I think I have a parting problem........

Turn the tool upside down and reverse the lathe.

Bob

I've got to try this. The nice thing about is that, is I would be able part off small machine screws without them unscrewing themselves from the bolts that are used to hold them in the chuck. Thanks for the suggestion.
 
I have a Harbor Freight 8x12, my starter lathe. I couldn't part for nothing, speed didn't make a difference it would chatter constantly. I don't know which forum I read it on, but someone else was having the same problem. Another forum member recommended this, so I can't say it was my idea. I can now part at just about any speed regardless of the material. I do slow the lathe to around 140 rpm, but have parted small pieces off at 600.

Bob
 
I had the same problem/fear with my lathe. Parting would either break the tool or stall the machine. I found a youtube video that mentioned turning the tool upside down and running the lathe backwards, as Bob mentioned above, and now I'm able to part without any problems.

You could also try using a rear-mounted tool post if you don't want to run in reverse.
 
The best book on machining on small lathes is, IMHO, The Model Engineer's Workshop Manual by George H Thomas. It is sold by Tee Publishing in the UK. In it there is a chapter called On Parting Off. He used a Myford Lathe, which is pretty small and he used an upside down tool in a purpose built holder which cut from the far side of the work. Hemmingway sells castings for it. The tool post stayed on the cross slide but the blade could be rotated out of the way when not in use. Because it was running upside down, when the loads got high the tool would flex away from the work rather than digging in.

http://www.hemingwaykits.com/acatalog/Retracting_Tool_Holder.html

I don't need such a holder on my super 11 because with an MT5 sized spindle it is pretty rigid. It is all about rigidity. I always hand feed because I can feel when it is cutting well and I can vary the pressure to get the cutting action that I want, which is chips rolling up into curls. You need a good firm oressure, not too light and not too heavy. If the cutter is just plowing or ripping off material I stop and investigate, sharpening or realigning the tool if necessary. Obviously it has to be at center height. If the groove is getting filled with curls I stop and brush them out. I apply cutting oil continuously.

A lot of cheap chinese lathes and older worn machines won't have good contact on their moving slides. This will make them prone to flexing and chatter. I have seen some where the carriage casting, when sitting on the bed ways, will teeter back and forth due to a the casting having warped after being machined, orobably because it was never stress relieved or seasoned. Good luck parting off with that!
 
I have a MaxiMat7 which is a light lathe. I part in a conventional way with a standard parting blade. I just use lots of ATF fluid. But then I use ATF for everything, cutting, tapping, drilling, etc. I have a Sherline in which I use a parting tool mounted upside down at the back of the lathe.
 
I agree with above comments on upside down tools, I also like to do threading upside down and in reverse. Magic. If a screwed chuck make a rear toolpost and fit a chamfer tool as well, you won't regret it.
Could I also suggest locking the compound AND the sasddle and making absolutley certain that the blade is at 90 degrees.
I'll post a pic of toolholder from scrap that works for me this evening.
In the meantime have a look at this vid.


Cheers,
- Barry.
 
Yes lock everything down. In the book I describe above George Thomas (GHT) shows a locking cross slide and compound. Anything that helps reduce flexing. A sharp tool on center is also essential. GHT used to demonstrate parting off 1" cold rolled steel at 300rpm at the Model Engineer expo in England just for "showmanship". He does admit yhat on his furst lathe, which was foot powered (!) he was reduced to using a hack saw. As his lathes grew in spindle size his success at parting improved as well.
 
Parting seems to be difficult for almost everyone, including me, until you learn the "secrets" (which are really simple steps that must be followed to a "T" in order to be successful.) I respectfully disagree with those that say parting cannot be accomplished on a small lathe. I have successfully performed parting on my 1943 Logan 10" lathe as well as my dad's old worn Atlas 10" bench lathe, a model which many say is not very rigid. On both lathes I have been able to reliably part by both hand and power feed. I am an amateur machinist with no claim to expertise, however these are the keys to success for me:

#1 Rigidity of the work. The cut should be made as close to the chuck jaws as possible. Certainly no more than two diameters of the work away from the chuck jaws. Closer is preferable. DO NOT attempt to part between centers in an attempt to achieve rigidity. You will likely break your parting tool toward the end of the cut or damage the work..

