johnsmachines

machines which I have made, am making, or intend to make, and some other stuff. If you find this site interesting, please leave a comment. I read every comment and respond to most. n.b. There is a list of my first 800 posts in my post of 17 June 2021, titled "800 Posts"

Tag: lost PLA casting

July 20, 2021

How to Pour the Bronze Barrel?

I have given some thought to how to manage the bronze pour for the barrel of the Ottoman bombard. It will be at the size limit of my potter’s oven for the PLA burnout.

The red 3D printed PLA is the barrel. The breech, although significantly shorter, weighs almost exactly the same, but being shorter, should be less problematic. The wall thickness of the breech is greater than the barrel.

I had thought that the steel cylinder would be adequately long to cast the barrel, but it is about 50mm too short when I take into account the bronze feeder reservoir which will be required. So I will add a 50mm length, probably by arc welding another bit of tube to one end. It wont matter if it is not a perfect join. I will make it waterproof with duct tape. The tape will burn off during the investment melting/burnout.

I will cast the barrel with the threaded end downmost. The molten bronze feeder reservoir will be 60mm deep which I hope will provide adequate pressure and extra molten bronze if required during cooling contraction.

I decided that the usual rubber pouring funnel (pictured above at the bottom of the steel cylinder) would not have an adequately large bronze opening or reservoir depth, so have 3D printed one in PLA. The PLA will disappear during burnout, but will leave its shape in the investment medium and become the funnel and reservoir.

I wont be using the rubber cap/funnel mold. I hope that the 3D printed one works. Despite appearances the thin flat base is watertight. As shown here the funnel is positioned as it will be when the investment medium is poured. The cone seen is not a funnel. It is a distributor for the molten bronze. When the bronze is poured the cone will be point up.

Hard to get your head around that one? It certainly was for me when designing it.

You (and I) need to remember that any space around the PLA will become investment medium. The PLA will disappear and become a void which will be filled with bronze.

This screen shot of the 3D diagram might help. This is the pouring reservoir in the pouring position. The wide disk at top is to position the print on the steel flask. The skinny cylinders are vents to release air during the bronze pour. The stepped cylinder in the middle sits on the inside lip of the barrel. The inverted cone will be solid investment to direct the bronze into the barrel void. Confused?
Maybe this will help. This is the position the PLA parts will sit inside the steel cylinder during the pour of the investment medium and later during the PLA melt/burnout.
During the bronze pour the PLA components will be voids, which the bronze will fill. (except for the widest disk at the top which will disappear.)

The 3D print took over 8 hours. 0.2mm layers, 210ºc extruder temperature, 3000mm/min. I will need to do a similar 3D print for the breech. If either or both pours fail the whole process will need to be repeated.

Still in lockdown. Cannot visit my workshop due to the 5km travel limit. So 3D designing and printing at home is fairly good use of my time.

Leave a comment
July 16, 2021

Lockdown No. 5

A few more small jobs out of the way, and some underway.

I showed you the “large” Kant Twist clamps recently.

Now I have finished the small ones. Same pattern, just reduced by 1/3. And a different handle.

Brass pins again. Machine cut knurl was simple with a CNC rotary table. No grooves machined into the jaws. They can be done later if required.
Lathe chuck spiders are not new. I have made them from steel in the past, but I never seem to have the correct thickness. Fellow GSMEE member John Bernoth brought 3D printed versions to the last meeting, and it seemed like a great idea, so I have been printing up some too. The chuck has 20mm deep jaws, so I have printed 10, 5, 2.5 and 1.5mm thick examples. I discovered that levelling the printer bed is absolutely critical to getting consistent thicknesses. Best so far is the one at bottom which is within 0.02mm. The top one was an early one, and is only within 0.1mm, so will be redone. Notice the honeycomb infill. It has 10 surface layers top, bottom and edges, plus the infill. PLA. Quite strong, but very light. Seems a good application of 3D printing technology.

And back to the bombard. (The rib pain is easing). A 14kg crucible is on the way from UK. And I have the PLA models for the mold. I do hope that I do not need to reprint them.

