Is a "bright HSS" 5-40 tap rigid enough to tap O-1 tool steel?

[twhite]

To keep the tap perpendicular make a simple tap guide. Drill a hole in any piece of material as long as it is parallel to the major dia of the tap. These work great. Or you can purchase them also. Example

Cutting oil is my blood.

 

[RJSakowski]

If you are using the lathe to make the part, mount the tap in a Tee style tap wrench and a spring loaded tap follower in the tailstock. If you don't have a tap follower, (you should!) a center mounted in the tailstock can be used. Turn the tap by hand and back out every half turn to break and clear the chips. Advance the tailstock quill as you turn the tap to maintain contact with the tap wrench.

 

[cazclocker]

I am terrified of 5-40 taps! It will be important to maintain the correct geometry to prevent breakage. Do you have a drill press (or some other method) you can use to keep the tap aligned?

Fair question. Yes, I have worked out a reliable way to keep the tap aligned using my Rivett watchmaker's lathe. The tap will be held in place with my lathe's tailstock. I should probably show pics (which I don't have just now) but I had a special tailstock runner made for me with a 1"-deep 0.1253" centerhole with 3 tiny set screws to hole any size carbide PCB drill I need. I've worked out a way to securely locate the butt end of a tap in the end of the runner with my right thumb & index finger, while I push the end of the runner with my right pinky. That leaves my left hand free to rotate the lathe's headstock a little at a time. I keep the tap from rotating with a little wrench on the flats of the tap's butt end.
It sounds ridiculous, and it is, but it works just fine.

 

[cazclocker]

Hi Doug,
For some threads drilling the hole larger makes a huge difference.
It depends on material, tap condition, and final use.

I like this tap chart form Little Machine Shop:
https://littlemachineshop.com/reference/tapdrill.php
(notice the link at the top of the page to download a printable PDF version..... I laminated it for shop use)

I have found 50% threads in steel work great for me and make tapping almost a joy.

That chart indicates that for a #5-40 tap:
to cut 75% thread in softer material (aluminum, brass and plastic) to use a #38 (0.1015") drill.
to cut 50% thread in harder material (steel, stainless, iron) to use 7/64 (0.1094") drill.

That's why @twhite asked above about the final use of the thread. If it will be used often and therefore subject to wear (especially if softer materials), you should leave more material for thread strength.

I'd recommend trying both hole sizes above. There's noting like seeing for yourself to prove the point.
You might even decide to tweak the values up or down based on your experience.

Brian

Hi Brian,
Thanks for that. I've printed out the LMS tap chart, it seems useful, but I still have a couple of questions:
I keep seeing references to 50% and 75% threads. What are those?
And, what is a clearance drill?

 

[RJSakowski]

The 50% and 75% refer to the amount of thread engagement. The clearance drill size is the size that provides a clearance for a screw to pass through,; slightly larger than the major diameter if the screw.

 

[cazclocker]

The 50% and 75% refer to the amount of thread engagement. The clearance drill size is the size that provides a clearance for a screw to pass through,; slightly larger than the major diameter if the screw.

Thanks, I appreciate your help.

 

[Karl_T]

Biggest thing to break a small tap is a side force. To stop this, drill your tap hole then use a pilot tap with the pilot in the drill chuck.
This does more to prevent a broken tap than anything else.

https://www.amazon.com/Pilot-Wrench-Spindle-Thread-Drill/dp/B07JRD787Z

 

[brino]

I have been searching for a good diagram of thread depth (preferably a cross section view), but can't find a good one.
Anyone have one to share?
Thanks,
Brian

 

[twhite]

I have been searching for a good diagram of thread depth (preferably a cross section view), but can't find a good one.
Anyone have one to share?
Thanks,
Brian

Thread profile? Like this?

Thread depth is what ever you need.


Cutting oil is my blood.

 

[brino]

Sure, but one that shows say 50% thread depth vs. 75% thread depth.
Maybe a cross-section of a bolt in a nut showing the mating profiles.
Something to really help show the difference.

Brian