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
21/64" hole.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!
Michael Stoic
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 egg-shaped hole which does not tap well, and can leak. Get the bit and the pipe locked completely stationary 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 it right, 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.
-- Tom
Drill at a relatively low speed and use some oil for lubricant. Shouldn't have any trouble. Most radiators have a lug cast on with a dimple in the middle, drill there.
Dan
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.
Good luck,
Jeff
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?
Frank
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 run.
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 compensate.The taper on the tap looks tapered enough to fit into a 5/16." Any objections to this plan?
MS
Use HSS drill bit at 850 RPM. No lubricant is required for cast iron. Use drill press or clamp drill to ensure rigid setup.
Should be no problem.
If this is something I could not afford to botch, I'd drill undersize and invest $10 in a reamer (or perhaps 2!) to bring the hole size up to final requirements.
Jonathan
Low speed and lubricated is interesting.
What happens with high speed, and low feed pressure? (High speed, low feed woulda been my first guess.)
The cutting tool rapidly dulls and / or burns up due to your running it at too high of a surface speed.
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 streel.
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.
Jim
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. No surprises.
Here are photos, if anyone wants to see the results.
http://209.197.228.45/~podjars/rad.htm Thanks again,
Michael
Machine shop 101 project grade A+. Well done.
Frank
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