Parting On a Hobby Lathe

[Johnboy25Participant @johnboy25]

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

[Bob Brown 1Participant @bobbrown1]

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

[Larry Coleman 1Participant @larrycoleman1]

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

[Ady1Participant @ady1]

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

[blowlampParticipant @blowlamp]

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

Martin.

[Larry Coleman 1Participant @larrycoleman1]

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

[Ady1Participant @ady1]

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

[Bob Brown 1Participant @bobbrown1]

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

[Larry Coleman 1Participant @larrycoleman1]

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

[Russell EberhardtParticipant @russelleberhardt48058]

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.

[Larry Coleman 1Participant @larrycoleman1]

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

[mark costello 1Participant @markcostello1]

I have a 14 1/2" South Bend lathe, paring off with a normal parting tool in the front is an exercise in futility. An upside down parting tool on the front side makes it painless, I even have a screwed on chuck and am being careful.

[JasonBModerator @jasonb]

Posted by Larry Coleman 1 on 08/01/2015 12:57:25:

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.

Larry

Not a problem if using tipped tools

1 . The tip itself tapers back from the cutting edge

2. The holder is quite a bit narrower than the tip

Also I would say at least 80% of hobby lathes don't have a coolant system

J

Edited By JasonB on 08/01/2015 16:34:05

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by JasonB on 08/01/2015 16:32:50:

Also I would say at least 80% of hobby lathes don't have a coolant system

I use one of these to get suds into the cut for parting steel. At £0.99 including postage anyone can get a coolant system!

Russell.

[JasonBModerator @jasonb]

Lid off a spray can free, pack of 10 brushed for a quid off the poundland tool sellers works for me.

But thats not flood coolant.

[David CambridgeParticipant @davidcambridge45658]

Thanks everyone for the input on this. I tried to get the inverted parting tool holder from eccentric engineering, but unfortunately they didn’t have any in stock so yesterday I made my own. What a result! It was magic and I parted off a 1.5 inch aluminium bar without issue.

David

Home made parting tool holder :-

[VicParticipant @vic]

Any more pics David, perhaps with the blade in it please?

[David CambridgeParticipant @davidcambridge45658]

Vic (and everyone else)

My original parting tool came from RDG tools and had a 4 x ½ x 3/32 blade

 

When the blade is upside down then the cutting edge is at the wrong height. The home made parting tool is similar, but with the section that clamps in the lathe at a different height to correct for this (see below)

 

I used a tilting table for the ~25 deg angle at the top, and a 3/32 slitting saw for the groove the blade sits in at the bottom.

David

Edited By David Cambridge on 26/01/2015 18:32:47

Edited By David Cambridge on 26/01/2015 18:33:35

Edited By David Cambridge on 26/01/2015 18:34:19

Edited By David Cambridge on 26/01/2015 18:42:09

[VicParticipant @vic]

Thanks for that David, very helpful. I actually started making mine a couple of weeks ago but have had other things to do. Mine has to have the main body in two parts due to lack of stock material! I'll post a pic when it's done.

[Author]

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