milling insert damage

So why is the back of the insert broken and the bottom of the toolholder damaged to what looks like the same depth as the slot being cut?

Depending upon the supplier I wouldn't be in the least bit surprised if the toolholder rubbed. You've only got to look at the grinding on some of the cheap milling cutters to realise that they'd never cut properly.

Andrew

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Hope you don't mind me cropping your photo, Dave

MichaelG.

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 The arcuate marks on that insert look uncannily like a knapped flint.

... Likely sign of impact loads ?

[ naysayers will doubtless wonder how many angels I think were dancing ]

Edited By Michael Gilligan on 05/09/2018 07:56:18

Edited By Michael Gilligan on 05/09/2018 08:00:56

The fracture could just have easily started from the front and propogated towards the rear of the insert, I'm sure I have a few chipped cutters that show similar fractures after catching the cutting edge rather than the back

Assuming the insert failed during a cut then the tool was still being fed as it failed so the holder would then have been run into the previously cut edge of the slot and produced the marks seen. Don't know how the holders can rub as the part that holds the cutter is smaller than the nominal size, if you look at this 12mm shank and 12mm cutting dia SECO one you will see the end is about 11mm dia so should not rub. You can also see this on Michaels enlargement wher the 12mm shank is just visible above the 11mm ish end

Yes that is why I asked where the holder and inserts came from, I'm sure one from SECO would be better than one from the likes of Bangood.

Edited By JasonB on 05/09/2018 07:52:11

I think we're on the same wavelength, Jason

MichaelG.

To be fair, I've got a Seco insert holder that I tried when the ML7 was as close as I got to a milling machine. Similar damage occured and wrote the insert and the holder off.

I have a suspicion that the problem might relate to too high a feed and too low a rotational speed. 3000 rpm and 3 inches per minute might be closer to what the tool was designed for.

Edited By Mark Rand on 05/09/2018 12:04:06

Hi Jason,

Your link goes to the very tool I'm using, doesn't say Seco anywhere on it though. Can't find any info using the part number. The insert is the one supplied. I got the 20mm version as well might try one of it's coated inserts.

I expect it's the flaking from the insert clearance flank that's damaged the holder.

The curved marks were both there before setting the insert in it's current location, makes you wonder if all the damage was caused from the first chipped point.

I'm feeding by hand and ten turns a minute with my restricted rpm is a bit slower than I was feeding, but not greatly. All I did was just keep the weight of my hand on the handle to keep up with the cut.

10 turns per min on what is more than likely a 10tpi leadscrew should give you chips of about 0.03mm rather than 0.22mm or did you miss a zero off? You would need to be cranking at 80turns per min to get 0.22mm chips.

The holder is of decent quality then, don't know what the insert would be though and could just be an odd one that has been fitted for sale purposes

Hi Jason,

Re checked the chips, 0.18mm so still cranking too fast.

Tried a fresh one from the 20mm cutter, (they look the same) with only 0,5mm depth of cut and trying for the ten turns/min. The cutter is fine until you get to the end of the slot then I get chatter with 3mm depth and minor flaking from the back of the insert, the cutting face is still fine.

What is the reasonable depth of cut assuming I get a proper milling insert please?

About 3/4 of the insert height. A little bit more once you are happy that you aren't fracturing the insert edges (don't want to turn a 3 sided insert into a 1 sides one)

Get yourself a modern insert / holder system such as the AP** which have much better geometry and are supported with a vast range of modern insert materials, coatings, chipbreaker geometries and applications. There are many reasons why TP** inserts are essentially obsolete today. Ironically, AP** inserts are more suited to older and less rigid machines than TP**, as they generate lower cutting forces.

You can get yourself a 12mm AP** holder for 20 quid and matching inserts for under £3. If you are happy to take your chances on Banggood, AliExpress, ebay etc you can do even better but you need to have an idea what you are buying.

As mentioned, that insert has no obvious parentage, so could be made of sugar or flint. Whatever you do, get inserts of known / decent origin before you get too baffled at what you are finding.

Murray

My first choice milling cutter for a lot of work uses AP** inserts. It is a two insert 16mm cutter.

Martin C