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

[cazclocker]

I want to drill/tap into a short piece of 0.250" drill rod, presumably O-1.
Is a 5-40 plug tap rigid enough to effectivel cut nice clean threads in drill rod?
My plan involves using a carbide end-cutting burr to make a small flat on the rod, then spot with my carbide spot drill. From there, drill with a carbide #38 AWG drill, and then follow with a HSS tap.
My question is whether you guys think HSS is enough to make it through the drill rod cleanly enough to make good threads. My taps are brand new; never used.
Thanks,
Doug

 

[twhite]

As long as the O-1 has not been heat treated you will be fine. Make sure the tap is sharp and you use tapping fluid. Break that chip every half turn. Do not be afraid to open the hole slightly to make tapping easier. Only if it is a one time assembly. If you plan to repeatedly use the thread try to keep max material.


Cutting oil is my blood.

 

[cazclocker]

As long as the O-1 has not been heat treated you will be fine. Make sure the tap is sharp and you use tapping fluid. Break that chip every half turn. Do not be afraid to open the hole slightly to make tapping easier. Only if it is a one time assembly. If you plan to repeatedly use the thread try to keep max material.


Cutting oil is my blood.

Thanks for that. The O-1 has not been heat-treated. The taps I bought are brand new, unused yet. I understand breaking the chip every half turn, but what do you mean by opening the hole slightly? Do you mean drilling a slightly larger pilot hole?

I plan to use the thread rarely, but whenever needed. I'm making a "support flag" for my little Rivett watchmaker's lathe. It's just a short stub of O-1 drill rod with a 0.032" slit cut maybe 3/16" deep on one end. The intact end fits in my tipover tool rest, and the slit is to accommodate a little piece of brass strip. I want to drill/tap the slitted area so I can lock down a piece of brass strip. The strip will have small holes drilled into it - it's basically just a "poor man's steady rest". It's for small bits like clock wheels and such.

 

[benmychree]

There should be no reason to use carbide tools for your job, HSS should be fine.

 

[twhite]

Drill size for 5-40 is a #38. You could go to #37 it tapping is rough going. Being they are new taps. Stick with the #38. O-1 in its un heat treated state is fairly soft.


Cutting oil is my blood.

 

[epanzella]

Nothing special about tapping O1 in it's annealed state. A sharp quality HSS tap will work fine. I have had cheap taps that had would be problematic tapping O1 or any other steel harder than beeswax. Harbor Freight comes to mind.

 

[rwm]

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?

 

[GeneT45]

Get your tap started straight and, with proper tap etiquette you should be fine. If the tap is brand new I suggest tapping a 1/4" thick through hole in some mild steel before hand - some taps, even good ones, have burrs or something and seem to wear in and cut better after a few turns. I don't notice this with coated taps, but have definitely noticed it with plain HSS, most recently with a tap from McMaster - presumably not garbage.

GsT

 

[markba633csi]

#4, #5 and #6 taps are very easy to snap when tapping steel. Go slow, don't rush. Use tapping fluid. One size larger tap drill will help.
Best wishes
-M

 

[brino]

I understand breaking the chip every half turn, but what do you mean by opening the hole slightly? Do you mean drilling a slightly larger pilot hole?

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