TWEAK the JIG
Just my 2 cents on the Harbor Freight Dovetail Jig and others like it.
I have had one for a long time and they work great as long as you TWEAK the JIG.
Any time you have a jig that the parts are held down by a cam device the part will move when the cam is locked down.
In the Habor type, the jig parts move to the right and the left when you slide the blank stock into the jig. (1st error)
To start with screw the jig to a base board,then put side blocks ,so the front plate can't move more than .002 " from side to side when you push the stock under it.
Then put sand paper under the hold down parts (3M spray glue works great for this job on the back side of the sand paper) then press in place and trim.
Stock size is a real key to get the jig to work right and not put out any firewood parts.
Use stock that is 2.500",3.500",4.500",etc. wide.
Side NOTE** Bob and Rick had a small clip showing how to use this type of jig on the Router Workshop..(PBS) I don't recall the date,but it was this year. about 01/2006.
Most will do the front/back and the sides at the same time ,but don't until you have the jig setup on the left side 1st.just right and then use that part to setup the right side with a .500" setup block.
If you want to see a snapshot of the jig I use and the tweaks, Just ask and I will post it.
Once you make a great dovetail box keep it for a quick setup next time.