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Is 25 thou backlash enough to cause parting problems? (15" Colchester)

JasonPAtkins

Hot Rolled
Joined
Sep 30, 2010
Location
Guinea-Bissau, West Africa
I have trouble breaking parting blades on my 15" Mk 1.5 Colchester lathe. The lathe is fairly stout - I know a floppy machine can cause problems in parting. It seems like there's enough iron here to be able to part correctly, though. I'm still pretty new to lathe work, and this machine is the main one I've learned on, so I don't have a lot of other points of reference.

I have trouble with parting of all types, but seem to do a little better with HSS than carbide inserts. I break a carbide insert about one in every two parting operations. I'm often parting 3" diameter solid rounds near the chuck, so workpiece rigidity isn't the problem. I think I'm using a roughly correct SFM, though it's been a little while so I don't have sample numbers.

Generally, I start the cut and have some chatter. At some point in the cut, it starts squealing (I've tried both flood coolant and oil dabbed on with a paintbrush). I'm not sure if the squealing is because of work hardening, or because the chatter has dulled the point on the blade, but then if I try to push through it, sometimes it works and sometimes (often) it snaps the insert/blade.

My lathe work isn't my main job, so I don't know if I am doing it wrong, just don't have the feel for it, or if this backlash in the cross slide is enough that even if I knew what I was doing, I'd still be having trouble.

Thanks in advance!
 
Without knowing what you are parting it's hard to say...but I've found parting is especially sensitive to tool height.
 
Parting sucks! I've been parting for nearly 40 years and I still have trouble with some stuff. A lot depends on the material. For one offs I never use flood coolant - I always dap with a paint brush - mainly because I hate the mess.

I like to cut with hss whenever possible - carbide sucks unless you're super rigid - as in a turning center with lots of mass and a ball screw.

I grind an aggressive hook on the blade so that the cutting edge is presented to the work piece in an extremely positive fashion.

With a hss blade cutting low to medium carbon steel I would be around 50-60sf. For tool steels or higher carbon steels I would be around 20-40sf.

Did I say parting sucks?
 
Somewhat comforting to know it's not just me. I will confess that for non-critical stuff (like the v groove casters I make for heavy rolling gates pretty often) I just part the first 1/4" of the depth, then go to the bandsaw for the rest of the cut, then back to the lathe to face, lol.

Still - do I have any hope of this working with .025" of backlash in the cross screw, or should I tackle that problem before I start working on this one?
 
Still - do I have any hope of this working with .025" of backlash in the cross screw, or should I tackle that problem before I start working on this one?

That much backlash on a machine of that age isn't bad. and in itself shouldn't be the cause of any problems you're having with parting. Are the cross slide gibs adjusted OK -if they're slack that won't help with the chatter problem as the toolpost will be more likely to oscillate.
 
Tool tip on center height, tool holder dead square to the work, coolant on, engage power cross feed. Done.

Almost all the folks I have seen having trouble with parting, did so because they were trying to finesse it, rather than putting the machine to work.

I have parted off hundreds if not thousands, of shims made of 17-4PH Stainless and Al-Bronze, as well as assorted other materials including a couple flavors of Al., and Ti.

I found that a carbide insert tool esp., will tolerate a very large speed range difference between the start and finish of the cut, but it would not tolerate inconsistent feed rates worth a darn.

Backlash should only factor in, if the tool geometry allows it to be pulled into the work rather than being pushed into it, yeah? Work that thing like it owes you money!

Cheers
Trev
 
That backlash isn't at all bad. Nobody likes parting off really. Best results I've ever had was when I was setting up some capstan lathes for women operators.

Without wanting to be too critical most of them were pretty un skilled and had a touch like an angry rhino. They were on piece work as well !

We had a rear mounted tool post at the back of the cross slide specifically for parting off. We mounted the HSS parting off tools upside down so we didn't have to change spindle rotation.

These women wound the parting tool in by hand like there was no tomorrow but very rarely broke a tool. Having said that we were machining mild steel most of the time.

Regards Tyrone.
 
