I did a search and found no questions for this. It has always been my understanding that you should never bottom out a router bit in the router and always leave a gap (to prevent heat transfer). I don’t know if that is fact or fiction. For the Carbide3D CNC routers, should we place the bit all the way in or leave a 1/8" (or other) gap?
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Yes, you should not bottom out an endmill — my understanding is that doing so can result in thermal expansion causing problems.
This article notes at least 80% of the collet bore should be filled at a minimum:
https://cncrouterbits.com.au/technical_collet_use
Some collets are made so that no clamping occurs at the top, so arguably the endmill shouldn’t reach all the way.
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It seems that one problem with having the end mill all the way in the collet is that chatter can really shake the end mill around, and if it’s pressed up against something solid it can end up pushing the end mill down a bit.
That actually happened to me a couple days ago with my HDZ — I think I had a loose v-wheel somewhere, and when the job was finished I noticed that half the pocket was about .005 deeper than the rest after experiencing a bit of chatter (for some reason it’s always when the machine is moving back and to the left, still trying to figure out where it’s coming from). All the vibration seemed to cause the end mill to slip a bit, but if it wasn’t all the way in and up against something solid, I don’t think it would have budged very much at all.
So is there a Non metal material that is acceptable for a spacer that could be put up in there and left there so that a bit would never be bottomed out? Seems there has to be something that would not melt and stay put so that you never bottomed out. When I put the bit in, I (was) inserting the bit, pushing it all the way up, the finger tighten the nut using push button lock, then use my set of wrenches to lock it down.
Which brings me to another question. What is the proper torque to tighten the collet nut? I know this seems like a childish question but I have often wondered “how tight is tight enough”. If I was to use a torque wrench to know the proper tightness, then I would just know the feeling of that moving forward and using the the regular flat wrenches would be all I needed.
Thanks for the feedback!
I keep hearing “monkey tight, not gorilla tight”, which is a great rule of thumb. Kinda subjective, though 
And yeah, what I usually do is push the bit all the way in, then back off just a tiny amount so I don’t risk that deflection. I also use the button and wrench to tighten, but I tend to also do this for the final tightening too, rather than two wrenches. Maybe someone will tell me that is a bad idea.
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An old trick I have always used in conventional router bits, is to get some “O” rings that are a little smaller than the shank. Put an O ring on each bit shank to act as a stop for the bit when inserted into the collet.
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Not a spacer, but depth rings are the industrial solution here:
https://www.precisebits.com/reference/depth_setting_rings.htm
The router bit should be at a minimum fully the length of the collet. The bottoming out can cause vibrations because the bottom of the router shaft may not be smooth from machining. The bit must be fully seated in the collet. Any further up inside the router shaft is good but not required. The further out the router bit sticks out the bottom of the collet helps magnify run out. My Dewalt 611 has about .8" of collet length. A lot of router bits/endmills are 1.5-2.0" over all length. So if you have a 2.0 OAL bit you should insert the bit in the Dewalt .8 inches leaving 1.2 inches stick out from the router. Most of the router bits have between .75 and 1.0 inches of cutting depth. You can cut deeper than the flutes but you can run into burning around the top edge of a pocket and can introduce vibration for a straight sided bit. If you have a 1/8" bit that has a 1/4" shaft then never cut deeper than the cutting depth specified by the OEM. If you cut deeper you can break the bit or possibly start a fire from the bit rubbing on the wood.
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I was watching a video on cabinet making a few years ago by Mark Sommerfeld on youtube. I believe that’s his name if I remember correctly but regardless I’m new to this whole cnc thing and I’m not sure a spindle would be the same as a router but he instructs those that buy his cabinet making kit to simply put an o ring that will fit in the collet just enough so it doesn’t fall out and push it to the bottom in the router. He said to just leave it there. He claims the router bit would never bottom because the O ring would cushion it so it was never metal to metal and it would keep your bit from vibrating loose. I did it to my router I was using at the time which was a 3 1/4 hp hitachi using half inch bits. I’m not certain I ever bottomed the bits out as I was told it wasn’t good to do so but I never stressed whether it was bottomed out or not as long as I had the O ring in place. He refers to them as rubber shims in some videos and and O rings in others. I plan to get a new spindle soon so I may have a go at testing this old palm router, an old bit and an O ring out just to see if I can bottom it out and put it to the test!
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https://www.youtube.com/watch?v=klv0jzWD26w This video he explains it in around the 10:50 mark and shows you what he uses. He has a dual purpose for using it as well in cabinet making.
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My understanding is that bottoming router bits prevents standard router collets from fully tightening because the bit normally gets pulled slightly deeper as the collet gets tightened. Having to use the wrench twice during bit removal is a manifestation of that. ER collets probably don’t do that but bottoming bits with them (even with rubber spacers) would likely still negatively impact runout.
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