[Jim27]

Wanting to get some advice in using cobalt tools in 4140 and 4140 PH. Mainly interested in the PH as the other should be easier. Have some cobalt drills and will be running them on a machining center with coolant. Is that sufficient or should I be thinking of cutting oil? Peck drilling of course. speeds and feeds?

Another question is roughing end mill - I have a 1/2" cobalt roughing end mill (uncoated). Is this reasonable to use on the PH or is it going to have a short life and I should go to carbide?

[joeavaerage]

Hi,
I use 16mm tools in steel. I started out with uncoated HSS, but without tons of coolant got very little, several hours use out of them. Then I went to coated carbide now
I get thirty plus hours with coolant and much more aggressive cuts. The carbide is about three times the price but I get 10 times the use. Value for money in my book is: carbide, hands down,
in all steels from mild to hardened 4340.

Craig

[pctree]

Im also machining some 4140 with a 1/4" coated carbide ball end mill. I was planning on using flood coolant and was hoping to get some feed back about what kind of cutter speeds and feed rates to use. Im trying to get proficient with machining but it has been hard as my machinist and best friend died at the end of last year so Im kinda struggling.

[joeavaerage]

Hi,
for an uncoated carbide end mills about 100m/min surface speed for steels, maybe 50m/min-75m/min for stainless. Being coated you can probably push those speeds out by another 30%,
so 130m/min steel, 65m/min-95m/min stainless without degrading tool life. I always use coolant, as much to flush the chips away but I get vastly better tool life with coolant than without.

So 1/4 inch endmill:
1/4 inch= 6.35mm or 0.00635m diameter and 0.00635 x PI= 0.0199m circumference
100m/min /0.0199=5025 rpm.

This is about the right rotational speed, you can increase it a bit because of the coating, or decrease it a bit if you want really good tool life.

Now all you have to do is decide how big the chips you peel off. You can use a too low feed rate but that tends just to 'give the material a good rub' and is not good,
you need to at least force some sort of chip to take away the heat. I'd say you'd not want to drop the chip thickness below 0.01mm. You can push the feed rate up peeling
off even thicker chips and the job goes faster, but the power required from the spindle goes up to and so do the cutting forces on the tool. Eventually you'll either break the tool
or stall the spindle. I'd start slow and work your way up until your machine is comfortable.

Lets say you want to start gentle at 0.01mm chip thickness at the aforementioned 5025 rpm:
0.01mm=0.00001m
1 rotation = 1/ (5025/60)
=0.01194 seconds
so the feed rate is 0.00001/0.01194 =0.0008375m/s or 0.0525 m/min or 50 mm/min

Note this is for just one tooth, if you have a two flute tool that would be the same as 100mm/min and a four flute tool would be 200mm/min.
Within the exception that you can go too slow, the idea is start slow and work up until you find the right chipload for your machine.
The surface speed is about the heat generated, you can go faster but you risk damaging the tool with excess heat, going slower it not a problem.

Craig

[pctree]

Craig, thanks for sharing your knowledge. for 35 years I have been a tree climber and since retiring due to injury I am trying to manufacture a couple of inventions that I have for tree workers. Bit of a change in pace to say the least. Your post was greatly appreciated