and is from knowledge?
Parting tool chatter
- Thread starter OrangeAlpine
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Yes the tool did tuch.
Richard Hed
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Absolutely! My 8tpi even with the setscrew set tightly will come loose and make a disaster. However, I am waiting, praying and waiting for a new lathe with which I can test this rear cutoff idea out. In the mean time, some times I can cut maybe 100 thou in but I have to make a cut about half the blade thick extra, that is, I use blades about 50 thou so naturally, I needs a cut about 75-100 thou wide which virtually eliminates side friction problems. Also, sometimes, I have a problem (where I needs to eliminate all the probs spoken of above) in which I must be some how work hardening the material for it acts as if it has a tough skin. Once I gets thru the "skin" it cuts well but I have to feed faster than I would like. When I first started doing cut offs, I broke more blades than I could count (in Chinese anyway.)
Anyways, Orange Alpine, can you tell us more characteristics of the chattering? Slow speed?, special type of metal? cutting oils? any thing else?
I use very thin blades
It gives better cut. The down side is the blade may brake if move the carriage
Dave
It gives better cut. The down side is the blade may brake if move the carriage
Dave
One thing that I have both read and experienced is that sometimes chatter can be eliminated not only by slowing the rpm but also by increasing the feed. This is of course a matter of balance! Someone else mentioned using power feed for consistency, and I've read that elsewhere ... but haven't had good results doing that on my lathe. I do better with hand feeding, being able to push it more when I need to "dig under the chatter," then pull back to clear chips.
On the rigidity of an upside down parting blade - this is something that gets beat to death, with proponents for and against. On the one hand, using the parting tool right side up seems like it would be more rigid because the pressure is down onto the bed. On the other hand, there is an argument to be made that the lifting pressure of a reverse mounted setup increases the wedging action of the crossfeed dovetails, thus making it more rigid.
I would tend to say, try both and see what works on your machine. My suspicion is that this is not a one-size-fits-all decision.
On the rigidity of an upside down parting blade - this is something that gets beat to death, with proponents for and against. On the one hand, using the parting tool right side up seems like it would be more rigid because the pressure is down onto the bed. On the other hand, there is an argument to be made that the lifting pressure of a reverse mounted setup increases the wedging action of the crossfeed dovetails, thus making it more rigid.
I would tend to say, try both and see what works on your machine. My suspicion is that this is not a one-size-fits-all decision.
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There lay the learning process. Is a fine line between too aggressive a feed and digging in and to timid a feed and scraping which result is chattering.
Think of the tool/part as the Fiddlestick/String relationship. A gentle rubbing plays the violin, I have no experience on what a heavy paw does to the violin.
Like everyone starting out I was chattering, digging and busting tools to no end until, by a slow learning process I learned to grind the tool right, slow down and keep that curl coming out. It was a white knuckles, clenched teeth experience until learning the stay in the right conditions envelope.
We can write and share but is one of those thing one has to learn the feel.
As for the rear post or equivalent Upside down/reverse method the principal issue has nothing to do with altered rigidity of the set up.
Consider the tool post as being hinged down on the cross slide toward you.
By pressing on the cutting edge the tool goes Down AND Advances toward the part. This increase the downward pressure increasing the dig in until either the chip is cut or the tool breaks in the former case the cycle restarts.
With the tool upside down, an increase pressure lift the tool and pushes it away from the part until an equilibrium is reached.
In other words the two situations can be explained in term of positive feedback versus negative feedback.
Speed should be 2/3 of your cutting speed, don’t go slow, it makes it worse! Check your chuck to ensure you have zero play in the job, you would be surprised as to how many lathes have small amounts of play in the head bearings and have bellmouth chuck jaws. Use a TDI to check, and put some decent pressure on the job to check it. I’ve probably broken 2 part off tools in 40 years of machining , center height and crappy jaws. If you cant get rid of the play then you will need to set up a rear mount parting off system, it’s the only way you can get old lathes to part off sometimes. As to feed to part off at, that should be the same as your facing speed, just slow it down near the last part. Use a catcher to keep the parting off safe, I.e. a tool up the middle for a bush, or a bush over the outside for a solid part. Tools angles? I’ve seen nearly every angle you can imagine, but high front clearance (7 to 10) is good with a top rake to suit the material. Center height is critical, I go slightly high to allow for tool flex in older holders, lathes etc, but only half a thou. Hope this helps. At high speeds the tool loads quickly so you need to feed to maintain that load. Any play in the cross slide will kill your tool.
"Consider the tool post as being hinged down on the cross slide toward you.
