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The Ultimate Trick: Cutting off with the Mini Lathe

Pcdoctor

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#1
I have a Weiss 180 7x12 lathe, and had had a lot of problems cutting of, probably because of lack of rigidity. Then I fall over the above video on YouTube, and my problems were over.
I don't know if will work for everybody, but it works for me, and as I know a lot of people have problems with cut off, I hope it will help you too.
https://www.youtube.com/watch?v=-RZRq0olsxM

Happy New Year

from Denmark

Pcdoctor
 

Ulma Doctor

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#2
thank you Doctor!!!
from another(imaginary on my part)Doctor...:jester:
 

TTD

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#4
Thank you for the tip!

I too own a 7x12 mini and admit that parting off with the little guy has always been a bit troublesome for me...:banghead:. I'm definitely going to give this a try!

Happy New Year!!!
 

Micke S

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#5
I have difficulties to understand why the reverse cut in general is less prone to jam than a normal cut. The explanation that the tool is pushed away in reverse and sucked into the workpiece in normal turning sounds OK. But this would also happen or not for normal turning if the tool is above or under the centerline. The angle of the tool will also have a great effect.

So, it is easy to understand that the cutter location vs. centerline and it's tilt angle will have a great effect on releasing or jamming the cutter. The cutting force will however be reduced the lower the tool tip is and the more it is tilted down during normal turning. So it seems to be a balance of these parameters independent of the turning direction.

I don't buy the explanation that chips falling down by gravity will be a significant factor, unless the lathe is very weak like a mini lathe.

Someone who understands the fundamental reason(s) for reverse cutting better than me is very welcome to explain. I need a big cup of coffee now to reset the brain :))
 

Karl_T

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#6
I have a rigid lathe, so I've thought flipping the cutter over was BS. But I have seen the sickening "crunch" when a light lathe binds on cutoff. His simulation explains why that would be far less likely cutting upside down.

BTW, the insert he shows works WAY better on any lathe rigid enough to use it.

FWIW, my personal experience on the 7" asian lathes: cutoff with a hacksaw, then face your part.

Karl
 

Micke S

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#7
The simulation would have showed the same difference for normal turning if the cutter would have been over alternatively under the centerline. It is not a matter of turning rotation. Well maybe it is, but I don't get it very well :thinking:

The coffee I've just drunk didn't help at all...
 

chuckorlando

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#8
Think of it like a tire and a wood shim. Place the wood shim behind the tire. In one direction the wood has to travel under the tire (sucked in) and thrown out the other. In the other direction it simply gets pushed off the tire.


I sure whould not run a threaded chuck at speeds like that in reverse. When it unscrews it's gonna do it super fast
 

MozamPete

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#9
I sure whould not run a threaded chuck at speeds like that in reverse. When it unscrews it's gonna do it super fast
Yes! If you have a threaded chuck much better to build a rear tool post for the cross slide. Parting tool still goes in upside down but you keep the lathe rotating in the 'right' direction.

Would hate to contemplate the possible damage as the chuck unscrews, pushing and snaps the parting tool, then drops onto the bed ways.

PLEASE DONT TRY THIS WITH A THREADED CHUCK
 
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Micke S

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#10
That is very true but I'm not sure it applies for the turning cut. It would be like pushing the tool upwards and having the cutter on the lowest part of the workpiece and this is not the case. It works great for cutting cheese slides though. The phenomena seems to be related to the better rigidity for tangential cutters, but in a opposite way.

I do also avoid reverse turning by the reason you say. A rotating chuck that come loose is very dangerous, especially if it is a heavy beast and there are not many threads to hold it so it can happen within a fraction of a second.
 

chuckorlando

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#11
Well bubba, the more I think on this the harder it is for me to reason it. I think I might have to agree
That is very true but I'm not sure it applies for the turning cut. It would be like pushing the tool upwards and having the cutter on the lowest part of the workpiece and this is not the case. It works great for cutting cheese slides though. The phenomena seems to be related to the better rigidity for tangential cutters, but in a opposite way.

I do also avoid reverse turning by the reason you say. A rotating chuck that come loose is very dangerous, especially if it is a heavy beast and there are not many threads to hold it so it can happen within a fraction of a second.
 

TTD

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#12
FWIW, my personal experience on the 7" asian lathes: cutoff with a hacksaw, then face your part.
^^X2...that's what I've been doing for awhile now, especially with steel. A little more work/time, but way less nerve-racking and easier on the machine.

Although, after seeing this thread I just tried the "upside down/reverse" method a few minutes ago on some scrap pieces of brass/aluminum and have to admit it worked like a champ...not even a hint of chatter or diggin' in like before.

