spoilboard vs bottom cleaning vs surfacing router bits

Mike_C said:

I would assume there is some issue with chick clearance? or maybe the ones with multiple cutters are easier to sharpen?

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I could see where theoretically the 4 edge could be better with chip clearing, for me being only a one man shop with a 3HP dust collector on my CNC machine that I rarely run faster than 300 ipm(usually never faster than 175 ipm) I find the collector clears the chips faster then the bit can produce chips at the rate I cut anyhow.

For people cutting at 600ipm or 1200 ipm using larger bits with deeper cutting paths then chip clearance may become an issue.

Most chip clearance discussion and issues are actually for the pros. In my experience no type bit, lessor quality or designed bit, hardly any issue at all with a CNC, can't be over come by doing one thing and that is slowing down.

Even at 60 ipm being one guy I can never outwork the machine for my work. Clearance and all those calculations I used to fret over as a newb now after 17 years in CNC never enter my mind. Most home or garage CNC workers or smaller shops will tell you this, that the sound the machine makes during the test cut tells them to immediately to increase or reduce the depth of cut or tool path speed and they do it by ear on the fly. I use the bit I have on hand I think is best for the job. Then I make a test cut and use sound to tweak the settings for speed and depth to get the best cut, indirectly giving me the correct chip clearance for the job.

I think many guys here feel what I am saying, they can hear the right point for their CNC machine and that's better than any paper calculation on chip clearance ever can give. Chip clearance is a starting point at best for our type machines(well mine anyhow).

In short, regarding the type bits and their differences for any profile, I would go with the best deal, least expensive over all that look to give the same edge we want. I do not think too much about chip clearance when purchasing the bits anymore. Many bits I have do the same exact thing, but use different tech, just like these 2 bits. I generally purchase the cheapest bit with decent reviews and my brand loyalty has lessened over the years too.

For planing I prefer to use a dish cutter as it's more tolerant to slight tilt of the router, whereas bottom cleaning bits that I used to use can cause lines where the corner digs in.

harrysin said:

For planing I prefer to use a dish cutter as it's more tolerant to slight tilt of the router, whereas bottom cleaning bits that I used to use can cause lines where the corner digs in.

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That works, but for a CNC Spoil board I do not recommend using that type bit for the sole purpose of stopping ridges.. Those bits cut flat, they don't leave ridges. It is not the bits that cause those ridges and if they did who would buy them and what purpose would they serve. I suspect as long as the bit is not defecteive for the msot part the ridges come from jigs, fixtures or CNC machine that are not set up perfectly, trammed or otherwise. Or from a the routers themselves that simply do not have balanced enough collets or have slop in the plunge mechanism or jig or fixture mechanism, I am talking very small tolerances. The wider the bit the tighter the tolerances need be.

If I get a ridge on my CNC spoil board I know for a fact my machine needs tramming. I adjust and adjust until that ridge disappears and I never had an instance where I could not get that ridge to disappear with a cnc machine with a decent quality spindle.

Now on a jig or fixture sometimes even a regular router right from the factory the tolerance just aren't there for most to adjust out a small .003 ridge due to imperceptible flex, or the router not being 100% perpendicular to the surface or very slight movement of the entire router as the router moves along the fixture or the collet not being balanced enough etc, etc.

I always say if it works to get the end result we want, then do it. There is no right or wrong in woodworking to me as long as an operation is safe. If using the bowl bit works for a given project to make it flat great.

For a CNC table application I go for the dead flat tables(as flat as I can get using my machinist levels). Believe me I have skipped that step and not done enough to ensure the table had no ridges or wasn't flat and too many times in the end it came back to bite me. I never found it to be the bit causing the non flat or ridge issue on my table either. If the use of a dish bit is to hide an accuracy or precision issue, ie hiding a CNC table's true flatness that isn't actually dead flat(which is actually an indication most times there is another issue somewhere else on the CNC machine), some other project, probably 3d bas relief or projects where you want to cut through your material and not cut into the table itself(or as cut in as little as possible)are going to be effected most. Ask me how I know that. Hand routing with jigs and fixtures I don't see any issue using the Dish bit for most work though.