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Rear Toolpost Parting Off on C3 Mini Lathe?

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Gordon W04/04/2010 13:04:54
2011 forum posts
Thanks for all that mgj, more or less what I was thinking, normal tool mounting puts the load into the machine bed, which is where it should be. So have stopped making upside down holder and back to normal. Interestingly I never had much of a problem parting off, until I started reading the ME literature, but only did small bushes and the like. A tecnique I use for deeper cuts is to withdraw the tool, move over slightly to widen the cut, then go in again, this of course gives more side clearance. I feel lack of side clearance is often the problem.
mgj04/04/2010 15:23:50
1017 forum posts
14 photos
Gordon - you can do, but that problem is overcome completely by the indexable tipped tool.
 
The "blade" or carrier is say 2.5mm wide and the tip will be 3mm so there is a clearance from just behind the cutting edge of the tip. (Tthey are only full width for about 1mm max, all the way back.) So the set up only touches at the tip and is much less critical of grabbing. Also the tip will fold the swarf up, so coolant stays on all surfaces. So its much less prone to jamming. Add to that a shallow back rake, and there is no component  to draw the tool in. So they don't ajm and you don't need to move the tool sideways to gain clearance. Part off 2-3"bar at one slice with ease on a Myford.
 
You can perform a similar trick with ordinary ground blades, but all of them that I have seen are ground with a considerable back rake. Straight away there is that inward pulling component or vector. Add a smidgen of backlash, and the thing pulls forwards into the clear space allowed by the back lash in the feedscrew, and straight away you are in trouble. If the tool is above centre at the front or below centre at the back the cut thickens instantaneously and uncontrollably and a jam up ensues.
 
Its not clearance or wobble in the bearings - its excessive rake and inadequate clearance that causes the problem. Rectify that and you parting problems will disappear.. 
 
I tend to use 2.5 mm ISCAR GTN2 clone tips mostly. Nice convenient size.
 

Gordon W04/04/2010 17:04:04
2011 forum posts
I've been looking at the tipped tools ,which I think you are describing, not cheap but probabley worth saving for. How do you part 3" bar ? Must be min. of 1 1/2" overhang.
mgj04/04/2010 20:11:03
1017 forum posts
14 photos
Yes- I have a Sandvik blade which takes the same tips as the Kit Q cut.  - standard Sandvik tip. Its actually a blade for a 3mm tip, but you can shoehorn a 2.5mm tip in there.(cheaper)
 
Bring the tailstock with a rotating centre up, add blade overhang as you go along, lots of coolant as a jet direct into the slot!. 
 
if you are looking for a good blade for the smaller machine, the Glanze clamp type from Chronos is a good one. The tip they provide is pretty naff , but Zenit do decent coated ones fairly reasonably. Thats an ISCAR clone, and it works very well.
 
Thats a better bet for the smaller machine than the Kit Q Cut (which is excellent) but it takes a 2.5 rather than a 2mm tip, and the narrower tip is better if one is a bit short of horsepower. Sandvik do do 2mm blades, but the assembly with clamp is a Father Xmas job. the Glanze is not.
 
Personally, I see no point in getting the holder type parting off tool. The tip is the same but it doesn't have the reach of a blade, so it is limited. You can always retract a blade and run it with little overhang (good), but once the pinch type have reached their limit, thats as far as they go.
Stub Mandrel17/05/2010 21:04:40
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4318 forum posts
291 photos
1 articles
Ok Guys, to put the cat amongst the pigeons - look at the 2 1/4" EN1A starter pulley on this:

That groove was made entirely using a 3/32" HSS parting tool on my mini lathe. Not a scarp of carbide in sight, nor rear toolpost. Yesterday I was parting off  3/4" medium carbon steel with the same tool and it was just rolling up into neat round spirals.
 
I think the main thing is having the courage to really feed the tool into the work, instead of just gingerly poking it. If only the mini-lathe motor had the grunt to keep going at modest speed and high power without overheating
 
But it proves the basic lathe is solid and sound.
 
Neil
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