By pressing on the cutting edge the tool goes Down AND Advances toward the part. This increase the downward pressure increasing the dig in until either the chip is cut or the tool breaks in the former case the cycle restarts."
I have a Drummond Roundbed lathe.
Discuss....
By pressing on the cutting edge the tool goes Down AND Advances toward the part. This increase the downward pressure increasing the dig in until either the chip is cut or the tool breaks in the former case the cycle restarts."
I have a Drummond Roundbed lathe.
Discuss....
Timehunter
Member
I agree with Awake and tornitore45, plus good ideas rom WOB, Fabricator, gpldstar31, Peter Twissell, and smith door except for the thin blade part.
The OP has not posted again so I wonder if he found the problem or is still having the problem.
I always use slow to really slow rpm and hand feed.
For cutting fluid I use an acid brush dipped in dark thread cutting oil and hold it on the part just above the parting blade.
Make sure it is not sticking too far out the chuck and the tool as close to the holder as possible.
I hate having to use (and seldom do), a live center in the tailstock for support as when you get close to the end things get dicey.
And can not stress the importance of what was previously posted as to keeping your machine well oiled and adjusted.
Good luck!!!
The OP has not posted again so I wonder if he found the problem or is still having the problem.
I always use slow to really slow rpm and hand feed.
For cutting fluid I use an acid brush dipped in dark thread cutting oil and hold it on the part just above the parting blade.
Make sure it is not sticking too far out the chuck and the tool as close to the holder as possible.
I hate having to use (and seldom do), a live center in the tailstock for support as when you get close to the end things get dicey.
And can not stress the importance of what was previously posted as to keeping your machine well oiled and adjusted.
Good luck!!!
justintime
Active Member
Primary importance is properly adjusted spindle bearings including preload. Solid tool & holder with “textbook” grind ( On Center ) will help you get the best results. Good luck.
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Nothing wrong with using the tailstock center, but make sure you pull it back before cutting trough. If the cut off piece is heavy protect the ways with the protector you have built. You did build a way protector for when changing chucks, did you?
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I am not familiar with the construction details of your lathe but the hinge I was referring to is not a real hinge is just a mental picture. The actual hinge is distributed all along the "beam" that connects the bed to the cutting edge. No matter the shape and construction, the beam flex under strain.
In the round be lathe, the 'virtual' hinge axis is directly below the spindle axis, so downward pressure on the front mounted tool tends to push it away from the job. Conversely, upward pressure on a rear mounted tool would tend to push it into the job.
The location of the virtual hinge axis will vary with the design of the machine and the relative stiffness of the components, so it is not surprising that we see a variety of individual experiences with the benefits or otherwise of rear parting tools.
The location of the virtual hinge axis will vary with the design of the machine and the relative stiffness of the components, so it is not surprising that we see a variety of individual experiences with the benefits or otherwise of rear parting tools.
goldstar31
Well-Known Member
I would disagree. This is one of the jobs which a fixed steady is recommended
Having had the experience of production parting off 4" solid mild steel stock with a 3/8" blade on a "monster" Bardons & Oliver cut off lathe down through various size autos to my Chinese 20mm spindle hobby lathe - that can barely part off anything - let me add my 2c worth.
Firstly the laws of physics are out to get you - cutting forces reduce with increasing velocity - that creates the very propensity for all cutting operations to chatter - hence you need rigidity to avoid flexing.
If the whole shebang flexes (there will always be some) then the cutting speed reduces and the cutting force force increases - thus further reducing the speed and increasing the force until the machines' resultant forces "win" and the reverse happens - hey presto chatter.
So obviously anything that reduces the forces and increases rigidity is going to help.
Use the narrowest blade that will safely do the job with the minimum of stick out for the job.
Part off as close to the chuck as you can - my experience tells me you get less chatter on a 4 jaw and less again on a collet chuck or profiled soft jaws - but I'll be hornswoggled if I can give a rational explanation for that. I suspect three pressure points induce vibrations that just simply go from bad to worse - more support - less propensity to vibrate ?
Upside down (backslide) or reverse parting can help inasmuch that it helps to clear swarf - but you can achieve the same thing with a swarf breaker - the double beveled edge on the top of the blade can be left as is - i.e. don't sharpen the top but instead use a holder that gives the top rake by holding the blade at that angle.
I went to the trouble of making an upside down (backslide) part off holder and my Chinese lathe positively hated it - some slideway designs won't cope with it either.
