Parting Tool leaves domed surface

[picclock]

Hi

I just bought a 10mm shank parting tool from Chronos. Its a little unusual as it uses the lathe toolholder post to clamp the blade. What is more interesting however is that it appears unable to part off in a straight line, leaving a domed surface. On the 17mm aluminium part the centre (excluding burr) was 0.6mm thicker than the edge. The remaining stock in the lathe was dished similarly, like a satellite dish.  I believe this is caused by  a 1/3rd blade width shamfer which causes uneven pressures to be applied to the side of the blade making it bend during parting. I have other parting tools but none have exhibited this behavior.  Pics attached, (17mm Aluminium with 2mm central hole) : 





Best Regards

picclock

[srm_92000]

Hi.
I have a similar type tool (8mm) and it works great.
From your pictures it looks like the blade has been ground wrong - the top should be flat with slight relief on each side formed by the blank geometry meaning only the front relief needs grinding to sharpen. both the blade that came with mine and a spare purchased separately are ground in this way, I'd send it back.
Of course make sure your carriage is locked when parting too.

[picclock]

Hi srm_92000

I have a spare blade and that is ground exactly the same way, if anything its slightly worse as the angle is 1/2 the width of the blade. I think its time to bin it and invest in a different type, hopefully one which is flat across the top. I might try grinding the top flat for now, but it is very annoying. Clearly not fit for purpose.

Best Regards

picclock

[Stilldrillin]

Hi Picclock.
Sorry, but it looks fine, to me!

The blade's chamfers are to locate with the holder's dovetail.
The blade needs it's initial grinding........ Front clearance, and top surface/ chip breaker.

Make sure the blade lies at 90* to the work axis.
My toolpost is set slightly askew, to acommodate my version of this parting blade.

David D

[DavidA]

I was having similar trouble when facing across.  The problem turned out to be cause by the top near corner of the tool not being sharp. It was trying to push the whole table towards the tailstock. I have no way of locking the crosslide other that engaging the leadscrew or tightening up the gib strips.
I re-ground it and the problem went away.

Dave.

[90LX_Notch]

It's hard to tell from the picture, and I'm not familiar with that holder,  but it looks like the holder is upsidedown.

[DavidA]

More likely that the blade is upside down in the holder.(which will make the whole thing upside down.)
Dave.

[90LX_Notch]

Exactly what I meant.  If the holder is upside down, the tool is mounted wrong, making everything wrong.

[picclock]

@ Stilldrillin
I have thought about grinding the top flat but that would have to be done for some length of the blade else the blade would be wider on the unground section. Just seems like a poor design to me.

@ DavidA
Crosslide is always locked when I'm doing facing or parting. I modded the lathe so its only a quarter turn of an allen key which is always fitted so I don't have to find it 
Sharpened blade with India stone on receipt of tool. Its good and sharp and cuts really well. If the blade had been ground to have an equal shamfer on either side it would work a treat. Used my larger parting tool on the same piece of stock and the surface is dead flat with minimal machine marks. I wanted a thinner blade because I figured when parting steel there would be less load on the machine. Its a 7x14 Real Bull.

@90LX_Notch
 The holder is completely symmetrical. What you can see in the picture is the packing I used underneath to get the correct tool height.

I have ordered a different type with an equally thin blade which is advertised for a myford so it may need a bit of adjustment. For now I'll just carry on with the big one (1/2" x3/32 blade).
The part I used it on is effectively scrap which is a pain as it has a 60 degree taper 1.50mm thick for retaining an oring. Quite a tight tolerance part as the oring only protrudes 0.2 mm. 

Best Regards

picclock

[NickG]

That looks the same as my tool, you only need to grind the rake angle on for about 1/4 to 1/2" in length and it'll depend what you are parting. Aluminium about 40 deg rake, steel 20, brass, cast iron 2 or even none. Think tool will be fine.

[raynerd]

I`m with Nick, does that tool not need some rake grinding on it??

Chris

[Fergus OMore]

If it is a domed cut, the tool is either not at right angles to the lathe bed or the tool is ground or fitted wrong.

What you should be looking at is a top which has a female- not a male curve ground in.

This is GHT stuff but that is the way that mine is ground so that any swarf literally narrows as it comes off- and does not bind or jam.

Frankly I'd dump it and fit a rear tool post with a cutting tool inverted and pointing at 7degrees.- to avoid having to grind in a curve along the top.

That's what I had and have had unchanged through a series of lathes- and I'm still on the same blade. Others can comment or disagree. Fine by me

Norman

[Jonny]

Theres no relief on the one side and barely enough on the other.
Have or did have a much larger one than that and did exactly the same if kept ploughing in, me i didnt as could see quickly the tool was flexing an all too common thing from cheap asian rubbish.

No substitute lash out for Iscar, Ceratizit, Arno, Seco etc you will wonder why you struggled for decades.
I now have 5 or 6 permanently setup for different tasks ie left hand facing cut, neutral, varying widths, depths etc.

[awemawson]

I agree that an insert style parting tool is a dream to use, but there is nothing wrong with the tool the op has, it just needs grinding.

The blades as supplied are blanks that need the front and top rake and clearance ground. This is very easy to do on an off hand grinder. The design is a copy of the Eclipse ones that have been sold for decades, and again they needed finish grinding. After all you will want it ground differently if you want the fair face to be on the stock removed  than if the stock in the chuck needs the fair face. ie right or left cutting. With the insert style you still need to choose left, neutral or right hand when you buy them.

[rdhem2]

Mine cuts a dome also if I don't get it real square with the work.  I agree with the grinding.  The softer the material, the greater the dome.  Mine is a quik change, one is YAMASA, the other PhaseII, both interchangable.

[powderhorn01]

I got some parting tool blades from the Harbor freight cutting bit assortment.  Had quite a problem with them leaving a domed finish when parting.  Here is what I found when I finally figured out what was causing it.  These blades had the side taper on them.
When you looked at them new, the side profile looked like this /---/  end profile looked like this \/ on one end, other looked like this /\ .   If you mounted it the right way, the V is right side up, and no doming, mounted the other way /\ upside down, dome city.  Blades should have been side profiled like this /---\.
My 2 Cents.