drill bits, but they would catch. A step drill bit works ok, but i found a cone shaped bit on the dremel opens them up real nice.

They are looking good!
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