Cutting curves in plastikard

[River Lin]

Hi all.
How would members of this estimed group make semicircular cuts in thin plastikard. I am thinking of making my next project a 16mm model of NWNG No. 1 and the beading has semicircular cuts ranging from 5 - 9mm diameter.
Are punches available in that size or does it have to be a careful knife cut?
D.
PS. I would have attached a pic but the option does not seem to be available.
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[pskipper]

You can get sets of jewellers punches which will do the job, I've got a pair of engineers focussed that I sharpened so they will cut plastic but its more rudely than using punches.

[GTB]

there are punch and die sets available that can do that sort of thing. You punch a round hole in each corner of the panel, then use a scalpel for the straight cuts to join the curved corners.

Jewellers use them in your size range, but your eyes will water at the prices jewellery suppliers charge.

The model engineering trade have lighter duty ones available which will do the job at rather less cost.

http://www.arceurotrade.co.uk/Catalogue ... -Set-18-34

Unlike the punch and die sets used in the jewellery trade, these have a clear acrylic top plate to guide the punch, so you can see what you are doing when positioning the part before punching it.

Don't be tempted by the cheap wad punch sets you see in craft and hardware shops. The way they are sharpened forms a fairly clean disc, but leaves a distorted hole with a ragged edge if you try and use them to punch holes in polystyrene sheet.

Regards,
Graeme

[River Lin]

Hi all.
Hope this works. This is the coach.

IMG_20170327_235823.jpg (33.26 KiB) Viewed 6273 times

David.

[River Lin]

Hi all.
More pics.

IMG_20170329_154602.jpg (57.29 KiB) Viewed 6270 times

IMG_20170329_154727.jpg (85.09 KiB) Viewed 6270 times

My summer project.
D.

[JCSteam]

I would drill the corners as holes and then score between them finishing with a file/sandpaper to straighten everything out

[LNR]

Do you have access to a mill, I have used one for something similar. With that many circles, a mill run slowly with the right sized cutter will cut them all in a straight line perfectly. Just a suggestion.
Grant.

[-steves-]

Do you have access to a mill, I have used one for something similar. With that many circles, a mill run slowly with the right sized cutter will cut them all in a straight line perfectly. Just a suggestion.
Grant.

Great idea, I have a mill and I never even thought of putting plastic or wood in there, that will make things easier for me in the future

[LNR]

As long as you run the cutter slow enough to not generate much heat, and support something like the above carriage work with say timber top and bottom before actually cutting it to width (ie full sheet or similar) it works very well and of course is very accurate. You can site along the line and every line is exact, something harder to achieve with a file.
Grant.

[River Lin]

That sounds good. I have a dremel type machine with drill stand so might be able to rig something up.
D.

[LNR]

Hi David, I think a drill stand arrangement would lack the traversing ability that allows the accuracy of the cut to be obtained. Not sure that a Dremel can run slow enough, but not familiar with what you have. The suggestion was a long shot, not knowing what equipment you have available.
Grant.

[JCSteam]

I think that cutting into the plastic with a dremel will generate too much heat, and the plastic will melt, I think accurate drilling then clamping the plastic between two pieces of steel angle will give you a line to work to, the holes can be cleaned up with a file and you get a neat straight edge, the curves will take some dexterity to finish with a small round file, I have done it before, but take on board what's been said above, its not easy and is time consuming

[GTB]

Dremels work more or less for roughing out holes in polystyrene sheet, but forget any sort of precision work. Even with a speed controlled one, by the time the cutter is running fast enough to cut, the plastic will sooner or later melt, stick to the cutter and the hole will suddenly and rapidly get a lot bigger. That or the plastic will grab and wrap itself around the cutter...........

Since I assume we are talking about making polystyrene lace overlays in thin sheet like 0.020", drilling a large hole will be problematic. There's a lot of force involved in drilling and polystyrene sheet isn't very strong, so will be likely to break where holes are close together.

An ordinary twist drill in the 5mm to 9 mm size range, when used to drill sheet less than 1 mm thick, will just snag the sheet as it breaks through and then tear out a chunk, or wrap it around the drill bit.

I agree with Grant, the only way to accurately place a hole in thin material is to use a milling machine and a slot drill. Twist drills wander and aren't designed for drilling thin stuff anyway, not that that stops anyone.

If you know someone with a laser cutter, that is also a good method to make these overlays. Polystyrene sheet cuts nicely with a laser and needs little cleanup afterwards. Unlike machining methods, the cutting places no strain on the workpiece, so there is little risk of the fragile lacework breaking up when cutting openings that are close together.

I don't know if anyone has read the writings of the late David Jenkinson on the subject of building carriage bodies in polystyrene sheet. His output of models was prodigious and his G1 carriage panelling overlays were made from 0.015" polystyrene sheet, marked out in pencil and cut out with a scalpel. For round corners he just cut straight across the corner and filed back to the curved line with a round needle file.

His book 'Carriage Modelling Made Easy', published by Wild Swan, is out of print, but it's a good reference for building British style passenger stock in the larger scales. Fairly easy to find secondhand though and several copies are listed on AbeBooks.

The only thing I do different when working in polystyrene sheet is to run a light scalpel cut along the pencil line before starting so I don't lose the line while working on the piece. In thicker material like carriage sides, I prefer to rough the openings out with a piercing saw instead of a scalpel.

Regards,
Graeme

[River Lin]

Hi Graeme.
A very interesting read. I have worked with 0.03" but for this project i will probably try 0.02 and a sharp scalpel and file. I can make the beading in small units so a damaged piece will not wreck the whole.
Thanks again for your interesting and informative input.
David.

[River Lin]

Thats how i see it with a drill in the tight corners..
Thanks.
David,