I read a few posts about concerns regarding threading the soft material that MDF is made of. Different folks had different ideas that all made sense, but I didn't see this suggestion. Modelers strengthen balso wood fo threading by putting a few drops of super glue down a threaded hole, then rethreading the expanded wood once the glue has dried. I've used the trick a few times myself so decided to try it on a JGRO I've been building. Talk about NICE!

The threaded holes have sharply defined threads and the threads hold very well. I can thighten the screws to a mild to mid GRRRR and the threads hold and parts are held together very firmly. The bottle cost $8.60 at my Hobby shop and probably much cheap at a place like Tower Hobbies on line. It was enough to do all of the threaded holes wth aboout 1/3 left, and I'm very happy with the results.

Hi,

I just used standard PVA glue as per a couple of other posts and it seems to work fine.
I think, after doing it, all you need is PVA. as you really done put too much force on the threads.

I tried serveral different ways to get the PVA in all theads. the easiest way i found was to pour some PVA down into the tread and then start winding in a bolt or allthread. at the same time hold your finger over the other end of the hole. This make a small amount of pressure in the hole and, i guess, forces the PVA into the threads.

then just wind the bolt out and let it set.

the most surprising thing i found was that if i left the thread as it comes out the bolt/all thread is VERY tight. If i ran the tap through it again i found it was loose.

Just My 2c.

Chris

What are you calling PVA glue?

Hi there,

Sorry after closer inpection its called
"Selleys Aquadhere"

"Woodworking adhesive.
Polyaliphatic cross-linking PVA.
Fast setting, easy sanding,
Weather Resistant
Water cleanup"

the quip says "if its selleys it works.", bit corny but in this case it is 100% right. this stuff sticks to MDF like S*&t to a blanket. :)

CH

You think Gorilla Glue would work? It expands quite a bit.That means you would have to retap the hole.

Hi,

I guess that glue would work. But i think you would want something that will penitrate the mdf. I dont know if your one does that.

But in the end the threads you are going to use it on dont see that much use or pressure.

So anything is better that nothing.

CH

Hi all,

I'm new cnc but have worked with vacuum clamps for 5 years. I use gorilla glue. I drill and tap my hole apply the glue to the my hose barb, screw it in let it dry for 24 hours and your ready to go. When it gets loose just reapply.

Max

Are we talking about using bolts to join MDF parts instead of screws?

John

Yep. We are talking about tapping and threading MDF and screwing them together. Seems that by using glue in the threads and retapping will make the threads stronger. Any thoughts on that?

Super glues (cyano acrylaic?) work well with MDF because of their ability to wick into the fibers. This wicking is fairly deep (1/8" or deeper) and when it wicks in it swells the wood requiring a retap of the hole. Other glues may work, but they don't have this wicking ability to penetrate deeply.

Yes, I bolted my MDF together with 1/4-20 hex head bolts (with flat washers) and tightened them MUCH tighter than I'd have been able to without the super glue reinforced threads. I honestly don't think other glues including Gorilla Glue can offer the strength and penetration that super glues do for tapped holes in MDF. MDF to MDF gets Gorilla Glue <-- Two thumbs up!

My first cnc router was made using MDF also I have used MDF very frequently in my 25 years working in the timber industry.
I have never used bolts to joint it and can say never had a problem with weak joints. What I do is this; I put a couple of standard wood screw into the face of the part I am screwing to either side of the intended fixing screw. Then screw the joint as normal, I do use longer screw then I would for timber or plywood. Also it is worth the extra effort imo to also use dowels in some situations as a well doweled joint will withstand twisting effects far better than just screws. The latest machine I have built used multi ply in the main and would say if you are planning to use your machine for a long time (not just as a learning tool as I did with my first router build) then go with ply not mdf.
I still use the same method to joint ply as I can use tighter pilot holes for the screws and get them very tight without the risk of splitting.

One last point; you may get a nice tight joint with your discussed method but could find that in use the joint may split and become useless requiring disassembly and new parts having to be made. Just something else for you to think about.

Give it a go on some scrap and see what you think.

John

When my application requires threading mdf, I use thin CA (superglue, cynoacrilayte). It wicks into the mdf making a hard strong thread. The thinner the better. I drill, tap, then use CA, then retap when it has dried.

home depot sells threaded inserts for that purpose ,they would have a far better hold , you can find them in the hardware section