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threading a block too heavy for the table


Jan 22, 2022
how would you guys go about drilling and tapping some holes 1"-8 in a block of steel that is 16"x16"x18" tall? I have a typical bridge port style clone grizzly. I know I could turn the head and do the work next to the table, but wondering if anyone has done this and a good way to set it up without it moving on me.
A mag drill would make the holes, don't know if it would tap them. Tapping by hand wouldn't be fun, but 1300lbs of steel would probably stay pretty still while you did it.
About 10-15 years ago Sam Mesher Hardware in Portland still had barrels of NOS large (like 1" to 2.5") carbon steel taps from WW2 ship construction. I imagine using one of those puppies was either a two-person job with a 6 foot tap wrench or a pain in the neck with a 35lb pneumatic driver, a bulky air hose and a reaction arm hopefully braced on something solid.
20", floor stand drill press! Swing the table to the side and mount the block on the base with some timbers to raise it as needed. Threaded rod and steel members (angle or channel or whatever) to clamp it down to the base. Another wood or steel member clamped to the side and tied/clamped against the column to prevent rotation.

I would suggest a tapping head, but you may not have one. Try the slowest speed and use a good tapping fluid, like Tap Magic. It will lubricate the tap so it goes easier.

Anyway that's how I would do it.
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I’ve never found rotobroach cutters in tapping drill sizes, shame that, your stuck with a drill, I’d prefer an anular on big sizes, but big taps are without question hard work when you get up to 1” I’m on the edge of what I can do with the biggest tap wrench I have, getting old!
Sorry gotten old!
You certainly know when the tap is dull
I hand tapped 1 1/2 Whit holes in a cast iron jig plate 6'x12' and weighing 4 tons....I used the largest size tap handle ,probably meant for hand reamers ,anyhoo it was about 3' across ,and held the tap square no trouble.
I cant imagine using a mag drill to cut the threads. they are on/off and like 250 rpm and up correct? seems like it would be a nightmare. I dont have a drill press, just the mill. I do have a 3 ft tapping handle, but havent had to use it haha.
I've tapped both 1"-8 and 1-1/4"-7 by hand when I was much younger, and it wasn't fun back then. I wouldn't attempt it today. I have some BIG taps here Matt sent to me. They will only make unusual door stops.
You need one of those giant vintage tap handles of the sort that Adam Booth has managed to find. Then get two people and walk around the part. However in the spirit of procrastination, I just worked out that at 566kg, and being a clean steel block on a clean steel surface with a coefficient of friction of around 0.5, the block will move at a torque of around 30Nm or 22 ft-lbs. I'm pretty sure that running a 1"-8 tap in will require more torque than that, which is only to say that you'll need to stop the block from twisting.
i can clamp it between the forks on my forklift. Well, I guess i'll order up a mag drill and give this a try. I been wanting one for while anyways.
I should mention ,I only drilled and tapped the first four holes by hand ......I was then able to bolt down an old radial arm 'girder drill' to the plate ,and by using that drilled and tapped the two hundred or so holes in the plate .
If you have a second tap, grind off the full threads. Then tap with that to make an under size thread. Follow up with the full size tap. That way you are only taking off a little at a time.
If you have a second tap, grind off the full threads. Then tap with that to make an under size thread. Follow up with the full size tap. That way you are only taking off a little at a time.
+1 for that.

Effectively a DIY version of serial taps where the first two of a set are undersize. Great things, especially for larger threads.

Biggest issue with hand tapping large threads is keeping the tap square to the job. The relatively high helix angle tends to pull things out of line and long handles on the wrench aren't the best for holding square. No chance of eyeing things up either on a job that size like you can with itty bitty bits in a vice.

If I had to do that by hand I'd put a smaller tapped hole in first at one (or more) points and bolt a guide block in place to help hold things square. If I had a lot to do I'd make up a bracket device with a screw to support the end of the tap so I could use a long handle ratchet rather than a tap handle. Tighten the screw every 1/4 of turn so all stays in line.

This old fart has finally realised that it's usually quicker to "waste" time at the beginning assembling stuff so the job goes easier rather than diving in in an attempt to bull through! Hate the "Oh S**t, now what." moments when everything comes to a grinding halt half way through or the even worse realisation that its gone wrong badly enough to be scrap.

If you can tack weld or magically hold a tapping block in place it really helps with big taps. You will have no abitlity to judge or maintain level once you start wrenching it in.
I made some out of 1” flat bar and a 31/32 hole which I did tap. The tapped block self starts.
You don’t really need a difficult-to-find special big tap wrench to drive the tap. Just bore a hole in a piece of 2x2 flat bar large enough to clear the square end of the tap. Then mill a 1.5” long by loose 1/2”w x 1/2” deep slot on each side of the hole spaced to allow the tap drive end to slide between key stock pieces dropped into the slots. Use 3/8” set screws to pinch the keys on the tap square drive. Weld two 1.5” pipe arms as long as you like onto the block. Use the guide blocks suggested above. Invite a friend over and walk around the hole.


Napkin Drawing if my hand waving above is unclear. A standard dimension across the flats of 1" tap is .6" and the shank diameter is .8"


And, yes, I am practicing my CAD drawing ;-) But, for me it is not an idle exercise. I often find ways that general ideas need to be refined when I actually draw them out. For instance, I saw that a 2" length of 2x2 is not long enough for enough meat to have good purchase by the set screws. 2.7" is much better.




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Hardinge tap driver collets just bore the hole shy of the tang depth, then mill the necessary width slot straight across from that end; deep enough to intersect the round hole.

I use a mag drill and while i no longer look forward to drilling/tapping 1", it is not onerous.
Consider how much thread engagement you really need. 65% taps much more easily than 75%

You can put a center in the drill and press down to get the tap started and use a wrench or gear wrench if one fits.