How do you shape the edge of a circle?

[Shamrock]

I'm trying to avoid the tearout associated with routing against the grain as you rotate the circle counter clockwise. (I'm shaping the edge of a 20" circlular table top with a bearing guided round over bit on a router table)

I've had luck in the past with dividing the circle into 4 parts and climb cutting each section but just wondering what you guys have done. Is there an easier way?

Also, what speed would you guys run a 3/8" roundover bit at for this process?


Thanks in advance for your ideas.

 

[PeteM]

Have you tried Charles' "Bump Cut" method:
http://www.youtube.com/watch?v=-l4suyL84-w&feature=email

p

 

[mlzettl]

Mike,

I have had the best results by trying to rough cut the curve as close to the finish cut line as possible. The less material that there is left to remove with the router, the cleaner the cut tends to be. Climb cutting can help, but when you are going across the grain, a certain amount of tearout is almost inevitable. It also goes without saying that the bit needs to be very sharp. That is where the extra money paid for the best quality bits really pays off.

As far as RPM's are concerned, it is really dependant upon how fast you are feeding the router. The higher the RPM, the faster the feed rate needs to be. Intuitively, one would think that if you run the RPM's up, and slow the feed rate, you will get a smoother cut. The problem is that when you do that, you also increase the heat generated. That can result in burns in the wood, and worse yet, overheating the carbide in the bit which drastically reduces but life. Overheating probably causes more premature dulling than any other cause. Ideally, you want to produce a certain chip size, or as it's called by bit manufacturers, the chip load. Some manufacturers have charts on their websites that show the ideal chip load for a given bit, and a formula to calculate the RPM and feed rate necessary to create the proper chip load. Onsrud has extensive charts available on their website, downloadable as PDF files.

One seat of the pants way that you can judge if you have the RPM and feed rate right is to see if you are producing little shavings or chips or dust. If it's dust, something's wrong. Also, if everything is OK, you should be able to touch the bit immediately after cutting and find that it is just warm. If it's too hot to grab on to, then either the feed rate is too low, or the RPM too high, or both.

Hope this helps.

Matt

 

[Dan Bowman]

I've never tried it, but maybe you could use Charles Neil's bump cutting technique: http://www.youtube.com/user/InTheWorkshop#p/u/42/-l4suyL84-w

 

[Mike Davis]

I have a Stanley circular plane if you want to finish by hand.

 

[Shamrock]

Interesting with the bump cutting! I might just give that a try

Thanks Matt as always for the info, I have been surprised how many recent resources have detailed how to monitor chips to tell how your "insert cutting device name here" is working.

I think I need some new bits! That is probably some of my problem.

Mike-I did actually end up hand shaping the edge of the last table like this I built-the blades still have to be sharp though. Looks like there is no easy way to do this with dull tools ::gar-Bi. One way or another it looks like the answer is sharper tools!

Any other ideas?

 

[Matt Furjanic]

Hi Mike, try wrapping the circumference with a few layers of masking tape (only where the bearing will ride). Rout at high speed with a 3/8 bit, then remove the masking tape and rout again, this time removing just a bit of material as your final cut.

Matt...

 

[BWSmith]

Not really sure of what you're intending....Me thinks you're wanting tips on edge routing,to which you're gettin good advice above.But gotta say,I'd be lost without an edge sander......we do a considerable amt of curved wrok.They are just so effiecient at getting those last few .001's.Good luck with your project,BW

 

[fergy]

Hi Mike, try wrapping the circumference with a few layers of masking tape (only where the bearing will ride). Rout at high speed with a 3/8 bit, then remove the masking tape and rout again, this time removing just a bit of material as your final cut.

Matt...

I do this at times, especially for plastic routing, when chip load is especially important and I need a perfect finish. However, sometimes the bearing chews up the tape, especially on old bits when the bearing is less than optimal.

The other trick I do is to keep a set of different OD bearings, and change out the bearings, using a larger one for the hogging cut, and the flush size for the finish pass.

The other trick, instead of tape or bearings, is to use a piece of .040 or similar plastic sheet or "bearing strips" for this very purpose. I've got a bunch of strips of polystyrene for this. They hold up a lot better than the tape. You could do the same thing with veneer strips if you've got them laying around.

The "bump cuttin' " method looks very interesting. I hadn't seen that before, and it makes a lot of sense.

 

[Joe Scharle]

The bump edging technique is similar to what I do for pie crust edges. My warning is to not try and take too much because the bit could catch and rip off a splinter. DAMHIKT. Just like making a pie crust, it works, but don't be overly aggressive.
I tried the tape shim just once. In my case the tape peeled off; caught in the bit; then dove into the work;.....fast!
For a long time I've used progressive bearing steps. It's comfortable to use, but it's also time consuming. However, usually at this point in a project, I'd rather make a pass; get a coke; change a bearing; make a pass; catch the weather; change a bearing......you get my point.
I also tried using an edge guide and incremental steps down to the bearing. I can see that with some practice it would be my favorite. Although, in my case, doing this once every 2-3 years I'm not likely to get enough practice.
In any event, I recommend a heavy router with a big base both for balance and control. Speeds and feeds for me is by ear. The router is working, but not laboring.