Metal printing of a 5" gauge locomotive blast pipe

[AVTUR]

I met metal printing at work just 20 years ago but never thought it would catch up with me in retirement. I am slowly making an LBSC Speedy loco (5” gauge) and reading recent accounts of metal printing decided to see what it would now do. I chose the blast pipe since this is a complex silver soldered assembly with the likelihood of more than one attempt needed to get a satisfactory result. The blast pipe is made of ½” copper tubing and sits between the frame plates and passes through the bottom of the smoke box and saddle. The blast nozzle is screwed on after assembly into the locomotive.

Knowing a little about the subject (very dangerous) I ventured forth. I had already solid modelled the blast pipe using FUSION360. This model was modified to increase the tube thickness to over 1mm so moving it away from the printing limitations. The geometry was tidied up and a thread put it for the blast/nozzle. Because of my FUSION360 limitations the thread was ½” x 28tpi (UNEF) with local thickening of the pipe [picture attached].

On 23 August 2026 I registered with CraftCloud and sent them a .STEP file of the model. I chose a cheap option for manufacture, a Chinese company, and fast delivery by FedEx. I specified 316L stainless steel as the material.

The printing company came back giving the manufacturing tolerances, warpage ±0.3 to 0.7mm and manufacturing ±0.2 to 0.5mm and a price to finish the thread of $91. Model was corrected for manufacturing and warpage tolerances by thickening the flanges (this later gave difficulties). I said that I would finish the ½” UNEF thread. The cost would be manufacturing £49, express shipping £29, V.A.T. £16, coming to £94.

These I accepted and manufacture started on 7 September. The finished item was dispatched a week later and arrived on 16 September. FedEx gave very prompt parcel tracking information which made interesting reading. Meanwhile I ordered a small amount of 316 stainless for machining trials since I had no experience of the metal.

AVTUR

[petertha]

Cool. I think 3DP metal has opened many doors. Because I did a similar project, I'm curious
- how you handled the mitre where the tubes come together. Typically it would be a sharp-ish corner on the inside joint. Did you fillet that corner in your CAD model or did they do some post-processing on their own?
- you show some threads, assume you will do that with a die? Is the OD pretty good as-is or will you machine it to a spec OD beforehand?

[petertha]

What I mean by an inside mitre corner if my description was confusing

[AVTUR]

petertha

I built the solid model in parts: Individual curved tube (suitably scarfed) with flange [attachment 1] and the exit tube. Two curved tubes were joined together followed by the exit tube to give the solid model [attachment 2]. Since the steam from the cylinders enters from the flanged ends the internal condition of the mitred join is not important (unlike an IC engine manifold). Also the internal diameter and surface finish is not that important since the steam velocity is low. The steam is accelerated though the blast cap [attachment 3 (thread for blast pipe not shown)] to produce the sub atmospheric pressure required in the smoke box. The nozzle area is about a quarter of the pipe area.

I did think about putting a fillet radius externally on the mitre to reduce the stress concentration but that was beyond my solid modelling capabilities.

The thread at the top of the pipe was printed. However because of the printing process it needs finishing (the next episode).

The blast cap will be conventionally machined and silver soldered, probably from bronze.

AVTUR

[AVTUR]

The result looked good with a nice finish and no defects [attachment]. The thread obviously needed finishing so a set of ½” UNEF taps and die was bought. The holes in the flanges were below the required diameter of 4.0mm and the distance across the outside flange faces was 105.28mm compared with the required 103.17mm. This I did not expect. The manufacturer had worked to the maximum tolerance while I was expecting them to use the minimum so using less metal powder (I believe most of the cost is the metal powder). Anyway, I now had to machine back the faces.

Finishing the thread, using the die as a chaser, proved easy. The die picked up the thread with ease and Morris’s thread cutting lubricant (grease) used.
 

AVTUR

[petertha]

Oh you incorporated the threads in the 3DP part, bonus points.

[AVTUR]

Oh you incorporated the threads in the 3DP part, bonus points.

No, bonus points to Fusion360.

