I used to do this all the time between NW and LH flights, on both outbound and return trips. IMO 1.75hrs should be fine.

The DL flight will arrive in FRA Terminal 2; you will get on the SkyLine train to Terminal 1 (concourses A,B,C) which is LH land. Once there you pass through immigration and customs and, as another poster accurately described, you will weave your way through the warren of LH T1. Parts of that terminal are ridiculously bad. But, in true German fashion, things run pretty smoothly and it shouldn't take you more than 30 minutes to get from the DL arrival gate to the LH gate. Important: you can get a BP at a transfer desk or the gate. I've done both, it just depends on the lines at the transfer desks.

On the return you will do the reverse, however you do need to exit security and check-in at the DL counter in T2 (as if you were starting your trip in FRA). If I recall the Skyline from T1 dumps you just upstairs from the T2 check-in area, so it's really pretty easy. But I would add another 20-30 minutes of extra time going in this direction.

p.s. I was carry-on only, but I'm pretty sure interlining will be fine; just show them your bag claim (bar code thing) when checking in for the 2nd flight.