Another Artillery Tractor

[Ray Dunakin]

What a great model!

The 100hp tractor is quite a beast, with a very unique appearance.

[Peter_T1958]

Thanks gents!

There is a long way to go before I will/can start with the Dürkopp. I am a very slow modeler, and as I am would like to do this specific vehicle found by American troops in 1919 I have to dismantling one or another part (e.g. the bonnet). But a vehicle in rainy conditions could be a new challenge... But first: I have to build the Büssing now!

[Bill Gill]

When I look at those photos Hannibal's elephants and Star Wars AT Walkers ponderously slogging through the mud come to mind.

[Lawrence@NZFinescale]

The thing that gets me is the crank handle to start the thing!!

[Peter_T1958]

I am often astonished over absolutely perfectly built models presented in various social media i.e. facebook. Yes, it will always be easier to talk one's own successes but to me failures accompany us at every step in our hobby and those are the experiences that move us forward. Nevertheless, at the moment they are rather boring , so here a new chapter in the  story of a (to some extent) novice in the matter of 3D designing.

Looking from the distance, progress on my «Büssing» doesen't look so bad (and I am still happy with the subject)!



But I wonder what happened here:


...and here:


The printed parts are often warped (dash board / cabin floor/ rear wall) or full of cracks. I have read, that it's able to righten such problemn in heating up the resin parts in hot water. But what means hot water : 50°C, 60C° or even boiling water?

[Lawrence@NZFinescale]

Hi Peter

Your experience is typical.

Unexpected artefacts are a bit of a hazard in 3D printing, especially if pushing the envelope a bit.  They tend to be particularly noticeable on mechanical and fabricated things, less obvious on the organic.  Quite a bit of my time is spent optimising models to address this.

Such things are mainly due to the forces involved in parting the model from the vat membrane at every layer.  Those forces (I'm told) are a 4th power factor of the area (ie double the area and the forces increase by 16).  Layer area is constantly changing in prints like yours (and mine), sometimes significantly, which can cause distortion and layer shifts. All of this also imparts stresses to the cured plastics, which can cause distortion over time. Ironically, tiny models can often be easier than large models as a result. I think some commercial processes work a bit differently and thus produce a more inherently stable product.

Much is made of the rigidity of the 3D printer Z axis, but I'm unconvinced.  The reality is that many models are relatively flexible during printing, which makes the structure of the machine moot.  Model orientation and support has a huge bearing on all of this, but every approach has it's pros and cons and the best option is often something of a compromise.  More support will generally give better shape fidelity, but then you need to deal with extra supports - which may, or may not, be problematic in each case.

Choice of resin is very important and the condition of the vat membrane also has a significant impact.

Resins vary, but you can generally straighten them with hot water.  Boiling is often fine, but the model can get too soft.  Frustratingly, heating can be a short term fix, with parts reverting over time. I imagine you need to heat sufficiently to relieve all the stresses in the print for long term success. In my experience best results for this are achieved by having a jig to hold the model in the desired position during heating and cooling.  I have 3D printed such jigs, but obviously they need to be of sufficient mass and strength so that it is the model that moves rather than the jig.  There are heat resistant resins that probably make better jigs too.  While I haven't made a study of it I suspect a good jig might sometimes over do the correction so that when the model relaxes it assumes the desired form.

Personally, when it comes to modelling sheet metal or thin section I prefer to go to etched brass.  3D prints are fantastic, but not the solution to all problems.  Most significant to me is that they do not handle sustained forces well.  A resin bogie with pinpoint axles will tend to spread due to the outward force due to the pinpoints.  This will obviously depend on the weight on the vehicle as well.  For this reason I used an etched structure for bogies with a 3D printed cosmetic sideframe.  Mixed media can be very useful in stabilising  prints.

[finescalerr]

After reading all of the above, I'm glad I chose a laser instead of a 3-D printer. As Clint Eastwood muttered in (I think) Dirty Harry, "A man's got to know his limitations." -- Russ

[Lawrence@NZFinescale]

After reading all of the above, I'm glad I chose a laser instead of a 3-D printer. As Clint Eastwood muttered in (I think) Dirty Harry, "A man's got to know his limitations." -- Russ

Dirty Russ...?

It's not that bad.  Every medium and technique has it's pros and cons.  Plenty of pros to resin.

[Peter_T1958]

Hello Lawrence

First, I really appreciate that you took the time to answer my questions in detail!

Ironically, tiny models can often be easier than large models as a result.

More support will generally give better shape fidelity, but then you need to deal with extra supports - which may, or may not, be problematic in each case.

I hope that I will not be repeating myself, but this is an point I feel is most important. As I wrote before, I have to combine parts on request of my producer to reduce the opportunities for copyists. This results in rather large resin parts.



As I was told, the market for such rare niche products is highly competitive, especially taking into account that reserach work is is extremely time-consuming (Ok, I have studied these tractors for about 30 years, so that's quite some basis). Obviously, in this way it's almost impossible to do a scan of some parts to create a new product in a simple manner.

Personally, when it comes to modelling sheet metal or thin section I prefer to go to etched brass.

According to the producer's philosophy these kits are done strictly without etched parts. I agree completely with you, that some parts would better be done with sheet metal or etched brass. So, for my own kit I am building now and fort he «Dürkopp» I have already ordered a sheet of PE parts.



You may see, I m subject to significant rigidities in many cases, but on the other hand this way I get my own model printed.

What I will do now is trying to righten those warped parts with (almost) boiling water with the help of a good jig as you describe it further above !

 

[Lawrence@NZFinescale]

I get it Peter, I really do.

[Peter_T1958]

Hi gents
In the course of discussions my two projects are now totally jumbled up. So I will move along this thread, as both vehicles are essentially military tractors.

While the « Dürkopp » is en route to the customers, I am still struggling with my first own kit, the «Büssing».
The following pictures show how many parts have been broken due to the very brittle resin and had to be repaced with plastic parts. In many cases I have to add tiny parts made of brass, as only soldering allowed sufficient stability (i.e. etched chains).
And yes, even for myself progress is very slow. I managed to build the mounting brackets of the ventilator only after five unsuccesful attempts or in other words: after two weeks...






Just one small personal remark yet. As for all my works there is a weird thing - I have to vistie the place where the origin had been photographed. Some might
consider it a quirk, but form e it is essential to see the area where all that happened. So as it happened, on the way  to the Netherlands last week, my family and I passed the small village «Oberkail» where the photo was taken in 1919 

[finescalerr]

Even at this stage we can tell it will be a terrific model. -- Russ

[Lawrence@NZFinescale]

... how many parts have been broken due to the very brittle resin and had to be replaced with plastic parts. In many cases I have to add tiny parts made of brass, as only soldering allowed sufficient stability (i.e. etched chains).

Looking good.

Mixed media is the way to go in my view.  No one material or method has all of the answers.  Manufacturer's will generally try to use fewest materials on economic grounds of course.

Resins are not created equal either.  If you have the option, there are resins that will be less brittle and produce more robust parts.  There may be a trade off in print speed, cost or some other print parameter though.