AVTUR

[Jasonb]

I do find holes come out a bit undersize typically about 0.2mm for those sort of diameters.

[Minh Thanh]

The result looked good with a nice finish and no defects [attachment]. The thread obviously needed finishing so a set of ½” UNEF taps and die was bought. The holes in the flanges were below the required diameter of 4.0mm and the distance across the outside flange faces was 105.28mm compared with the required 103.17mm. This I did not expect. The manufacturer had worked to the maximum tolerance while I was expecting them to use the minimum so using less metal powder (I believe most of the cost is the metal powder). Anyway, I now had to machine back the faces.

Finishing the thread, using the die as a chaser, proved easy. The die picked up the thread with ease and Morris’s thread cutting lubricant (grease) used.
 

AVTUR

  Nice!
----------------------

A little off-topic
  if I wanted to cast a similar exhaust pipe in the future (with an inner diameter of about 6–8 mm), are there any specific considerations for creating the internal casting core? (The material is aluminum.)

[AVTUR]

The result looked good with a nice finish and no defects [attachment]. The thread obviously needed finishing so a set of ½” UNEF taps and die was bought. The holes in the flanges were below the required diameter of 4.0mm and the distance across the outside flange faces was 105.28mm compared with the required 103.17mm. This I did not expect. The manufacturer had worked to the maximum tolerance while I was expecting them to use the minimum so using less metal powder (I believe most of the cost is the metal powder). Anyway, I now had to machine back the faces.

Finishing the thread, using the die as a chaser, proved easy. The die picked up the thread with ease and Morris’s thread cutting lubricant (grease) used.
 

AVTUR

  Nice!
----------------------

A little off-topic
  if I wanted to cast a similar exhaust pipe in the future (with an inner diameter of about 6–8 mm), are there any specific considerations for creating the internal casting core? (The material is aluminum.)

I hope I am answering your question. The item is printed, there is no casting involved (forgive me if this a language problem).

The printing broker (Craftcloud) advises a wall thickness of at least 1mm. I made it thicker just in case. FUSION 360 allows you to make a pipe by extruding an object along a sketched path. Initially it may seem complex but rapidly becomes second nature. I imagine most solid modelling packages have a similar facility. I don't think there are any other specific considerations. However I have a feeling that copper alloys cannot be printed at present (possibly because of their high thermal conductivity).

Now to see how easy it is to machine 316 stainless.

AVTUR

[AVTUR]

First, I attach a picture of the finished thread [attachment 1]. I will find out how good it is when I have made a gauge.

The mounting holes drilled out to 4.1mm without difficult using CORA B cutting oil.

I knew the machining back of the flange faces would be difficult. My little milling machine is about 20 years old and well used. The lead screws have quite a bit of backlash so climb milling is out of the question. Therefore, I elected to flycut the faces on the lathe [attachment 2]. A trial with a short length of 1” diameter 316 stainless steel bar gave a good result, but when it came to the actual job all I did was to polish the faces [attachment 3].

I am now trying to work out how to remove about 0.5mm from each face. I think I am on a steep learning curve but there are a few options.

AVTUR

[Jasonb]

Try an insert with a small 0.2mm tip radius rather than the round one.

[AVTUR]

Jason

It was very much a case of "what have you". Since today's postal delivery I am in a better position to face back the flanges.

AVTUR

[petertha]

AVTUR I'm intrigued by the 3DP thread. It actually looks better than I would have expected, but I really have no basis to compare. So you are going to run a die over these threads? Is the picture showing that finished result quite yet or this is still 3DP? Did you specify threads in the print job to make the die work easier - as opposed to ommitting the threads altogether & just running the die conventionally?

[AVTUR]

I had a long chat with knowledgeable members at the model engineering club last night about facing back the flange faces. So a lot of thinking.

petertha - I modelled a thread, 1/2" UNEF, to see the result. This was specified and printed. The picture of the threaded end is after being chased using a die. The original finish can still be seen at the tube end. I have yet to make a gauge to check the thread.

AVTUR