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Boring with carbide inserts

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Hugh Gilhespie23/12/2010 09:14:40
130 forum posts
45 photos
I am about to use a boring tool on the lathe for the first time. I have used a boring head on my mill without any particular problems but not tried it on the lathe yet. I will be using a boring bar from Cutwel with a CCMT09 insert. The recommended cutting speed for this insert on free cutting steel is 250 metres per minute with a range of 150 - 350 m/min.
This seems awful fast - I will be boring a 32 mm hole and this translates to about 2500 rpm. Not only won't my lathe reach that speed but I really wouldn't want to be in the same room with it even if it did.
 
The recommended feed rates and DOC are fine, 0.07 - 0.22 mm per rev and 0.1 - 2.0 mm DOC.
 
So, my inclination is to use something like HSS surface speed around 30 metres per minute - 100 fpm in old money - which needs about 300 rpm. Will this do a reasonable job or should I pluck up coutage and go for 1500 rpm? The lathe is a Colchester Student 1800 so reasonably beefy and I am using a 16 mm diameter boring bar.
Ian S C23/12/2010 10:57:07
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7468 forum posts
230 photos
   Hi Hugh, I think I'd go for the lower speed, say 300/500rpm, I do a regular job of boring a 35mm hole in 8mm plate that has been gas cut to 33mm, and the trick with that is getting under the hard skin. On the machines that we build there are 10 of those to do.  For that job I use a commercial boring bar, with a brazed on tip, in the past I'v used a home made tool with an old indexable tool brazed on a bar, and that worked quite well also. My lathe has two ratios, low goes up to 500rpm, high goes to 1200rpm.  I rarely use the high ratio.   Not sure about the feed rates, but the DOC is about what I use.  Ian S C
KWIL23/12/2010 16:41:09
3681 forum posts
70 photos
Go for a lower speed, watch how it cuts, if OK stay with it or move up a speed until the cut is producing quite warm swarf, at that point you will be just about right. Too low a speed will leave a rougher surface.
PekkaNF23/12/2010 18:34:21
96 forum posts
12 photos
I totaly agree with the previous posts. My limited experience with carbide inserts shows that the most important (and hardest to get, coming from extensive use of HSS tools) is to avoid going under recommeded minimum feed and DOC.
 
With the speed I go down until I'll get good stable cutting. I know that speed is too low when I get welding with aluminium or rough cut with steels.
 
Often increasing the feed/speed seems to improve finish. I get hardly never (even with 06-size inserts) close to max. feed/doc or max. cut cross section. Most of the time I get better finish when swarf is violet/blue/black and hot as hell.
 
I use machine feed 100% time with carbide. Carbide is not happy when it is rubbing. And fragile tips are easily chipped with manual feed.
 
Internal turning has it's own set of problems with chip evacuation. Trough cooling  boring bar has worked sometimes better with blasting air.
 
One thing which is even more importat than with HSS is to set tool height just right. It's whole lot more critical with carbide and even more with internal turning. This is where I had most problems, hopefully new expensive set of german made boring bars will be big ofering to great swarf spirits and I start to get this right every time....
 
I don't have extensive experience on this matter, but I have chipped/cracked/dulled  quite a few inserts - I'm learning slowly how not to do it too often.
 
Most of the time I get away even with whole lot less than text book setting. Your lathe is relatively big and a 16 mm boring bar is stable by our normal hobby standards. Come to brag us back after you get shiny bits out on your assemby line.
 
PekkaNF
David Colwill24/12/2010 00:32:01
782 forum posts
40 photos
    I have some alpha mill cutters from cutwel which are milling cutters with tct inserts. I have never had very good results with them and the inserts would chip really easily no matter how "careful" I was.
    When confronted with a job which required quite a lot of work with a 20 mm end mill I decided to have 1 last try with them before giving up in disgust. I looked on line at the hertel site (they produce similar cutters) and found a speed and feed calculator. I entered all the details in and Bingo 1500 revs at 250mm / min. Way faster than I would have gone in both speed and feed. The result.... For the first time the cutter sounded "happy" doing what it was made for.
    So I would now tend to stick with the recommended speeds and feeds and drop down if things don't sound right.
Hugh Gilhespie24/12/2010 18:13:06
130 forum posts
45 photos
Hi Everyone and Merry Christmas to you all. I think David gets the extra helping of turkey for his reply. I did a few trials today and the finish was not great below about 1000 rpm but when I went to 1400 rpm, what a difference! A really nice finish with no visible tool marks.
 
Actually, Ken also deserves a second helping , he was spot on about the warm swarf. At 1400 rpm it as coming off straw coloured so obviously nice and toasty.
 
Thanks again, Hugh

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