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Backside parting tools

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mickri

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#1
I have been reading a lot about parting off and the problems associated with parting. To be quite frank I am not very good at parting off. After getting less than stellar results whenever I try to part off I usually end up resorting to my trusty hacksaw. Not pretty but it gets the job done and facing off the rough cut is no big deal.

A common solution echoed on this forum and others is to part off from the backside. I have searched high and low without much success looking for ways to mount a parting tool to cut from the backside. All I find are lathes that come with an extended cross slide with slots on the backside to mount a parting tool. But I have found nothing for your typical lathe like my craftsman 12x36. Not even DIY/homemade solutions.

The only thing that I can think of is to make a part that would fit on the dovetails of the cross slide to hold the parting tool. But how would you go about attaching this to the cross slide? Would it need gibs? Has anybody done this?
 

Rooster

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#2
Greetings, try parting with the lathe in back gear. I have an Atlas 618 and have no trouble parting everything from alum. to drill rod.
 

T Bredehoft

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#3
I have one of the notorious Mk2 Clausing lathes, its a 6" by 18 or so. I removed the compound, fabricated a piece of 3/4 by 4 by 8 or nine" 1018 steel to fit on the boss left behind on the cross slide. On top of it I mounted a four way tool post of my own design, and cut T slots in the back for rear mounted tooling. If you do this, study the clamping of the original compound on the boss, and duplicate it. I'''ve since found, after replaciang the plastic compound gibs with steel ones, that my setup is solid enough for parting from the front. I use the rear mounted tools for chamfering.
 

Dabbler

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#4
Mickri - parting can be frustrating, and requires some skill to do well. on large lathes like you see on the popular youtube channels, parting from the backside and mounting a tool makes a lot of sense. On a small late like the 12X36, even if you mounted a backside tool, chances are that your results won't be much better...

This is because the cross slide is fairly light weight in relation to the forces involved, and the backside parting puts upward pressure on the dovetails instead of the flatways. I'm sure you'd get more vibration, not less.

so... what to do? - well there's a lot of things to try and check - but the first places to look are:

1) is the tool exactly on, or a tiny bit below the centre line? if you have an adjustable QCTP, this is one place to experiment with.
2) Are you using a very sharp HSS parting tool? On a small lathe like a 12", using carbide puts more pressure on the machine, sometimes causing problems.
3) are you parting at the right speed? for a 1" piece of mild steel, anything over 200RPM can cause chatter, even on a bigger lathe. I'd recommend 100RPM to start.
4) always ensure your carriage is locked and that your gibbs are tight. that helps a lot as well.
 

mickri

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#5
I have tried both front and back gear. Just about everything I do is in the back gear and at the slowest speed. I typically make a mark on whatever I am trying to part off and try to advance the cross slide no more than .001 for each revolution of the mark. Nothing I try seems work very well.
 

T Bredehoft

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#6
.001 is too slow. You gotta push the tool. Keep putting more and more pressure on the tool, all of a sudden it will stop chattering and turn out a neat coil of steel. RPM slow, plenty of lubrication.
 

Boswell

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#7
I used to have a tough time with parting. pretty much only work in Aluminum and 360 Brass. I would break parting blades and nothing seemed to work. Then I broke down and bought a indexable parting blade that holds an insert similar to this one from Aloris https://www.mscdirect.com/product/details/09061888
It was very expensive however all of my parting problems ended the minute I started using it. Parting is no longer any thing that I worry about so while it was expensive, it was money well spent.
I do realize that parting seems to be one of the areas that what works for one person seems to not always work for others. Good luck in finding what works for you.
 

mikey

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#8
I have one of the notorious Mk2 Clausing lathes, its a 6" by 18 or so. I removed the compound, fabricated a piece of 3/4 by 4 by 8 or nine" 1018 steel to fit on the boss left behind on the cross slide. On top of it I mounted a four way tool post of my own design, and cut T slots in the back for rear mounted tooling. If you do this, study the clamping of the original compound on the boss, and duplicate it. I'''ve since found, after replaciang the plastic compound gibs with steel ones, that my setup is solid enough for parting from the front. I use the rear mounted tools for chamfering.
I would explore Tom's idea here. Without getting into a protracted discussion about how forces are distributed when parting from the rear, my experience suggests it is the best way to part on a small lathe that doesn't allow you to run in reverse (chuck limitations, etc). I have parted 2" OD steel on my little Sherline at 1200 rpm with a rear mounted parting tool and a little P1-N blade so little lathes definitely do benefit from a set up like this and I would look into it.
 

MrWhoopee

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#9
.001 is too slow. You gotta push the tool. Keep putting more and more pressure on the tool, all of a sudden it will stop chattering and turn out a neat coil of steel. RPM slow, plenty of lubrication.
This.

Chatter is caused by the tool deflecting downward and springing back. Feed it more, it goes down and stays there. It's harder to do when feeding manually.
 

macardoso

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#10
What you are missing here is the powered cross feed. Makes parting a breeze. I too invested in an indexable parting blade (Shars .087 GTN-2 1/2" height ~$35) and I have been very pleased with it. It is a little more limited in the max diameter to cut off compared to HSS blades, however I rarely need to part a 5" bar :).

