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Under-size drilling for a tapped hole. Bad idea ?

I was thinking about SS threads. What if the threads could be cut in a way to leave the
finish burnished and hard to cut later. Like when parting-off and suddenly the tool just stops plunging in.
That type of finish would be resistant to galling. I mean a work-hardened surface than doesn't need a thread lubricant.

Instead of the finish where it looks good to the eye but up close the surface looks like brain matter. No wonder SS locks up easily.
For a manual machine I don't run more than 400 rpm at .005 steps. The hand controls are too slow for fast rpms.
YOU !"£$%"£$%^£$%^&"£$%^ WHAT? ......stainless is the ABSOLUTE LAST metal Iwould even think of using smaller size tapping drills
 
Using a forming tap would cold work the SS and increase the strength. Probably to the point of tap failure.
If you really need SS threads that don't gall, put helicoils in and use Nitronic 60 material. Just buy the inserts don't price compare with standard ones, I will not be responsible for your heart.
 
Tapped holes do come in different classes of fit. They do sell taps that are slightly undersize. While that may be off topic, it might be related to the OP’s intent. Many threads in automotive applications are tight fits; some require a torque just to install the stud originally.

BTW, for metric tapping, I always use the rule: tap drill = thread dia. - pitch. Has worked for me for 50 or more years, mostly with automotive applications. Never found it hard to tap using that formula.
:-) Unless you're arithmetic challenged just do the same for all threads. :-)
...lewie...
 
304SS forms well so form tap it to get those burnished work hardend threads.

I would think the less load the tap has the better the surface it will leave.
 
Another vote for form tapping, it cold-works the material making it harder and stronger. Presumably it also refines the surface finish.
 
In thin sheet a smaller hole will provide a little extra pullout. With only a few threads tap breakage is not too likely. I will leave the fine or corse pitch debate in thin stock alone.
Bill D.
We were taught that a minimum of 4 threads was desirable for pullout strength so that would have bearing on thread choice. Sheet metal screws are in a different league.
 
From the chart:
tap size = 3/8
drill size = 5/16 (.3125)

Would using the next down size drill = N (.3020) alter the surface in any way? Are the charts that close to optimum?
Obviously going too far will break taps. The question is really about taking a greater depth and how that can effects the surface.
Down to things that a electron microscope could see.
.
not sure why you worry about surface finish of tap drill diameter, drill tip tends to bounce around
a bit especially longer drills. tap chart usually based on low carbon steel 2/3" depth minimum to diameter
and cast iron tapped hole depth at least deep as diameter of hole to match strength low carbon
steel screw. obviously high strength screw need more tap depth to match screw strength
.
1) drills drill oversize depending on runout, how even or centered point sharpened, cutting
fluid and or built up edge formation, etc. obviously uneven point hole can be .005 to .050" bigger.
have had oversize holes before many use pin gage to confirm hole not too big, i have seen
chipped drill drill way over sized holes before. picture shows damaged .409" drill that drilled
12ea .050" oversized holes and not break the carbide drill.
2) many taps even cut taps can form burr making tap drill hole smaller, obviously checking
with pin gage not unusual to see over .001 dia change hole smaller after tapping
3) hole straightness most holes are not straight that is deeper holes a pin gage can go in and
get tighter after a inch or 2, seem multiple parts where pin gage went in beginning parts, nogo
a few parts, then pin gage go in last parts. usually hardness variations causing drill point drift
to softer side
 

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They don't give those thread depth recommendations for surface finish reasons... After a certain percentage, any deeper threads just don't make that much stronger of a hole. And they are a terror to tap. If you really want me to delete the thread I will.
We do 125% of the major O.D. in depth as a minimum for most of our parts.
 








 
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