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Built Review
135
Sherman Skink turret
Sherman Skink turret
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by: Tom Cromwell [ BARKINGDIGGER ]

Background
The Canadian Army developed an anti-aircraft turret for the M4A1 Sherman "Grizzly" that came to be known as Skink. It mounted four 20mm Polsten guns that were synchronized. The new tank was obsolete even before it was finished, but a single prototype was sent to NWE for a short time near the end of the war, where the lack of enemy planes to shoot was compensated by an abundance of infantry targets on which to release storms of 20mm hurt.

There were two versions of the turret - one with a full-height bustle as modelled some time ago by a Brazilian company (Panzer Resin Models) in 1:35, and a later version with the bustle floor raised to presumably offer clearance for the hull crew hatches. This new 3D-printed turret is the later type.

contents
Available from our very own Iain "Pave-Hawk" in his Spectre Scale shop at Shapeways.com, this new turret comes as three main parts. There is the turret shell, a choice of two turret rings (DML or Tasca/Asuka depending on your choice of hull kit), and the four guns. The shell is also offered in two varieties with the base ring moulded in place (for DML or Tasca/Asuka), but oddly these one-piece shells are slightly more expensive than the two separate parts due to the odd way Shapeways calculates prices. The guns also have two variants - with barrels or without.

All the parts are printed in hard translucent resin that needs a thorough wash in warm soapy water and a good rinse in clean tap-water to get rid of residual oils and wax from the manufacturing process. Be aware that thin parts like the lift-rings can be extremely brittle compared to plastic and prone to snapping off under pressure, so try not to knock them.

review
I followed Iain's efforts here on Armorama over the past year or so, and he was ably supported by comments and reference materials along the way, so I for one am very happy with the look of the finished shell. The only big omissions are the barrel-like lock for the front hatch, and the prominent "D in stop-sign" casting mark seen on the rear of the bustle in some pics of surviving turrets - these will be added on mine with a little plastic rod and some Archer transfers.

There is a rough surface caused by the support wax used in printing, and it has caused odd streaking down the sides of the turret. This is easy to smooth out with a sanding stick. The hatches are meant to hang inside the shell by a tiny sprue - mine had broken off but were in the package, so no biggie. The only issue is the lack of underside detail, so I will have to either button-up or do some scratching of periscopes etc if they are posed open. The base doesn't have any mounting "ledge" inside the shell, so to get a firm joint I glued strips of plastic inside the shell first to form a flange against which the base can be seated. (I had to do a light sanding of the front edges of the base to make it fit - the printing process can leave little burrs on parts.) Note that my DML base does not have any locking tabs to hold it onto the hull - I'll need to make some from plastic scraps.

The four guns are designed to mount in holes in the gun rotors that are part of the shell. These holes need a gentle reaming out - I used a 1.4mm drill bit for this. Or, by filling the holes and drilling new ones the guns can be posed pointing skyward! Be careful when removing the nub from the sprue attachment on the side of each gun, as the sides and bottom of the "tray" around the recoil spring are not very thick, and only touch the gun itself in a few places, so it is a tad wobbly to sand down. (It would have been better if these were made solid below the halfway line for stiffness, since the underside of the springs cannot be seen.) The barrels are ok on my example, but I bought the full guns knowing I could always snip off the barrels myself to replace with brass tube if needed.

conclusion
This is a much-welcome conversion for the venerable Sherman to make a Skink. Like all 3d-printed resin products it needs a little TLC, but should give a truly unique model!

Disclaimer: I have nothing to do with Spectre-Scale, other than being a paying customer...

SUMMARY
Highs: Only Skink turret in town! Looks pretty good.
Lows: Printed resin needs a bit of clean-up. No interior details.
Verdict: A good rendition of a rare variant - worth the cost and effort.
Percentage Rating
90%
  Scale: 1:35
  PUBLISHED: Mar 15, 2018
  NATIONALITY: Canada
NETWORK-WIDE AVERAGE RATINGS
  THIS REVIEWER: 83.86%
  MAKER/PUBLISHER: 90.00%

About Tom Cromwell (barkingdigger)
FROM: ENGLAND - EAST ANGLIA, UNITED KINGDOM

A Yank living overseas on a long-term basis, I've been building tanks since the early '70s. I relish the challenges of older kits (remember when Tamiya was "new"?...) because I love to scratch-build.

Copyright 2019 text by Tom Cromwell [ BARKINGDIGGER ]. All rights reserved.



