HSS Endmill dulls quick

[faavs730222]

I hope to graduate someday from the beginners forum. (Try to ask advanced questions, but I am still struggling with the fundamentals)

Hi I have a problem: I want to make a holder for a TNMG carbide tip. I have a old boring bar and I milled 2 x 30 degree angles and a 7 degree neg rake. With the second 30 degree angle the end mill started eating the bar. The tip of the mill is blunt. It is a new industrial quality end mill (FEW). The end mill is 14mm The speed is 710rpm with a slow feed. I did not use coolant. The bar looks to be mild steel. I used the tip of the end mill most of the time(plunge cutting?). It is HSS. What am I doing wrong? Hell at the price of end mills I cannot burn it so quick. Would it be better to use Cobalt Cutters, I see in the Cromwell Catalogue they are almost the same price as HSS. Would it be better to use coolant? Oil? I thought it would go better when u chucked the Chinese mills and opted for the better (much more expensive) mills.

 

[ericc]

Hi. Are you sure that the bar is mild steel? Where did you get it? You could estimate the hardness by using a spark test and a file test. If it is hard to file, it will be difficult to mill with HSS. What is the feed? It could be too slow.

I got myself in a similar situation, but at least I knew what I was dealing with. I had cheap Chinese endmills, as forged 4140, and a Bridgeport clone. The poor mill was really shuddering and the end mills were getting dulled. I made the cut by doing a sub-critical anneal and roughing it out with a custom made small diameter flycutter made from a cobalt lathe bit. The cobalt lathe bit cut great, but it did get dull. The advantage of a flycutter is that it can easily be sharpened. The disadvantage is that I had to make a custom tool. This was made out of mild steel, (really) and it cut easily with HSS.

 

[faavs730222]

Hi ericc, I think you may be right, the steel i am milling is a broken brazed bit boring bar. In hind sight I can see that it is much harder than mild steel.

 

[Robert LaLonde]

You may be able to mill it, but imagine the headaches you will have when you try to drill and tap it. You might be able to anneal it, machine it, and reharden it. You might not. The thing is you have mystery steel so you can't just look up the best heat treat. You would just have to wing it.

 

[ericc]

I have a broken off brazed carbide lathe bit. It sparks as A-2. This is definitely NOT mild steel, and it will be tricky to cut. I have found out that haphazardly using junk steel in place of known stock can be a waste of money and time. My mistake was finding this bit in a box of garage sale bits and trying to use it to cut steel. For some stupid reason, I thought it was a vintage simple carbon steel, and I tried cutting steel. It would last a little while and just get dull. I know how to handle old fashioned carbon steel tools, and I use them routinely, but this one was terrible. It turned out not to be full hard. But it was probably hard enough to ruin even a good end mill. A good end mill will cost $10 or $20, and you can ruin it in seconds with this kind of unknown steel. Annealing A-2 is very tricky. Unless you have years of experience heat treating tool steel, it is not a good idea to try to mill it with HSS in an unknown state of hardness. Most of my friends who use A-2 will not machine it unless it is in factory annealed condition.

 

[Robert LaLonde]

You know handling most tool steels is pretty well documented right? There are some modern exotics which might be tricky, but all of the old tool steels have pretty reliable charts readily available. The only issue in this case is the OP doesn't KNOW what steel it is. A microwave analyzer will tell him most of the metals in the alloy, but it won't tell him how much carbon there is.

Unless you are doing a bunch of pieces at once you will use more energy in your furnace/kiln/oven than the tool is worth.

NOTE: This is for A2 other tool steels may be much less particular.

From http://www.hudsontoolsteel.com/technical-data/steelA2
Annealing
Annealing must be performed after hot working and before re-hardening.

Heat at a rate not exceeding 400°F per hour (222°C per hour) to 1550°F (843°C), and hold at temperature for 1 hour per inch (25.4mm) of maximum thickness; 2 hours minimum. Then cool slowly with the furnace at a rate not exceeding 50°F per hour (28°C per hour) to 1000°F (538°C). Continue cooling to ambient temperature in the furnace or in air. The resultant hardness should be a maximum of 235 HBW.

That is very similar to what I have read from other sources. Its not impossible to do accurately for anybody who wants to take the time to do it. Its just a lot of energy for little return when only doing one piece. I did read one reference that seemed to think you could get away with burying it in vermiculite for a very slow cool down, but even if that works the ramp up to temperature still burns a lot of juice.

 

[Reeltor]

I think that you are running the endmill too fast. A quick look at a speeds and feeds says that you should be around 340 rpm not in the 700's. Slow the rpm down and take a bigger bite so that you don't rub the cutting edge on the work dulling the endmill.
I am definitely not an expert, but this is what I would try; actually, I would slow the rpm to about half what the speed calculator gives.

Just my opinion and you know what they say about opinions

Mike

 

[4GSR]

I agree with Mike, slow your endmill down. Your'e not doing production work! I also recommend using some kind of cutting oil, too! It all helps.

 

[Reeltor]

I also recommend using some kind of cutting oil, too! It all helps.

I can't agree more, be sure to use a cutting oil and not oil. The big box stores sell a cutting oil in their plumbing departments for not too much money. The two stores are directly across the road from each other, one is more than twice the cost of the other. I don't remember who is cheaper, Lowes or Home Depot.

 

[Silverbullet]

YUPP cutting oil always helps. Speed does seem high.