cutting a thread in a hole plumb

ARC-170

ARC-170

Jeff L.
H-M Lifetime Diamond Member
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Oct 17, 2018
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I need to cut a 9/16-18 thread in a hole in 1/4" thick steel. It needs to be as plumb as possible. What is the secret to getting the tap as plumb as possible so the threads are at right angles to the plate? This is a piece to hold a QCTP in the tool holder on my lathe. I can cut the hole with a drill press, but getting the tap plumb is not easy. I thought it was straight, then I threaded the bolt in it and saw how crooked it was. Any methods, advice would be appreciated. Thanks!
 
Use a tap follower in the drill press chuck to keep the tap in line. The tap follower will also work in the lathe, but unless you can hold the work in the lathe securely you are better off with the drill press. Best for a lot of tapping work in flat material is a milling machine, you can start the tapping with the tap mounted directly in the drill chuck. A spring loaded tap follower works best in my experience.
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On a drill press, I would clamp the work and drill the hole and then tap with a tap follower, as Bob suggests. If multiple holes are being done, apion the same diameter as the drill can be used to center each hole before reclamping. In the mill with a DRO, I clamp the work and note the coordinates of each hole so I can repeat the position when tapping.

For free hand tapping, I use a tapping block to guide the tap. The block consists of a clearance hole for the tap shank. Blocks for this purpose are commercially available. If you do a lot of hand tapping, there is a tapping jig which keeps the tap aligned and has a hand crank to make tapping easier.
 
Thanks! I learned something new today. A tapping guide is not expensive, so I will be getting one.
 
There are a few different ways to use the drill press for this. I have found the tap followers mentioned above to be a bit too fiddly, and moving the table to get the extra clearance means losing the center of the hole (at least on my cheapish floor drill press).

What I have been doing instead is to put the tap in the chuck, less than fully tight, then turn the chuck by hand. Once the tap has entered deep enough to cut a couple of threads, remove it and use a standard tap wrench. In general (so this may be too imprecise for your needs), it is necessary only to get the tap started "true". Once started, it should keep itself true for the remainder of the tapping.
 
ThinWoodsman said:
There are a few different ways to use the drill press for this. I have found the tap followers mentioned above to be a bit too fiddly, and moving the table to get the extra clearance means losing the center of the hole (at least on my cheapish floor drill press).

What I have been doing instead is to put the tap in the chuck, less than fully tight, then turn the chuck by hand. Once the tap has entered deep enough to cut a couple of threads, remove it and use a standard tap wrench. In general (so this may be too imprecise for your needs), it is necessary only to get the tap started "true". Once started, it should keep itself true for the remainder of the tapping.
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I do the same. Depending on the metal being tapped, I sometimes start the tap under power. Year ago, I hooked up a foot pedal switch, so I can easily "jog" the power.
 
I sometimes use a block of aluminum as a square, first one way and then at 90 degrees away- back and forth until the tap starts. Then as ThinW mentioned it will stay straight the rest of the way
 
Making one would require a working lathe. I needed a tapping guide to make something for the lathe so I could use it. I'm almost done with restoring it. I'll put it on the list!

I used my mill and the drill bit to align the tap. Worked pretty well.
 
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