Water jet tolerances 5 inches thick

I'm quoting some parts that that are shaped like a boomerang 5" thick from 6061 T-6. They will have to be machined all over afterward, but I'm thinking of having the blanks cut by water jet (subcontract, I don't have a water jet).. For that thickness, how much material should I have them leave on to machine after water jet cutting the blank ? There is a 12" radius in the the center, and the part is 20" by 20", and will average 1 3/8 wide when finished. We have had very little ever cut by water jet, so I don't have any reference.

I don't run a water jet myself, but I asked the guys here who do. According to them, the blank will flare out at the bottom by only a little. They claim that 0.03" should be enough to leave for cleanup, but leave 0.06" if you're unsure.

Or you could ask the folks at the place where you will get the work done.

*disclaimer* Not a machinist.
I'm just a welder, but I work on a bunch of parts that start as waterjeted blanks and end up machined after. Some are 1"-2" thick done in house. I have seen 1/16 tapers through the thickness, and have seen other edges square enough to not show light through the cut edge and my import 123 blocks and a blanchard ground welding table (I hope that makes sense, again not a machinist...).
From talking to the guy who runs the jet, speed is one of the biggest factors in terms of cut quality/squareness. Fast causes taper, slow takes too long...
W.A.G to your situation... Plasma cut them? Unless you are constrained by thermal issues. If that's the case, give your self a 1/16"+ a smidge all around and let the jet people rip and tear through as fast as they can...
Again, not a machinist, just a welder that works with and hears them bitch about the parts we are working on...

plasma 5" ?!

I can't say for sure how much allowance you need--you should talk to the place that will actually do
your work--but I can tell you that waterjet machines can be set for various levels of edge precision.
Run at the highest speed they're actually pretty coarse but slow them right down and it's scary how
sharp and square the edges can be. Of course cost goes way up in proportion to how fine you want
the cut to be...

Since you're likely to semi-finish, then finish the profile, I'd be generous and leave at least a tenth on all sides. Won't make much of a time difference during the semi-finish pass, and could save a lot of cursing if there's a spot where taper or other "excursions" cuts into the finish envelope.

5 inches thick in ALUM well they can do it on a water jet but it will be slow time is money on the water jet the one i used to run was charged out at $3 a minute cut time.
You can program the shape and thickness and get a cut time from the machine so quoting is really easy.
It will flare quite a bit on that thickness corners turn out bad if you go too fast on thick items so i would allow 5mm each side as even myself is not exactly sure on someone elses machine, programs by another and i have not cut 5 inch thick only about 4 inches so to machine it off it not a big deal but you cannot add it on once its cut if a blocked nozzle or garnet interruption happens and mucks up the cut. It does happen at the most awkward of times sometime, you can junk the part if you don't have enough allowance on it.

Better to er on the safe side.
See what the operator of the machine says about 5mm each side and go from there he knows his machine better than i do.
My best guess is they will take about 45 min per part to cut by programming the machine will give you a very accurate cut time and quote should be accurate. Time depends on setting in the machine and machine power. 5 inch is nearing the upper limit of what i would like to cut and near the limit of the z height as well on the machine i ran.

I have a OMax WJ at home I play with. I don't have the A-Jet or Tilt-a-Jet head just straight up and down. I am not on the clock thus speed is not a factor and I have found I get about .0055, about .32° taper from top to bottom no matter what material I am cutting. Thickest so far is 3 inches.

See if you can get a sample from your supplier.

Here is 1 inch 6061-T6. A buddy of mine broke the shifter collar on his 57 Olds. Instead of buying a whole column I made the part and pressed it on the shaft.