How to bullnose MDF window board?

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Lamentable

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Hi,

I want to bull nose some MDF window board.

I understand I can buy bull nose board, but I also wanted to bull nose the sides (not sure what they are called) on the left and right of the board where it sits proud of the window reveal.

So I was going to get a router but I'm unsure what bit I need.

If the window board is 25mm thick, what size and type of router bit would I need?

Would it be better for me to buy window board that is already bull nosed and then just do the bits I need? Or buy the cheaper square edged board and attempt to shape it myself?

Thanks in advance for any help received.
 
Block plane and sand paper on the edges of pre-profiled bull nose board - that's the trade method. Most bull nose board has a larger radius than the thickness of the board, 30, 40 and 50mm belng commonly used. You can't get portable router cutters of such diameters so readily

With 25mm thick MR-MDF you can probably accommodate something like a 6mm (1/4in) or 8mm (5/16in) radius bursting guided cutter. To use a cutter like that you need to leave a flat "land" on the edge of the boards in the middle for the bearing to run against. This might suit your needs but wouldn't pass muster in my world (trade, and architects are picky)
 
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Hi thanks for the reply. I hadn't realised the radius on the bull nose would be larger than the thickness of the board.

I guess I could have a go with a bit of scrap to see if I could round it by hand.

This is actually to go on an upstairs bay window which doesn't currently have a board. I'm insulating the bay with PIR board which will create a ledge. The other option is not to bring the ledge around the corners of the wall but just finish it against the walls each side of the bay.
 
Lamentable said:
Hi thanks for the reply. I hadn't realised the radius on the bull nose would be larger than the thickness of the board.,
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AFAIK it's the norm. It has to be partly because you get less cutter wear on large radius profile cutters than small radius profile ones as well as less chamnce of overheating and damaging the cutter, also less chance if scorching, etc

A lot of tradesmen buy-in 22mm or 25mm MR-MDF, pre-profiled on one edge ("window board") then simply profile the short edges, which is a fairly simple task

Lamentable said:
I guess I could have a go with a bit of scrap to see if I could round it by hand.
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One trick is to make yourself a cardboard template of the profile and use that to help you generate an accurate facsimile of the original. Remember that a joiner would rarely attempt to bull nose the long edge, just the two short edges ar the ends, which are in any case less noticeable if they are slightly wrong
 
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I do it by using a router with a chamfer bit - to remove the bulk of material.

Then use a block plane on the points created, then sand round with a ROS.

It's possible to get it pretty good.
 
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