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Cutting curves in plastic

3438 Views 16 Replies 13 Participants Last post by  Pete Thornton, 
Pete Thornton
I'm about to have to fit a sand dome on a new boiler, and to cut down the smokebox saddle on my new project loco. Both the dome and the saddle have to fit a curved surface about 1+" radius, (2.375" diameter actually. It's a ten-wheeler conversion, so the dome is moulded in to the top of the old boiler.)

I can think of a few ways of doing this, but I'm wondering if you clever guys know any other way besdies the following:

- using a coping saw (hand version of a band saw,) carefully cut to approx the right diameter and then file or sand smooth,

- using a large curved file, just file away until it fits,

- buy a grinding bit for my drill and grind the plastic away.

How would you make a sand dome fit a new boiler?
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Pete Thornton
Guys, an amazing response - many thanks.

I noticed on the adjacent thread the "dremel 1830 18" scroll saw", which is an automated coping saw, I guess, and would probably do a great job - for $205 at amazon.com.
http://www.mylargescale.com/Community/Forums/tabid/56/view/topics/forumid/24/Default.aspx

The plan is to finish the curve using sandpaper on the boiler. The 'masterclass' technique of drilling around the cut mark is a good idea, but a bit tricky as I don't have a clear shot at the cut - there's bits of the old boiler moulded into it! See below.

The Forstner bit idea seemed interesting, but maybe overkill, and them bits are expensive - especially 2 1/4" ones.

I picked up a small scroll saw for $3.49 this afternoon, so I'll try that and then finish with the sandpaper.

Thanks again.

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Pete Thornton
Gor Blimey - that's some scary math - and I have a Math Degree too!

I did the rough cut, as suggested, and used some 60 grit sandpaper on a piece of spare pipe. Apart from it being not-quite perfectly centered [the curve is offset to one side slightly] it looks OK.

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