I want to cut a dado into the surface of my crosscut sled to perfectly hold a T-track flush to the surface. The sled is made out of 18mm-thick MDF. I would only be leaving behind 3mm thickness of wood to allow a dado deep enough for the T-track. Is that enough? I will glue in the track to help strengthen it.
The sled is made out of 18mm-thick MDF. I would only be leaving behind 3mm thickness of wood to allow a dado deep enough for the T-track. Is that enough?
My brain works in Imperial units, so I'm going to convert. Three millimeters is about equal to 1/8". MDF is not a particularly strong material in bending, especially not with that thin of a section. You will probably find that it will want to snap off at the thinned section when you put any kind of load on it. Personally, I do not think that this is enough thickness for your sled to stand up to the use expected of it.
I will glue in the track to help strengthen it.
This won't really have the effect you desire. Wood glue will not bond to aluminum all that well since it's designed to adhere porous surfaces together. It may help keep the T-track in place, but it will do nothing to strengthen the section.
Instead, if you are dead set on using the 18mm-thick MDF, I would suggest using a five-minute epoxy to glue the T-track in place. Scuff the surface of the T-track first so that the epoxy adheres better. This way, you have a positive bond between the T-track and the MDF, and the T-track will actually strengthen the weakness in the MDF dado.
As a side note, I would avoid using 18mm-thick MDF for this application. If you can, double the thickness by gluing two sheets together. Otherwise, I would use 3/4" (19mm) or thicker Baltic birch plywood. The plies of the plywood are pretty strong in bending and will stand up better than MDF at that thickness.