I'm sure to leave something out that someone will ask me. This is also my first post to this forum so hopefully it's in the right place.
I'm making a .5" diameter, slightly over 4" deep flat bottom hole in 6061 aluminum. I have a Harvey Tool 3/8" relieved shank stub flute square endmill that has a reach of 4-9/16" that I can't quite get a handle on speeds and feeds. I'm also open to what other tools might do it better. The only kicker is I am drilling a 3/8" hole first that is also about 4" deep that has a center point very close to the center of the .5" hole. Making it difficult to drill both holes out. So I had to take a pick as to which one I'll drill and which one I'll have to mill out after. The bigger hole seems like the better choice of the two.
I have measured run out on the end mill. At the top it is less than .0005" but I only have a .0005" indicator right now. Towards the bottom, near the flutes I'm getting more like .001" and I have tried rotating the tool and collet and get the same result. I ordered a new collet for a better a TG collet holder we have but it won't be here until tomorrow. I'm sure it would help to get the run out down under .0005" at the bottom as well.
I run the tool at 10,000RPM and rough out the hole at 33IPM using a helical ramp. By helical ramp I mean the code is outputting a circular interpolation that also uses the Z. This surface finish is OK, atleast for a roughing pass. For a finishing pass I've tried what seems like everything. Different RPMs(slow and fast) as well as feeds that vary. Very rarely I feel that I have encountered an endmill that can't be run at a fast spindle speed and slow feed in 6061 for a good surface finish. This tool however just can't handle it. Even at 10,000RPM and feed of 1ipm and radial load of .005" the chatter is unbelievable. I've also tried loading it better with 10,000rpm, a feed of 40IPM and a radial load of .036" as per recommendations. Still terrible finish and chatter.
We have an Iscar multimaster with a 3/8" shank and a 1/2" ball insert that is able to easily give me a great surface finish at similar speeds and feeds. This tool is also 4+" long. I'm not quite sure what I'm doing wrong or if there is a different approach or tool/tools I should be using instead. When you start getting into these long end mills things get pretty hairy. Although, I've machined stainless with .062" 12X diameter end mills with good success. This big 3/8 cutter however is giving me nothing but grief. I know Harvey makes a great tool so I don't know where to point.
To double check speeds and feeds I've put the numbers into the FSWizard calculator on CNC Tricks Home and with a feed of 33ipm, .5" DOC and .036" radial cut it tells me that I should see .005" of deflection! These are feeds that I'm testing directly from Harvey. Typically when I use this calculator I would shoot for deflection of .0005" or less. However, I was told I was not properly loading the tool with feeds as low as I was using this way. It does seem to react fairly well to the higher feeds, especially on the helical ramp down.
It may just be that I cannot get this tool to any side milling at all. That's what it seems like. The ramp down it sounds pretty good. Surface finish isn't what I need it to be but not the totally messed up finish I get from trying to take a finish pass. I may also need to try and track down this .001" of run out which can be a lot for this length tool. I feel as though I've tried every speed/feed combo possible. If I choose to drill it out and follow with an end mill to get a flat bottom that will work but then I'm left milling out a 3/8" round "Keyway" which would mean using a 1/4" endmill of similar length.
If anyone managed to read all that and has any input I'll be eager to hear it!