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Willy
01-06-2018, 11:36 PM
We get a lot of threads here related to issues when parting off, especially in regards to the smaller lighter weight lathes.
One of the key recommendations regards increasing the rigidity of the parting tool setup. There are various factors here that can lead to chatter if one is not diligent when addressing any number of sources of slop or flex in how the parting tool and it's attending hardware are setup in order to increase the overall rigidity.
I realize this is not as much of an issue in larger more rigid lathes but anything one can do on the smaller lighter lathes has got to be an advantage and a step in the right direction.

I stumbled upon a Youtube video this afternoon that looks like it might be an easy fix for a lot of folks that are looking for another tool to use in the fight against chatter. Who knows some here may have already tried this technique.
The link to the video below describes the use of a support used underneath a carbide insert parting tool used in conjunction with a simple support plate that straddles the lathe bed.

https://www.youtube.com/watch?v=lGPM_XuufLk

danlb
01-07-2018, 12:00 AM
That's an interesting technique. I don't think I would use it.

Both of his parting tools had a lot of overhang, much more than was needed for a 1 inch bar. Something was not very tight in the carriage/cross slide / compound areas. The tool was not bending, but you could see the tool post dip to the side. When mine did that I tightened the crossslide gibs and it stopped that behavior right quick. :)

MattiJ
01-07-2018, 02:26 AM
That's an interesting technique. I don't think I would use it.

Both of his parting tools had a lot of overhang, much more than was needed for a 1 inch bar.

Common problem with small lathes and insert parting tools. The Shars tool shown seems to be particularly bad.

JCHannum
01-07-2018, 08:57 AM
He also appeared to be hand feeding. Use power crossfeed for a consistent feed and learn to "ride the chip". You should have a nice ribbon continually curling from the tip of the cutter. It can be done by handfeeding, but use both hands and keep the tool moving with a smooth, aggressive feed.

Lew Hartswick
01-07-2018, 10:14 AM
And his tool was NOT on center. The bump left on the stock was quite obvious . I get the kids in high school to do better after a few tries.
...lew...

Dan Dubeau
01-07-2018, 10:25 AM
Another mousetrap...

metalmagpie
01-07-2018, 10:38 AM
I greatly improved parting on my last 9" South Bend. I installed the T-slot table and rear toolpost holder from MLA. Then I fitted an Armstrong parting tool upside down, held in the rear toolpost setup. It really worked a lot better.

metalmagpie

Willy
01-07-2018, 10:42 AM
I too noticed that there was definitely an issue with both his technique and his tool. For example he uses a shorter chuck in order to get closer to the headstock then uses a poorly designed parting tool that sticks way out into space. Ive seen that type of tool on these pages before so I'm sure he's not the only one to have bought one.
The point to posting his link though was to show that his answer to the chatter issue did make a huge difference in the quality of the cut, in spite of the other issues.
It should taken as an example of another possible tool that can be used to quell the chatter when the possibility of doing so exists. It should not be taken as a verbatim guide as to how to part off.

rickyb
01-07-2018, 07:33 PM
I greatly improved parting on my last 9" South Bend. I installed the T-slot table and rear toolpost holder from MLA. Then I fitted an Armstrong parting tool upside down, held in the rear toolpost setup. It really worked a lot better.

metalmagpie

Success on my 10" Logan began with the purchase of an aloris tool post and hss cutoff blade holder. Just what the video said doesn't work. Final move was to grind a small trough in the top of the blade. Trough folds and rolls the chip which keeps it from binding. Soft materials like brass and aluminum I cut at full speed. Steels at half speed including drill rod. All manually fed. No issues of any kind. Cut off is now a goto method rather than a cringing last resort method.

BCRider
01-07-2018, 08:09 PM
Start by fixing the obvious stuff. Much of it was mentioned above already.

Things that will help in no particular order are;


Adjust the gibs on the cross slide and compound slide if you use one so they are snug but you can still move them. The compound in particular should be noticeably stiff to turn the wheel.
Move the compound back so that your tool post is well supported down through the dovetails and directly into the bed
Consider removing the compound and making up a large solid spacer to mount in its place. The compound on smaller machines like shown in the video can be surprisingly flexible even when the gib is correctly adjusted. Another member here is the one that champions the idea of switching to a solid spacer replacement for general turning where you don't NEED the angle travel of the compound.
Be sure the lathe is bolted down to a good solid bench or stand. This not only aids with the rigidity issue it also gives you a good basis for truing up the bed so it cuts parallel
Run as slow as you can.
Use a fairly narrow parting tool. Convert less metal to chips and the machine will see less torque and other forces from the cut. I'm actually a fan of HSS for this reason. But it has to be a good blade in a good holder to be a good option.


