Stainless Steel Tapping Best Practices

Hi All -

This is my first post. I am planning on printing some parts in stainless-steel (SS) but I will need to do a bit of post machine-work to clean them up afterwards. In particular I will need to tap an internal tapered pipe (NTP) thread. I've read through the forums and I see some people suggested "printing threads." Others suggested under-sizing holes and then drilling them out afterwards and finally some suggested printing out a full sized hole and simply tapping that. There are reportedly issues with the latter approach due to pieces occasionally being under or over-sized.

I've also read about how difficult it is to machine Shapeway's SS and some people say carbide tools are necessary but others say quality high-speed steel is ok as well.

Finally, some of the posts I read were a bit old and I wanted to see if best-practices had evolved since then. So...

Bearing all this in mind I was wondering if I could get some recommendations on designing parts to be tapped. Specifically:
1. Will quality high-speed steel taps work or is carbide tooling the only feasible option?
2. What are the best practices for designing holes to be tapped?
3. Does under-sizing tap-holes for a 50% (or less?) thread make taping any easier?

Also, while not terribly important I could not find any information on heat transfer coefficients for SS. I'm assuming its somewhere between bronze and 420 SS but if anyone has a reference to heat-transfer coefficient data points I'd greatly appreciate it.

Thanks very much in advance

You probably do not need Carbide. Try looking into Cobalt steel or Vanadium steel. These are not cheap but should last practically for ever with occasional use.

gNSortino,

from your post it sounds like you are going to do some plumbing.

This sent me off to the Materials page and while SS is waterproof, I wonder if pressure has any relevance, and also about the expansion/contraction effects under heat/cold which might differ from the materials you are joining to. And then there are the potential electrolytic effects...

I have no answers but wondered if you had considered all this?

Thanks for the comments. Yes I had considered whether the material would hold low pressures and I definitely need to test that. However, based on the advertised properties of the material it *should* work.

Wait, water? You will end with a nice orange brown patina sooner or later.

You need to trust me when I say carbide tooling is the only practical way to drill, tap and machine this material. Slow feeds and plenty of lube. Playing with hole diameter would help. Expansion contraction is minimal. Probably better off silver soldering pipe nipples onto printed body. Two other issues you need to be aware of for your application. Yes there will be some electrolytic corrosion but this should stabilize after an oxide layer forms. Also, "waterproof" is NOT always absolutely watertight with this material. Under pressure this could be a problem without some form of internal sealant coating. The overall size and design of the part contributes to this factor. But finding out for sure will only be by trial and error. Not sure if any precise "heat transfer" data is available. But your intuition is correct, it's somewhere in between the two materials. Definitely not like copper or aluminum.
-G

Hi Glen -

Thank you for the candid advice on carbide. Also, until you mentioned it I hadn't thought about soldering but that does seem like a good idea. While I was researching this question I had stumbled upon another forum-post you made:

https://www.shapeways.com/forum/index.php?t=msg&goto=671 98&&srch=stainless+steel+silver+solder#msg_67198

It mentioned that Silver Solder works well on Shapeway's stainless steel. I will do some experimenting to figure out which approach is best.

Kind Regards,
Graham

My own small experience:
I was able to run a HSS reamer through an undersized 4mm hole... but drilling with HSS was noticeably difficult, and I found it impossible to tap by hand with a HSS tap... I ended up drilling through and bolting.

Maybe consider the new brass? SW is only offering it gold plated, though many are demanding UNplated. i.competitive service will do plain brass though (come on SW, catch up!)

Hi There -

Here is another update on this subject. I decided to test out the feasibility of tapping further.

What I did was create 3 small Shapeways Stainless Steel (SS) pieces with a .339", .34375" (11/32) and .375" (12/32) diameters. I put a slight taper on each of these to make it easier to start the tap.

I also wanted to test a straight imperial (non-tapered) thread to see how that worked as well. For this test case I created a 50%, 25%, and 15% thread for a 1/4"-28 UNF machine screw. This equates to a diameter of .2281", .23905", and .24343" respectively. Each of these also had a slight taper to help with threading.

##Setup##
For my tapping equipment I used a standard ratcheting tap wrench and the following taps:

1/4"-28 Cobalt Steel (McMaster Part # 25215A61)
1/8"-27 NPT Cobalt Steel (Mcmaster Part # #2662A26)

I chose cobalt steel taps because they have a Rockwell hardness between C30 and C45 whereas Shapeway's Stainless has a hardness of 20-25 ( https://www.shapeways.com/rrstatic/material_docs/mds-stainle ss.pdf)

I also used a drill press to align the tap in the hole. Very similar to this process here: https://www.youtube.com/watch?v=KmtbzL8YlxM. This was the first time I've used a drill press to align a tap and I found it did a very good job.

##Test Results##
I did the imperial 1/4"-28 (UNF) taps first starting with the 15% thread and moving up to the 15%.

All 3 threads were fairly easy. The easiest being the 15% thread and while the 50% was noticeably tougher although it was not what I would consider difficult. I was not concerned at any point the tap was at risk of breaking. As would be expected the thread on the 15% was barely noticeable. However, it would still mate with a machine screw and if screwed all the way in it would hold pretty well. The 50% created a really nice thread and the 25% was somewhere in the middle.

Next, I did the NPT taps starting with the 12/32" diameter and working my way to the smaller diameters. The 12/32" was very easy to do. It only produced a slight thread but it would mate with male NPT thread. The 11/32" was noticeably more difficult towards the bottom of thread. I stopped tapping with about 6-7 teeth still visible on the tap, which I believe is typical. This created a pretty well defined thread that mated very nicely. The .339", which is a standard "R" 1/8" NPT tap drill size was about the same difficulty as the 11/32".

##Closing Comments##
Based on these tests I would say that tapping Shapeways SS is definitely doable. I cannot say how much of an effect the cobalt steel taps or using the drill press for alignment had but I suspect they both helped significantly. I don't think the slightly tapered start had much of an impact but I also don't think this hurt things either.

While tapping I noticed that the chips were much finer that when you would typically see when tapping free machining metals. They were almost like large grains of sand. The material made a grating sound as it was tapped and even though the chips were smaller they still had to be broken (eg. back 1/4 turn for each turn forward). Also, the material did not get very hot like you would typically see when tapping something with a low heat transfer coefficient. I suspect this was due to the 40% bronze content.

Great to see the documentation of your testing! I'd say fixturing and using a drill press definitely makes a difference between sucess and that bad ping sound as your tap or bit snaps off .
The properties of this particular SS printed media are actuall very complex. This is a sintered binary material and how the two components interact is not yet fully understood or documented. Knowing the general specifications of the two components is important but in a composite state (ss print) many of these rules do not apply. Bravo to you for jumping right in, using your intuitions and not relying on charts, tables or software to solve a problem.
-G

thanks I appreciate that. Its good to see others are interested in using Shapeways materials for more engineering type purposes as well. I looked at a of your posts too while researching this topic and I'm starting to see some good information on this subject.