Fastest, most efficent way to cut a dado.

[mission17]

I might be doing a long term job that requires cutting a  1-1/2" wide by 3/8" deep  dado.

Parameters:
-pressure treated southern yellow pine.
-one cut will be 18" long in 4x4  
-one cut will be 24" long in 5/4x6 
-1-1/2" wide by 3/8" deep dado.
-1000 pieces in an order. 500 of each size.

  I bought and tried a Diablo dado set and was not impressed with the finish. Much too uneven.

What options would give me a cleaner cut at a high rate of speed?

[packerguy®]

straight router bit and a jig

something like this that slides onto the board.

https://s3.amazonaws.com/vs-lumberjocks.com/m1pwbgh.jpg

[Steve N]

You will likely get a lot of furring up on PT pine. Not just soft wood, but very wet soft wood makes for furry cuts. The adjustable width dado jig PG referred to will allow for contained multi pass, but for the amount of work you need to do, it sure won't be fast, and each piece will be multi pass. I'd say you will need an industrial Shaper and buy good quality cutters, because cheap will be aggravating.

These Freeborn might work

Hope you didn't bid low to get the work.

[Bit_Fiddler]

try a dado blade with more cutters than the Diablo.  It only has 2 cutters per chipper and leaves a somewhat crude cut.  Frued makes a Dado set with 4 cutters per chipper and Infinity has 6 cutters.  Both will give a much better cut quality and should cut faster than a router.

[fredhargis]

I agree with then TS and a higher quality dado set. The Infinity should do the job nicely and I'm guessing it will be faster than the router bit, though it will still be a 2 pass cut. Note Steve N's comments about the soft wet wood, that will make this a not-so-fun job. If there's any way you can let that wood dry out a while before you start the work, you'll be way ahead.

[CARYinWA]

I don't let pressure treated lumber anywhere near my good tools.  The stuff is wet and corrosive.  I vote for a router and fluting jig.  It will be multiple passes(even with a dado blade).  Set up for one pass and do all of them then reset for the other cut.  Two routers if you have the resources.

[jteneyck]

A shaper with the cutter sized for that cut and a power feeder would be the fastest way I can think of.  It could likely be done in two passes.  Short of that, the TS with a dado set, but it's probably going to take 4 passes with that approach. 

John

[JGrout]

agree a TS with a much better dado set is a good start add a ZCI to reduce the fuzzing 

then add power feed to take the tedium out of the job and speed up the processing for consistent results 

IOW you need to upgrade ($$$) the tooling  to gain the desired results

[JGrout]

A shaper with the cutter sized for that cut and a power feeder would be the fastest way I can think of.  It could likely be done in two passes.  Short of that, the TS with a dado set, but it's probably going to take 4 passes with that approach. 

John

 I have a shaper knife that would make the cut in one pass  but without a high production runs 20K + cost effectiveness  is pretty steep

[blackhat]

One needs to look at the whole job before deciding, do you need to load all these pieces, haul them to your basement shop and then deliver them or could they be done at the customer's yard and let the chips fly?