Lets talk additive manufacturing

[joanlluch]

Hi All,

I wanted to bring this thread to life again for a while because I have some news with regard to the parts I had directly printed by ShapeWays.

To put this in context again:

One month ago I ordered some 3D Printed parts in stainless in order to check whether the technique was suitable for making complicated ones which otherwise would be difficult to make using conventional subtractive techniques. 

One goal of my test was to check whether direct printed parts were (1) strong enough for use in mechanical parts, and (2) whether they could be further machined to get them to perfect accuracy. One possible scenario for my particular case was to make my locomotive cross-heads as 3D printed parts.

The parts I used for testing (loco buffers) were very strong, so they were ok as per (1), but unfortunately they were too strong for any subsequent mechanical operation on them, so (2) was not met. This implied that direct 3D printing was not suitable for parts that required further machining or work.


I have now decided to give it another round and decided to bring the parts to thermal treatment in order to anneal them. The parts were made off 60% 420 stainless steel powder and then infused in melted bronze at high temperature. I got the suspicion that after the parts are soaked in bronze they are just brought to quickly cool in air. So that might explain why they are so hard. 420 Stainless steel does harden. So subjecting them to Annealing should (possibly?) make them softer.

The following picture shows a set of them before (left) and after (right) annealing 

Topall Revingut by joan lluch, on Flickr

Apart from a very noticeable colour change, I found that the annealed parts (at right) did actually reduce hardness significantly and were more workable.

As supplied by ShapeWays (left) it was very difficult to drill a hole in them, and it was not possible to make a thread on them. After the thermal process (right) a thread was attempted and it was relatively easy to perform using a sharp new tap. See this:

Topall Revingut by joan lluch, on Flickr


This means that directly printed metal parts can be optionally machined if they are subjected to an annealing thermal treatment. This was suggested elsewhere but I didn't find a clear confirmation of it actually working before.

That's good news because this opens the interesting opportunity of making complicated single parts, which can me machined, which still keep a high mechanical strength, and without having to follow the route of wax printing, moulding, and casting.

[Roger]

That's very interesting indeed, I'm pleased you'll be able to use those part, I hate to see money wasted.
I'm not sure whether you mean that you used a company to heat treat them or whether you did that yourself. I'd be interested to know what the process was.
What I don't understand is why they were so hard in the first place if the grade of Stainless Steel used doesn't harden. Bronze surely doesn't harden to the sort of hardness you originally had so I wonder what made them like that. I can understand these things when an alloy is formed, but I didn't think that was the case with these.
I wonder if heating them and then quenching would make them hard again?

[joanlluch]

Hi Roger,

The money would not be wasted anyway because on these particularly simple parts I could use loctite in the central hole instead of making thread. However, now the door is open to make other more complicated parts using the same procedure.

The grade of S.S. used DOES harden. It is 420 S.S. Apparently that's the grade used to make knifes or other cutlery. You can google that. My take is that for mere productivity reasons the parts are just quickly cooled down after bronze infusion and this leaves the stainless steel in a hardened state.

The processing was made by a professional heat treatment service so this should be totally reproducible. I asked for the treatment that would bring 420 stainless to readily machinable state, but since I am not expert on heat treatments I did not bring attention on what they would particularly do. I will inform myself about the exact temperatures and temperature/time patterns that they applied and will let you know. I should know that by Monday. I know that the parts have been heated at some temperature above 500 C for some time and then cooled down slowly in a furnace, but better wait for me to ask again.

Yes, I have been told that after heating and quenching, the parts should become hard again. This certainly works for 420 S.S. But in this case this would have to be tested because the material is in fact a sort of composite Stainless Steel + Bronze. It this is right, it seems that we have a lot of possibilities.

[Roger]

Ah, I read that wrong in your original post! That makes sense now. If the process is anything like it is for High Carbon Steels, then you don't need to do anything that sophisticated to anneal them, just get them to a dull red heat and let them cool while insulated. You have to cool things fairly quickly to prevent them from changing back to their soft state. To guarantee the softest possible state you obviously have to be more scientific, but I think you'll find it easy enough to do.

I agree, this does indeed open up a lot of possibilities, certainly for your crossheads.

[abby]

I showed a photo of a printed pattern earlier in this thread , it is for the inside motion bracket casting as fitted to a 5" gauge castle class engine.
This is the real thing.



and this is the casting , still to be fettledn of course.





I think if you are building this model you will agree that the casting is an easier proposition than the fabrication shown in the drawings.

[Roger]

That's a cracking job Abby, but most people don't have the 3D modelling software to define things like that. If the files are made available for sending to a foundry or 3D printing company, then I'd agree with you.

[ejparrott]

No I agree, it is the easier option. Whether or not you have the files or the ability I don't think is the question, I think the question is primarily cost, as always, and whether the builder wants a casting or to fabricate.

[abby]

I don't want to be a "dog in the manger" here Roger but this thread was/is about additive manufacturing techniques ALL of which require some form of 3D cad software , you seem ready to comment negatively on almost everything posted , which of course is your perogative, however most of us refrain from replying to a post unless we have something to say which is actually informative or helpful.
Many builders want to see their engines finished before they go to the great shed in the sky , far to many projects finish up on ebay or similar as "my late husband's/father's etc".
for those who believe their remaining time is more valuable than money , these manufacturing services exist , just like CNC machining and laser/ water jet cutting.
Personally I couldn't give a toss how something is made but being anal retentive it has to be as perfect as I can possibly make it and I have to say that no way could I make the fabrications for this engine anywhere near the accuracy and quality of my castings.

[Roger]

Oct 22, 2015 12:23:16 GMT abby said:

I don't want to be a "dog in the manger" here Roger but this thread was/is about additive manufacturing techniques ALL of which require some form of 3D cad software , you seem ready to comment negatively on almost everything posted , which of course is your perogative, however most of us refrain from replying to a post unless we have something to say which is actually informative or helpful.
Many builders want to see their engines finished before they go to the great shed in the sky , far to many projects finish up on ebay or similar as "my late husband's/father's etc".
for those who believe their remaining time is more valuable than money , these manufacturing services exist , just like CNC machining and laser/ water jet cutting.
Personally I couldn't give a toss how something is made but being anal retentive it has to be as perfect as I can possibly make it and I have to say that no way could I make the fabrications for this engine anywhere near the accuracy and quality of my castings.

I don't think you've interpreted my posts in the spirit in which they were written.

[Roger]

Adam brought this Kickstarter project to my attention, and I couldn't resist adding my pledge in the hope of getting my hands on one. It's the first time I've seen a liquid based 3D printer that's affordable.
Anyway, I thought this was a really clever idea, and it ought to produce parts without those ghastly layer lines on them. Ok, this is only a small unit, but it might come in handy for valves and such like.