Carving an 8 foot P-51- questions

[GeneMpls]

I have been a Sbotter for a year now- on my second SB- a 2007 PRS and
having a ball. I am making an 8 foot long Mustang... found a model on
Sketchup and got it into Aspire and split it on the centerline and sliced it into
one inch layers. I have learned a lot and this far it has progressed nicely.
The Mustang fuselage is quite slab sided and lends itself to this.
At this scale (about 25%) each half of the fuselage is 4 inches wide. The
first inch on each side will be an thin wall square aluminum tube armature-
the first layer outside the armature is 2" Duna HDU and the outermost layer
is 1" Duna. So far so good.
I machined the 1" layer first and that went just as planned. The highest
point on the model is the surface of the foam- so not machined. The problems came when I started on the 2" layer. I used a .5 inch round nose for roughing(and finishing)- roughing went well. When I tried the finishing pass I had problems with the bit (which has a .75 cut depth) slamming into pieces of the foam which was left by the roughing- causing the bot to loose its zero.It seems like the roughing left way more material than it should have.
I had to abandon raster cut and use the offset strategy to avoid the bit slamming into foam that was not roughed out. And beyond that problem-
the nice flat surface that mates to the next slice was machined down a .25
or so- which did not show in the preview.
I (miss)spent 30 years of my life repairing and restoring Corvettes and am
not concerned about the large stepover BTW- a DA sander and polyester
prime will make quick work of that. Thanks for any thoughts. Gene

P.S. I see that picts don't show much detail- roughing was at a .05 machining allowance and I did not use the boundary vector offset- made my own offset
manually.

[Brady Watson]

Gene,
Next time do a profile pass around the boundary of your parts - either all the way through or say, 90% through tool to create a little gully around the part, depending on hold down. I like to remove all ambient material before the finishing pass.

You typically do not rough with the same diameter tool as you finish with. Usually the rougher is larger than the finishing tool, so that it creates some added clearance around the perimeter. If you offset the profile vector to the outside of your part 1/2 the dia of your rougher + .02", and use it for roughing, it should eliminate that material at the end of the raster, as long as you choose the profile vector as the machining boundary.

-B

[GeneMpls]

Thanks Brady- but I still don't understand why the flat portion
of my slice was machined. Here are some more picts- the
preview of the machine path (which is what I wanted/expected)
and the trace of the tool path ( I assumed that the tool was
passing clear of the surface of the work- but it was cutting at
least .02... could be more) and a pict of what the 'flat' surface
actually looks like. Thanks Gene

PS- I changed the tool path to raster with a high stepover to make the trace easier to understand

[GeneMpls]

I posted on Vectric as well and a nice gentleman suggested
that I view tutorial F6, which of course answers the question
completely. And he was nice enough not to state the obvious-
that I should watch all the tutorials! Thanks Gene:o

[Brady Watson]

Thanks Brady- but I still don't understand why the flat portion
of my slice was machined.

Yes - watch the tutorials.

-B

[GeneMpls]

I loaded them all on my tablet and have been watching them
in order. Sometimes we are too busy to do things in the correct
order. And as I tell my employees- you can learn more when things go bad than when everything is going well. Thanks Gene