The only inherent problem with parting is that as the grove gets very deep, it can become difficult to keep the chips ejecting cleanly and not fouling. Other than that, it's really no more of a problem than any other operation if using proper technique. As already stated, good tool geometry is important, as is a consistant feed, heavy enough to produce a continuous chip and keep loads constant.
I like a lot of the carbide inserts that tend to curl the chip along it's width so it is a little narrower than the slot it is coming out of. HSS is fine as well. High positive rake is good in most materials as long as you either have a small flat right at the cutting edge or limit clearance on the front to keep the tool from being pulled into the work.
Because of the nature of the cut, the work wants to climb over the tool more so than in other operations, combined with the fact that you typically have a very flimsy tool with a lot of hang out means that during cutting the tool is going to flex (so will the work) and be deflected below center significantly enough to cause problems. If the work already wants to climb the tool, having it at all below center is not good. That's why I set the tool slightly above center to start, maybe .010-.015" depending on the conditions.
As far as backlash goes, by itself it absolutely meaningless. You can have an inch of backlash and work just as accurately as if you had .005! Excessive backlash is sometimes indicative of excessive wear/poor adjustment of the machine as a whole.
 
Years ago I was operating a Myford Super-7 and I made a rear toolpost for it, a massive CI block that sat permanently bolted down to the backmost slot on the crossslide table. Tool usage as per Tyrone, above. It made parting off routine.
-Marty-
 
I use a parting tool regularly. As a guess, I'd say your setup holding the parting tool is not solid enough. Toolpost moving / compound flexing or moving. Is the parting tool perpendicular to the lathe? Do you have the compound backed up tight to its end of travel / stop? ( the compound hanging out in space with the tool post only supported by the top part of the compound adds all kinds of movement and flex. )
 
Dunno what the issue is.....

I part off on my personal machine, a 10" Logan, with whatever tool is handy, parting tool or a grooving tool, or whatever. Seems to be fine.

Backlash is not an issue UNLESS your grind is a "hook" or "raked" grind that can "suck-in" the tool.

Part off CLOSE TO THE CHUCK OR COLLET

have the parting tool SQUARE TO THE AXIS

Have the parting tool ON CENTER

use oil.

Lock the carriage.

Either use crossfeed or hand feed, but if using crossfeed, get the feed right.

Do that and the parts should just fall off, and your biggest problem should be finding them in the chip pan. OK, parting 6" solid stainless is likely to be an issue. I don't do that, most of what I need to part off is 2" or less.
 
Tighten the gib on the compound so that you have difficulty moving it. Looseness at that point might be the problem. I've never had a lathe that I couldn't part with (using 1/8 /3mm wide inserts), but I do have all my compound rests tight.

Parting with carbide (I use SECO because their blades are made of HSS and the pockets don't distort from heat when parting basically dry on a manual lathe) requires one to have the rpms up. It doesn't need to be 400 SFM the whole way but that is a good starting point. On a manual lathe, your reasonable and safe spindle rpm choice is limited. On 3" stock, you will need to shift gears at least once to speed up when you are half way done the cut. You can immediately feel how much better the insert penetrates as soon as you get the rpm back up.

Feed it firmly. If you cannot beat chatter right at the start with a firm feed, then you need to tighten up something on your machine. Quite often this will mean improving your chuck jaws by grinding or boring them straight with preload applied so they actually grip the part for the length of the jaw.

I don't saturate the part with cutting fluid, I basically brush the cut with lubricant for the sake of the chips coming out of the groove. That creates plenty of stinky smoke as it is. The carbide does not need 'cooling' in particular if the SFM is right, and unless it is flooded, it is not getting enough coolant to do any good. Flood coolant is great to lengthen the tool life on a CNC, but then the mess is controlled there.
 
Just a couple of things:

1. I'm guessing you throw the lathe in feed and stand back!
2. What kind of holder? I use an Aloris with a T style HSS blade and don't have problems
3. What is your SFM on the HSS and carbide?
4. How close are you to CL with the tip?
5. How close did you indicated the holder in to parallel?

I'm guessing it's your set-up and trying to power feed. For trying to part a 3" dia, you can't just be close, you have to be dead on!
JR
 
People are worried about tool height but I've always found that large diameter stock will part at almost any height until the diameter becomes small enough that the chordal height of the diameter gets too far out of whack.

If small stock (say 1") parts off without any undue difficulty my best guess would be poor chip evacuation and welding. You could be over feeding the tool as well, most parting tools I've seen have suggested a .002-5" FPR.
 
How much material is hanging out past the part off and at what point does it pinch the blade and break it?

Haney
 
Iscar inserts, flood coolant or oil in a constant drip, right tool height, and cross feed.
That's my combo, and I enjoy parting off. :)
Stainless steel, mild steel, tool steel, you name it. Same recipe.
 








 
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