# Cutting curves in plastic



## Pete Thornton (Jan 2, 2008)

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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## SteveC (Jan 2, 2008)

Pete

Maybe this will be of help, it's from the Mason Bogie MasterClass.

[url]http://archive.mylargescale.com/articles/masterclass/mc2/mc2-03/mc2-03-14.asp[/b][/url]


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## vsmith (Jan 2, 2008)

Masterclass technic, and trial and error, usually cutting slowly and test fitting alot, cut, test, cut, test, till its looks right, finish file till it fits tight


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## dltrains (Jan 2, 2008)

Sand paper on the boiler works well


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## Semper Vaporo (Jan 2, 2008)

Try this...

Using Forstner bits:

Bore a hole the diameter of the dome in a block of wood.

Bore a 2nd hole the diameter of the boiler at 90-deg to the first one so it center crosses the 1st one.

Insert the dome in the 1st hole, sighting through the 2nd hole to get the bottom of the dome at the correct position and afix it (wedges, tape, hotmelt glue, etc.) so that it won't move while performing the next step, but can be removed without damage when you want to.

Drill the 2nd hole again, or use a file to adjust the bottom of the dome to the curvature of the 2nd hole.

Then remove the dome from the block of wood and attach a piece of fine grain sandpaper, rough side out, to the boiler and rub the dome on the sand paper to fine tune the curve.


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## altterrain (Jan 2, 2008)

Forstner bit in a drill press - 










-Brian


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## jimtyp (Jan 2, 2008)

Micro Mark has a contour gauge that might help: 

 Contour Gauge


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## chrisb (Jan 3, 2008)

I might mark the cut line on the dome, then buy a sanding drum for a drill press as close to the boiuler diameter as you can get. start sanding and keep checking that that you are square with the boiler. Then as you approach the line rap the bouler with 150 grit paper and finish it that way.


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## Robert (Jan 2, 2008)

Steve the link was very helpful, but how was the original "wavey line" created


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## peter bunce (Dec 29, 2007)

Posted By Robert on 12/18/2008 12:04 PM
Steve the link was very helpful, but how was the original "wavey line" created

Hi Pete, The original was in a PDF that David Fletcher did as part of the Master Class, send me a PM and I will send it to you.


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## Pete Thornton (Jan 2, 2008)

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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## DKRickman (Mar 25, 2008)

Pete, 

I'd suggest starting by cutting away the remains of the old boiler. Just cut it fluch with the edge of the dome, effectively making the dome a little taller. That might make firther work easier. 

I made my domes from PVC pipe by drawing the "wavy line" on a piece of graph paper (using a method called parallel line development), marking the dome, then drinding the rough shape with a sanding drum in a Dremmel. I then finished the cut with sandpaper wrapped around the boiler. It's a slow process, especially making the paper template without a computer, but it's about as simple and inexpensive a process as you can get, and it works perfectly.


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## Torby (Jan 2, 2008)

I pondered that a bit while planing the 1/6 scale loco I never got to build. The boiler, domes and the like would be made of PVC pipe. I figured on chucking a hole saw in the drillpress and sawing holes in the boiler to fit the stack and dome pipes, then using a putty or something, handn't decided what, to fare them in neatly.


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## SteveC (Jan 2, 2008)

Posted By Robert on 12/18/2008 12:04 PM
Steve the link was very helpful, but how was the original "wavey line" created
Robert

The original template for the Steam/Sand dome base was part of the drawings provided by David Fletcher for the MasterClass 2002 Building a Mason Bogie.

However, you should be able to create one for any specific need.









Draw an end elevation of the boiler & dome base as depicted in the diagram on the left. Draw a quarter-circle of the diameter of the dome (bottom of diagram on left), cut the arc into three equal parts, extend vertical lines parallel to the vertical centerline of the end elevation so that they extend from the baseline thru where the two segment lines cut the quarter-circle until they intersect the boiler arc.

Mark out a line equal to the circumference of the dome base (right diagram) cut it into twelve segments equal to the cord of quarter-circle segments, the height of each line is equal to the respective numbered line extending from the baseline to the boiler in the left diagram.

If I've got it wrong somewhere along the line I'm sure that Rick Marty can straighten me out.


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## Robert (Jan 2, 2008)

Excellent Steve thank you. That'll work well. 

Robert


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## astrayelmgod (Jan 2, 2008)

For the Master Class locos, the domes are made from scratch out of plastic pipe. I used a Dremel tool with one of their carbide bits (possibly #9901, or something like that). Then I cut the shape freehand. The first one took me about an hour, but now it takes about 10-15 minutes.


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## Pete Thornton (Jan 2, 2008)

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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