parting tool geometry

[brendanh]

Ive been trying to part of some 35mm steel in my lathe, im running at about 150rpm and got the auto cross slider feed on 0.029mm/rev and it just isnt happening, stalled the lathe, tool dug in even snapped the blade off today. i did try to slow the feed rate down to no luck, which brings me to is the tool geometry right. whats the correct geometry for parting tools?

thanks

[simonl]

I used to have all sorts of issues with parting off. Similar to you, snapping blades and ripping the workpiece out of the chuck being the biggest issues. Then, there was a time when Stuart (stustoys) and I were talking shed stuff and he mentioned about reducing the back rake so that the tool is not so inclined to be pulled in to the work. I now grind my part off tools with very little back rake and it's pretty much fixed all my issues. I use a few other precautions too though. With large stock requiring deep cuts, I often part off in two places right next to eachother to create better tool clearance. I also withdraw the tool frequently to clear swarf, err on more feed rate rather than less, especially with stainless and I always use a form of lubricant, use the least amount of tool overhang as possible to get the job done and do it as close as possible to the chuck.

If you are using a part off blade that is thicker at top than the bottom, make sure you don't grind it upside down or it will not have the required clearance and it will grab. I don't use carbide part off blades, no need for my uses and they seem so expensive compared to a seemingly endless piece of HSS.

I still get nervous when parting large parts because of what used to happen, things would often seem OK and doing well and then at the last minute BANG! Bits everywhere! :doh:

Simon

[KBs PensNmore]

Any chance of pics to show what you mean, please Simon.:2tsup: Always interested in learning something new.

[.RC.]

lubricant, lubricant and more lubricant is a major secret of successful parting off with HSS...

[brendanh]

I used to have all sorts of issues with parting off. Similar to you, snapping blades and ripping the workpiece out of the chuck being the biggest issues. Then, there was a time when Stuart (stustoys) and I were talking shed stuff and he mentioned about reducing the back rake so that the tool is not so inclined to be pulled in to the work. I now grind my part off tools with very little back rake and it's pretty much fixed all my issues. I use a few other precautions too though. With large stock requiring deep cuts, I often part off in two places right next to eachother to create better tool clearance. I also withdraw the tool frequently to clear swarf, err on more feed rate rather than less, especially with stainless and I always use a form of lubricant, use the least amount of tool overhang as possible to get the job done and do it as close as possible to the chuck.

If you are using a part off blade that is thicker at top than the bottom, make sure you don't grind it upside down or it will not have the required clearance and it will grab. I don't use carbide part off blades, no need for my uses and they seem so expensive compared to a seemingly endless piece of HSS.

I still get nervous when parting large parts because of what used to happen, things would often seem OK and doing well and then at the last minute BANG! Bits everywhere! :doh:

Simon

have you got a pic of the tool?

i was using lots of cutting oil but still had no luck.

[simonl]

I'll duck out to the shed at take a pic of my blades. Bare in mind that there are many people far more experienced than me when it comes to (successful) parting off.

Simon

[brendanh]

I'll duck out to the shed at take a pic of my blades. Bare in mind that there are many people far more experienced than me when it comes to (successful) parting off.

Simon

no worries mate, see what the others have to say.

[Techo1]

It could have more to do with the Lathe rather than the tool geometry, what type of machine are you using?

[brendanh]

It could have more to do with the Lathe rather than the tool geometry, what type of machine are you using?

its an old herless, basicly an al960b sold by hare and forbes

[simonl]

Here are my 3 blades I use. Looking at them they are a little tired and probably need a re-sharpen before next use but the geometry can be seen. The second pic is end on to show how the wedge shape provides clearence from the rest of the tool… assuming you don't do what I did and sharpen upside down so that the cutting is done at the narrowest (thinest part) and then siezes when fed in!

I also used to blame the quality of the steel too. But you can see that one of the blades is made in India and the others are China. Not top quality stuff but it still works for me. Also, I rarely use auto feed. I like to feel the tool cutting and vary the feed rate until it feels nice. It should feel like your cutting butter, if it doesn't then there is something not right. I find that feeding not enough can be worse than too much especially with stainless steel, it needs a nice positive feed rate or the tool will just work harden the work and rub and go blunt.

Simon 347741 347740

Edit: I just read Techo's response. The other things that you may want to look at are spindle bearings, cross slide and compound backlash and gibb adjustment but my experience showed the vast majority of my issues were user related such as tool geometry, feed rates, lubrication and tool rigidity. Also, don't forget to wind in your compound all the way. I do this because my Chinese lathe has a less than perfect compound and winding it in all the way adds to rigidity.

Cheers

[kwijibo99]

I've had similar issues, I reckon everyone has fun and games when they first use a parting tool, it really fires the pucker reflex when the tool grabs and shatters with a bang.
I generally have good results with the following:

For steel or staino no top rake at all and keep the tool lubricated with oil, not the milky soluble stuff, proper cutting oil if you can get it, I use the Trefolex CDT in a spray can. Never let the tool rub on staino or it will work harden. For 35mm free cutting steel 150-200 rpm will be good to start but speed it up as the cut gets deeper, should be around 400 by the time you finish. Lower speeds for harder steel alloys and slower again for staino, a VFD is at its most useful when parting.

For brass or bronze a bit of negative or no top rake, I don't use any lube and rpm can be a bit faster.

