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[machinist] What's the secret to using long cutters without chatter? : Machinists
- From: Lawrence London <lfljvenaura@gmail.com>
- To: machinist@lists.ibiblio.org
- Subject: [machinist] What's the secret to using long cutters without chatter? : Machinists
- Date: Wed, 20 Nov 2013 01:01:41 -0500
What's the secret to using long cutters without chatter? (self.Machinists)
submitted 4 days ago by reddipeg
Right now I'm finishing up a project which requires a pocket to be milled 1.25" deep into a workpiece. I've cleared out all the material right up until a certain point where the quill bumps into the side of the part I'm machining preventing me from going any further. I figured to give myself more clearance I'll just buy a long type end mill to reach down into the pocket. Only now it chatters like mad and I'm only trying to taking off less than 0.050" before it begins sounding bad. So I figured it's a rigidity issue due to it's longer length.
I'm manually machining 6061-T6 using a 2 flute 3/8 HSS end mill with an OAL of 3.5 inches and a flute length of 1.75". I'm running the cutter at approximately 2900 RPM and using Safetap cutting fluid. The results are the same regardless of the tool holders I use (end mill holder or collet) And to make matters worse the cutter still isn't long enough and I'm going to need the 4.5" OAL version. The good news is I won't have to cut very much material with it but I also don't want to snap my tooling in the process. Hopefully somebody can recommend a solution!
[–]SilentPeaShooter 2 points 4 days ago
Good luck with your lower. Worst case scenario, just stop a few thou short of finish dimension and you can sand the the chatter marks out. The interior pocket is very tolerant of surface finish, all the critical alignment is handled by your pins.
[–]reddipeg[S] 2 points 4 days ago
I accidentally used the wrong measurement in the post (I'm doing a 0.63 DOC not 1.25). The rear take-down pin pocket is actually the issue where the spindle is bumping into the buffer tower and ultimately causing the clearance issue. I've cleared out as much of it as I could until I ran out of length. The rest of the pocket looks great especially after the finishing pass. However i just don't have enough length on my cutter to clear out the remainder... it's SO close and so little material to remove and it's driving me nuts...
[–]SilentPeaShooter 1 point 3 days ago
I could be reading my dimensions wrong, but it looks like you only need about 2" total between the bottom of your end-mill and the lowest part of the spindle to clear the buffer tower. Your collet should be able to support a .250-.300 protrusion. I just checked the HF mini mill, and as long as you've removed the little chip window I have 2.5"+ clearance from bottom of cut to hitting the spindle. (That's with the 7/16 mill with 1 3/4" of cutting flutes.)
Ahhh yes, my lower is chattery as fuck in that rear takedown pin pocket. Used a long ass 3/8 carbide endmill that was real dull. But the dimensions are within .002 for the hole location and it works great. Well I did have to do a little filing on the mating boss on the upper but its a damn tight fit and I like it. Shout out to /r/ar15.
[–]Nevil_Clavain 3 points 4 days ago
Switch to a 3 flute carbide with only an inch or less of flute. You can get one with an extended shank. Grind .010-.020 relief to the depth you'll be going or order progressive shank for the depth you need.
Slow the speed to around 1500-900. Slooow speed and feed. .01-.03 width of cut.
Using a diamond stone gently hone the cutting edges to kill some of the chatter.
You might try mixing your cutting fluid 50-50 with kerosene or lamp oil.
[–]Katapesh_Express 5 points 4 days ago
Go with a much shorter flute length. My company does injection molding so it's deep pockets like that all the time. They will get a semi custom endmill that's 1/4th in diameter 4.5 inches long then transitions to 1/8th inch ball for the last two inches.It only has a a short 1/8th to a 1/4 inch flute length.
[–]Katapesh_Express 3 points 4 days ago
I had another idea. Use a boring bar adapter in a standard R8 collet. You can use a standard end mill, get the rigidity you need and make one quick.
[–]TheRedditMachinist 3 points 4 days ago
Grind some clearance on the end mill. Leave about 3/8 (a guesstimate for the rigidity of a Bridgeport) of flute on it and step it down. The proper way is to circle grind it with a spindexer. but a bench grinder and a careful hand would be effective also. We do features like that all the time, I usually take maybe .02 off the OD.
[–]garthnet 2 points 4 days ago
I'm assuming that the design of the part restricts you from using a smaller endmill... a larger diameter tool would obviously be stiffer and therefore less prone to chatter (a result of tool deflection.) Try using a solid carbide endmill, it's going to be a lot more expensive than a HSS cutter but much more rigid. Try reducing depth of cut also. It wouldn't hurt to use a mist coolant setup to keep the tool cooler as well. Cutting oil at 2900rpm is probably not doing much.
[–]reddipeg[S] 1 point 4 days ago*
Yes that's correct, and I'm also restricted for diameter. The pocket/slot I need to clean out is 7/16. Now my machine is only a 1/2 HP bench top unit with a max of 4200 RPM. Is it still advisable to use carbide tooling in my situation? I assume my cutter would then need to run at a much higher RPM than the machine would be able to provide or do I have this backwards?
[–]mnmachinist 1 point 4 days ago
Yeah, I've been impressed how hard some tools can be pushed.
- [machinist] What's the secret to using long cutters without chatter? : Machinists, Lawrence London, 11/20/2013
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