The 350mm SS cylinder which will hold the PLA model and the investment medium, and eventually receive the bronze. The red barrel 315 x 107mm, and the black breech 240 x 107mm. There is almost the same weight of bronze in the barrel as the breech. The breech wall is much thicker, despite the smaller OD.

Now I am thinking about how to funnel the bronze into the mold, and where to place some air vents. Also have to work out how to drain the melted PLA during burnout cycle.

Will need to make some tongs for the new big crucible.

And for my non Australian readers, we in Victoria and NSW are in Covid lockdown again. So I have plenty of time for planning for the bronze pour.

Leave a comment
June 11, 2020

Lost PLA Casting – 3rd pour

Today I attempted another aluminium casting session with trees that I had made 2 days ago.  More wheel forks, and barrel trolley brackets.  16 parts altogether.

And this time I installed air release vents, following my previous poor results, and at the suggestion of reader Rob R.

I also made some 50mm extensions of the pouring funnel, to increase the head of melt pressure.  The extensions were “add ons” rather than designed into the system, and the molten aluminium leaked between the extension and the main flask with the tree, so I doubt that they were very effective.

BUT!  Of the 16 parts on the trees, 14 were good to excellent, and only 2 showed any voids, and I assess one of those as repairable.  So, 15/16 is very pleasing.  I feel that I am closer to getting good results every time, if I can make an effective system of increasing the delivery pressure of the molten aluminium.

PANA3978

These are the extension pieces to the funnels on the investment flasks.  The shape was made with the plastic funnel.  If I had positioned them before the investment plaster had set hard they might have worked better, but as they just sat on top of the already hardened plaster, the join leaked molten aluminium rather badly.  I have a different system in mind for my next pour.

PANA3980

Previous failures were cut up and thrown into the melt.

PANA3994

PANA3997

See the tiny silver dots surrounding the central funnel.  That proves that the air vents functioned as intended.

PANA3999

The aluminium trees.  Not very pretty, but delightful to see.  10 parts on the top one, 6 on the other.  It is odd to see the wax spaghetti turn into aluminium spaghetti.   I will separate the parts tomorrow.

And while the investment burnout and baking was proceeding, I worked on previously cast parts.

PANA3993

The centre columns have beep painted with etch primer.  A little more filling required, then I will use the best 2 on the models.   The 2 bracket and column assemblies on the right were initially considered unusable due to large voids, but I used some aluminium solder to fill the defects, and they might possibly be OK.  The 2 on the left just need some tidying, machining removal of  melt tubes, and minimal filling.

PANA3989

I will probably remake this one, but will continue to salvage it and see how well it comes up.  Note the solder fill on the RHS.   That will not be seen on the model.

One more melt and pour, and that should be the last of the castings made for the model Armstrong cannons.  It has been a challenge, and lots to learn, but very interesting and very satisfying.

IMG_8657

Finally for today’s post…  I noticed some black marks on the normally pristine white wall above the casting bench.  They extend about 4 meters above the floor.  Do you know what they are?  The paint has been melted off the wall by bits of flying molten brass, resulting from the steam explosion 2 days ago!

Many thanks to Rob R for his spot on suggestion about the air vents.

9 Comments
Video | June 8, 2020

Video of Casting Small Complex Cannon Parts

This video was taken and edited by my daughter Eleanor.  I was doing an aluminium pour of some parts for the Armstrong RML cannon, explaining the process to her.  I was hardly aware that she was videoing, so the interaction is conversational.

Although the pour was not a success because none of the parts were good enough to use, it does show the process as seen by someone who previously knew nothing about it.

 

 

PANA3965

 

 

 

 

There is also a 20 minute video of the whole process which I will add to this post when it is available.

Reader Rob has suggested that the positions of the defects suggests that air entrapment is the cause of the voids and that the fix is to position some vents at the positions at risk.  I will try that with my next pour.  Thanks Rob.

Here is the 22 minute video.  Just as recorded.  Not planned or edited.

 

 

 

 

 

 

 

3 Comments