If you must use the blade flat, you can grind in the rake on the corner of the wheel to hollow it out - both methods "narrow down" the resulting swarf thus allowing large clock spring swarf to accumulate without seizing. I prefer the "hollowed out" to the "double bevel" approach to "narrowing" the swarf.
Carbide part off tip - with hollow configuration to reduce the swarf width to less than the groove width. Hobby lathes are typically not powerful or rigid enough to use such tooling.
Parting off on an under powered and insufficiency rigid hobby lathe is a somewhat hit and miss affair. Typically I pull the bade out if I sense the swarf is being "chomped" down onto the blade - I then offset the blade with the compound slide ±0.2mm so that I'm working a part off groove wider than the swarf - left to right in a series of cuts with only the last cut to part off being on size (if at all - for once-off parts you are normally going to turn it around and second op. it).
A "pip-free" front clearance angle increases the cutting width and projects the swarf towards the component - neither of which are desirable - but sometimes it works well but not always - that said it is my default part-off configuration - in a production environment you want to mass produce pip free parts - but I will switch to a straight face on problem materials.
Typically cut off at half or less normal SFM and going slower or feeding harder is my go to when it starts to chatter - powerfeed if you have it.
Use lubricant - a PITB with no coolant pump and handfeeding - you need more hands.
On some materials (Stainless) an aggressive front rake works well but normally to be avoided.
Not everything works every time - let experience be your guide.
Regards - Ken
Firstly the laws of physics are out to get you - cutting forces reduce with increasing velocity - that creates the very propensity for all cutting operations to chatter - hence you need rigidity to avoid flexing.
If the whole shebang flexes (there will always be some) then the cutting speed reduces and the cutting force force increases - thus further reducing the speed and increasing the force until the machines' resultant forces "win" and the reverse happens - hey presto chatter.
So obviously anything that reduces the forces and increases rigidity is going to help.
Use the narrowest blade that will safely do the job with the minimum of stick out for the job.
Part off as close to the chuck as you can - my experience tells me you get less chatter on a 4 jaw and less again on a collet chuck or profiled soft jaws - but I'll be hornswoggled if I can give a rational explanation for that. I suspect three pressure points induce vibrations that just simply go from bad to worse - more support - less propensity to vibrate ?
Upside down (backslide) or reverse parting can help inasmuch that it helps to clear swarf - but you can achieve the same thing with a swarf breaker - the double beveled edge on the top of the blade can be left as is - i.e. don't sharpen the top but instead use a holder that gives the top rake by holding the blade at that angle.
I went to the trouble of making an upside down (backslide) part off holder and my Chinese lathe positively hated it - some slideway designs won't cope with it either.
If you must use the blade flat, you can grind in the rake on the corner of the wheel to hollow it out - both methods "narrow down" the resulting swarf thus allowing large clock spring swarf to accumulate without seizing. I prefer the "hollowed out" to the "double bevel" approach to "narrowing" the swarf.
Carbide part off tip - with hollow configuration to reduce the swarf width to less than the groove width. Hobby lathes are typically not powerful or rigid enough to use such tooling.
Parting off on an under powered and insufficiency rigid hobby lathe is a somewhat hit and miss affair. Typically I pull the bade out if I sense the swarf is being "chomped" down onto the blade - I then offset the blade with the compound slide ±0.2mm so that I'm working a part off groove wider than the swarf - left to right in a series of cuts with only the last cut to part off being on size (if at all - for once-off parts you are normally going to turn it around and second op. it).
A "pip-free" front clearance angle increases the cutting width and projects the swarf towards the component - neither of which are desirable - but sometimes it works well but not always - that said it is my default part-off configuration - in a production environment you want to mass produce pip free parts - but I will switch to a straight face on problem materials.
Typically cut off at half or less normal SFM and going slower or feeding harder is my go to when it starts to chatter - powerfeed if you have it.
Use lubricant - a PITB with no coolant pump and handfeeding - you need more hands.
On some materials (Stainless) an aggressive front rake works well but normally to be avoided.
Not everything works every time - let experience be your guide.
Regards - Ken
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The big problem : is with lots of helpers but the questioner has solved the problem yet ?
I saw some people ask questions and then disappear.
I saw some people ask questions and then disappear.
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BaronJ
Grumpy Old Git.
Yes this seems to be the way of the world ! Few reply with thanks for solving my problem.
I found most do not want to grind rake angle to the top of cut off blade.
After running turret lathes and screw machines this was only way to take out chapter. This also works on wide form cutters too.
Dave
After running turret lathes and screw machines this was only way to take out chapter. This also works on wide form cutters too.
Dave
