Creating a plastic card disc with accurate radius and neat edge what methods do you use ?

[Merlin]

Hi,

Just wondering if there is a better way of creating plastic discs of a non standard diameter (circle template etc not suitable)

I have posted a request for a quality metal circle cutter with fine adjustment of radius by central wheel between arms or maybe beyond one arm.

Armed with a quality device, there is then the question of how does one get the part cut disc on 40 thou or 60 thou or 80 thou out of the sheet, as at these thicknesses one cannot cut through easily, even 30 thou I dont plan on 'cutting through'.

 

Method 1, (my only method in fact)

make a few turns of the compass blade, then cut the circle out of the sheet with 5mm or more plastic around the circle. Cut off any corners to equalise the lengths of the about to be cut 'petals'. Cut 'petals' from the circle outwards with one strong stroke, on an imaginary line from circle centre outwards, repeat again and again, spacing the lines so as not to expect the break (in our next phase) on the circle line to have to go in a curve. narrower the petals, better the resulting edge. Break the petals apart with a folding then twisting action, then work around the circle bending petals off at the circle cut. Despite keeping the petal widths as small as we dare, the resulting circle edge will have a rough nature to it, then take a file and knock these 'corners' where the petal edges were, off, working around the card as if card is wheel and file the road ! It will be as perfect a circle as ones petal widths and file work allow. Edge of the disc though isnt perfect perpendicular to surface and can see underhangs etc.

 

How does one improve on this method ?

 

If plastic card could be mounted in a lathe and a blade adjusted with lathe controls, would give the precision, and could refine the edge. Mounting the card in a lathe, any ideas ?

 

After the petal method, ensuring the disc is a bit oversized, mount the disc and lathe the edge good, sandpaper or file etc. How though could a plastic disc be mounted in the lathe ...if centre must not be harmed except for the needle indent ?

 

Any other ideas ?

 

Merlin

[bootneck]

I struggled with that problem for many years and, as I got older, the hand cut circles turned to elipses and so I had to invest in a cutting machine.  There is a fairly decent explanation of what these cutters can do here.  Probably a bit overkill for just a few circles; however I have achieved loads more scratchbuilding by having one.

 

Mike

[Merlin]

Hi,

Thank you so very much for that link.

Took me a moment to realise what that was about, blimey, strewth, wow !!

just look at that railway building roof edge valance against the mm ruler. and the thin plastic strips , bridge, etc.

 

I will study that more.

 

I can cut perfect circles, its the getting them out of the card.

 

I see that PC cutter can accomodate thickness up to 40 thou at most, and above 115thou needs petal snapping out etc, or whatever method posters will come forth with.

 

That would be awesome also for decal masks, canopy masks etc.

 

Merlin

 

 

[CarLos]

A possible update to your method: after making the circle with the compass, use a small drill bit to pierce the card (make sure it is vertical); Then score another circle in the other side of the plastic. This way you can cut circles on thicker plastic card.

 

Carlos

[Marklo]

2 Methods. Use a compass cutter or attach a rough plastic  blank to one of the Dremel   bits for holding polishing disks( I actually have a LIDL Dremel knock off, but you get the idea) and at slow speed and being careful use a blade/hand file to turn it down to the right size, periodically checking with a vernier. Also good for forms for molding etc. I've used this sucessfully to make a new shock cone for a MIG 21 I was building and to make mold forms for wheels and cowlings from balsa.  Not necessarily the safest method but wear safety glasses and go slowly and it's fine.

[bhouse]

On 6/8/2018 at 11:19 PM, Merlin said:

How though could a plastic disc be mounted in the lathe ..

Use double-sided tape and stick it to a sacrificial wood face plate. No centre dot either!

[DMC]

Just curious, how big a disc?

 

Dennis