# Perfect ratio for pilot hole



## Patrickgeddes14 (Nov 27, 2018)

Fastening to the edges od 3/4 mdf, whats best drill bit diameter to thread diameter to not split and still be snug?


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## RichT (Oct 14, 2016)

I don't recommend screwing into the edge of MDF. Its construction makes it weak for that. Screwing into the face is pretty strong, but the core is soft and the threads don't bite very well. Under stress, they'll be prone to loosen. If all you're doing is attaching some trim, that will work, but I'd still use glue to strengthen it.

That said, there's no ratio involved. Say you're using a #10 screw, do a search for what diameter bit to use for a pilot hole. There'll be two; one for hardwood, and one for soft wood. You'll want to use the soft wood size if you insist on screwing into the edge of MDF.

While you're out searching, you'll find many charts that show what bit to use for different screw sizes-again, there will be one for hardwood and one for soft for each screw size. I printed one out on a Brother label and stuck it to the back of my box of drill bits.

Edit: I hesitated to bring this up because it might sound rude, but under the circumstances I have to. I assume this thread is referring to the MDF you mentioned painting in your previous thread, that was part of a mudroom bench project. You're asking some very elementary questions for someone who is building a bench to sell to a customer. Do you fully understand the structural issues required for something that is going to support the weight of a person? This isn't a plant stand or a bookcase. Someone is going to sit on this and in the course of putting on or taking off boots, or whatever they do in a mudroom, this bench is going to be subjected to a number of different stresses. The fact that you're screwing into the edge of MDF in its construction sets off alarms in my mind. That's just the sort of stress I mentioned in my first paragraph that screwing into the edge of MDF is a bad choice for.

A great way to ruin your business reputation is to have your bench collapse and possibly injure someone. It's also a good way to become the target of a lawsuit.


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## CaptainKlutz (Apr 23, 2014)

+1 Screws into edge of MDF are not very strong.

Suggest you need some education or reference materials in your shop for mechanical stuff? 
This one has pilot hole size by screw information: 
https://www.amazon.com/Pocket-Ref-Thomas-J-Glover/dp/1885071000

+1 There are massive number of web pages on proper size of pilot hole for screws, and/or best screw for MDF if you prefer web over reference books:
https://lmddgtfy.net/?q=Best%20screw%20for%203%2F4%20MDF
or
https://lmddgtfy.net/?q=pilot%20hole%20size%20for%20MDF


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## robscastle (May 13, 2012)

shot your question over to LBD he is an expert on screwing stuff, he has some special taps to do exactly what you are needing.

I will see if I can find his post and link it

Also do a search on threading wood lots of other LJs have contributed related stuff as well.

I tried it and gave up not smart enough!!


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## therealSteveN (Oct 29, 2016)

> Fastening to the edges od 3/4 mdf, whats best drill bit diameter to thread diameter to not split and still be snug?
> 
> - Patrickgeddes14


Using MDF, what are you attaching, to what, and will it have weight bearing, or push/pull load? It may not be the best material for the job.


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## MrRon (Jul 9, 2009)

One way is to drill a hole for a hardwood dowel. Glue the dowel in the hole with epoxy. drill the dowel for your wood screw. If you are trying to add a trim strip along the edge, just use a rabbeting bit in a router and route the edge of the MDF Cut the trim strip to make a snug fit in the rabbet and glue in place with Titebond glue.


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## HokieKen (Apr 14, 2015)

I have successfully screwed into the edges of MDF *in non-weight bearing applications*.

Find a chart like Rich referenced in the first reply and use what's recommended for soft wood. Then put your mating parts together and run a screw in until it's barely snug. Now, remove the screw and drip some CA glue into the hole. Make sure it covers all the threads and let it cure. The thin CA glue will make your thread form harder and less likely to crumble under light torsion load from the screw and will also wick into surrounding areas of the core material and help bond it better to distribute the load over a larger cross-section.

This^ is my method that I gleaned many years ago from someone else online. YMMV. As I said though, *this is not for load-bearing applications*. Only cosmetic ones. And in my case, it was only to aid in dry-fitting and clamping prior to doing a glue up.

A little more information on your application may get some better/more specific suggestions. Good luck with your project!


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## SMP (Aug 29, 2018)

I worked in a shop for a few years where we made custom subwoofer/speaker enclosures out of MDF. Some people would glue and air nail. I did too for a while. But had better luck with drywall screws and glue, i always just used the drill/driver bits that you can flip to pre-drill and drive, like belowmine was set of 3 sizes but similar)

https://www.homedepot.com/p/DEWALT-8-Drill-Flip-Drive-Complete-Unit-DW2701/202579928


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## Sylvain (Jul 23, 2011)

use the bed bolt principle.
(the same basic idea like MrRon) but with metal. see 
exists also in 16 and 12 mm long.

If you use a wood dowel:
- use a dowel diameter 3 times as large as the screw one;
- orient it in such a way that the screw will be perpendicular to the grain.


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## Redoak49 (Dec 15, 2012)

When I have had to brew into mdf from the side, I will drill a pilot hole and then drop some super glue into it. This really strengthens the hole. This is better than nothing but the overall strength is still not great.


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## fivecodys (Dec 2, 2013)

> One way is to drill a hole for a hardwood dowel. Glue the dowel in the hole with epoxy. drill the dowel for your wood screw. If you are trying to add a trim strip along the edge, just use a rabbeting bit in a router and route the edge of the MDF Cut the trim strip to make a snug fit in the rabbet and glue in place with Titebond glue.
> 
> - MrRon


This is how I do it as well. This gives the screw something to bite into.


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## Heyoka (Oct 21, 2018)

When edge screwing into MDF I drill a relative large pilot hole and use very long screws….oh! and never screw near the corners.


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