using a router to turn a shallow concave??

[HSS]

Hi! I've potentially got a job coming up and thought a router may be the most efficient way of achieving consistancy. I've done a bit of a search and saw that someone had mentioned a router on a swinging arm being used to hollow large 1m diam bowls.

I will need to turn a shallow concave (300mm diam and only 5mm deep) in a 400mm square stool top (20-25mm laminated American walnut boards).

I could hand turn the concave (like a shallow bowl) but the order is for 8 and since it is such a shallow curve I think any variation would be very noticeble. Also, I need a good finished cut to reduce the amount of sanding since it's side grain and I want to avoid any hard-soft grain variation.

I was thinking of mounting a curved template to the bed on the tailstock side of the stool blank. I would then have the router base plate (or an extension of) ride along this template. The router would be orienated with end of the router bit facing the headstock.

A single pass would probably be ok given the shallow depth (5mm max at centre).

I thought of a swinging arm but given how shallow the curve is the radius may be a couple of metres.

Any thoughts...

[TTIT]

How about parking the toolrest across the ways with a template attached to it and rig a depth stop to a large round nosed scraper. Doesn't seem worthwhile making anything fancier for 8 items :shrug:

[RETIRED]

Hmm. I don't think a router is the way to go for this one.

The problem with doing it with a router is that it tend to tear the grain on the quarter, much like turning.

If you can't hand turn it which is the way I would go, set up your compound rest to cut a taper starting at 4mm - 4.5mm deep at the centre.

Cut inwards from the rim in a straight line and (dare I say it) use 80# paper on a styrene foam* pad to sand the curve in.

*Foam fruit box type stuff.

[sjm]

One way I've seen of doing this is with a radial arm saw - fix the blade so it can't move front to back, then swing the blade through an arc from left to right, and it will cut a nice shallow dish shape. The only problem is the size, you're going to need a pretty big blade!

[NeilS]

the problem with doing it with a router is that it tend to tear the grain on the quarter


+1

[HSS]

Thanks for the replies. I might try fixing a depth limiter to a scraper somehow. I'll let you know the outcome if the job comes through.

Cheers,

John.

[dogcatcher]

This could be modified to work. Low Cost Duplicator (http://www.sawmillcreek.org/showthread.php?180582-Low-Cost-Duplicator)