Parting On a Hobby Lathe

[VicParticipant @vic]

I have an 8 x 14 Lathe and it doesn't really have the power for insert parting tools and I can't fit a rear toolpost either.

When time allows I'll be making an upside down front tool holder to take the Chipbreaker parting blades from Chronos. I'll post my results when it's done.

[IanTParticipant @iant]

Parting seems to be a very personal preference thing – everyone has different views. I can part 2" mild steel with ease on my (somewhat worn) S7 with one of these….

I think there are two main factors that really help – one is the rear mounting of the tool and the other (and perhaps most important) is the shape of the tool tip – which is curved. It curls the chip in on itself and helps avoid jams in the deep groove being formed. They just fall out underneath the work.

In the main, I prefer to use HSS tools but this is one of the exceptions, simply because of the tool point geometry. I still use HSS parting tools from the front post for smaller diameters but (for all but the smallest diameters & tube) I tend to cut in stages, pulling the tool back, moving it over and widening the gap before going in any deeper. This works well enough but is a fiddle and care is needed to get a usable finish on the back of the work. Of course, you have to get as near the chuck as possible and extend the tool no more than absolutely necessary (essential if your machine is less than 100% 'rigid'  ).

GHT advises how to 'shape' the tips of HSS parting tools in his workshop book but the inserted tips make it so much easier to get the tip geometry just right – and with the above set-up, I can part off with one long cut.

Regards,

 

IanT

Edited By IanT on 03/01/2015 10:40:37

Edited By JasonB on 03/01/2015 13:21:51

[VicParticipant @vic]

Nice job Larry!

Edited By JasonB on 03/01/2015 13:22:20

[mechman48Participant @mechman48]

As for parting on my WM 250V-F , I use both a front mounted insert type (RDG.. usual disc. ) plus a HSS home ground tool, on Ally, Brass & BMS ( up to approx' 20mm ) anything larger in BMS I have used my home made rear parting system. Larger dia' in Ally, Brass I can use the front mounted inserted tip holder (2mm wide tip ).

Some 2mm x 25mm dia. large BMS washers trialled when I finished constructing my rear tool post system…

This shows I have used a front mounting holder upside down as it proved that the original design meant that there was too much flex in the original blade .. too much unsupported overhang… creating a concave/convex surface.. this is much more rigid..

The front system link is..

**LINK**

As mentioned, Make sure that everything is rigid, compound slide snugged up, saddle locked, tool square to the X axis, tip dead on centre height, plenty of lubricant ( flood or brush fed ) &, once started keep the infeed going, I parted the above washers using approx' 260 rpm with the auto crossfeed at the machines lowest setting, have just recently ( yesterday ) parted off 16mm Brass @ approx 600rpm, lowest set auto feed, with the RDG tool system.

What was once a job filled with dread now is now confidently done.

George.

[Ian S CParticipant @iansc]

The tool needs to be no wider than 2 mm, and the depth as big as you can fit, I see that the parting tool blade for Sieg lathes for the 8 mm holder is 8.3 x 1.5 mm. with a HSS tool I would suggest that the 125rpm is on the high side, this is were a back geared lathe is the thing, with rpm of 60 or less.

Ian S C

Edited By JasonB on 03/01/2015 13:23:08

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Les Jones 1 on 03/01/2015 10:10:31:

Hi Michael,
I did not know what the "special 2MT collets" were but the ones I found with Google look like they work on the same principle as ER series collets. < etc. >

.

Les,

Thanks

For info, they are shown on this Myford list [section 2]

Allegedly a Myford patent, 'though I have never located it.

MichaelG.

[blowlampParticipant @blowlamp]

A video of my Mini Lathe parting with 2mm wide GFN inserts.

**LINK**

Pictures are in my folder of the blade I made to use with these inserts.

Martin.

[colin hawesParticipant @colinhawes85982]

Occasionally the chuck can indent the workpiece during machining operations and so reduce its grip on it. Rigidity everywhere is important. If you anticipate heavy parting it pays to ensure the chuck is tightened properly first. If the chuck is getting bell-mouthed a shim at the tip of the jaws can make a big difference during the strain of parting off.

[Larry Coleman 1Participant @larrycoleman1]

One easy solution is a power hacksaw. I am surprised how many people tend to go for a bandsaw.

