Can a round-over router bit do 50% of the thickness while resting on the cut profile?

Question

I think I now understand just enough of how router bits work, but I'd like to confirm I got it right.

I will use a router to round the edges of shelves. The thickness of the plywood is 3/4". Initially I was hoping to get a perfect half-circle and use 3/8" round-over carbide bits. But now I gathered that when doing the first side I'll be able to rest the router bit on the edge just fine, but after turning each piece over to work on the other side, there will no longer be anything to rest on. I'd then need something a lot more elaborate for my spartan beginner set, to be able to finish the second side, and so the top part of the illustration is not doable.

Instead, I'm gathering I can only aim for the less attractive 1/4" round-over on each side. as illustrated in the bottom part of the illustration.

Did I get it right? Does it sound to you like I understand what I'll be doing?

The profile I'll be working on looks partly as shown below.

Answer 1

Adding a full round-over edge (bullnose) to plywood of the type that you have pictured is questionable in my mind. Typically such edging is applied to solid wood pieces. The plywood may very well have tearout along the edges where the grain is at odd angles. This can be somewhat mitigated by using the very best router bits that are extremely sharp.

You will want to fix up parts of the edge, such as shown here, before you attempt to cut the radiused edge. You will want the transitions to be very smooth and once cut would be much more difficult to correct.

Rounded edging on plywood is usually confined to the type of plywood that has many more layers such as this baltic birch plywood with void free core.

And finally the rounded edge that would most often be found on such material will use a different profile than a full round bullnose.

Answer 2

You can use a 1/4" roundover bit from both sides, but it must be guided, perhaps by a built-in guide. These look like this:

You might insist on the nicer perfectly round contour shown in the top image.

You cannot use a 3/8" roundover bit. When you turn the board over, the guide will rest on the now-curved support. The outcome will not be exact. I haven't tried it to determine whether a little sanding will make it acceptable.

You can find on the market a roundover bit that solves exactly this problem, but they will gouge you. The one I found on the market was selling for 5 times the usual price. I can't recall any useful search keywords. I'll add a picture if I locate it again.

Michael's answer may still ahead of this answer in one respect: The wood should be full, not plywood. If anyone knows the result of using a router bit on plywood, please comment or add an answer. If you experimented with sanding afterwards, please also mention (or, ideally, show us) the result.

Answer 3

A little late answer, but this picture shows how it works when you make the second pass. You have to be very accurate about the depth settings of your bit

Small imperfection can be smoothed out by light sanding. That's a lot of work if you have many pieces to do.

Actually, it's not just two passes. Usually we take out wood in small chunk. Thus on each side, you would do multiple passes, each taking a bit of wood off. You do that by adjusting the depth of your bit progressively until it reaches the midpoint as pictured.

That being said, another option is getting a bullnose bit that cuts both sides in the same pass. They look like this https://www.amazon.com/Yonico-13515q-Bullnose-Router-4-Inch/dp/B078HS7GK6. I personally don't like using the bullnose bits because they are not versatile (e.g. not a good investment) and much harder to use. When I cut wood, I like to see what I am cutting especially for precision cuts. With this bit, I can't see the bottom face.

Answer 4

Adjusted properly, you can do a full "bullnose" round-over, or very close to one, using a round over bit that is guided by a pilot bearing.

But as noted in the other answer, common construction plywood is unlikely to give good results when radiused. The more common approach is to attach a piece of solid wood to the edge of the plywood and shape that ... Or to attach a piece of off-the-shelf molding, which is even easier since it makes the router work someone else's problem.

Answer 5

I was experimenting with a few different techniques. I am making two ship tables out of Sipele marine plywood, The scrap piece experiments turned out this way.

1. Climb routing(right to left) with very small passes on a piece that I put polly on and masking painters tape. It seems to be the best way so far. The polly not only acts as a “harder, but gives the painters tape better adhesion. Tearout was very very fine and un noticable.

2. Another way ts to doubleside tape a sheet of lauan 3/16 inch plywood on top. It splintered but the Sipele didnt eventhough I routed left to right instead of climb routing. the rounder isnt a true roundover, but it was splinter free.Any advice would be appreciated. You MUST use downward pressure on the router when using the luaun, and I use a Woodpecker mounting plate on my Elu router for more surface My ways so far are costly or time consuming. However seem to work. It may just be the Sipele making me look good. Again, ant suggestions would be gladly received.