When youre porting cast iron heads, do you use a single cut or double cut bit? And how (ie how many ports/heads) do your bits last? What kind of grinder speed do you use?

What about on aluminum?

The bits for cast iron and aluminum are different - coarse for aluminum, fine for iron.

Not sure what you mean by "double cut".

You'll need an assortment of cutters as well as different lengths.

Use a good carbide bit and they last - should be no problem doing a set of heads with a single set of bits. Use cheapies and you won't get a single port done.

Electric grinders spin about 25,000 RPM. The problem is they get HOT. Also, I've found the bearings go out of them really fast because of the side loads and speed.

Air grinders are a bit slower and stay cool BUT you need a serious compressor to keep them going.

I don't do any jobs as big a heads any more so I stick with the air grinders

http://www.eternaltools.com/blog/7-facts-about-tungsten-carbide-burs-and-how-to-use-them

I don't have many answers for you. Never ported anything iron.

Used an assortment of burrs on my intake manifold and air grinders.

single cut is for soft material and will cut faster and grab the material, more for use on aluminum, double cut is for harder material and won grab as bad and also leaves a smoother surface on both iron and aluminum but alum will clog them up, I use WD40 while cutting alum.

Yep... plus I use both air and electric for doing
port work.. I have a reaostat that I can dial down
the rpm.. I use the WD-40 also... on my alum burrs/bits
I use wax also(if I have it.. keeps them from loading
up.. as to rpm.. dont know what it is... but you will
know when its cutting well.. dont over heat your burrs
or they will get dull in a blink.. spend the money on
quality burrs... I just bought 2 new ones the other day..
they were $22 each for the good ones from a abrasive compan
(the short shank ones).. my long shank ones range from
6" long to 12" long.. I only have 1 12" one

I ghecked a couple of my air motors and they are rated at 25,000 rpm at 90 psig.

I regulate my air grinders with the trigger.. a
finger under and over the trigger to get less RPM

I use air, got a valve in the line to keep max speed under control easier and I run 2 air compressors, my big 80 gallon one will get behind after a while so I just piggy back my little 10 gallon one into another shop hose and they keep up great together.

For aluminum I use the aluminum bits with wd-40 and finish em with the iron cutter and lots o wd-40. I buy bits from mac tools guy, he warrantys them when they chip a tooth. Expensive but worth it.

I use carbide, I have a front exhaust die grinder. WD-40 helps. If one does get clogged with aluminum just grind some scrap steel and it will clean it's self.

My claim to fame? is that I started a machine tool business for the owner almost 20 years ago. Wonder of wonders, we are STILL in business! LOL. At any rate, to answer your question: Single angle carbide burrs are for aluminum. The flutes are wide apart and deep to evacuate the large chips that Aluminum and other "non ferrous" materials produce. A "double cut" is a carbide burr with shallow flutes, spaced close together. This is the burr to use for cast iron which produces a small, powder like chip. Also with harder materials, you want to have more cutting edges to take the maximum amount of material possible per revolution.
As for speed, a rule of thumb is 400-500 surface feet per minute in steel and iron. 1,000 surface feet per minute to 6,000 surface feet a minute in aluminum. The enemy of the tool is heat.
To calculate surface feet per minute (SFM) to revolutions per minute, use this simple formula: SFM X 3.82 / diameter. Example: 1/4" diameter burr in cast iron. 500 x 3.82 = 1,910 / by diameter (.250)= 7,640 rpm.
Aluminum: 1,000 x 3.82 =3,820 / .250 = 15,280 rpm. Now you know why your die grinder turns 30,000 rpm! If you stay within these recommended parameters, you should get the maximum life from your expensive carbide burrs. Also note that aluminum tends to stick to the carbide, gumming it up. In computer controlled CNC machines, we use a special coolant to help with this. If you could get a gallon of this from a local machine shop,(pre mixed right out of the sump of their machine) and put it in a spray bottle, it will extend the life of your tools. But, beware! It will sling coolant everywhere! especially at 15,000-30,000 rpm!

Myself.. my alum bits never touch steel or iron..
if I clog a alum bit I stop and let it cool and spray
it then pop the alum out of the bit.. they will load
up with heat... also I dont use my steel bits on alum..
if I want it a bit smoother I will hit it with tootsie
rolls.. but not fine ones.. unless its the exhaust side..
I like the exhaust side SMOOTH

I dip my aluminum cutters into a small cup of atf while grinding, the chips will fly right off, can be messy if you use too much though, just a quick dip every once in awhile, I also use an air grinder, they seem to last longer and are lighter than electric.

Joey

I'm confused. So do you all agree with this statement of the link above?

1. Carbide burrs can be used on many materials

Tungsten Carbide burrs can be used on most hard materials: metals including steel, aluminum and cast iron, stone, ceramic, porcelain, hard wood, acrylics, fibreglass and reinforced plastics.

Different cuts of carbide burs will be best suited to certain materials, see the next point below to find out about the different cuts.
2. Carbide burrs commonly come in two cuts; single cut and double cut

Single cut (one flute) carbide burrs have a right handed (Up cut) spiral flute. These tend to be used with steel, copper, cast iron and ferrous metals and will remove material quickly with a smooth finish.

Double cut or Cross cut carbide burrs tend to be used on non ferrous metals, aluminum, soft steel, plastics and hard wood. Double cut or cross cut (2 flutes cut across each other) will leave a slightly smoother finish than single cut due to producing smaller chips as they cut away the material.

I am not understanding what they are saying.. a
alum burr has wide cutter on it(single cut)... I
will put up a link for both alum and steel/iron burrs..
a double cut is used on steel/iron
EDIT
When I'm doing alum I get it close with a single
cut burr.. then I fine tune it with a tootsie roll
thats medium to heavy grit(it will vary on how close
I was with the burr).... on iron I will get it a lot
closer with the burr(double cut burr) then fine tune
it with a course tootsie roll.. you can getthe burrs
in multiple styles and shapes... I like egg shape and
tree shape.. in different sizes and cuts
Also.. the carbide or tungsten carbide is just
the material its made out of.. you can get the same
burrs in either material

https://www.pferdusa.com/products/202b/index.html

this one is for steel.. its a double cut style
http://www.mscdirect.com/product/details/85284982?item=85284982&fromRR=Y

Hemi Ragtop has good assessment of each type and
the chip off of each

So what normally causes the burr to skip off the iron? Too little RPM or too much?

Dull burrs.. also if you dont have good control
of the grinder... high speed and sharp burrs will
will cut nicely but I dont like the high speed...
if it bites it will fly around the port multiple
times before you get it stopped.. I prefer a slower
speed and good control on the grinder.. on my electric
one the switch is on the end which is a push to turn
off... if I bump it off it will bite... but I tend to
keep my one hand right near the button/switch and the
other hand well forward.. this gives me better leverage
and control... I like the electric unit so I dont have
to listen to the compressor running.. and its a BIG
compressor
EDIT
Dull bits/burrs people tend to push harder and it
causes heat.. thats when things get weird.. if you
are leaning on them you tend to have less control..
sharp bits/burr you dont lean on them to get them to
do the work.. so with less pressure you tend to have
better control

I prefer using burrs recommended for aluminum (wide single cut) when porting iron heads.

as with any cutting tool, be it drill bit, lathe tool, or cutting burrs, slow and steady will give the best results as well as tool life. are there exceptions ? absolutely. but we are [i think] talking hobbyists using hand tools, not your modern, state-of-the-art, cnc machines or shops. just my observations being around shop equipment 40+ years.

i bought a set off summit racing years ago and they still cut good.