Ron, I believe the 40-640 is the same model as I had where you need the special tool to open and close the bottom clamp. There is not room enough to install the Quickset clamp on the bottom on that saw. Thus you can only bottom feed. I sold mine and purchased the Delta P-20 3 years ago and really like it. I used the old one for 11 years. I liked it accept for that bottom clamp, I never had any problems with it.

 

 

Mick,  Delta- P-20