Tapping a thread in resin

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aquaholic

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Is it possible to tap a new thread into a resin? If s which type of resin s best to use, also will it take a self tapping screw instead?
 
Did an epoxy course at West Systems and they say you can tap a machine thread into their resin. of couse it will be weaker than metal but OK if lightly loaded. Cannot remember if they said to use a particular mix - suggest you ask them direct
 
A lot depends on the resin and the reinforcement. It's definitely possible to tap a thread in most resins - just use a tap as if it was metal, but how much load it will take is a different question. Self-tapping screws are more difficult because you need to be careful you get the pilot hole just right. Too small and there's a chance you'll crack the resin. Too big and the threads will pull out. If you can get back with a description of what sort of resin, how thick, whether or not there are any fibre or powder reinforcements, and what sort of magnitude and direction the forces acting on the fastener will be, it might be possible to have some further thoughts. Is this a block of resin that exists already or that is yet to be made? If the latter, there are a great many other options available.
 
This info from WEST might be useful:

http://www.westsystem.com/ss/bonding-hardware/

Instead of tapping a hole for the screw I would enlarge the hole, fill it with epoxy and cast the screw in place (wax the screw if you might want to remove it later) .

This method is very strong. I saw it demonstrated on a test rig during a WEST training course and the timber to which the fastener was bonded broke first.
 
If you want to be able to remove and refasten the screw often then maybe use a helicoil thread insert like you would with aluminium (in a non-marine environment). Helicoils are stainless steel so if the screws are also there would be no corrosion issue.
 
Last edited:
CreakyDecks said:
If you want to be able to remove and refasten the screw often then maybe use a helicoil thread insert like you would with aluminium (in a non-marine environment). Helicoils are stainless steel so if the screws are also there would be no corrosion issue.
Click to expand...

+1
Thread inserts can work well used in this way (although my preference is for Recoil rather than Helicoil...simpler system), but the load-bearing capacity will not be as great as an insert or a tapped hole in metal. They're best used in an epoxy/microfibre mix.
 
Depending on the application, a rivnut might be more reliable than a tapped thread but, for high loads, I'd agree with those who suggest casting a nut in place.

I've never needed to do it, but I think plan A would be to grease a bolt, thread the nut on it and cast the whole lot in position. Once the resin's gone off, unscrew the bolt.
 
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macd said:
+1
Thread inserts can work well used in this way (although my preference is for Recoil rather than Helicoil...simpler system), but the load-bearing capacity will not be as great as an insert or a tapped hole in metal. They're best used in an epoxy/microfibre mix.
Click to expand...

I like that idea but how do you keep the epoxy out of the inside of the Helicoil?
 
parsifal said:
I like that idea but how do you keep the epoxy out of the inside of the Helicoil?
Click to expand...

The mix is allowed to cure, then tapped (for the insert) in the normal way. No harm in putting a smear of epoxy on the outside of the insert before inserting, then after locking it, running a lightly oiled nut through the insert. Alternatively, run a tap through while the mix is green.

I'd stress that this is not appropriate for very high loads, but makes for neat 'blind' fixings.
 
You only need the same cross sectional area (CSA) of threads cut in the epoxy, as a 'cast in' nut CSA to achieve the same strength. It therefore follows that if you have sufficient depth of epoxy you should be able to achieve a very strong fastening where the primary substrate is epoxy. Clearly though over drilling an old metal stud hole and filling it with epoxy is not going to have the same strength as a metal thread. However, if the tapping into an epoxy base, say GRP mount for a cockpit spray hood, then tapping the GRP maybe good enough if the depth is good. I suspect in many instances there is insufficient depth of GRP and a cast in nut is probably more robust when it comes to frequent removal and insertion of the threaded item

West does have figures, from tests, where wood screws (in solid wood and ply) had the screw holes wetted out and the screw inserted into wet epoxy and then backed out before they set i.e. are not epoxy glued in place. In some pull test cases the head of the screw sheared off before the cured epoxy soaked threaded holes striped out.
 
macd said:
The mix is allowed to cure, then tapped (for the insert) in the normal way. No harm in putting a smear of epoxy on the outside of the insert before inserting, then after locking it, running a lightly oiled nut(?) through the insert. Alternatively, run a tap through while the mix is green.

I'd stress that this is not appropriate for very high loads, but makes for neat 'blind' fixings.
Click to expand...

Understood - thanks for the advice.

ps I guess you mean 'screw' not 'nut' :)
 
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