I think I have a parting problem........

sanddan

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I think I have a real problem with parting on the lathe. When I got my fist lathe I bought a HHS parting blade and installed it into the holder that came with my QCTP. I started with a thinner parting blade, around .087" thick I think, and watched a bunch of video's before attempting to actually part off a piece on the lathe. It went ok for the first several attempts but then BAM! The tool jammed in the cut and busted about a third of the blade off and left a chunk buried in the part.

WOW, that got my attention. It was some mystery steel I had for practice so at least I didn't ruin anything important and I wasn't hurt so back to watching video's looking for the cause of my misfortune. I thought, maybe I should get a thicker blade, less likely to break maybe?

Some time goes by (where I avoided the parting tool like a bad dream) and then I needed to make some aluminum washers for a project so I had to break out the parting tool again. No avoiding this time.

You might see this coming based on the title. Apparently 2 hp can quite easily break a 1/8" parting blade.

Bummer.

More video's later I decided I needed one of the blade style tools that uses carbide inserts. I got a nice setup from Shars and about the third time I tried it BAM. Actually it broke without the drama that the solid HHS blades displayed, but ruined one of the two ends. This is starting to get expensive.

So I went back to avoiding the parting tool and used the bandsaw instead. I thought my problem might be flexing of the blade so when I saw a cool looking setup that looked very heavy duty I spent more $$ hoping for parting success.

I had to make some plastic washers so thought it was a good time to try out the new tool. After aligning the tool I made several washers without issue (keep in mind this is plastic now) and thought, maybe this is the answer.

Then I had to make a 1" OD by .100" thick washer with a 1/2" hole in steel. Shouldn't be too bad right? You might see where this is going about now. More carnage.

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I think I will go back to the HHS blades as they are a lot cheaper. Ruining a very expensive tool is starting to get old about now.

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If you are doing everything right and are still breaking tools, then something has to be flexing a lot.

On the other hand, this is the only parting tool I own. It hangs where I can get to it easy.

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Dan,

Are you hand feeding? What RPM are you turning? last question, is the insert clamp tight on the Kennametal?
 
Hi Dan
I feel your pain.I went through the same process starting with HSS blade then GTN2 and GTN3 bits in blade holders with similar results. Making relief cuts to stop binding helped a bit. However I think GTN bits are very short which does not help to stabilize the bit in the holder
I finally changed to MEGHR1616-2 holders with Korloy MGMN200-G H01 or NC3020 Inserts. What a difference!
Day before yesterday I was parting 15mm deep (the limit of these holders into 54mm dial steel with no problems. I used the uncoated H01 inserts and they cut the steel like cheese. 460rpm and just hand feed. Didn't seem to matter how fast or slow I fed the cutter the cut just proceeded with no drama. I did take the precaution of making sure the blade was exactly square to the cut, dead on centre and made a clearance cut after entering about 10mm. I'm totally convinced!!!!! I suppose there is a chance with my luck that next time will not go so easily but from my current observations I dont think so.
As mentioned above the only problem with MEGHR1616-2 holders is they are limited to about 30mm diam rod.
Have just ordered some MEGHR1616-3 holders to try and looking for a holder with greater depth of cut. Both holders and Korloy inserts are available on AliExpress very cheap
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The next photo compares the inserts. Note the jump in size between MGMN200-G and MGMN300-G and how short the GTN inserts are. I think the MGMN300-G inserts will be much more robust. I haven't received my MEGHR1616-3 holders yet but I am hoping they will have a greater depth of cut.
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Top - MGMN200-G NC3020
Next -MGMN200-G H01
Next - MGMN300-G NC3020
last GTN2 Unknown grade.

Don't give up you will get there
Ron
 
Owning a flexi 9x20 I went through the same woes as your self.
I have cured my problems with parting to the point where I just sling the tool on and go.
A couple of things I had to do to get there.
1, a thin brown paper shim under the axa tool post so it wouldnt turn around the post.
2, tighten the cross slide gibbs. Holding my finger on the join between the cross slide and saddle I could feel the two separate when cutting. A new brass gibb strip cured this.
3, watching the considerable flex in the tool post when parting I decided to remove the compound replacing it with a solid plinth. I now only replace the compound to cut small tapers.
4, use a continuous supply of cutting fluid to the tip of the parting tool.
The combination of all these has allowed me to part with no worries and to the point where I dont even have to ensure the tool is EXACTLY at 90 degrees to the job.
I've parted a couple of stainless and mild steel parts with a slight cone/curve. (I dont advocate this but it does do it if I'm not exact and works)
With the continuous supply of fluid the swarf just peels out in long tight curls.
I forgot to add I only use high speed steel parting blades.
 
Most if not all small LATHES will never part off a piece of metal. My reason for saying this they lack the strength to keep the parting blade from flexing. First the tool rest moves , compound flexes , the cross slide moves the ways even torque over. If you ever expect to part you will need the tool blocked up from the ways with a roller bearing to support the blades movement up to the point of just touching the piece to be cut . Rigid as possible no flexing at all then the blade still will flex and may break. I'm in the design stage of making a rotating saw blade mounted to the tool post , like a grinder , only mounted in a qctp with air fitting . My thinking with the lathe turning and blade turning no flex of any amount should make much difference. I'm letting the cat out of the bag but I'm sure it works.
 
I am not sure I agree. My small south bend lathe does very well at parting. Admittedly it was learning how to properly part. My problem was being a fraction too low, thinking it would cut better. I learned after a few mistakes that being low was bad , being low often caused a disaster since the piece would lift and all hell would break loose. Being high by a small amount was acceptable until it stopped cutting, it would not lift.

Being dead on, would be best. But there might still be a lift danger .. So being high by about .0005 might not be a bad thing. Being dead on, I still worry .. Being low.. never.
 
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