Just take the bar out of the lathe and wack it in the power hacksaw.

I have used that type of parting tool with the replaceable tips.

When I was an apprentice our workshop had those 45 Deg parting tools but they had a spring steel blade with a replaceable tip and regardless of how hard you wound the cross slide you could not jam it. that tool I made holds the tool to ridged and when I get some spare time I will make a spring steel tool with a solder in tip.

Does anyone know if you can buy them now.

Larry

Edited By JasonB on 03/01/2015 13:23:35

[NobbyParticipant @nobby]

Hi
Parting from the front digs in From the back digs out
Nobby

[David CambridgeParticipant @davidcambridge45658]

After a bit of practice I’ve managed to part a 1.5 inch diameter aluminium bar with an HSS blade in the normal orientation (i.e. not upside down). Although it went well it seemed very tedious and took a very long time.

The procedure was to raise the parting tool slightly higher than centre and start off with the blade very short. As I moved towards centre the tool would start to dig, and at the point I found if I backed it out I could see the cutting surface was gumming up with ‘aluminium foil’. Progressing about 1 mm at a time, and continually backing out and cleaning seemed to do the trick. I also found it necessary to remove the shims to lower the tool and extend the length the closer I got to centre. All the time I was dripping in cutting oil.

All in all , successful but very slow. I must be doing something wrong to make it so labour intensive. I watched BlowLamps very helpful video and it took only a couple of minutes. 

Come next week I’ll get in touch with eccentric engineering’s UK wing and see if I can sort out the upside down tool (although before I do I’ll investigate the brilliant idea of a shim under the tool post to raise the height).

David

Edited By David Cambridge on 03/01/2015 22:11:58

[MuzzerParticipant @muzzer]

Dovetailed slides aren't really designed to support significant upward ("reverse&quot forces. Rather than discounting the experiences of users who find success with upside down toolholders, I suspect that some machines (Myfords for one) are safest used with some degree of compliance in the tool holder.

As Nobby suggests, there is more compliance with the tool upside down tool holder than when held "normally". When a tool starts to dig in or a lump of swarf jams in the slot, something has to give. If you don't have a sufficiently rigid setup (arguably the ideal situation), you need the tool to move in a way that doesn't lead to catastrophe. At the very least the tool needs to have some margin of safety (strength) to avoid that deep thunk sound that you feel deep in your bowels.

Personally, I part off in the forward direction and prefer to use inserts. Power feed gives a consistent tool load which is surely helpful. Rigidity also seems to me to be a helpful quality. Ultimately it's each to his own but I try to understand what's going on rather than simply identify something that seems to work and stick to it.

Murray

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Muzzer on 04/01/2015 00:31:14:

… I try to understand what's going on …

.

Well said, Murray.

MichaelG.

[JAParticipant @ja]

I note that both Andrew and Murray use power cross feed when parting off. I used to and found that it eased the work and gave good results. However after reading many times, on this web site and other places, that this is a "real no-no" I gave up and now use hand feed.

However the more I think about it the less I understand why you should not. One always has a big red button. Perhaps it is due to the fear of parting-off?

JA

[mechman48Participant @mechman48]

'I note that both Andrew and Murray use power cross feed when parting off. I used to and found that it eased the work and gave good results. However after reading many times, on this web site and other places, that this is a "real no-no" I gave up and now use hand feed

Odd ?.. Why give up after you state that it gave good results, just because someone says this is a no-no doesn't mean you have to stop what works for you, I used to be the opposite, now I part off under power feed when I need to & am quite happy & confident to do so… Ref. my post above..

George.

Edited By mechman48 on 04/01/2015 09:39:13

[Ian S CParticipant @iansc]

David, try using either kerosene, or WD-40(or similar) rather than cutting oil on aluminium.

Ian S C

[Anonymous]

Posted by JA on 04/01/2015 08:41:02:

I note that both Andrew and Murray use power cross feed when parting off. I used to and found that it eased the work and gave good results. However after reading many times, on this web site and other places, that this is a "real no-no" I gave up and now use hand feed.

However the more I think about it the less I understand why you should not. One always has a big red button. Perhaps it is due to the fear of parting-off?

