Backside parting tools

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.
 
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.
 
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.


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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
 
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.
 
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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jdedmon91 said:
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.
Click to expand...

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

mickri said:
I'll give the powered cross feed a try. What feed rate should it run at?
Click to expand...

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)
 
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


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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?
 
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