Types of blade clamps
This was the early clamp used for plain end blades. It was basically just two pieces of thin metal with a bolt and wing nut setup. It worked because the saws speeds were not very fast so a lot of stress was not transferred to the blade and clamp.
Next came the New Rogers type of clamps. The blade actually slid into an opening between two stationary metal pads and then a thumbscrew was used to apply the pressure to hold the blade in place.
Next was the use of the pins on the blades so they could be hooked into a notch on the arm to hold them.
Then the idea was used that the notch could be made in metal and attached to a wood arm, commonly used on the earlier saws.
This old Delta rigid arm saw used the same style of clamp as the New Rogers saws of the 1880's
The pin end blades are still used today as shown on this Central Machinery saw.
Saws like the Delta 15" and Hegner use the block type blade clamps. The blade is inserted into the clamp and then the clamp is placed into the saw.
The Delta now uses a " Quick Clamp" that is a lever lock system to lock the blade into the clamp. A simple flip of the lever releases the blade, or clamps it in place.
I use the Delta and really like the blade change on it. Once you set the clamp for a blade size it's just a flip of the lever to open the clamp or close it. The initial setting for the blade size consists of turning the thumbscrew until there is about 1/4" of throw left to lock the blade in the clamp.
Other saws like the Dewalt use a thumbscrew type setup. Just turning the thumbscrew loosens the blade, tighten it to clamp the blade in place.
Some of the other saws like the Dewalt boast that there is no adjustments for the different blade sizes. Well when you just unscrew the clamp until the blade fits into it and then screw it back in until the blade is tight , I guess that could be called no adjustment. But going from a small blade to a large blade you will unscrew the clamp more to initially get the new large blade in it, then tighten a small amount to lock the blade in. So there is actual at lot more turning of the thumbscrew to open it farther to get the blade in initially, compared to opening the clamp and putting the same size blade back in.
On clamps like the Hegner the clamp is opened full every time then screwed close until the blade is locked in. So this type clamp you completely open every time then close it until it locks the blade.
Think of this whole thing as walking through a door. Do you open the door just enough to walk in or do you open it all the way to the door stop before you walk in. Either way you still get through the door, but opening it all the way took a split second longer. Doesn't seem like much, but now doing fretwork you may be doing this 100-500 times.
You ask about the slot in the table to feed the blade into after it is in the clamps. For that style of clamp it is looked at as a neat feature. Without the slot you would just stick the blade through the hole in the table. Is this like walking over to the neighbors house, do you follow the sidewalk out to the street and then to their sidewalk, or do you just walk across the yard on the grass from your door to their door.
Most all of the blade clamp systems on all of the saws work good. Some are easier for some people to use than others. But the one I like you may find difficult, same as the one you like I find difficult. The best thing to do is to actually replace a blade in every saw that you are thinking about, so you can see for yourself how easy it is. There is no perfect clamp or everyone would be using that one.