I saw that video a while back and my first thought was that the idea was fairly clever. I'd have done the support slightly differently but the idea is actually quite decent. It's a kludge solution of course. But it's a kludge that could greatly aid those with small lathes with the same problem of flexing due to their light weight. Especially when equipped with QCTP's that tend to accentuate the overhang issue like the one in the video does But be sure that other stuff, the proper gib screw settings in particular, are done first.

JCHannum, not a lot of smaller lathes have a power cross feed so it may not be an option.

JCHannum
01-07-2018, 08:21 PM
JCHannum, not a lot of smaller lathes have a power cross feed so it may not be an option.

Which is why I made the comment regarding hand feeding. The object is to keep the tool moving in the cut.

BCRider
01-07-2018, 08:29 PM
Which is why I made the comment regarding hand feeding. The object is to keep the tool moving in the cut.

Good point.... which I missed.... :)

thaiguzzi
01-08-2018, 12:55 AM
Start by fixing the obvious stuff. Much of it was mentioned above already.

Things that will help in no particular order are;


Adjust the gibs on the cross slide and compound slide if you use one so they are snug but you can still move them. The compound in particular should be noticeably stiff to turn the wheel.
Move the compound back so that your tool post is well supported down through the dovetails and directly into the bed
Consider removing the compound and making up a large solid spacer to mount in its place. The compound on smaller machines like shown in the video can be surprisingly flexible even when the gib is correctly adjusted. Another member here is the one that champions the idea of switching to a solid spacer replacement for general turning where you don't NEED the angle travel of the compound.
Be sure the lathe is bolted down to a good solid bench or stand. This not only aids with the rigidity issue it also gives you a good basis for truing up the bed so it cuts parallel
Run as slow as you can.
Use a fairly narrow parting tool. Convert less metal to chips and the machine will see less torque and other forces from the cut. I'm actually a fan of HSS for this reason. But it has to be a good blade in a good holder to be a good option.


I saw that video a while back and my first thought was that the idea was fairly clever. I'd have done the support slightly differently but the idea is actually quite decent. It's a kludge solution of course. But it's a kludge that could greatly aid those with small lathes with the same problem of flexing due to their light weight. Especially when equipped with QCTP's that tend to accentuate the overhang issue like the one in the video does But be sure that other stuff, the proper gib screw settings in particular, are done first.

JCHannum, not a lot of smaller lathes have a power cross feed so it may not be an option.

You've mentioned 6 points.
Points 2 & 6 in particular are important on the less rigid lathes out there (Southbend 9 et al) and help out a lot...

Arcane
01-08-2018, 08:28 AM
When I bought my 9" South Bend it came with a lantern style tool post and a parting tool holder like this

https://images.machineryhouse.co.nz/products/L073/375/Main.jpg

I was never satisfied with the way it performed so one day I bought a Sandvic blade and holder.

https://engineeredtooling.com.au/image/cache/data/parting%20blade-350x350.jpg
https://engineeredtooling.com.au/image/cache/data/parting%20tool%20holder-350x350.jpg

I made a substantial plinth to replace the compound and it mounts to the cross slide in the exact same manner. I attached the blade holder solidly to it and it is a great deal more rigid then what I had. Parting is now easy-peasy.

I can exchange it with the compound in a very short time, less time than the fellow in the video took to attach his support to his cutoff tool.

BCRider
01-08-2018, 11:41 AM
So that would be the big solid spacer suggested by some.

It's worth nothing that a plinth or spacer to replace the compound would likely benefit by having the tool post mount off center a bit to the front and tail of the lathe. So when the tool holder is mounted and the tool extends out that it sticks out past the spacer by only a little instead of a lot. And "a lot" would be the case if the tool post sat on the spacer in a way that was centered.

Benno
01-08-2018, 01:15 PM
Willy, one thing I noticed right away was his cut off speed was way to high. On a small lathe I would not go over 120 RPM, actually I would try to keep the speed by 100 RPM and use a steady drip of cutting oil while cutting and be easy in bringing the cut off blade toward the cut off material. The cutting edge of the cut off blade should kept close to the rigid tool post, but this was indicated nicely by "danlb"

BCRider
01-08-2018, 01:42 PM
Some of the smaller lathes don't have the benefit of a back gear though. Although the South Bend in the video with the support leg certainly does. And you're right, he should have been using the slower speed.

It would not have made up for the cutter being pulled down by the lack of rigidity though. Flexing like that is never a good thing and it can easily lead to the cutter being sucked under the work when getting near to the end of the cut. Those points I made above would help but on something fairly flexible it won't be a total cure.

One option that could help is to loosen and remove the QCTP and replace it for parting operations with a custom made tool post that is dedicated to parting. With that sort of design the cutter can be well back in a tool post "block" that has a semi circular cutaway so that up to some reasonable diameter the tip of the cutter is supported over the extended foot of the cutter block. And the whole block AND cutter is located within the area of the compound or replacement solid spacer.

