I am trying to make sense of the books that give equations for speed (rpm) and table feed to cut aluminum (with a Bridgeport and 1/2" 4 flute endmill).
From what I gather, it is:
rpm = fpm*12 / pi*D
so for a HSS cutter of 0.5", a 750 fpm, that gives an rpm of ~ 5900 ! (this seems too fast)
Feed = ftr * T * rpm
using ftr of 0.016 , 4 flute end mill gives a feed rate of 367 inch/minute
are these correct?
what values are "reasonable"?
(these formulas are from John Walker's Maching Fundamentals)
lastly, if I use a TiN coated end mill, do I HAVE to go faster, or is this "allowed" to go faster?
1. Years ago, I learned RPM=CS*4/D. This you can do in your head and it's close enough for most work. CS for 6061 aluminum should be around 250 FPM with a HSS cutter. So, RPM=250-4/.5 or 2000.
2. Feed rate is something that you can look at any book, but it won't tell you enough. If you're using a HSS end mill, do you have regular or high helix? If you have a regular helix end mill, you can usually get .001" per tooth with aluminum (maybe more, that's conservative). So, .001*4*2000=8 in/min. This is a starting place.
3. TIN, ALN, and other coatings are put on cutters to make the edges last longer. There are 2 ways of looking at this, run same speeds and feeds as uncoated and get 10% longer life on the tool or run 10% faster and get same tool life. For most home shop use, I don't think that coatings are worth the extra money.
Depends on the type of aluminum. Also a 4 flute will load up faster than a two flute if the aluminum starts to heat up. A few drops of oil will help but not prevent this from happening. Also the size of cut is a huge factor. The bigger the cut, the more heat generated, the greater the risk of loading. If you are programming for production jobs and need a set feed for rate standards then go by the book. For other jobs go with what you feel comfortable with. Learn to read the material to see how it acts. Running coolant allows a higher feed and speed. Start out around 1500 RPM's and 10" IPM and adjust accordingly. Smaller cuts= faster feeds=faster feeds. Larger cuts = the opposite. If you start getting chatter, slow the rpm's down or increase the feed rate slightly to put more of a load on the endmill. The best thing is trial and error.