For ally or plastic, grind a bit of positive top rake (2 o 3 degrees) no lube for plastic and a bit of kero, diesel or cutting oil for ally, again higher rpm is ok.

In general, keep the front relief small to support the tip, I usually feed the tool straight into the grinding wheel and let the curvature create the front relief. Make sure tool is set 90 degrees to work and on centre or even a little below but never above. Keep tool extension from holder as small as possible, extend tool out as cut progresses if necessary. Again increase rpm as cut progresses, if speed is too low it will be more inclined to dig in.

Cheers,
Greg.

[brendanh]

Thanks for the pics. Ill try have a go at that. Because i mainly use carbide im not really cluded up on what back rake ect ect is. Could somone help me out?

[.RC.]

My other take on it is front clearance should be kept as small as possible, this is what will stop the tool from digging in and is sort of you depth of cut restrictor..

A real learned person like Steamwhisperer may have the most parting secrets to give up...

I gave up with HSS and went to carbide inserts... :D

[simonl]

Thanks for the pics. Ill try have a go at that. Because i mainly use carbide im not really cluded up on what back rake ect ect is. Could somone help me out?

Have a look at these pics and then compare to my pics and it will give you an idea of rake angles and front relief. You can see that my tools have very little positive rake angle. Greg suggests zero or negative for brass or bronze but I rarely change my angles specific to material type, mainly cause I'm too lazy.

rake.jpg (http://ixian.ca/pics9/rake.jpg)

Toolbit.gif (http://littlemachineshop.com/Reference/images/Toolbit.gif) This is for a general lathe cutting tool but the front relief is the same concept.

Cheers,

Simon

[brendanh]

So thats the back rake..front releif i guess is the angle on the front of the tool

[cba_melbourne]

Parting depends a big lot on the lathe you are using. The rules that govern parting on a +500kg lathe are very different from those that govern a -100kg lathe. And a worn or not ideally adjusted lathe may behave like if it was a 5 or 10 times lighter machine.

I am assuming you are using a light hobbylathe, and not an industrial lathe with these tips:

- if your lathe has roller bearings, check that there is sufficient preload. Loose spindle bearings are often a cause (or the cause) of poor parting experiences.
- make sure when parting your saddle and topslide are locked. If there are no locks, consider adding lock screws.
- make sure the cross slide gib is adjusted for minimal play
- on nearly all bench lathes, parting is a LOT easier if you install the parting tool upside down and run the spindle in reverse. In this configuration, the parting blade is deflected out of the cut when it binds, not into the cut. On lathes with threaded spindle nose, you may achieve the same advantage with a rear tool-post and the parting upside down with the spindle safely running forwards.
- parting blades with a T profile (the blade has a T shape when you look at it from the front) are a lot easier to use on light bench lathes. They cost more though.
- if you can afford it, a carbide insert parting blade is the best there is for ease of parting on a light bench lathe. They cost a lot more though.
- the wider the parting blade, the harder the parting on a light bench lathe. It is like using a form tool, you quickly exceed the rigidity limits of a light lathe.
- the blade needs to be exactly on center height, as exactly as you can adjust it. If you must err, err on the low side.
- part off as close to the chuck as possible
- if your lathe has variable speed, you have a great advantage. Use it: when beginning the cut, search for a speed that does not chatter, then increase rpm on the fly as you advance c loser to the center of the workpiece to keep cutting speed approximately constant - always avoiding any speed range that causes chatter.
- on a not so rigid and possibly worn out lathe, avoid stopping the feed until the workpiece being cut falls off. Stopping or slowing the feed is the situation most likely to cause the tool to bind and break.
- Use coolant for difficult materials and for deep cuts.
- make sure your tool is sharp.
- make sure your tool is perpendicular to the workpiece, not angled.
- if the workpiece you are parting off is supposed to have a through hole, drill this before you part off, it is easier
- stay away from incredibly cheap parting blades. I have personally had the pleasure of owning several Indian made parting blades that were not ground to the same height front to back - depending on what sort of clamping holder you use, that can cause the blade to vibrate as it cannot be clamped full length. I ended up chopping these blades into short pieces that were good enough for grooving.
- my personal experience: if using a normal, simple parting blade, never grind it on the top. If you feel like the top needs grinding, some other factors are probably wrong.

[Dingo Dog]

Parting off scared the poo out of me for years after seeing a big Colchester lathe at TAFE loose the top slide when an errant adult trainee allowed the swarf to build up in the cut. A huge bang, heads turned and gathered, top slide torn out of its mount.

Watching a youtube video earlier this year, I seen a yank talk through the correct speed, lube and design of the tool. Tried his method on solid round and was very happy with the result. No top rake at all, he gave warning to make sure your tool bit is kept sharp, lube wise, have been using thin hydraulic oil, works for me.

DD

[brendanh]

Thanks for all the tips ill give them all a go. So front releif angle keep to a.minmum. neutral/no back rack?

[simonl]

Sounds the go. Minising the front relief angle so as to control/minimise depth of cut is a good idea that had not occured to me.

Simon

[brendanh]

Thanks for the tips everyone, i ended up getting a t shaped parting blade. I parted of with great sucess, locked everything up made everything as ridgid as i could. Power feed in at aprox 100rpm and at chatter at all. I think the blade i was original using was just to thin, was about 2.5mm. New blade is 4. Thanks again