Helicoils in acrylic?

Conrad Hoffman · Jul 26, 2010

I'm making some replacement insulators for a piece of electronic test equipment and want to use clear acrylic. The fastening screws are #10-32 and I was thinking about using Helicoils because acrylic isn't very good when it comes to fine threads. This isn't a high stress application or I'd use something else. Two questions. Has anybody ever tried Helicoils in acrylic? Does anybody know the actual tap size for a #10-32 Helicoil? I only have a few to do, lots of inserts, but no tap. Figured it would be easy enough to make a tap for plastic if I knew what size to make it.

Limy Sami · Jul 26, 2010

It works a treat in acetal (delrin) so can't see any prob with clear acrylic no idea on tap spec, FWIW in MSC/J&L UK a 5mm helicoil tap is about £5 say $7 - 8

2nd thought;- IME Acrylic tends to grab taps, so they need a bit of backing off (form relief) 7 bucks could be cheap

Clive603 · Jul 26, 2010

The mechanical properties of clear acrylic materials vary significantly depending on source and mode of manufacture.

Between 25 to 35 years back I routinely hand tapped goodly numbers of 2 BA (0.185" OD, 31.25 TPI so near as dammit the same as 10-32) holes into ICI supplied "Perspex" of 1/4 and 3/8 thickness using whatever tap was to hand in the lab workshop. I don't recall any problems in cutting or in thread durability. Done dry to avoid contamination. The taps were unlikely to have been in pristine condition, picking alloy pinning out before use being normal along with rounding up, sorting and sharpening drills before any job could be done.

I seem to recall suggestions that helicoils aren't best practice in brittle plastics, such as transparent acrylics, which can be snapped along a scored line due to stress raiser issues at the end once the insertion tang has been snapped off. Bush type inserts being preferred, which was a major reason why I ended up with a nice Timesert 5 to 12 mm kit!

Clive

Ray Behner · Jul 26, 2010

Does anybody know the actual tap size for a #10-32 Helicoil?

Conrad,
That is an "STI" tap. Screw Thread Insert tap. Same tpi plus the insert size. You can get a cheap one for around 5 bucks, or US made for double or more. I think it'd be kinda trickey to guess the size for making one.
Ray

Bruce Johnson · Jul 26, 2010

Another common technique for putting threads into acrylic is to get some brass or aluminum threaded standoffs, as are used in electronic gear. Drill a round hole and glue them into the acrylic.

wippin' boy · Jul 26, 2010

i have no track record with your application, but i have found no place where these

would not out perform helicoils

McMaster-Carr

Forestgnome · Jul 26, 2010

My opinion is just tap'em 10-32. I've seen many plastic insulators with this thread size. I think inserts are going to cause you more headaches than they're worth. Maybe try making afew and test them first.

CougarMountain · Jul 26, 2010

Are the screws going to be installed once and left, or removed often? The threads in the acrylic should be fine for a "screw it in and leave it alone" application. Helecoils are great for muti use, and for thread repair.

eddieirvine · Jul 26, 2010

Use what Wimpin Boy suggested but without the pins you drive in you won't need them just use some blue loctite screw them in and you will be done! Don't use helicoils on your operation, the bad part about them they can unwind themselfs and it takes a special size drill bit comes in the set you buy when you buy a repair kit! The ones Winpin boy is showing are called keenserts I am going to guess other names will be the same product! very easy to put in and will make a strudy great looking job when you are done!

Conrad Hoffman · Jul 26, 2010

The recession must be over- you guys just aren't frugal enough! Spend five bucks on a tap? You think money grows on trees? I suppose you think there's a better use of an hour or two, like profitable work or something?

I've got this big box of #10 Helicoil inserts from a surplus buy, just looking for a place to get used. OTOH, the screws will never be removed so now I'm thinking just tap 'em and leave 'em be.

FWIW, the original parts were way over designed, with a big cross drilled hole and a brass plug that was then tapped from the side. Structurally it could handle a nuclear blast, but the phenolic they used became electrically leaky over time.

Thanks!

machinistrrt · Jul 26, 2010

If the tapped holes are used only for fastening, consider 10-24. The coarse thread will have more strength in the soft parent material. That said, Helicoiled holes will be considerably stronger in acrylic than ones just tapped, the load is spread on a greater surface area by the insert.

Use a #7 tap drill for 10-32STI tap. Also, use at least 1.5x dia insert length, 2x is better.

Conrad Hoffman · Jul 26, 2010

Since this is a replacement part, I need to stay with the original screws. In fact, one of them is trapped and can't be removed from the associated assembly at all. There's a huge amount of engagement, so just tapping it is probably OK. My inserts are the extra long style, so they'd be good too, though I agree with the above poster about the solid type inserts being better here. If you want to see one of these things that I've rebuilt, I have a page here.

wippin' boy · Jul 26, 2010

i think i would avoid the acyrlic and stick with lexan or pplyn
i have issues with acyrlic cracking and if its that big a bitch gettin that one screw in and out i would not want to do it twice

avivz · Jul 26, 2010

Insert is better

There are special brass inserts for plastic/wood (often pressed-in). This would be much better solution comparing to helicoil.

Limy Sami · Jul 26, 2010

Conrad Hoffman said:
The recession must be over- you guys just aren't frugal enough! Spend five bucks on a tap? You think money grows on trees? I suppose you think there's a better use of an hour or two, like profitable work or something?

Thanks!

OK, no data, reverse eng it, wind a helicoil insert around a male thread & mic OD, that'll give you your Major dia.

IIRC Looking at a section of helicoil it's like 2 male thread forms back to back.

Though knowing what I do now, the thread having progressed, I'd just tap em, and if I could (engineering etc etc etc) keep away from acryllic.

specfab · Jul 26, 2010

I (also) would just tap the material directly, no inserts. If the electrical characteristics are suitable, I would also stick with polycarbonate rather than acrylic. The geometry of intersecting holes close to the edge of the material, coupled with the tendency of acrylic to suffer brittle cracking/crazing failure is stressed during machining or installation, would make polycarb a more durable choice, if machined correctly.

jim rozen · Jul 26, 2010

I personally tap acrylic like that for the application. Yes actually do it a lot,
using the standard no. 21 drill.

UNDER NO CIRCUMSTANCES USE ANY LOCKTITE PRODUCT ON
THIS MATERIAL.

If you want to find out why to avoid this, do a test piece and let it sit
overnight. Put it in a shoebox so you don't have to sweep up the pieces
afterwards.

Conrad Hoffman · Jul 27, 2010

Yes, I suspected Loctite might be a bad idea in acrylic. FWIW, I never realized that Helicoil puts all the necessary data in their on-line catalog for the tap dimensions, if anybody wanted to make one.

Gary E · Jul 27, 2010

It wont matter what you screw it together with....it'l only last so long anyway...
What do you expect, your making it from PLASTIC !!!

Conrad Hoffman · Jul 27, 2010

Well yeah, but the ideal material is fused quartz and I'm just not set up to work with that! I've heard tapping it is a b***h.

Helicoils in acrylic?

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