Carbide Insert Speeds and Feeds

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EOsteam
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Location: Pendleton, Oregon

Carbide Insert Speeds and Feeds

Post by EOsteam »

Most of the my machining has been with HSS and while I have some inserts and holders the finishes with inserts were pretty much terrible. The latest project involved turning a part out of some 3" round 4140. After just about giving up on the inserts because of the awful finish on the part, an attempt was made to be much more aggressive with both the speeds and feeds. The first thing that was discovered was that my speed was way too slow. 220 RPM was where the experiment started and 630 was were it ended up. The next part was to find a good feed speed and depth of cut. Turns out when using the TNMG 332-CM CM14 (Cobra Carbide) inserts it didn't really matter what the depth of cut was but if the feed into the part was less that 0.0055"/rev the finish suffered. In fact the best finish was around 0.0076"/rev. The only bad part was that I couldn't get the chips to break no matter what the speed or feed. It either wanted to create really long tight spirals or a big rats nest (the finish suffered when a rats nest formed). Is there a particular type of chipbreaker on the inserts that will work better? Also, the same speed and feed into the metal longitudinally produced similar results as facing off. I don't have any 1018 at the moment but what change to speeds, feeds and depth of cut could be expected relative to 4140? I have never really gotten a good finish on 1018 using HSS so will the results with carbide be similar?

Merry Christmas!

HJ
John Evans
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Re: Carbide Insert Speeds and Feeds

Post by John Evans »

It is hard to get a good finish in 1018 no mater what you do. You have discovered that carbide likes 2-3 times surface speed compaired to HSS and digressive feeds. 4140 does like to make rats nests BTDT ! did your insert have any chip breaker?
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EOsteam
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Re: Carbide Insert Speeds and Feeds

Post by EOsteam »

Yes, it has a molded in Chip Breaker.
SteveM
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Re: Carbide Insert Speeds and Feeds

Post by SteveM »

Take a look at this video from NYCCNC (Saunders Machine Works).

https://www.youtube.com/watch?v=NpuZFCG2JBg&t=11s

It's interesting to see how just a speed or feed adjustment turns strings into c's.

Also interesting to see how much he can get out of a simple T insert.

Steve
Jawn
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Re: Carbide Insert Speeds and Feeds

Post by Jawn »

https://littlemachineshop.com/mobile/speeds_feeds.php

I don't claim to be an expert, but I have found that calculator handy for getting a starting point. Choosing turning alloy steel 3" diameter gives 890rpm.

I use CCMT/CCGT style 21.5x in a 10" bench lathe - works well for quickly turning metal into chips. Not great at taking away small amounts of metal, though. Finish definitely gets worse if trying to remove less metal or at slower speeds. For example, facing a 3" piece of 1144 the outer 3/4" or so will look great, the inner part will be noticeably rougher.
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Harold_V
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Re: Carbide Insert Speeds and Feeds

Post by Harold_V »

EOsteam wrote:Yes, it has a molded in Chip Breaker.
Kick up the feed (I'm assuming you're already taking a deep cut). If you have enough hp and rigidity, you should be able to take .012", even more. And, consider what you think is a good depth. With a rigid machine, you should be able to remove .100" per side without issue, and more, depending on the machine.

I've machined more than my share of chrome moly, both 4130 and 4140. Never failed to get chips to break when roughing, and I commonly use TNMG inserts with a molded chip breaker. They do not yield great results for finish cuts unless you have wide open tolerance and can take a deep finish cut.

Don't expect chips to break with modest depths of cut. Anything less than .05" (per side) may not break as hoped.

H
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EOsteam
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Re: Carbide Insert Speeds and Feeds

Post by EOsteam »

Let's stick with 4140 as the metal and carbide inserts. What do most folks use for finishing if there is less than 25 thou to remove?

HJ
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Harold_V
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Re: Carbide Insert Speeds and Feeds

Post by Harold_V »

I, typically, switch to positive rake. I use an HR holder, with diamond ground inserts. They're worthless for roughing, but yield a reasonable finish with chrome moly.

Regardless of surface speed, you can't get a super finish with chrome moly using carbide*, as it displays similar characteristics displayed by mild steel. The difference being that the tearing isn't as deep. Depth of cut is critical to cutting without tearing. You can get beyond that problem by increasing surface speed and using ceramic (think Cermet) inserts, however. Beyond that, you have two options. You can polish for finish (and sizing), or you can resort to precision grinding, which yields excellent finishes and makes size control a breeze.

*If you increase surface speed beyond reason, finish will be improved, but the cutting edge will fail quickly. If sparks come off the cut, you're running too fast.

H
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EOsteam
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Re: Carbide Insert Speeds and Feeds

Post by EOsteam »

This may be a dumb question but what is an HR holder?
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Harold_V
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Re: Carbide Insert Speeds and Feeds

Post by Harold_V »

Not a dumb question at all. HR made holders for inserts that were precision ground. The holder in question accepts a modified triangle insert, ground with positive rake. Bear in mind, this holder is from about 1970, and I've been away from commercial machining since 1983, so I'm not up to date on what's currently available.

I have a little literature on HR, which I'll try to locate in the next day or two (wish me luck). If I do, I'll scan and post so you can get an idea of the holder and inserts involved.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
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