316 Question

I've generally used Nickel/Silver but wanted to try 316 for a through-tang knife pommel. The problem is I've broken two taps trying to thread it. Even drilling was a chore - almost seemed brittle not smooth cuttings. I checked the hardness on my tester and it was Rc 54 which seems to be double what I saw online. It's not magnetic and there are no sparks when I grind it.

It's a chunk of 3/4 x 3/4 bar stock I got from the cutoff bin at Metal Supermarkets in Edmonton. They told me the colour code for 316 was green/yellow and this has that so I'm assuming it's 316. I don't have any experience with this steel though. Is this normal? Anybody got any suggestions for softening it up?

Thanks for any help you can give me
Dave

 

316 stainless steel can't be softened its carbon content if something like .08. I thing problem with 316 is the Nickel it makes the steel tough or gummy that's why your bits are breaking. 416 SS is what most guys use, as it's a "free" machining steel and better suited for drilling and taping operations. I use 316 SS because its all I can easily get locally, and I've gotten much better at resharpening broken drill bits.

 

At Rc 54, I'm pretty sure you don't have 316. Trouble is, I didn't think metal supermarkets carried any high hardness stainless. You could try etching with some acid to make sure that it is stainless. Otherwise, I would throw it in the 'interesting pile' and buy some new 316 (it's pretty cheap).

Magnus

 

Thanks for the information. I took a closer look at the bar and saw somebody had written 6706 in magic marker - very faint and you could hardly see it. I'm not sure what that means but maybe it isn't 316. I got a couple of other smaller bars so I'll check them out. Anyway, I'm all out of taps and our Canadian Tire is under water right now so it's a mute point for the time being. Interesting that you can tap 416 but not 316 though.

Thanks again
Dave

 

Dave,

I know it's a biatch to tap. I have broken my share of small bits and #8 and #10 taps in 316. It gums up threads pretty quickly and binds easily. It also work hardens easily with dull machining tools.

416 has the addition of sulfur to make it easier to machine. Even grinding/sanding goes noticeably faster with 416. Interestingly, annealed 416 doesn't reach its full corrosion resistance until after heat treating it. I don't know any knifemakers that heat treat 416 though.

I always go back to the selection guide from the Nickel Institute. Great washroom reading.
https://www.nickelinstitute.org/med...theselectionanduseofstainlesssteels_9014_.pdf


Dan

 

Thanks Dan - good reference to have. Once things settle down a bit I'll try and track down some 416.

Dave

 

I get my 416 bar stock and 416 pins from Rob at knifemaker.ca. It's nice to know the pins will vanish into the same material.

 

Thanks, I'll send him an email. The website doesn't show any in stock but maybe they haven't updated the site.

 

Many stainless steels can work harden pretty easily, this makes them weakly magnetic but does not change the spark pattern any.

Simply drilling at the wrong speed and feed rate will work harden the hole making follow up work a real pain. It is possible to anneal the stainless if the process is followed exactly.

 

Thanks John. I figured out you can't be hesitant with this stuff - no pilot holes, sharp bit, enough pressure to cut and give it - no stopping. What I haven't been able to figure out is how to hand tap it. I've broken two 1/4" NC 20 taps so far. They just seize up. I'm thinking I don't need full contact thread so I'm going to try a slightly bigger hole and see how that goes cutting less metal. Does anybody know if a finer thread taps easier than a coarser one. I just chose NC 20 because I had some 1/4" threaded rod.

 

Find a drilling tapping fluid specific to stainless steel or check in the machinist handbook they used to have recipes.

When tapping think aluminum and how it gauls and sticks now imagine something worse, I make it a habit when tapping by hand to use 1/4 to 1/3 turns instead of half to one turn. keeps the pieces breaking off small as it work hardens you want much less in the die for cutting

 

Hi Dave,

Breaking 1/4-20 taps? Shite! What size hole are you using? I don't have a complete number set, so I use fractional 13/64" (0.203") for both 20 and 28 threads, but in theory a slightly larger hole is spec'd for the fine thread tap. Like John mentioned, once the tap is oiled and the threads are starting to bite I go about 1/2 turn or less, then backwards to clear the chips. Squirt some oil in periodically.

Why do I have a sneaking suspicion that you have some 400 series SS that's been hardened?

Dan

 

Hi Dan

It was out of the cutoff bin at Metals Supermarket so probably anything's possible but it's not magnetic so I don't think it's 400. They told me the paint code on it was 316 but I measured the hardness to be around 54Rc which seems high. I can't remember the drill size but it was what the chart on my set recommended for 1/4-20 and I was using a tapping fluid. I'm away right now but when I get back next week, I'll play around a bit more.

Dave

 

Update: May 16

When I checked the hardness of the blocks I broke the taps in, plus another test block, they 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.

