Oversized taps

K

kmanuele

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Not sure if anyone has experienced this but:

I am building one of those steam engine kits. Started to tap some #5-40 holes in a cast iron piece, and the tap just would not go in. So, drilled the hole one size over and still was a struggle for any of the set (of 3). The drill sizes were correct. Taps are fairly new, seem to be sharp.

Ordered a set of Hertels from MSC and they went right in like butter -- even in nominal size hole.

Not sure how to properly measure a three-flute tap, but the originals seem to be about 5 or 6 thou larger than the Hertels.

K
 
Taps are sized by GH sizes . Every number up or down is .0005 on the pitch diameter . You can also order oversize taps . Usually .005 oversize . Used them for re-tapping dies loaded up with carbon .
 
kmanuele said:
Not sure if anyone has experienced this but:

I am building one of those steam engine kits. Started to tap some #5-40 holes in a cast iron piece, and the tap just would not go in. So, drilled the hole one size over and still was a struggle for any of the set (of 3). The drill sizes were correct. Taps are fairly new, seem to be sharp.

Ordered a set of Hertels from MSC and they went right in like butter -- even in nominal size hole.

Not sure how to properly measure a three-flute tap, but the originals seem to be about 5 or 6 thou larger than the Hertels.

K
Click to expand...

Maybe A1 Cardone is selling used brake caliper bleeder taps?

I don't know how you'd properly measure those either, but here's what I do. Find a piece of material that'll take just four or five threads from the tap, and tap it with both. Screw a screw (the same exact screw) into each hole, and see how much it wiggles. If the taps are off enough that they fight to tap the threads in a correct hole, you'll know it. I have seen that a couple of times over the years. I first noticed it when Vermont American got so bad it got dropped from the DIY parts stores, and they all started selling Century. In hindsight it was probably a good decision on their part at that time. Anyhow, I've seen on size taps ground in some wrong way that just wouldn't tap a hole without a four foot snipe. Geometry or angles or something. And I've seen taps that were truly WAY oversized. Not like a tolerance descrepancy, but flat out crazy oversized. The bolt in a SHALLOW through hole thing (using the SAME bolt, so that's not a variable) will not "measure" the difference, but will give you insight as to what the issue is for sure.

Or just try running one through a hole made by the other, see if it makes chips... If it's on size, it's a good candidate to make a bottom tap out of, or to save for cleaning rusty holes that you wouldn't go near with a good tap. Or if it's truly oversized beyond any reasonable tolerance, put it in the kit that you loan your neighbors when you're training them to not borrow tools.....
 
benmychree said:
Plus '005 taps are used in die work, where heat treatment swells the steel so that a normal fastener would not screw in; I had a set of them for any work that was to be heat treated.
Click to expand...
You betcha ! Kinda hard to open up a heat treated thread in a die . :grin: They're still a PITA to clean out even being oversized . :cussing:
 
As mentioned there are quite a few uses for oversized taps. They are commonly used when materials have to be plated. The proper depth plating will return the tapped hole to the correct size.

The standard GH numbers go from 0 to 12. 0 being exact size with no clearance between the male and female threads H1 is .0005” larger than the nominal size H12 is .006” oversized. The G means the thread on the tap is ground rather than formed.

A similar system exists for metric taps. If I remember correctly they use a D designation.

The H or D number should be either etched or engraved on the shank. The most commonly used tap size is GH3, or .0015” larger than the nominal size of the male fastener. This allows appx. 75% hold.

If the tap has a STI identification on the shank it’s oversized for Screw Thread Inserts.

If it has no flutes or a very thin channel down one side it’s a roll form tap. These taps don’t remove material. Rather they push it out of the way to form a thread. Consequently they need a larger drilled hole. There is a chapter in the Machinery’s Handbook about roll form taps and the calculations needed to determine the proper drill size.
 
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Are the taps really oversize, or are they poor quality? I’ve used poor quality taps before and they would be difficult to tap, but once I switched to good taps, I never had a problem tapping a hole. I think your problem might be the taps you bought are poor quality and not their size.
 
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kmanuele said:
Not sure if anyone has experienced this but:
Click to expand...
Yes.
There is a lot of experience to be gained from cutting threads in cast iron with poor quality or off size taps.
Also learned a lot about cutting fluid in the process.
 
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