Shaper cutters

[michael gates]

Would there be a noticeable difference in cut quality between a 3/4 bore and the bigger 1 1/4 bore cutters.

[Bob Wingard]

I would expect a smoother cut if the cutters themselves are larger radius. A larger bore in the same size cutter will have no practical effect on the cut quality. For example .. I have some 1 1/4" cutters that I COULD run on a spindle as small as 1/2" with reducers ... now, THAT might get a bit hairy, but mostly because the spindle might twist on a heavy cut, but, otherwise, no difference ... the tips are still cutting the same arc, and, at the same speed.

[Peter Quinn]

Would there be a noticeable difference in cut quality between a 3/4 bore and the bigger 1 1/4 bore cutters.

Maybe. Assuming you are comparing apples to apples in terms of cutter geometry and build quality, then the bore is not of itself indicative of cut quality. Things that do have an impact are the size of the cutting arc, feed speed, tip speed, and depth of cut. I find generally at the same feed rate that 1 1/4" cutters tend to tear out less, and this I attribute to the gentler exit angle of the cutter on the larger bore. Pretty much without exception 1 1/4" bore cutters are 1"-2" larger or more in diameter than 3/4" cutters, so there is your gentler exit angle. Think about attacking a piece of wood with a chisel, which is essentially what a shaper cutter is. You can pare plugs flush quite effectively with a sharp chisel and a very slight angle of attack, or you can lift the chisel way up and carve out chunks from a rough blank. Same idea, but primarily on the exit of the cut. Similarly a 3/4" bore shaper cutter is typically much larger than a router bit and they tend to tear out less than routers given the same depth of cut conditions too.

Another aspect of cut quality is chatter. The smaller spindles are more prone to deflection under load, and generally shapers that max out at 3/4" bore have a less robust build quality in terms of bearings and spindle cartridge assemblies than larger shapers. Given the same feed rate and depth of cut, you can expect the 1 1/4" bore spindles to deflect LESS than 3/4" bore, and that results in less chatter, or better cut quality. So, less tear out on average, less chatter, often the bigger spindles win out, particularly in a high feed rate production environment.

So what can you co to improve surface quality with a 3/4" cutter? First, match the spindle speed to the cutters diameter. I like to spin most of my 3/4" cutters at 10K RPM's, my 1 1/4" cutters generally spin at 6K-8K RPM's. Spinning the 3/4" cutters faster results in higher tip speed, a smaller chip load at the same feed rate (the size of each slice being taken is effectively smaller and thus less prone to tear out as the cutter exits the work, much like setting a hand plane for a very shallow depth of cut), and the CPI, or "cuts per inch" are increased, which putts the chatter lines closer together, makes the valleys in between smaller, and gives a better surface appearance.

The third strategy I use with the smaller cutters and spindles (the two are really a system and that fact should not be separated in your mind IMO) is to reduce my feed rate. My main 3/4" spindle shaper has a grizzly feeder, grizzly feeders and many others are made by comatic, and comatic sells a slower gear set than the standard set that comes with most four and eight speed feeders. I bought the smaller gear set and can set the feeder to a virtual crawl if I find it is required for surface quality. I always run the larger spindles at a higher feed rate and never bothered to get a slower gear se for those feederst.

Other strategies? Climb cutting can help greatly, but if you are not familiar with that I wont start that discussion here. If you are not familiar with climb cutting, make sure you fully understand what you are doing before ever trying that. DAMHIK! A last resort is often to take multiple passes. At the upper end of a 3/4" bore shaper's capacity and on more dense species it may be necessary to take several light passes rather than on deep one. This can be accomplished very quickly by setting the fences for final depth of cut, then shimming them out with 1/8" or 1/4" MDF or hardboard fence plates which are well secured to the main fence plates. Remove them in succession, your final light pass will certainly be an improvement, and far less sanding.