The other thing I did was join the PracticalMachinist.com forum and posted my questions. Very helpful group over there with a lot of good suggestions - highly recommended. The only problem is that the stuff they have ready access to in the US is either unavailable or crazy expensive to me here. That drives me nuts and has been a sharp stick in my eye for a long time now. I have a few other hobbies I do and this sourcing issue makes them a lot more difficult (and expensive) than they need to be. Now I'm all wound up - I'll be stewing all morning.

Have a good long weekend
Dave

 

Dave I find it hard to believe that the hardness could be between Rc 48-55. I use 304 and 316 all the time for bolsters and butt caps. Yes it will work harden if your cutting tools are not sharp.
The biggest problems with working with 300 series stainless are.
- the wrong or not using cutting fluid ( I use rapid tap)
- dull or improperly ground bits (drills and taps) I resharpen my drill after drilling each hole
- when drilling not applying a constant pressure (this will work harden the material and just about instantly dull the bit)
- drilling at the wrong speed, most of us have cheap drills the were basically made for woodworking and many times they just do not turn slow enough for metal working especially when it comes to larger bits.
- when tapping only tap about 1/8 of a turn and back off to remove the chip and then advance another 1/8 turn.
- Aluminum is a soft and easy to cut materiel, but when it comes to tapping small hole like 2-56 and 4-40 in thick pieces without the proper cutting fluid and taking it 1/8 of a turn at a time your are going to break a lot of new taps.
- then you have people say you should not use 300 series for bolster because it does not buff very well. In my opinion it buffs easier than 400 series and with a lot less work . You just have to have the right compounds and apply the right technique and you can get a perfect shine on 300 stainless.

Hope this helps.

 

I was surprised too. Ran several samples to double check. The general idea on other metal/machinist forms is this barstock might have been cold rolled and work hardened. In any event, it annealed back to an expected hardness for 316. I haven't tried tapping it yet though to see if it makes a difference.

Took a while but I figured out the drilling - sharp bits, constant pressure, don't stop and good cutting fluid but hand tapping is where I'm having trouble. I got lots of posts that say under no circumstances stop and start for the same reasons as drilling. I also got lots of posts that said go slow and break often like you point out. Obviously both camps must have something that makes their procedure work for them.

In any event, I'm still waiting for my replacement taps to come in and then I'll try the annealed blocks. As far as the buffing goes, I did try a test sample and it was great - lots of shine and you couldn't even see the pin.

Dave

 

Cold rolled bar will be free of the darker mill scale finish and rougher than cold rolled. A few suppliers carry it since it looks like less work to get a true surface on the back but the trade off is it is harder to drill & tap

 

Thanks John. It annealed to a much lower hardness ok but I'm still waiting for new taps. Hopefully they come in soon.

 

Problem solved. Here's the summary that I posted on the Practical Machinist forum - great forum if you ever need some help.

"Eureka - I did it and thanks to all of you for your help. As you all pointed out, quality is important and I ended up having to wait until I got to Edmonton to find the right materials. Got back last night and tapped the pommel in 316ss this afternoon. Hard to believe how well it went considering how it all started out a few weeks ago.

I'm sure all of you have the techniques that work for you but here's what I found that helped me.

1) Good quality cutting tools and taps
2) Annealed my stock to lower the hardness (1900 deg F for 1 hr & quick quench)
3) Drilled my pilot hole for 50% thread (7/32 cobalt drill for 1/4-NC20 in ss) not 75% which is the bit you get with the sets. I found a tapping chart that indicated 75% is for plastics, brass and aluminum with 50% for SS, steels and iron. Your experiences and needs might be different but 50% worked for me. Thanks ChipSplitter for first pointing this out to me.
4) Used a good ss tapping fluid (Tap Magic XTra-Thick)
5) Used my drill press for the tap and turned chuck by hand (slow)
6) Started the thread with a 1/4" plug tap (only a couple of turns in short 1/8 turn segments)
7) Mounted a good quality spiral flute bottoming tap for ss (this is a blind hole), more fluid and continuous slow and steady tap to bottom (no stopping)"

Here's the link to the thread if anyone wanted to follow it as well as a link to an Imperial tap drill chart

https://www.practicalmachinist.com/vb/general/hand-tapping-316-ss-need-help-377635/#post3549477

https://www.armstrongmetalcrafts.com/Reference/ImperialTapChart.aspx

Hope you're all good
Dave

 

Good work Dave. Are you making a threaded SS butt cap?

For blind holes, I actually grind some of the tip of the tap off to make it a bottoming tap. I find even three or four tapered threads at the end is wasting valuable depth. I have to go a little easier on starting the threads and make sure everything is dead straight in the hole, but it works pretty good with a bit of oil and a gentle touch on the tap wrench.

What I have also done from time to time is drill through and tap through the butt, then cut/grind the tang bolt a little short of full threads. Clean out the hole with acetone and insert a piece of pin stock. A few whacks of the ball pein hammer and it fits nicely in the hole. I usually use 10-24 SS machine screw for the tang bolt and 3/16" pin stock fits snugly in the tapped hole. When the pin splays out it really bites into the threads. Grinder to finish.

Dan