I "kinda-but-not-really" understand the theory behind it, but after trying it I can say that it definitely made a huge improvement...at least for me on my small, non-rigid machine.
 

Micke S

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#13
Well bubba, the more I think on this the harder it is for me to reason it. I think I might have to agree

I begin to think that this applies only for a tool holder or toolpost that swings to some degree as in the simulation. In such case it will combine hard cutting force with an automatic anti-jam function.
 

Karl_T

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#14
Although, after seeing this thread I just tried the "upside down/reverse" method a few minutes ago on some scrap pieces of brass/aluminum and have to admit it worked like a champ...not even a hint of chatter or diggin' in like before.
OK, have you got the balls to put a piece of 304 in your lathe? Then we'll know for sure. AND I haven't hurt my neighbor's lathe :rofl:
 

chuckorlando

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#15
The only thing I can think is it's pushing the tool post up instead of down which in effect makes the tool go back instead of in. Same principle as up side down tool on the back side of the work I believe.
 

MozamPete

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#16
It is working exactly the same as a rear mounted parting tool post which seems to be accepted as a better way to part off (at least on Myford lathes for which there are various designs around for home build ones and was available as a OEM accessory from the factory).

Other advantage of a rear mounted tool is that as it mounts directly to the cross slide you eliminate any possible slack/movement in the top slide (as well as obviously not having to run the chuck in reverse).
 
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TTD

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#17
OK, have you got the balls to put a piece of 304 in your lathe?
LOL...not having the cajones to try something new has never been an issue for me (not always a good thing!)...it's more the thinking it through/possible ramifications process I have trouble with!...:biggrin:

Being a newbie to machining I'm sure it has more to do with a lack of experience on my part, but ya, if I had some 304 I would try it, if for nothing else than to share a horror story with you all!:D
 

RVJimD

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#18
I have a little Unimat out in my unheathed shop that I ALWAYS had trouble with parting off! I may just bring that little sucker into the shop today and give this a try just to see. It would really make a big influence on my decision to use the little lathe.

stay tuned for a video...

jim
 

epanzella

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#19
The only thing I can think is it's pushing the tool post up instead of down which in effect makes the tool go back instead of in. Same principle as up side down tool on the back side of the work I believe.
I think you've got the answer. The only real difference is whether the toolpost leans towards or away from the cut under load. An inverted tool at the rear is the only way you can get the TP to lean away if you can't reverse the spindle (threaded chuck). With a camlock spindle an inverted tool in front with the spindle in reverse would act the same as an inverted tool in the rear with the spindle going forward. IMHO. They would both cause the load to lean the TP away from the work.
 

Wreck™Wreck

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#20
The only thing I can think is it's pushing the tool post up instead of down which in effect makes the tool go back instead of in. Same principle as up side down tool on the back side of the work I believe.
One of the advantages of tools on the back of the part (up side down) is chip control, the chip falls into the pan below the tool, it also allows more room for tools on the front side (on a lathe that only has one cross slide such as a gang tool machine).
 

basildoug10

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#21
Hi to all home machinist that like me, have a lathe that has diminished rigidity. Surprisingly, some folks are still not convinced that inverting the parting tool and running in reverse has no real benifitt. Well perhaps I can clarify this phenomenon. Think about your tool post and your parting tool that protrudes out on one side towards your work piece.
Now as the tool enters the work piece there is an immediate down force on the tool bit. Now because your tool bit overhangs on one side and is now being pushed downwards, the tool post bends towards the work piece and this forward movement increased the depth of cut, then the downward forces on the tool bit also increases, so the the tool post bends even more towards the work piece and you get this "run away" effect and it happens is flash, extreme chattering or a sheared off parting tool. Now let's invert the parting tool and run in reverse. As the tool begins to cut, there is an upward force now on the tool. This upward force bends the tool post away from the work piece and this effect reduces the depth of cut and this in turn reduces the upward force, and so on. So what we now have is a level of "self regulation". So without a doubt, on small lathes that don't have the rigidity of bigger machines, running in reverse with an inverted parting tool is a big plus. Please be careful as this procedure in NOT RECOMENDED for screw on chucks.
Regards Basil.
 

The_Apprentice

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#22
Congrats to the person who made an animated simulation to show this. This has confirmed the way I was trying to visualize this problem, after David Fenner wrote about this issue in his book for home machinists. From what I remember, when rear mounting tool posts came into fashion for larger lathes, this was a revelation. Especially for tasks of parting off. I believe he strenuously advises not to part off on a mini-lathe in the regular fashion, and this is specifically why.

It's one of many problems we will have to face with the mini-lathes... even a simple task of knurling has to be done with a specialized tool, as the standard practice would cause too much a load coming directly from the tool post.
 
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