Posted previously and had some good suggestions. New job, same issue. LM 2.5, SLB. Rough cut for 2.5D model no problem. Raster finishing cut (about 12 hours total) with Sienci 1/4", 1/16", Ball nose. Previously suggested slippage due to pressure on upcut bit (makes sense. I noticed deepening ramp when cutting more material, especially vertical). Also suggested changing collet. First was Makita, next 2 Sienci Precision Collets. Have tried my āstandardā tightening and āextra-forceā. Have verified that cutter has slipped looking at side ārampā of job plus when return to Home position. Am resetting Z, cutting for about 20 minutes, then repeating. Have verified F&S with Sienci. Suggestions? Possibly replace cutter?
I probably donāt have anything you havenāt tried but what the heck!
Iām sure youāve cleaned the collet but I think I found some stuff inside the shaft before so itās worth sticking a pipe brush in there to check.
Iāve heard of oil from the factory on the bit but that most likely only applies if the bit is new. I often clean the collet and nut just by unscrewing it all and tapping it out but maybe it needs a more thorough cleaning. Iām not sure of the best way to do that though. I just searched and I think Iāll watch this video, maybe Iāll learn something.
The other problem I had once was putting the bit too far into the collet, so far that it bottomed out, or maybe it topped out depending on how you look at it. As the collet nut is tightened it pushes the collet into the shaft. If the bit hits inside the shaft it can prevent it from moving up and squeezing the bit enough. That happened with a long bit and tall stock. I was trying to eek out some more Z travel and it didnāt work out for me.
EDIT: Just remembered that I had some chatter going on with that long bit but I donāt know which came first the bit slip or the chatter. Seems like a chicken and egg problem to me, each probably making the other worse. I know you verified F&S but if all else fails maybe slow down a bit? Iām not great at feeds and speeds though, I just do what sounds okay to me.
Depth of cut could be an issue as well. Iāve heard things like you can cut as deep as the bit is wide but Iāve never even come close to that. For me Iām more like a 1/3rd the width for the depth. Shallow cuts let me keep the feed rate up which is better for the bit than going slow and deep. My Mk1 is not as stiff as your Mk 2.5 though so maybe you can do a bit more than me. I think some of these ārules of thumbā Iāve heard as well as most chipload charts are for big boy machines and not applicable to my LM Mk1.
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Thank you. Have tapped, cleaned the collet/nut, multiple times, and used shop vac on shaft. Checked with mirror - clean.
Have tried minor differences on cutter shaft insertion (never full bottom). Have done some gentle sanding of shaft with 120 G paper.
Using 5% stepover for the cutter setting. Nice and smooth cut, butā¦
Had same problem several months ago on a different job, which I cut at 2/3 the rated feed of the cutter, so speed does not seem to be an issue. If I lower the feed, the cut time will go from 12 hours to ???
Have router set at 25K rpm, as per Sienci, and have tried going a bit higher rpm to reduce pressure on the cutter flutes.
Using W Oak, so should not be an issue with hardness, such as with ebony.
My guess at this time is that it is either the Makita router or the cutter is just too aggressive on the flutes.
I figured you had probably tried my suggestions but itās not a problem. I can type okay! On another note how did I know when I watched a video that I found when searching how to get oil of from a collet that he was going to use oil to clean the collet 
Again Iāve heard people complain about the oil but that doesnāt mean itās a problem per say. When we are having trouble and grasping at straws we tend to blame whatever we find, no? I was kinda thinking as I wrote about the oil in my first response āWhat about those machines that mill metal with what I assume is oil flying around the whole cutting chamber?ā.
Unfortunately Iām out of ideas to help but I wish you all the best with your project, sir.
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More infoā¦
Have contacted Sienci - requested measurements of my 3 collets and shaft of cutter - all at 0. 249. Sienci has asked for a video of my ātightening techniqueā. OK - do not claim perfection, but I have been doing this for 6 decades or soā¦ always willing to learn.
My Brother (Mech Eng), bless his soul, suggested two further options: bluing the cutter shaft, and applying some lock-thread compound. Will wait for Sienci response and will KYP.
Concerned about expensive wood spoilage in all this experimentation, butā¦ Need a totally dependable depth cut for my projects!