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Tapping 2.5" NPT

Hertz

Stainless
Joined
Apr 27, 2009
Location
Sudbury, Ontario, Canada
Good morning. Anyone ever cut with a 2.5" NPT tap?
I'm not talking about chasing a hole, but rather actual cutting.
Here's the thing, we have to make a 2.5" NPT thread in a part (Cast Iron) but it's too high for our VMC to threadmill, and we just have a manual boring mill otherwise, but it doesn't have a manual handwheel for power tapping. It's controlled through a hand held wheel like on a cnc.
I did some calculations and it seems we'd need about 1500 or so ft lbs of torque to drive a tap(not sure the accuracy of this calculation). I suggested sending it out to get done but it doesn't seem like the GM is on board. He doesn't believe we can't do it. Anyone have other suggestions before we send this out?
 

john.k

Diamond
Joined
Dec 21, 2012
Location
Brisbane Qld Australia
I d doubt your torque requirement........if the thing is plain cast iron ,it will tap very freely.........sg iron will depend on the tensile .....I once worked in a pipe and valve foundry and was always surprised at how little power plain grey cast iron took to cut.
 

Milland

Diamond
Joined
Jul 6, 2006
Location
Hillsboro, New Hampshire
A stiff spring-loaded center in the horizontal spindle bore to guide the back of the tap to keep it straight, follow movement of the tap manually with the spindle, and a milled square drive in some 3/4" steel plate with provisions for a strong tube handle.

Just like you'd do in a small lathe, but, umm, bigger...

Note: do not use an open-section drive (like a big adjustable wrench) on the tap shank, use a full square that envelopes the flats. To much risk of injury if the drive slips while heaving on it.
 

Milland

Diamond
Joined
Jul 6, 2006
Location
Hillsboro, New Hampshire
I was curious what a 2 1/2 NPT tap would go for, so I checked McMaster for S&G. A HSS tap is around $430, a lot but not crazy. Noticed they also offered an economy carbon steel tap, and the same size was over $620!. That's not the sort of "economy" I'm used to from McM.

From the same cheap line, a 4" is almost $1800. For that much, I make my own...
 

gmach10

Hot Rolled
Joined
Dec 27, 2008
Location
N.E. Illinois
Morning, this is where a sharp tap and a Cinncinati Bickford radial drill press come in handy. We had to tap 1 1/2" & 2" NPT holes at a former job. Using a shop made tap driver and a drill press set at a low spindle speed torque wasn't a problem. Just made sure wee had it clamped good and tight to the table.
 

Limy Sami

Diamond
Joined
Jan 7, 2007
Location
Norfolk, UK
As others have said - it's not as bad as it sounds.

While not NPT I've done 3'' BSPT (11 tpi) in CI by hand with little more than 3ft cheaters on the shop made tap wrench, was surprised how easy itwas. .....aka still hard graft but easily doble - better with 2 guys.
 

gmach10

Hot Rolled
Joined
Dec 27, 2008
Location
N.E. Illinois
BTW, I believe they make tapered reamers for larger pipe taps. Also there are "skip tooth" taps that lesson the amount of torque to turn them. I once had to drill and tap a hole in a fire hydrand cap for a Chicago coupler. Someone weldeed the weep hole closed and gave me the cap. I'll never do it in that order again. Not sure what grade of iron it was but that shit got hard as a rock from the heat.
 

DDoug

Diamond
Joined
Oct 18, 2005
Location
NW Pa
Good morning. Anyone ever cut with a 2.5" NPT tap?
I'm not talking about chasing a hole, but rather actual cutting.
Here's the thing, we have to make a 2.5" NPT thread in a part (Cast Iron) but it's too high for our VMC to threadmill, and we just have a manual boring mill otherwise, but it doesn't have a manual handwheel for power tapping. It's controlled through a hand held wheel like on a cnc.
I did some calculations and it seems we'd need about 1500 or so ft lbs of torque to drive a tap(not sure the accuracy of this calculation). I suggested sending it out to get done but it doesn't seem like the GM is on board. He doesn't believe we can't do it. Anyone have other suggestions before we send this out?
Can you set the horizontal boring mill's feed to match the lead ?
 

gbent

Diamond
Joined
Mar 14, 2005
Location
Kansas
I don't know your boring mill, but I power tap with mine by simply running the spindle in by hand while turning under power. For smaller taps, I will put a tap extension in the drill chuck and watch the amount of tap protruding from the extension. For 3/8 and larger, I just chuck the tap directly. I have seen tap drivers that have a sliding splined coupling to give a few inches of axial travel, I've just never felt the need to make one. Try some smaller holes and see how easy it is to follow the tap lead by hand.