In the end it's all about minimizing the overhang and flex in the machine. And that's the root issue. The smaller and lighter machines are like rubber bands compared to the big heavy stuff. So extra care is needed... or suitably effective tricks such as the support leg and cross bed plate shown in that video.

Willy
01-08-2018, 02:30 PM
As in every other aspect of machining, choice of tool holders is a compromise. The insert holder shown in the video is an "R" version, this allows the user to get close to the chuck which is a plus. However the offset of the overhang it offers is not conducive to stability. It would be interesting to see how the cut would progress with an "L" insert holder that centers the cutting forces. Even with the additional workpiece stickout I would be willing to bet that the rigidity would be much improved.
Learning to center the tool and feed it smoothly manually would help as well. As mentioned lots of factors to contend with while parting.

enginuity
01-08-2018, 02:37 PM
If rigidity is a huge problem just get rid of the compound.

I have a 10x18 import bench lathe. I part off regularly using HSS part off blades with no issues. I sorta took offense to him saying that his carbide Shars tool is better than any HSS tool he has tried. I'm not anti carbide - but in the home shop parting off smaller diameter material (like he was doing) a properly sharpened and honed HSS blade should have no issues. In fact I find a lot of carbide isn't nearly as sharp as it should be for use on light equipment.

You need to get the centre height properly adjusted, part off close to the chuck, slow spindle speed and constant feed. There is also a feel component to it. You can feel when something isn't right. Just jamming the tool in and hoping to part of is asking for trouble. Observe the way the chips are coming off, and how much force is required.

One thing to note is I've seen a few QCTP part off holders that are not machined properly to hold the HSS blade.

The upside down mounted part off tool works really well as well.

BCRider
01-08-2018, 03:12 PM
A lot of smaller lathes such as the South Bend in the video linked in the first post do not have a flat spot on the cross slide to allow for a rear mounted tool post to take the upside down cutter.

Cenedd
01-08-2018, 03:25 PM
Not sure if I'm helping or just adding noise but I looked at that same video after I'd snapped a 1.5mm tapered cobalt blade a few times. I bought myself an MGEHR part-off holder with MGMN200 tips - it wasn't a lot more than replacing the cobalt blade that was not too short to hold - but the root of my problem was backlash in the cross slide. I only found this out when I tried drilling by holding the drill bit in the spindle with the part on the tool-post (this may sound about-face but it was to get the hole in the part at centre height for a tool holder - thanks to BCRider for the tip) and found the cross slide slocked back and forth too much to do anything more than spot it for position. Now I've found the hidden adjuster for the backlash, I can part off (with the carbide tool, to be fair) with power feed and no nervous waiting for the bang!

Edit: Lathe is a Sieg SC4 by the way.

DancingBear
01-08-2018, 03:34 PM
The grind on his HSS blade that he didn't like had a ridiculous front rake. Little wonder he didn't get the results he wanted with that.

Lots of good points in the above posts as well. His solution did seem to work for him, but I'd be concerned with the effect it would have on the center height of the tool.

One thing I've learned is that for small lathes a narrow blade helps a ton. I use a .040" P1N blade on my 7x14 (not exactly the paragon of ridged machine tools) and I really don't have much trouble parting.

JCHannum
01-08-2018, 08:24 PM
A lot of smaller lathes such as the South Bend in the video linked in the first post do not have a flat spot on the cross slide to allow for a rear mounted tool post to take the upside down cutter.

Metal Lathe Accessories offers castings for extended topslides for South Bend and Atlas lathes as well as a rear mounted toolpost for just this purpose. These can probably be adapted to some other machines.

Andy Loftquist's casting kits are of excellent quality with easy to follow plans and instructions. Anyone not familiar with his offerings should take a look at them. Lots of good stuff for the HSM.

http://www.statecollegecentral.com/metallathe/index.html

J Tiers
01-08-2018, 09:05 PM
The Sandvik type insert, in either a blade as shown, or the older shank type holder, will run rings around most others.

That said, I usually use whatever I have that looks about right, even if the top is flat. If you get it in-line with the crosslide movement, and on-center, it will work well if it is sharp enough to cut. The special ones come into their own when doing very deep part-offs (which I prefer not to do to begin with).

MattiJ
01-09-2018, 01:43 AM
Willy, one thing I noticed right away was his cut off speed was way to high. On a small lathe I would not go over 120 RPM, actually I would try to keep the speed by 100 RPM and use a steady drip of cutting oil while cutting and be easy in bringing the cut off blade toward the cut off material. The cutting edge of the cut off blade should kept close to the rigid tool post, but this was indicated nicely by "danlb"
His lathe didnít look that much smaller than my 11x24 kerry and I usually part at 1500rpm for that size of workpieces. Took some experimentation to make it work but if I just lower the speed I have to go down to 150rpm or so to cure the chatter.