Gear cutter shank length

try cutting a scrap piece of metal. There are many variables involved including the stiffness of your machine, the quality of the tool holder, run out of the cutter and shank itself. I don't remember what tool holder you are using in your mill but the shank should slide up into the tool holder, Put it up as far as possible, shorter is better. Don't try to climb mill unless your machine is very rigid.
good luck with your machining education. :)
John

Yikes. If you mean the gear cutter is 8" away from the spindle nose, then yes, I think that is too long. I have made a stub milling arbor, and also bought one that gets the cutter down to about 1" from the spindle nose. You may actually need more, depending on the arbor the gear is mounted on and the rest of the setup. But, I would want the cutter no more than 3" away from the spindle, unless it is really heavy up to where the cutter sits. I have done some of this with 1" and 1.25" arbors, the 7/8" arbor is even more flexible than mine.
Jon

It's basically the same situation as turning with stock held in a lathe chuck. Stickout shouldn't be more than 3X diam. I don't think that Young's modulus is going to be much different for your arbor.
Bob

I agree. I will just cut it off.
i

You could make a shorter one, index it to the spindle and turn the cutter seat on the mill with a lathe bit in the vize. Then the cutter should run as true as possible and not leave revolution marks.
jsw

Is that 8" the overall length -- or just the length of the 7/8" diameter part? My following answer assumes that it is just the 7/8" diameter part.
Well ... the diameter of the stack of spacer rings will help somewhat. (What do those wind up at -- perhaps 1-1/4" for 7/8" bore?)
But mount the cutter as close as possible to the NMTB 30 taper end. For the 40-taper ones on my Nichols mill -- the other (free) end is supported in a needle roller bearing to more than double the stiffness.
Good Luck, DoN.

Overall
This tool has a shank, and no spacer rings. It is called "slitting saw arbor".
Yes, I do not have that support.
i

O.K. Not that bad, then.
Oh -- one of those. Does it have a key to prevent the gear milling cutter from slipping? The normal arbor for a horizontal mill has a keyway running its length and a key goes into it and the keyway in the holes in the cutters and spacers. Beyond that there is a threaded length and a big nut (typically two flats milled into a cylinder, instead of the usual hex nut) to clamp the whole stack firmly. Beyond the nut is the reduced diameter for the needle roller bearing described below -- supported by at least an overarm, and likely some diagonal braces as well to allow heavy cuts with minimal deflection. Really long horizontal milling cutters have an extra bearing spacer which goes between cutters (there are often many cutters stacked on there to do multiple parts of the job in a single pass -- such as cutting the two flats, the edges, and the two inverted V ways on a cast iron lathe bed in a single pass.
O.K. I was thinking in terms of a horizontal milling arbor. I know that I have several in various sizes for the NMTB 40 taper, and assumed that you found a smaller one in NMTB 30 taper designed for a benchtop horizontal mill.
Good Luck, DoN.

No. I misled you. The arbor is 7/8" thick, it is not thicker than that in any of its parts.
No key, unfortunately.
I just missed an ebay auction for a nice cutter arbor in the size that I needed, dang!