Hand Tapping 316 SS - need help

Dave Hodson · May 15, 2020

I don't have much (any) machinist knowledge unfortunately but I'm hoping someone on the forum can help me out.

I've been making knives as a hobby for a couple of years now. I mostly use 18% Nickel Silver for my bolsters and hardware but this year I picked up some 3/4" 316 stainless bar stock to try and I have to tell you, it's been a bit of a challenge. I finally figured out how to drill it to minimize work hardening but I'm really struggling on how to tap it - I've stuck and broke two 1/4-20 taps so far.

I hand tap using a cutting oil and the hole is the size recommended for the tap but it gums up pretty quick. Any suggestions would be appreciated.

Thanks
Dave Hodson

ChipSplitter · May 15, 2020

Try drilling your hole to 7/32". Make sure you have a good spiral point or spiral flute tap (OSG, Emuge, Guhring).

Kyle Smith · May 15, 2020

Make a block with a slip fit hole for the tap to keep you square with the world and hide any 4 flute taps in the back of the drawer for emergency use only. And don't be bashful with the cutting oil.

Edit: Added picture of tapping block. This is much fancier than you need, but you get the idea. The tapping block looks trivial, but they are actually a lot of work. Taps are roughly 0.003"-0.005" oversize of nominal, so you end up needing some odd hole sizes. I got close and then barrel lapped them to size. After heat treat I had to tune it with the barrel laps again.

Richard King · May 15, 2020

On parts that are not holding a lot of weight I also drill the hole bigger and get less % of thread. I am assuming your drilling the hole on a drill press? I "power tap" a lot and you can see that on you tube. If your afraid to try it, practice on some scrap. You can also drill the hole leave the knife in the same set up, remove the drill and chuck up the tap on smooth surface and turn the chuck by hand, don't forget to reverse now and then. Also buy a good grade of tap at a supply house and not some import tap from the hardware store. You could also tap it 1/4 28 as those threads are not as deep but they hold more. Oh and buy some taping fluid made to tap SS. A can costs about $5.00

TeachMePlease · May 15, 2020

A good quality tap is your best friend. A cheap, home depot bought tap is your worst enemy.

Good tap brands include: Guhring, Emuge, OSG, Balax, Nachi, and probably others I can't think of right now.

Bad tap brands: Anything you can buy at home depot or lowe's or ace hardware.

guest · May 15, 2020

Use a block to start the tap square as mentioned in Post #3.
Use a quality tap as mentioned in every post...
Use Moly-Dee and break the chip often- about every half-turn. Work it back and forth, don't let it bind up.
Pull the tap all the way out when you are halfway through and clean it, then finish.
Oh, and use Moly-Dee...

Dave Hodson · May 15, 2020

Thanks for all the suggestions and help - great ideas. Fort McMurray is a bit off the beaten path for getting stuff but I'll try to track down some SS tapping fluid. I'm guessing that everybody has personal preferences but maybe there are a couple of recommendations.

It's not going to take much load so I'll try a bit larger hole. I think I might have been trying to tap too much at a go as well (1/2 turn) so I'll try less. I was using 1/4-20 because I had that kicking around but the finer thread sounds good. Thanks also for the guide block and drill press ideas - starting by hand is definitely interesting. I am using the press to drill so incorporating the tapping makes sense.

I'd like to switch to SS so hopefully I can get this to work.

Thanks again for your help
Dave

Dave Hodson · May 15, 2020

When I checked the hardness of the blocks I broke the taps in, all three had a varying hardness between 48 & 55 Rc. My understanding is that's hard for 316 which, from what I've been able to find out, should be in the low to mid 20's Rc.

One suggestion I got was that the bar could have been cold rolled and work hardened so I found an annealing temperature (1900 deg F for an hour with a quick quench) and tried that. The new hardness is now similar between all three blocks and around 22-23 Rc.

Not sure if this will make the tapping any easier but at least the hardness question is answered. I'd gotten the bars from the cutoff bin at Metals Supermarket in Edmonton and I was wondering if maybe I'd gotten something else.

Dave

Dave Hodson · May 15, 2020

That looks terrific Kyle - great idea

Gordon Heaton · May 15, 2020

Kyle's tip in post #3 about not using 4-flute plug taps is good advice. In that size I'd use a 2-flute spiral point coated tap. My personal preference is Hertel - best bang for the buck. When using spiral-point taps do not make short turns or back up, just turn it straight through. These are through-holes, right?

Dave Hodson · May 15, 2020

Hi Gordon

I was trying for a blind hole for the pommel and some of the advice I got elsewhere was for short cuts and reverse to break the chips frequently. I did try a through hole though and didn't fair much better. I didn't back up in that one but it seized pretty quick and then I broke the tap.

I'm going to keep working to get a workable through hole procedure and then play around with the blind hole. I can always put a decorative plug in the end of the pommel.