#2 Rigidity: If the chuck's jaws are bell mouthed you cannot maintain rigidity of the work, no matter who much you tighten the chuck jaws. If you are using a lathe with a threaded spindle be certain there are no chips in the threads of the spindle or the back plate and of proper fit between the spindle shoulder and the chuck back plate. There should be full contact at those points if the backplate is properly fitted. Blue those areas and install the chuck then remove and look for blue that has not been removed. Those are areas with no contact, correct the fit. The portion of the spindle shoulder I speak of is the one that the back plate butts against, not area that enters the backplate bore. (Without getting into a huge debate about how back plate bores should fit the spindle I'll say that I subscribe to the belief that the threads, due to their 60 degree form, center the chuck on the spindle to prevent runout and that the shoulder the backplate butts against holds the chuck true to the axis of the lathe ways. Others may disagree, and that is fine, but that debate is like debates on which oil is best, circular and without end.)

#3 Rigidity: The tooling must be set up as rigidly as possible. If you are using a Lantern tool post ditch the rocker and make a solid steel ring of a thickness that puts the tip of the parting tool on center. With a QC tool post be certain there are no chips under the tool post and that it fits the compound flat without rocking. Compounds can have worn surfaces from years of use that will not allow tool posts to be rigidly mounted. Dings from dropped tooling will cause raised areas that allow minute rocking and ultimately chatter. Be certain that the parting tool does not extend any further from the holder than is necessary regardless of what tool post you use. If using a Lantern tool post choke up on the tool holder as much as possible. Keep overhang to a bare minimum.

#4 Rigidity: Think about every possible way for movement to creep into your setup and eliminate it as much as possible. Properly adjusted machine gibs, small tool overhang, tight tool posts, holders etc. Be certain your headstock bearings are properly adjusted if they are plain type or have proper preload if of ball type.

Up to this point most of what has been said is appropriate for accurate chatter free plain turning and boring as well as cutting off. Much is just general good practice for machine maintenance.

#5 Parting tool alignment. The parting blade must be perfectly perpendicular to the work and on center. Take great care in assuring this. I usually use the lathe chuck as my guide and put the parting bit right up against it. I use a white piece of paper or a light shined from below to adjust the tool holder/post until the light coming through shows an equal space at the blade tip and where it meets the holder at the outer perimeter of the chuck. I use the same care I would when I set up a threading tool for a threading operation using a Center Gauge. The parting tool must also be perfectly vertical. If it leans one way or the other it will rub on one side and track to the other.

#6 No back rake on the parting blade itself. It should be on centerline of the work and level. Be certain there is adequate front clearance or the tool will rub rather than cut and of course there should be side clearance on both sides of the blade. If the cross feed requires much pressure when attempting to part be certain to check front clearance, the tool is most likely rubbing due to lack of front clearance or is adjusted above center. The blade should be sharp and honed to a fine edge at the tip and sides. Some advise grinding the tip at a slight angle to eliminate the tit left on the work. I do not do this as it seems to make the narrow parting blades I use try to wander to one side. I grind my tip so it is perfectly parallel to the work.

#7 Once you have the cutoff tool position where you want the cut lock the carriage to the ways.

#8 Speed. I generally use speeds of about 1/2 to 2/3 that recommended for turning whatever material I am parting. Don't be afraid to try a faster or slower speed if chatter develops. Note that chatter is a sign of lack of rigidity. Check again to assure you have done all you can to assure a rigid setup.

#9 Feed. Start with hand feeding, feed slowly but steadily and with enough feed to produce a thin continuous chip (in steel or aluminum). Be aware of tool pressure. The tool should feed in easily if there is enough front clearance. The chip should come off as a curl when everything is right and will roll up on itself. When conditions are right I get a curl that looks much like what the curl of metal that is produced when using a key to open a can of sardines. There is a distinct sound when all is well. To me it sounds like a hissing or tearing sound.

#10 Lubrication. Continuous lubrication should be applied while parting. If you don't have coolant pumped (I don't) use a brush and flood the cut. I find that I can usually work the hairs of an acid brush deeply enough into the kerf to keep things lubed adequately. I use dark cutting oil on steel and stainless and WD 40 on aluminum although the dark oil will work on aluminum as well.

#11 Once you have gotten the hang of feeding by hand and knowing what RPM to use you can mimic the feed rate with your power cross feed.

#12 RIGIDITY! Truthfully that is 70% 0f success so it bears repeating. Do all you can to make everything as rigid as possible. The other 30% is tool alignment and front clearance.
 
WOW! Firbikrhd1 you have said everything that needs saying. Great stuff.

Hear's those photo's I said I'd post, please excuse the state of the lathe, it does actually get cleaned on a regular basis.
The toolholder needs to have countersink screws but it was knocked op in an hour with what came to hand from the scrap bin.
P.S. Scrap machinery like computers, washing machines, old microwaves are a good source for odd screws ect.
I hope this will be of help.

-Barry.
 

Attachments

  • Parting Toolholder.zip
    465.3 KB · Views: 44
Back
Top