Do you merely want the bed of the sewing machine to be level with the table top or do you want the whole machine to disappear below the table top when not in use?

To make the machine level with the table top would be easy enough and wouldn't require any mechanism, just a cutout and a lower shelf installed in your existing table top.

To have the whole machine disappear when not in use would be a more intricate job, but shouldn't be too difficult. Many sewing cabinets have the machine head hinged with special sewing machine hinges which have a flat plate recessed and screwed into the table top. The other side of the hinge has a short rod which fits into holes in the underside of the machine head and held on with set screws. In front of the sewing machine is a lift-up flap so there is clearance to lower the machine on it's hinges. Lift up the flap, lower the machine and then put the flap back down. You would then have to have a cover plate to fit in where the machine hole is but you would already sort of have that from when you cut the original hole for the machine.

Anyone who repairs sewing machines may have a set of the hinges, they aren't very large so if they were found online the shipping wouldn't be expensive.

I don't want the machine to disappear. I never use the desk for anything else but sewing.

>>To make the machine level with the table top would be easy enough and wouldn't require any mechanism, just a cutout and a lower shelf installed in your existing table top.Easy enough, if you know how to built shelves! I was hoping to find a ready-made shelf that could just be screwed on, sort of like computer keyboard shelves that can be added to older (pre-computer days) desks.

Before recessing a sewing machine in a table top, be sure the bobbin case is on top of the machine and not necessary to get to under the table top.