How to make bottom for a big drawer?

Question

I'm making a set of amateur under-bed "drawers" from scrap. Nothing fancy, just re-using materials from old closet. This is my first project of such scale.

There are intended to be 3 quite large drawers, 130 x 60 x 18cm on the inside. Walls are going to be made from veneered particle board (16mm thick), intended to be held together by 2 or 3 confirmat screws on each side. Bottoms are planned to be made from 3mm thick hardboard.

How do a I make bottom rigid enough in the case of such large drawers?

So far my plan is to:

1. add 2 narrow crossbeams on the bottom (something around 60 x 5 cm), effectively splitting large 130 x 60cm area into 3 smaller ones - 40 x 60 cm. Having crossbeams pop up from the bottom by 1cm does not worry me that much.

     |----------------------|
rear |      ||      ||      |  front
     |      ||      ||      |
     |----------------------|
           crossbeams

2. add grooves along all the sides and crossbeams (using router with 4mm bit) and slide hardboard into them.

|     |
|    _|
|   |_   grooves on walls where hardboard slides into
|_____|

If that plan sounds okay, what distance from the edge does the groove need to be and how deep can it go into 16mm thick particle board?

This is an example of what I want to achieve (construction-wise) and what to avoid (sag and breaking):

(taken from http://www.instructables.com/id/Fixing-Saggy-Drawers/)

Answer 1

Bottoms are planned to be made from 3mm thick hardboard.

This is the part you should probably change. If you make the bottoms from something much stouter it will self-support. You could go up to 6mm hardboard, which is significantly stiffer than 3mm, but something like 12mm particleboard/chipboard or plywood would be the superior choice. 12mm and even 18mm board is regularly used in better-quality kitchens for drawers, some of which will be filled with heavy pots and pans. If they can take that they can support the kind of stuff most people store under a bed!

add 2 narrow crossbeams on the bottom

This is also a viable technique. These crossbeams are referred to as muntins and they were the traditional means to support the centres of wide drawers not too long ago.

^{Source: Drawer-Building Basics, excerpted from Chests of Drawers by Bill Hylton, published by Taunton Press.}

add grooves along all the sides and crossbeams (using router with 4mm bit) and slide hardboard into them.

You would normally do this anyway with drawer bottoms. But with thinner material like 3mm hardboard in a wide drawer it probably won't prevent the bottoms blowing out (as in the picture you posted) if the drawer has anything weighty stored inside it.

Note that if the bottom is 3mm then a 3mm router bit should be used to form the housing grooves. A 4mm groove in the box of the drawer will make the bottom a very sloppy fit and provide no real support to it. If on the other hand the drawer bottom is a tight sliding fit* the groove will add significant support to the bottom, although it needs to be glued in place for this to work fully.

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*Tight enough that you need to tap it home with a mallet.