Comments

Raw material is relatively cheap for both 3D printing and injection moulding, depending on what material you choose for printing. Injection moulding requires large outlay to buy the initial moulds, and if you want to do a different model you need another large outlay to create the model and buy different moulds. The moulds also have a limited lifespan. Commercial 3D printing requires a large outlay for the 3D printer, after which you just pay the cost of model design, and each kit only cost the price of material for the manufacturers. There is no cost to have moulds machined. There may ongoing costs to maintain and service the printer BUT I don't think they would reach the cost to have a full kit mould done. Having said that, 3D printing isn't cheap, and in my experience parts are more closely priced with cast resin kits if you use similar materials. If you look, full 3D printed kits can get into the hundreds of dollars for even quite small kits. Costs can go up quickly if you choose to print in fancy materials like metals or extra strong plastics. Through services like shapeways, final cost is affected by how much profit sellers add on top of shapeways costs.
MAR 18, 2018 - 04:31 AM
As Iain says, injection moulding has a high start-up cost because making the steel moulds costs many thousands of pounds. The actual plastic sprues are cheap (pennies, really) but you need to sell lots of them to spread the tooling cost. It also means up-front costs for the tooling, storage of the unsold sprues, and shipping costs to the customer, so is only viable for companies with capital to invest. 3D printing ("rapid prototyping" as it was when it started) uses an expensive machine that can spit out parts from an infinite variety of CAD drawings, so the true cost of the machine is spread over lots of different customers rather than being landed on a single design. The downside is that the printing is not as cheap as I-M, so instead of each shot costing pennies it costs tens of pounds. Since the printers are so expensive, it makes sense to use a print-house like Shapeways - they own the machinery and do all the order-taking, printing, packing, and shipping. The traditional cast-resin process as used by Verlinden, Resicast, etc requires a "pattern" of the part, and then each casting requires labour that contributes to the cost. Each method has its uses, but the designer must balance the costs with potential sales and the effort of churning out & selling the product.
MAR 18, 2018 - 03:53 PM
Of course, that why I couldn't understand why Iain wanted to have this conversion in resin, which has the need for CA glue and avoidance of the toxic dust plus the lack of flexibility for thinner parts mentioned in the review, over plastic.
MAR 26, 2018 - 11:15 PM
Cost. Based on my estimates, I could make resin versions that I could sell for about $AU25-35 (not including postage which might kill from Australia depending where it went) and still cover the cost of resin with a small profit. That's bit cheaper than a shapeways print, and I would have control over the quality of the final product. Selling through shapeways is more expensive for the buyer, and I have no visibility over the finished product until someone tells me if it's good or bad. Very much Caveat Emptor, no matter how much work I do. Plus I am not the best 3D modeller around, so my work is close, but very much NOT exact, whereas I could get a physical model closer to the real thing. The advantage of shapeways is that I can make fast changes based on feedback, and produce different scales to suit different kits at effectively zero cost, so that the next purchaser can get an "improved" version, but again, I have to wait for feedback to ensure I didn't stuff something up during my changes. Ultimately, the reason I went with shapeways for this despite my preference for doing resin, is simply that real life prevents me from being able to spend the time that I would need to finish the master, produce a mould and get kits made up. So now I am slowly working on my next piece, an Australian 6x6 Land Rover chassis to replace the fantasy part that Black Dog produced.
MAR 27, 2018 - 08:01 AM
Ah, so Shapeways only offers resin, not plastic printing, right?
MAR 27, 2018 - 03:47 PM
Shapeways has a variety of different materials and different printing processes to suit each material. Each material has different levels of detail that it can reproduce. The higher detail materials tend to cost more than the basic plastics, and the metals are even more expensive.
MAR 27, 2018 - 04:27 PM
Yes, although they often refer to their acrylic resin as "plastic" in the generic sense of a mouldable material. Technically the "plastic" we see in kits is styrene, an oil-based product which is heated to a liquid and squirted into a metal mould to form shapes. Shapeways uses an acrylic resin "plastic" that can be printed droplet-by-droplet onto a build tray to make the parts - kinda like cutting out the shapes from layers of paper in a stack. I don't know anyone that offers 3D-printable styrene - it may not be possible to do it at all. And to make matters confusing, the traditional cast resin we see in aftermarket parts from Verlinden, Formations, etc is different to the acrylic resin printed at Shapeways! If anyone develops the technology to 3D-print styrene plastic parts, they'd make a fortune!
MAR 27, 2018 - 04:31 PM
I don't know anyone that offers 3D-printable styrene - it may not be possible to do it at all... ...If anyone develops the technology to 3D-print styrene plastic parts, they'd make a fortune![/quote] Styrene covers a whole family of plastic including HIPS and ABS. ABS is is one of the more common 3D printing plastics available and is compatible with the styrene used in model kits. I believe Plastruct brand materials are also ABS.
MAR 27, 2018 - 04:57 PM
I don't know anyone that offers 3D-printable styrene - it may not be possible to do it at all... ...If anyone develops the technology to 3D-print styrene plastic parts, they'd make a fortune![/quote] Styrene covers a whole family of plastic including HIPS and ABS. ABS is is one of the more common 3D printing plastics available and is compatible with the styrene used in model kits. I believe Plastruct brand materials are also ABS.[/quote] True - I should have said pure "polystyrene" for kit plastic! ABS is a styrene-based mix, but it doesn't react with the mild glues we use on PS kit parts. (I have fond memories of trying to get Plastruct girders to bond onto my plastic conversions as a kid...) And as far as I know, the use in 3D printing is mostly limited to the low-end machines that heat and melt a thread of ABS to fuse it onto the model as it builds up - these are different from the polyjet printers that make parts suitable for our modelling hobby. I'm not sure if something in the rubbery component of ABS might make it unsuitable for polyjet printing? But I still think the Holy Grail of 3D printing would be a polyjet machine using pure polystyrene! Just think of the possibilities...
MAR 29, 2018 - 02:06 PM
   

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