I am converting a HW Radiator into a one-pipe Steam Rad. I need to add
a vent valve. I'll need to drill and tap a hole. The tap wants a
I am wondering if someone can advise me on some cast iron drilling:
Is it better to drill a pilot (or several gradually larger pilots) or
go with the final size from the outset?
I use Cobalt bits - is this OK for cast iron?
I have a variable speed drill. What speed should I drill at?
Any advice is appreciated. Thanks!
Cast Iron is very soft. I would start out with a 1/4 then 5/16 then the 21/64.
You shouldn't have any trouble. Doesn't matter what you use for a bit. A cheap
hardware store bit will do as long as it is a highspeed bit for metal.
That is a fairly good sized hole. You should drill it with a drill press and
some sort of
fixture to hold the pipe. As the drill size gets larger, it becomes more
difficult to get a clean,
round hole because the bit walks around in circles, giving a three-cornered
which does not tap well, and can leak. Get the bit and the pipe locked
with respect to one another, drill slowly, and use cutting oil. My drill
press guide suggests
1000 RPM - but I would go a little slower. You will know when you are doing
as the walls of the hole will be square and sharp.
The same fixture can help you tap the hole - chuck up the tap, and turn the
shaft by hand
(unplug the drill press) while using the drill press level to lower the tap.
This helps keep the
tap square to the work. After getting the tap well started (3-4 turns
threaded), be sure to
back out the tap a half turn every turn or two to clear the chips.
Sounds like from that 21/64" diamether you're planning on tapping a
nominal 1/8-27 NPT pipe thread for which the normal length of thread
engagement is 0.263 inches.
Are you sure the spot you are going to drill through will be thick
enough to let you tap enough threads to get that?
Scratch my question if you are planning to drill into a boss that's
already been cast there for that purpose.
My name is Jeff Wisnia and I approved this message....
Cobalt is top of the line, no problem with soft cast iron. If they are
split point (4 cutting edges on tip) and you have a starting dimple you
should be able to drill easy enough if you use some kind of lubricant
and not too fast. Split points often drill better without a pilot hole
but you do need to keep the workpiece stable. Very curious, what size tap?
Thank you, Everyone, for your help!
The tap is, indeed, 1/8 27 NPT. The 21/64" drill size requirement is
stamped right on the tap shank.
There is a boss on the radiator (the manufacturer made provisions for
both steam and HW systems), but there is no way for me to clamp the
radiator, not use a drill press. I'll have to support the rad as best
I can and rely on skill to make the hole round. Based on the replies,
it sounds I can go directly with the final size drill on the first
I want to run one more idea by the collective wisdom:
Since I am going to have to drill freehand, maybe I should go with a
5/16" hole, instead of the 21/64." If there is a slight out of round,
my thinking is that - the cast iron being soft - the tap will
The taper on the tap looks tapered enough to fit into a 5/16." Any
objections to this plan?
The cutting tool rapidly dulls and / or burns up due to your running it at
too high of a surface speed.
Hint : the smaller the drill, the faster you want to spin it, and the harder
the material, the slower you would want it to spin.
I don't drill metal that often and forgot the rules yesterday. My
intention was to enlarge a 3/16 hole to 5/16 in some pretty hard
I put a decent bit in and cranked up the speed. When nothing was
happening I increased the pressure. . . .
I ended up with a 3/16 tip on my 5/16 bit in a blackened 3/16 hole.
All right, folks, thank you all again! I appreciate your comments and
have followed the advice that made most sense.
The rad is now drilled and tapped. All went well.
Summary: I used a 7/64" bit as a pilot and the 21/64" as the final.
Household oil lubrication, every 30 seconds or so. Variable speed
drill, no faster than 3/4 of the speed (on the "low" setting), with
medium pressure. You could see/feel when going too fast. Very clean
hole, in the end.
Good thing I did not go with 5/16" (a 1/64" undersize to what the tap
really wanted). The tap-recommended drill size was perfect for the it.
Here are photos, if anyone wants to see the results.
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