Taps and Dies...shouldn't they match?

[mklotz]

55% is about the "loosest" you can make the engagement without seriously affecting the strength of the threaded joint.

In difficult-to-tap materials (e.g., stainless and many types of brass) I always use a DOT < 75% to reduce the stress on (and consequent breakage of) small taps.

The conventional formula,

TD = MD - 1/P

makes the approximation that:

0.013 * DOT = 1

or:

DOT = 77%

which is fine for easy-to-tap materials.

 

[Foozer]

Actually tried some toothpaste on a part I was practicing on. Chose a 'whitening' formula and wondering if it's simply more abrasive than normal paste.

'Progressive'...thanks for the name.

Marv...working back and using your formula...I had used a #7 bit and that gives 75% DOT. Why 55%? Experience? Some other criteria?

Thanks.

For me, as it is the only chuckerhead I can speak too, I have found through the learning process, that settling upon specific favorites regarding hole sizes, tap and drill combos, etc becomes the norm. Try the standard tap-drill routine, break a few taps and adjust the DOT (drill size) to achieve success while maintaining some degree of positive engagement. Lots of room between the 75% and 55% curve. I'm not trying to build a small block chevy that's expected to produce 1000hp so adherence to engineering rules can be stretched in the realm of model building. I like 0.375 as example for crank shaft size, I can deal with that dia. 4-40 screws is the smallest that my finger touch can handle on a tap.

It is a bit of trial and error to see what works for you. As the comfortable factor increases the drifting towards the precise will naturally occur. For now its just fun to turn a hunk of metal into something you can hold with pride, standing, puffed as a roaster in the hen house announcing with boastfulness speech to the bride "Look what I Made"

Some just have the "Green Thumb" when it comes to metal working, mine are black and blue

 

[zeeprogrammer]

So much to learn...so much fun learning it.
Yes...like everything else...it's not all in the books.
And everyone develops their own techniques...the birthplace of tips and mods.
(Although there are the occasional inviolable rules.)
That's why I'm here on this forum...you don't want me to learn it on the street do you? You can pick up bad habits there.

 

[arnoldb]

I don't have _any_ experience with imperial threads, as we use 95% metric here. I've invested a fair amount in a range of metric taps and dies of good quality from 3 to 12 mm, and all the standard metric taps have freely available drills for correct sizes. Also,nearly all of the taps I have come in sets of three progressive taps. It may be a PITA to use the successive taps, but so far, I have not had an issue with getting taps stuck/broken (touch wood!). I follow a few simple rules:

Clean taps before & after use.
Use a good tapping lubricant suitable for the material you are tapping.
If at all possible, drill & then tap (manually!) in the same machine immediately after drilling. If this is not possible, take the time and make a tapping guide to help you tap when not in the machine. Right now, nearly all of my tapping happens in either the lathe or the drill-press. I drill the hole, then replace the drill-bit with the first tap, and using minimal pressure, feed the tap into the workpiece (or workpiece into the tap - depending on what I am doing), until I feel it just pressing against each other. Then, by hand, I turn the chuck, still maintaining a minimal force to start the tap. after a couple of turns on the chuck, it becomes self-feeding - so I thread to depth. To reverse, I just reverse the minimal pressure (on the lathe this is a full reversal, but on the drill press, it is using less pressure, as it is self-reversing), and turn the chuck in the other direction. I keep minimal reverse pressure until the tap is free of the workpiece - everything happens pretty much by 'feel'. Same story for each progressive tap - they take up the previous thread nicely doing this. This method leaves a tiny bur sticking up at the start of the thread once the last tap has been used. I normally remove the bur by just using a countersink in my hand and turning it "into" the thread. Makes for a nice and sure-start thread every time.

The only metric taps I currently have that don't use the three progressive taps are for 10 and 12 mm Metric Fine - they have only 2 taps each. I nearly had a fight with the salesman when I first bought the 10mm set; I accused him of trying to sell me an incomplete set! The reason is simple though - the metric fine requires less material removal to cut the thread, so only 2 taps are needed.

Also, another observation: A metric tap normally won't screw into a die. Besides for the difficulty in trying to get the them screwed into each other because of the gaps for the cutting edges, the nominal OD for a tap is larger than the nominal OD if a die, and a die has a smaller nominal ID than a tap.

Hope all this makes sense ;D

Arnold