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. It will be a designed 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 the sheet as it was easier. This was cut prior to fixing the sheet down and the tongue and groove of the sheet helps to locate and hold the cover flush at that end.
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 unobtrusive. In an attempt to minimise manual fiddling I chose to run the overlay boards across the open hatch as shown in image 3 and then to cut out the overlay as one piece using a flush trim router bit using the cut opening in the sub floor sheet as the template. To do that I just need to make a short starting cut (image 4) before the opening is completely covered over that will allow me to reinsert the bit later (image 5). Unless the end of a board lies within the cut width (image 5 next row) then I need to leave enough either side to accept a cleat, which is at least 100mm.
I'll use a 12.7mm diameter bit which will allow me to frame the cover overlay with a 12mm border strip.
The above will work fine where the (yellow plastic) tongue of the sheeting is in the hatch cover. Where the tongue is in the floor 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
6. First side cut
7. Second side and tape
8. Third side and more tape
After one mishap I began 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 can be removed, edges cleaned up and a hardwood perimeter inlay added to complete the hatch cover.
Cutting out the hardwood hatch overlay boards using a router
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 do 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 even light loading.
But I persisted a little longer and packed the shaft with some aluminium foil to eliminate as much play as I could then tried again with the (so far) unused second bit. 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 had already but was just 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.