Reply by Chriskmb5150

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Posted on Design help needed

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253 posts in 3045 days

#1 posted 01-09-2010 07:10 AM

the piece you describe is called a concentric reducer in the sheetmetal trade.
Look at figure 589 on page 6….

Draw centerline then make two marks at points D and C thatll be your 4” length.
then draw a 90deg lines to the right representing the radius’ of both ends of your reducer(points D to E being 4” and C to F being 7”).
Next use a straight edge across points E and F and let it intersect the centerline and scribe a line to create point H.
Set a divider on points H and E and scribe a 360 deg circle then set divider on points H and F and scribe circle.
multiply smaller diameter times pi to get circumference (25-1/8).
Set divider to 1 inch and count out circumference in 1 inch (set divider to fraction at end) increments starting at point H and going counterclockwise on arc/circle.
scribe straight line from point H to end of smaller arc and through larger diameter arc. larger diameter’s circumference will work itself out.
youll end up with a patttern that looks like the one DaveR posted with points E and F on the right side and points M and L on the left.
Cut it out and roll across you thigh till the ends meet. I do this for a living and could explain better by showing you so sorry to confuse if this doesnt make sense. The pattern to make a cone is similar.
Hope this helps

-- Woodworkers theory of relativity - the quality of your scrap is relative to your skill level

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