Help with Tapping Stainless Steel for Carbide Tool

[David Justice]

I am attempting to make my own carbide tools. Everything was going great until I got to tapping the hole for the screw to attach the cutter.

I am using 3/8 18-8 stainless steel keyway stock. I made a 1/8 notch on one end and drilled a hole with #29 drill bit, per instructions for an 8-32 thread screw, but have broken 2 HSS taps trying to tap it? I have not done much tapping, but I'm completing one turn then reversing about a half turn at a time. Is there something harder than HSS that I should be using for the tap?

Getting frustrated, I have everything done for these tools except the tap!

Thanks,

[Paul F Franklin]

Stainless is tough to tap. For one thing, it work hardens, so if it gets hot when you drill the hole, either because you ran the bit too fast, didn't use cooling, fed it too slow, or had a dull bit, it hardens, making it much harder to tap.

Always use lube when tapping. Some folks like to use grease for stainless rather than oil. A two flute tap has a beefier core than the typical 3 flute or 4 flute and will be less likely to break, especially in that small size. You could certainly consider going to 19/64 drill size.

I assume this is a through tap and not a blind hole?

[David Justice]

Thanks very much for the response. I will look for a 2 fluted tap. Is there a steel that I can use that will be easier to tap and still strong enough for the tool? I am planning on having around 5 inches in the handle and around 8-9 inches in front of the handle.

Thanks,

[Charles Drake]

David, I've built several of these tools using common steel bar purchased at Lowes and it taps fine and never have had a problem the tool bending (I use 1/2 inch square). stainless is tough as you have found out.

[andy bessette]

...I'm completing one turn then reversing about a half turn at a time...

This is why you're breaking your taps. A full turn is WAY too much! Should be more like 1/8-1/4 turn.

Also you could go to a spiral point tap.

[Adam Herman]

no need for stainless for this application.

for taping stainless, take small pecks at it, keep it cool while drilling and use a proper cutting oil. go slow to avoid hardening and double check your bit size and maybe even use a reamer, as a 2 flute drill will drill a triangle shape hole, esp. as the material gets harder.

[David DeCristoforo]

Key stock is insanely hard steel. Get a bit of mild steel as suggested and you should have no difficulty tapping it.

[Clint Bach]

Here's a trick for tapping I learned somewhere. I don't remember where I learned it. I haven't broken a tap using this method. Of course use lube.

Put the tap into a cordless drill with an adjustable clutch. Set the clutch to some setting that won't break the tap. Drill the tap into the hole until the clutch slips. Reverse the drill. Repeat until the hole is tapped. You may need to adjust the clutch...

This works like a charm and I haven't broken a tap using this technique.

Clint

[John K Jordan]

As others mentioned, stainless, especially some types, is horrible to drill and tap compared to mild steel. With the time you have invested so far with that piece of steel, one possibility is to just drill it out oversized and just put a nut on the end. Depending on your geometry, of course (angles, etc). Not very elegant looking but it might let you try out the tool and see how you like it.

JKJ

[andy bessette]

Also it is good and common practice to go a size over on the tap drill when tapping stainless.

[David Justice]

Thanks so much for all your help guys. I decided to scrap the stainless and get some mild steel to work with. The tap went much better this time. Here is a picture of the finished tool. 21" long overall with 8 inches in front of the handle.

thanks again for the help!IMG_1279.jpg

[Jak Kelly]

I made my own handle for holding carbide tips/cutters as well. I used aluminum, about a 10" long piece approximately 5/8" square, epoxied 5" of it to the inside of the handle, been using it for 4 months now.