JA

Among the professional machining fraternity there are those who part off under power, and those who hand feed. It's a matter of personal preference. However, I suspect there are vast numbers of bar feed CNC lathes around the world parting off under power without issues, so it is not inherently bad.

Like Murray says, it's a case of trying to understand what is actually happening. It's all about feedrate, especially with insert tooling. A lot of time and effort is expended in the design of inserts to control chip formation and breaking. If one operates outside of the parameters, normally too low a feedrate, then chip formation goes awry and a jam up may well result. As mentioned before I use insert tooling for parting off on the manual lathe. With a decent feedrate the chips are curled in on themselves and break into small pieces, so evacuation is no problem. Too low a feedrate inevitably results in chatter and erratic chip formation.

To summarise, use a decent feedrate and leave the chatter to the forum.

Andrew

[MuzzerParticipant @muzzer]

As Ian SC says, some form of oily lubricant is a good idea for parting aluminium. The last thing you want when parting is for the chips to weld to the cutting edge, as this can dramatically increase the load. A decent amount of WD40 seems to help.

One of the benefits of inserts is that the top face forms the chips into a curved shape which is narrower than the groove you are cutting. This reduces the chances of another type of jam.

I've now obtained some aluminium-specific parting inserts from Cutwel for my Korloy parting tool. I'm surprised that they are not highly polished and have an almost flat top surface with the same top rake as the steel ones, so I'm interested to see how they perform when I finally get back in front of my lathe later this year.

The above causes of jams (welding, mechanical) can cause large transient loads on the tool and would kill many of the ground HSS tools. The indexable tools generally have a large blade which is considerably more rugged than HSS. I'm not sue if they are made from spring steel but they don't flinch even when the transient loading is high enough to be audible. I'm sure that alone has been one reason for their longevity in my useage.

Murray

[Michael GilliganParticipant @michaelgilligan61133]

Nice 1954 cartoon, by B. Terry Aspin here

MichaelG.

[Neil WyattModerator @neilwyatt]

You'll note that Aspin was an Adept owner – see bottom left

Neil

[old AlParticipant @oldal]

following this thread makes me want to give up parting off altogether.

It is a simple machining operation that needs to be done correctly and with confidence. all of the good points have been highlighted and all of the bad point have also been identified.

If you have room, use a rear toolpost

tool on centre height

loads of coolant

everything else can be adjusted to your machine and budget. Must get myself a tipped parting tool, but don't have a problem with my hss ones (2 lathes, 2 back toolposts)

The lighter the machine the less rigid it becomes and parting off becomes less reliable.

love the Aspin cartoon, but notice that none were wearing safety glasses. Caught one of our members without a pair recently, even coolant stings

[colin hawesParticipant @colinhawes85982]

I have confidently broken several parting tools over the years. Colin

[mickParticipant @mick65121]

A bar fed CNC lathe would use G96 constant surface speed where the RPM increases as the diameter being parted decreases so the tip has constant machining conditions for the entire operation. If you use power feed on a manual lathe the tip is laboring the closer it gets to the centre of the bar, as the surface speed is in fact decreasing, so its always best to hand feed as the operator can compensate for the surface speed by decreasing the feed rate of the parting tip, or blade, as it approaches the centre of the bar and of course using a rear mounted parting tool will always give better results.

[JasonBModerator @jasonb]

But if you have a variable speed lathe you can wind up the wick as the diameter becomes smaller, using the power cross feed gives you a hand free to twiddle the knob.

I powerfeed mine all the time and if its a large dia will increase the speed as I go. Anything 1" or below I have teh lathe running full speed from the start.

J

[Neil LickfoldParticipant @neillickfold44316]

I use mostly a 1 mm wide hss parting tool in a dickson holder.The blade is hollow ground in top. As I mostly have the holder block true to the lathe axis. I don't use a rear part of because I have a router mount there for use with live tooling. For most things I part off at about 500 rpm and feed rate will be about 0.03 to 0.1mm per rev .I sharpen the front edge only on the tool when it either looks dull or the force required gets a little more than a newly sharpened tool. Steel, I sharpen a lot more than brass and ali. As for coolant, on Ali I us CRC or WD40 or cooking oil. For stel I only use cooking oil.

Never have issues parting things off.

Neil

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