All that said, first I'd try to make the thread mill fit, even if I had to order a special or modify a toolholder.
 

eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
It'll work. Done it a million times. Use a plain square driver and if your spindle is free-moving like it should be the tap will probably self feed. If the spindle is very difficult to move in and out or if you're that worried, use a spindle or table feed that correlates with your tap's pitch like Doug mentioned. If you don't have one that matches, just keep up constant pressure in the 'out' direction while the tap goes into the hole by hand. (Hand pressure if it's available - not with the feed handwheel). You can also stick a spring in the square driver between it and the tap if you have no hand traverse of the spindle other than the handwheel and a feed that's close but not quite where it needs to be.
 
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Ron Hofer

Hot Rolled
Joined
Jul 7, 2007
Location
Brookfield, WI
As others have said - it's not as bad as it sounds.

While not NPT I've done 3'' BSPT (11 tpi) in CI by hand with little more than 3ft cheaters on the shop made tap wrench, was surprised how easy itwas. .....aka still hard graft but easily doble - better with 2 guys.
Limy,
"It's not as bad as it sounds" is what Mark Twain said about the music of Richard Wagner.
 

Bill D

Diamond
Joined
Apr 1, 2004
Location
Modesto, CA USA
Buy or make a taper reamer to fit taper angle. Or step drill 3-4 diameter drills to differing depths.
My Father made a block to fit a ratchet pipe threader handle to hold a pipe tap. Just repalced the die head. His machine had one piece square dies no tricky cogs or such to match.
Bill D
 

Hertz

Stainless
Joined
Apr 27, 2009
Location
Sudbury, Ontario, Canada
It'll work. Done it a million times. Use a plain square driver and if your spindle is free-moving like it should be the tap will probably self feed. If the spindle is very difficult to move in and out or if you're that worried, use a spindle or table feed that correlates with your tap's pitch like Doug mentioned. If you don't have one that matches, just keep up constant pressure in the 'out' direction while the tap goes into the hole by hand. (Hand pressure if it's available - not with the feed handwheel). You can also stick a spring in the square driver between it and the tap if you have no hand traverse of the spindle other than the handwheel and a feed that's close but not quite where it needs to be.
Can't power tap it. I mentioned that in my original post. The machine doesn't have a handwheel and the spindle doesn't free travel. It's like a CNC but not a cnc. The handwheel is a hand held one with axis selection like a CNC.
 

gbent

Diamond
Joined
Mar 14, 2005
Location
Kansas
Then build a splined shaft tap driver. Look at smaller vehicle drive shafts. If you have 3" of travel that would be enough. You can follow it by hand if the travel isn't sufficient.
 

eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
Can't power tap it. I mentioned that in my original post. The machine doesn't have a handwheel and the spindle doesn't free travel. It's like a CNC but not a cnc. The handwheel is a hand held one with axis selection like a CNC.

If the table can be moved while the spindle is turning, you sure can. Just the way I described. Turn the spindle and follow the tap in with the table feed. Use a spring if necessary. Stop the tap and back it out to clear chips if they start to pack the flutes. You are blowing this way out of proportion to its actual difficulty level.
 

just Dave

Aluminum
Joined
Jan 18, 2023
Location
Kansas City
A one inch too holder, a 1” shaft with a point and a washer welded towards point end, a 1” ID die spring of stout nature and appropriate length , let shaft free slide in still spindle. Cheap quick HD tap guide.
 

Cole2534

Diamond
Joined
Sep 10, 2010
Location
Oklahoma City, OK
Then build a splined shaft tap driver. Look at smaller vehicle drive shafts. If you have 3" of travel that would be enough. You can follow it by hand if the travel isn't sufficient.
It needn't be fancy, either. Some square tubing with one size fitting in the other reasonably well, and proper end appurtenances should do just fine.
 








 
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