I agree with the constant pressure and deeper feed suggestion above. Also make sure to add enough oil to make the chip not bind in the slot. Alignment of the blade is critical. Run it against an indicator to make sure it is straight.
 

mickri

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#11
I have tried just about everything. I only use HSS for parting and I sharpen it every time I use it. Faster spindle speeds. Slower spindle speeds. Slow to fast feeds into the work. Tool at, slightly below and slightly above center line. Grinding tool square and with a slight angle. I am sure that one problem is using the atlas/craftsman parting tool in a lantern style tool post. It is all I have to part with right now. Other than my trusty hacksaw.
 

mickri

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#12
I'll give the powered cross feed a try. What feed rate should it run at? I have a craftsman 12x36 lathe with a qcgb. I have been trying find out how the feed rate on the carriage relates to the feed rate on the cross slide. Again without success.
 

jdedmon91

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#13
I always power feed while parting. I have a G0750G so it’s not a large lathe. I use GTN 3 tools because I’ve had the best results with the 3mm blades instead of 2mm ones. Of course the larger insert blades are thicker that supports the insert.

I rarely run over 70 rpm in steel because the next speed step is 200 rpm. Now in aluminum I do run 200 rpm I think the feed rate is .003 per rev. Use plenty of cutting oil even on aluminum to prevent chip galling.

Now parting something small diameter and flexible will give me fits. So I will saw that stuff.

I don’t do anything special as far as set up. Just the basics. In fact even on my older 9 x 32 Lathemaster parting wasn’t a problem of course I made a custom block to hold the 3 mm blades.


Sent from my iPhone using Tapatalk
 

brino

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#14
Hello @mickri,

I used to fear parting too. Nothing seemed to work, until I took it one step at a time to eliminate the flex in the system.
The single biggest change was to a quick-change tool post (QCTP) and proper holder for a parting blade.
Other things that helped me: locking down the cross slide, minimize tool stick-out, and tightening the top-slide gibs.

I have not tried it, but Metal Lathe Accessories does sell a replacement cross-slide kit for a few different lathes:
http://www.statecollegecentral.com/metallathe/S-4382.html
It has t-slots to allow the backside tool-post to bolt on.

There are a couple great threads here that cover nearly everything about parting.
https://www.hobby-machinist.com/threads/trouble-parting-off-mild-steel-southbend-9a.58928/
https://www.hobby-machinist.com/threads/parting-for-idiots.32938/ (no dis-respect intended; it's just the name of the thread!)
https://www.hobby-machinist.com/threads/parting.14556/

-brino
 

Dabbler

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#15
Another way to remove flex is to substitute the compound with a solid rest. A lot of work, but it fixes a lot of problems in a small lathe. It is my next project for my 12X36.
 

NortonDommi

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#16
Hello mickri,
As Brino suggested look at changing your tool post. Make one. Instead of a four-way make a three-way which solves a lot of problems or there are many other designs with free plans on the interweb. While at it make a rear mounted parting off toolpost.
Anyhoo, here's an article by 'Geomater' that explains a lot:
 

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macardoso

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#18
I always power feed while parting. I have a G0750G so it’s not a large lathe. I use GTN 3 tools because I’ve had the best results with the 3mm blades instead of 2mm ones. Of course the larger insert blades are thicker that supports the insert.
I completely agree that the GTN-3 inserts and holder are stronger, however they transfer substantially more force into the part. On a short well-supported component this is great. As the part gets further away from the chuck, the GTN-2 is preferable in my mind

I'll give the powered cross feed a try. What feed rate should it run at?
When I part off in aluminum (95% of what I do) I run the spindle at 50-250 rpm depending on how large the part is and feed somewhere between .005 and .008 per rev. The insert shape of the GTN-x is designed to curl the chip slightly in order to reduce its width and prevent binding, but you need to feed hard enough for this mechanism to work. Smaller feeds form tightly wound clock-spring-like chips which eventually get pushed out of the cut. Larger feeds form a much straighter chip which comes straight out of the work. At my lathes lowest feedrate (.0047) I find the cut can become discontinuous at times and start to cause chatter.

I just parted off some 5.5" diameter work (square stock) with relative ease the other week. 50 rpm, .0050 feed with a HSS blade. Snapped one due to poor tool alignment, but the rest went fine.

Use plenty of oil to prevent the chip from binding to the rough walls of the groove. You can also carefully use a slim tool to extract the chip from the slot (be smart here and don't use anything that could get pulled into the cut, or pull you into the lathe)
 

jdedmon91

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#19
Macardoso I only use one insert the ones I pick up either from Bangood or off eBay for parting in fact my blade is a Bangood one now. My chip form on aluminum is the curls just like you describe like you also I just use regular cutting oil and a brush to apply making sure there is a slight puddle in front of the chip.

I’ve parted some thick aluminum this way. Just be slow.

Steel just slower rpm. I don’t even have a HSS blade to do cut off with for this lathe


Sent from my iPhone using Tapatalk
 

mickri

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#20
While we are talking about parting tools my lathe came with an Armstrong No. 19 parting tool holder but no blade. I measured the slot and it is 0.480." I have 1/2" blades and they don't fit. I have searched for a .480 blade without success. Any idea where I might find a blade for this?
 

mickri

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#21
Found an old Armstrong catalog on the vintagemachinery.org website. It lists the No 19 parting tool holder and gives the blade size as 3/32 x 1/2. The catalog even had a part number for the blade. But then goes on to say the actual blade is smaller than the listed size.

Would it be a bad idea to carefully grind the bottom edge of a 3/32 x 1/2 blade to fit?

I like this No. 19 parting tool holder because the blade is held straight with the holder instead of being held off at an angle. I think that it would be significantly more ridged than the angled holders.
 

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#22

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#23
Usually the dimension for Anerican made blades was always measured pre-final grind, so they have ranged for me from .492" to .477 I got tired of all the variance and reground my holder to accept .500 to .465 high blades, the maximum the hold down would take.
 

mickri

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#24
Thanks for the info on blade size and the sources to buy. Just for grins I am going to grind down one of my 1/2" blades to fit in the No 19 parting tool holder. I do not have the skill nor the equipment to precisely increase the size of my No 19 holder to hold a 1/2 blade.
 
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