Parting On a Hobby Lathe

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

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

Nice 1954 cartoon, by B. Terry Aspin here

MichaelG.

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

Neil

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

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

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.

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

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

I too have consistently broken parting off tooling of various types over the years. I'm still on the Holy Grail to find the correct way to do this. Perhaps I'm not practicing enough! It certainly sorts the men from from boys! My friend who's a retired toolmaker wonders what all the troubles about - I'll just have to get him to give me a Master Class session.

John

One of the causes of parting tool breakages is backlash, as the tool advances there is a chance the tool grabs and the backlash allows the tool to dig into the work piece causing excessive load and ping there it was gone.

Bob

Edited By Bob Brown 1 on 06/01/2015 10:00:30

David

It really doesn't have a lot to do with skill. It has more to do with how rigid your lathe is and the shape of the tool tip.

If you consider the job at hand!!! You have a rotating piece of steel and a tool which creates a cavity trap for the shaving, Then a reducing rotational cutting speed. That is a lot of parameters to request from a small lathe.

I have quiet a large lathe and I still have the odd foul up. Now you would think after forty years as a Toolmaker I should have it down pat but I have resolved that the power hacksaw is the best option.

I am brave enough to admit you are not the only one with this problem so don't think it is just you.

Larry

You need 3 things for simple hobby lathe parting at the beginning

reasonable stiffness, a rear toolpost and backgear

A rear tool needs to be 'shoved' in more than a front facing tool because the work rotation is trying to push the tool tip away

reasonable stiffness means use the tailstock to support your early efforts

Once you get some parting confidence in backgear mode then you can move onto any system you fancy

Edited By Ady1 on 08/01/2015 11:02:38

I still don't understand the theory that a front mounted tool 'digs in', but a rear mounted tool is pushed away.

Martin.

Hey Andy

What the hell are you doing with that Mashedy.?

Now lets be real, I have used lathes from two foot bed to 100 feet long and parting can be a problem. Now I am not talking about brass or aluminium only mild steel to annealed tool steel and the only parting tool I have seen that works with any consistancy in all occassions is that 45 deg tool with the spring steel blade and tungsten tip.

Also if you have to engage the tail stock it means your head stock has movement in the chuck bearing.

Also how many real processes have gone to the grave and have not been passed on. I know of a few.

Larry

Edited By Larry Coleman 1 on 08/01/2015 11:38:21

The front geometry is different from the rear and you get dig-in problems

These dig-in issues become push-away issues at the back

This flexing is tiny, but significant at hobby lathe levels, don't forget that the cutting tip is also a point of resistance on the revolving workpiece

Edited By Ady1 on 08/01/2015 11:41:44

Martin,

I'm with you. in both cases the tool is being pushed away or if you have a lot of top rake this can cause the tool in both cases to be pulled into the work piece with the usual results. bit like climb milling when there is backlash.

My other concern with a rear tool post, if you turn the blade upside down, maintaining normal direction of rotation of the chuck, are you not then changing the directions of the forces acting on the cross slide and saddle. It would seem to me you are trying to lift the saddle which is not a direction it is designed for. It is OK if you are able to reverse the direction of rotation and have the tool facing up but that is not possible on lathes when using a screw on chuck.

I have always parted off with the parting tool in the main tool post and not had many problems, very occasionally breaking a parting blade but put that down to the nature of the operation.

Do not try to part off a long way from the chuck and I would certainly not use a centre in the tailstock as it can cause more problems as the part is actually parted off.

Bob

Yes I can see the sence in that and the swarf is acted on by ytivarg and it may help but most hobby lathes do not have T slots at the back of the compound slide. Also if it jams it will lift the slide and damage the dovetail.

Larry

Has anyone tried one of these ? With the groove along the top, and particularly if you grind a convex "v" on the front, the swarf will be narrower than the slot and should not jam. This is the tool geometry advocated by G.H.T.

Perhaps it's time to re-read the original post. The OP is using a WM 180 lathe, a light 3 1/2 in lathe without power feed or provision for a rear toolpost so advocating the use of either of these is rather pointless. Most other advice has been good.

Russell.

Russell

Yes I have one of those types of parting tools and there not to bad but hey will still jam on steel but that tool I told you about was absolutely unbelievable because if it jammed and it can it does move away from the cut. If you have a look at my album you will see the one I made but I still have to make the spring steel blade.

I think the beauty of that design is it can only move away. The blade made from spring steel would make it better and I think I will have to remove some of the bottom support to make the tool bend better.

In regards to what Andy is saying that method is used in turret lathes and Automatic lathes with good results but think about a jam the piece of steel being parted can still bend into the tool putting max power in a reverse direction to how the compound slide is supposed to operate. I mean the dovetail.

Getting back to actually parting I think water cooling is important because when steel get hot it expands and will reduce the width of the metal where you have cut and jam the blade. Now I have used sulfur oils and got better results.

I must ask my mate with the auto lathes because they part of every component after the process is complete.

That tool I used and I am telling you about was unbelievable and I wish I knew where I can get one.

Larry