Why and how
The hatches provide access to the crawl space under the floor but should rarely be needed. The goal is to achieve a clean unobtrusive look without attempting to hide the hatch completely. It will be a design feature of the floor.
Each hatch is slightly different in width but generally about 450mm wide by 600mm long. I chose to cut them out so that one side coincided with the edge of it's subfloor sheet. Cutting was done prior to fixing the sheet down and the tongue and groove of the sheet will help to locate and hold the cover flush at that end. It will also ensure that the cover, which is not hinged, always goes back into the opening oriented correctly.
1. Hatch cutout in sub floor
2. Hatch open
3. Planks across opening
The overlay boards afixed to the hatch cover will continue the overlay pattern of the surrounding floor, which will make the cover visually less obtrusive. In an attempt to optimize the process I chose to run the overlay boards across the open hatch (image 3) and then cut out the hatch overlay as one piece using a flush trim router bit using the already cut opening in the sub floor sheet as the template. To do that I just need to include a short starting cut (image 4) before the opening is completely covered over that will allow me to reinsert the bit later.
I'll use a 12.7mm diameter bit which will allow me to frame the cover overlay with a 12mm border strip.
Note 1: Unless the end of a board lies within the router cut width (as in image 5) then it needs to end at least 100mm either side to allow for it to accept a cleat.
Note 2: The above will work fine where the (yellow plastic) tongue of the subfloor sheeting part of the hatch cover, but where the tongue is in the main subfloor sheet (image 2) then the tongue will need to be removed first. Otherwise the flush trim bit will be following the wrong line along that edge.
4. Starter cut for hatch overlay
5. Hatch opening covered
Cutting out the hardwood hatch overlay boards using a router
6. First side cut
7. Second side and tape
8. Third side and more tape
After initially going in a clockwise direction I switched to working in an anti-clockwise [climbing cut] direction to avoid the tear-out that occurs when turning the corner from with-grain to cross-grain cutting.
Once the cut is complete on all four sides, the now detached boards are removed and the hatch base dropped back in while the overlay boards are aligned, glued and brad nailed to it. Then the whole hatch can be removed, edges cleaned up and a hardwood border inlay added to complete the hatch cover.
Problems with tools
I knew it was not ideal and that the router bits may be ruined after the job was completed but ...
Image 9 is as far as I got with the bit that I had, which was just too short in the cutting length. Nevertheless, for a cheap bit from a cheap set it did very well to last that long as it turns out.
I tried to source a half inch spiral flush trim bit but would have had to wait too long or pay too much. So I bought some cheap straight flush trim bits with the top bearing and more than enough cutting length, expecting to throw them away at the end. They were sold as a duo complete with ten spare bearings for less than AU$20. After just a few inches doing no more than cleaning out the slot cut by the first bit the bearing completely disintegrated (image 10). Several more bearings later and some actual routing I caught a bearing half way to completely disintegrating (image 11).
After some investigation I determined that even the new bearings had some play in them, both within the bearing itself and between the inside diameter and the shaft diameter on the bit. I would not trust these even for light duty because I'm sure the excessive play causes them to self destruct under the lightest loading.
But I persisted a little longer and packed the shaft of the (so far unused) second bit with some aluminium foil to eliminate as much play as I could then tried again. I clearly pushed the bit over it's limit again after which it faught back violently (images 12 and 13).
9. Struggled to get this far using a bit that I already had but was just a fraction too short in cut length.
10. Where did it go?
11. Partially disintegrated
12. Not good.
13. This takes some doing!
I've since managed to get through most of the four (of five total) hatch cutouts using a CMT bit (half inch shaft) without destroying its bearing, and only had to stop because the retaining hex screw flew off. It was lost under the floor until I could complete the cutout with another bit and retrieve it. The screw is an imperial TCEI 1/8Wx3/8 inch and appears to have a UNC thread, so none of the [metric] bearing retaining screws that were at hand from my other bits were suitable as a replacement. The thread pitch prevented them from getting more than one or two turns in before locking up.
Fixing the overlay boards to the hatch base and adding the edge surrounds
With all the difficulty routing the hatch openings every remaining edge needed trimming to produce clean straight lines. That process resulted in varying gaps between the main floor overlay boards and their matching hatch inlay pieces. Initially, for each hatch, the four edge strips will all be ripped from leftover boards a little oversized (width), then after all strips are fixed in place they will be trimmed for the hatch to fit snug in it's opening with the inlays aligned to their floor counterparts.
Images 14 and 15 show the completed den hatch, the first of the five to be done. The blue plastic strap will be removed when the flush pull handle is fitted. As can be seen, the hatch is not invisible but it is camouflaged by selecting boards with interesting grain patterns.
14. Completed hatch in the den
15. Completed hatch in the den