Thanks
Dave

Gordon Heaton · May 15, 2020

What kind of tap were you using? With a standard hand tap is is necessary to do short cuts and back up. The problem is, that doesn't often work well in most types of stainless. If you're going to go straight through you need a spiral point or spiral flute. The spiral flute taps are, in my opinion, weaker through the web and easier to break. Spiral point taps are not the best for hand-tapping, but if you can drive them in a mill or a drill press with a tapping head, they work very well.

I tap a fair amount of 304L stainless and have very good luck with those Hertel spiral-point taps but I do rigid tapping on a mill. I do use spiral-point taps in blind holes when I have room to drill deep. I also don't use anything fancy for cutting oil when tapping 304. I use 'Harvey's Dark' cutting oil from the local plumbing wholesaler! It works well, but I use a lot of it. When tapping a blind hole, I fill the hole with oil and run the tap in.

Dave Hodson · May 15, 2020

That could be one of my problems. I'm using a standard hand tap and not a very expensive one. I think I'll do a bit of research online and order some better quality taps. The other thing will be to figure out how to set up the drill press for it. Thanks for your help

metalpyro · May 16, 2020

How about a bench mounted hand tapper?

Shop Tools and Machinery at Grizzly.com

Dave Hodson · May 16, 2020

That's one of the frustrating challenges for us in Canada. There used to be a Grizzly outlet in Edmonton but it seems they're all gone. I did find this through Amazon.ca but they want $C638 for it which is nuts. Nice idea but a little much for me. Nice to see what some options are though. Thanks

MattiJ · May 16, 2020

Dave Hodson said:
When I checked the hardness of the blocks I broke the taps in, all three had a varying hardness between 48 & 55 Rc. My understanding is that's hard for 316 which, from what I've been able to find out, should be in the low to mid 20's Rc.

One suggestion I got was that the bar could have been cold rolled and work hardened so I found an annealing temperature (1900 deg F for an hour with a quick quench) and tried that. The new hardness is now similar between all three blocks and around 22-23 Rc.

Not sure if this will make the tapping any easier but at least the hardness question is answered. I'd gotten the bars from the cutoff bin at Metals Supermarket in Edmonton and I was wondering if maybe I'd gotten something else.

Dave

55 Rc sounds unrealistic for 316 so I'd vote for "you got something else" . 420 maybe?

Even still I'd follow the previous advice and toss the "regular hand taps" and get proper taps specially for SS.
For hand-tapping blind holes you might want to start with spiral point (aka gun point) tap and finish the last few turns with spiral flute bottoming tap.

AD Design · May 16, 2020

What everybody has posted is sound advice, two things are critical. Use good quality taps like the ones others have listed and the alignment of the tap to the axis of the hole. A small alignment block is what I've used when I have to tap by hand, it works much better than aligning by "eye". The Grizzly hand tapper at over 600 bucks (Canadian) is quite expensive. A similar rig made in your shop would help and be a lot less money. You've already found how to anneal the stock to lower the hardness, perhaps you could also do this before tapping. Also, a quick check with a magnet will tell you if it's 300 or 400 series stainless.

DanASM · May 16, 2020

Always tap as far as you need to in 1 pass with SS. Do not reverse out and continue to re-tap.

Tapping work hardens SS and you need to cut the material and remove the chip so the heat leaves with it. If you reverse and re tap the hole you are essentially trying to re tap hardened material.

I tap 316 SS all the time in high production runs. 316 SS does not like to be re cut. Dont ever try to cut it where you have already made a cut or had any type of dwell. SS work hardens at 500 degrees F. Which is not hard to achieve.

My advice is to use the best tapping fluid possible or tons of oil. Full Flood. Drill the hole without pecking and then tap right to the bottom in 1 pass then reverse out. 316 can be form tapped as well if needed.

It is tricky and you will break lots of tools. When you finally figure it out you will understand this metal and how it cuts. Take notes and learn from your mistakes.

Dave Hodson · May 16, 2020

Hi Dan

Thanks for information. I did try a one-pass, no stopping test with the hole flooded earlier but the tap seized up within a few threads and then broke but I think there's a few things I can do better. I've ordered some better taps and will try again on an annealed block when they come in.

I have to admit it's a bit confusing for me as a newcomer since there seems to be varying ideas regarding reversing and breaking of the chip with both for and against cases stated. I wonder if that has something to do with the type of tap used or the way it's being used. I'm hand tapping so obviously can't generate the cutting pressure a machine or press could - maybe that's a problem. Obviously people have had differing experiences that work for them but I think everyone has the similar view that 316 is tricky and all agree that work hardening is an issue.

I'll keep plugging away though. The one pass, full pressure, no-pecking, with a sharp drill worked to solve my drilling challenges so it stands to reason a similar approach should work on tapping. I just need to figure out how to get the pressure.

Thanks again and have a good weekend
Dave

jim rozen · May 16, 2020

Late to the game here but:

1) use a two flute gun tap. Designed to push the chip forward, as mentioned don't stop, don't back up with those.

2) cutting fluid: is there a home depot or equivalent near you? Plumbing section has small bottles of black sulfurized
cutting oil. Use that.

Hand Tapping 316 SS - need help

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