[How do I?] Parting with backlash

I never had much luck parting on my old SB9A--pretty much the same as you describe.
When first got my new Enco 12x36 lathe I was hesitate to try parting because of my experinence with the SB--but it parts like a dream using the same cut off tools.

I had also replaced the cross slide nut and screw--no difference, other than less backlash. I don't think backlash is the problem.
 
Tighten the cross slide jibs, lock the carriage, move the compound to its fully closed position (no overhang) lock the gibs and minimise tool overhang. If using a QCTP, check it for any play as well. Tool bang on centre height or slightly high if anything Make sure you have side rake on the tool. Having said all this, I hate parting off with a passion, but have been struggling along with my lathes in a tiny area where there was no room to fettle the machines. That is all about to change, and when I have moved them and set them up, I will have another determined go at parting. If you have a mount for a rear tool post at the back of your saddle, why not make a toolpost and mount your parting tool upside down at the back of the saddle, which will force the tool out of the work, instead of digging it in.
 
I hate parting off too, but I remember doing it successfully on a SB 9A (in fairly good shape) back in my high school days.
Mark
ps it was brass though not steel
 
Brass can be a bugger too, very grabby especially when drilling it, You can improve it by grinding tools and drills specifically for brass!
 
Hone the tip sharp, are you making the blades similar to theseIMG_0472.JPG
I think you can cut them out at different angles to reduce the rake
 
As others have said, they haven't had much luck parting on a 9" SBL. I too have a 9" SBL in very good shape and does have a little bit of slop in the cross feed screw and nut. Never had much luck using the "old school" parting tool. Finally started using a zero degree back rake parting tool, essentially a piece of HSS ground with about a .060-.075" width on the tip. I can only cut off about 1" in diameter with this bit. I only use it when I'm too lazy to take it to the 4 x 6 saw and cut the rest of the way off. Most of my parting off is done by starting a cut then taking the stock to the saw and finishing cutting off. Then go back to the lathe and face off to finish. Lot less pain and headaches. Ken
 
Still fairly new to machining myself, so take this for what it is…’advice’ from a noob! LOL

Totally different lathe than yours, but as far as rigidity goes (& overall quality for that matter) my cheapo Craftex 7x12 mini lathe is about as low as you can get on the “rigidity/quality scale”, but this is what works for me.

I too had horrible parting experiences when I first started out a few years ago, but after reading about the “upside down/reverse” method where you flip the parting tool upside down & run chuck in reverse, all my parting troubles instantly disappeared & have never had a problem since. No more digging in/jamming, broken blades or the wicked heart palpitations that used to be the “normal” procedure when parting. Since trying this method, I have parted aluminum, brass, mild steel (1144 & 1018 mostly), O1 drill rod & even 303 stainless with no problems at all….been using the same blade for almost two years now.

Granted, my wee chuck is held in place with 3 studs w/nuts, not screwed on, so no worries about chuck working loose & becoming a projectile. I’m not familiar with your SB’s configuration so this method may or may not be of any use to you. Although, even if your chuck is the “screwed-on” type, you could still use a variation of this ‘upside down’ method…just have to come into your work from the backside with upside-down parting tool & chuck spinning normally instead of reverse…provided you have the room, of course.

Hope this helps!
 
I use both carbide and HSS parting tools; all the insert parting tools that I have are zero back rake, with only a chip breaker forming any suggestion of back rake. I think your back rake on the home made parting tools is the problem, causing the hogging in and subsequent breakage. If it were me, I would not try to use carbide to part off on a small lathe. One thing that I have noted with my 9" Monarch lathe (virtually unworn) is that using QC tooling, the overhung tool tends to tip towards the chuck when parting, causing problems, this with the gibs adjusted up snug.
What to do? A tool block that would move the tool closer to the center of the compound would probably do the trick.
 
Carbide saw tips develop a radious as they wear on the front of the tip. After you cut it from the circular blade and before setting it up you need to square the carbide tip to remove the radius and to flatten the top of the tooth to remove the sharp point leaving a "slight" back rake for chip formation. If you leave the radius and the point on the tip, the radius will create more friction as it digs or drags into the cut made by the point of the tip causing blade failure . When you flatten the tip and create back rake , the radius is still present so you need to grind or hone a front rake angle, going far enough to create a straight edge across the front of tip. Do the same on the sides creating side rake so that top of tip angles are widest narrowing down to bottom of shank. I use a loupe and a cheap diamond hone set, HF sells sets and loupes. You have to tinker a little to make it work but after you picture it in your mind it's easy. The loupe lets you see what happening to eliminate drag etc. Homemade is usually faster, cheaper and better than waiting for store bought stuff, and relaxing!
 
It was virtually impossible to part successfully on my flexi 9x20 until I replaced the top slide with a solid plinth.
Now it parts with no problems, long rolled up curls in mild, stainless and aluminium. obviously brass sprays fine needles everywhere.
I no no longer worry about parting off to the point where I only eyeball the angle and sharpen the tip often not at 90 degrees and it still ploughs through nicely.
I do set the tip on exactly centre.
A couple of days ago I replaced the topslide to cut some small tapers and couldnd believe how much the whole assembly flexed, it was easily visible.
The backlash in my cross slide is around 1/16", been too busy to fix it.
 
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