Avoiding jitter when turning brass

[Rowan Sylvester-BradleyParticipant @rowansylvester-bradley37244]

I'm a beginner at this. I'm trying to make a couple of parts from 19mm a/f brass hex rod. It involves turning down a section to 15mm diameter, cutting a thread, and parting off the part. I am getting some jitter which is spoiling the part. I'm using a tool with no top rake, at the second fastest speed my lathe can do. I have tightened up all the slides, and can't feel any slack anywhere. I have tried no lubricant, and soluble oil.

What am I doing wrong? How do I get it to cut cleanly and leave a good finish?

Thank you – Rowan

[Rowan Sylvester-BradleyParticipant @rowansylvester-bradley37244]

[HopperParticipant @hopper]

What lathe? What speed? How far is the job sticking out of the chuck? Pics?

Try slowing the rpm. Try a tool with a little bit of top rake maybe about 5 degrees. Try a tool with a small radius on the cutting tip. Try rubbing the HSS toolbit on the bench oil stone to make a really sharp cutting edge. Try taking a deep enough cut to eliminate the hex in one cut so you don't have interrupted cut.

[John HaineParticipant @johnhaine32865]

Chatter? What grade is the brass?

[Martin KyteParticipant @martinkyte99762]

Sharper tool and less overhang.

regards Martin

[SillyOldDufferModerator @sillyoldduffer]

Posted by John Haine on 04/07/2022 07:40:18:

Chatter? What grade is the brass?

Most brasses machine well but not all. There are dozens of variants. I have a length of DIY store brass rod which is unusually hard and prone to chatter; it saws, files and polishes OK, otherwise difficult stuff. Their Aluminium is soft and sticky and their steel gritty too.

My DIY store Brass looks identical to the Brass rod bought from my local metal emporium, but his Brass machines with zero bother.

DIY store metal is easy to source, but expensive and liable to be uncooperative on a lathe or mill. It's sold for decorative purposes, not metalwork. For the same reason I avoid unknown scrap.  I'm happy to try it, but after painful experience able to recognise when it's the material causing trouble.

Difficult metals often can be machined, but they're unforgiving, requiring the operator get the tool, rpm, depth-of-cut and feed-rate close to optimum.

The Brass is a likely suspect if the lathe cuts other materials satisfactorily. However, I'd expect hex rod to be a machineable alloy, lube not required, so:

• use sharp tool with minimum overhang. Tool and don't wind the cross-slide out too far
• as hex rod starts with an interrupted cut, check the cutter with a loupe: it may be damaged
• keep length protruding from chuck short, or support with a live centre in the tailstock
• check chuck (worn jaws, loose jaw, dirt or distorted hex rod allowing rocking)
• loose bearings or gibs.
• saddle dipping due to bed wear
• hard spot in a belt
• bent shaft on the motor or elsewhere
• motor running rough
• if, to avoid off-cut waste, a long length of rod is passed through the headstock, make sure the far end can't whip.

I see from other posts the lathe is a Portass PD5, so at least 50 years old. Check for wear, tear and other age related problems. A 0.33HP motor is probably a little undersized for a machine of that size, but adequate – I doubt that's the issue.

Dave

Edited By SillyOldDuffer on 04/07/2022 10:28:03

[not done it yetParticipant @notdoneityet]

don't wind the cross-slide out too far

Can you explain that one, Dave? I’m guessing you meant to type compound, not cross?

[SillyOldDufferModerator @sillyoldduffer]

Posted by not done it yet on 04/07/2022 11:57:42:

don't wind the cross-slide out too far

Can you explain that one, Dave? I’m guessing you meant to type compound, not cross?

Your right! Hope it's not an age thing but what I think, say, type and proof-read are often all different. I was the same at school though!

Dave

[old martParticipant @oldmart]

I was always told to stone the tips of drills used on brass to stop them dragging themselves into the work. Reducing the rake of a lathe tool by reducing the angle of the front face of the tip would have a similar result.

[Rowan Sylvester-BradleyParticipant @rowansylvester-bradley37244]

Thank you for all the helpful advice. Here's a picture of my setup:

I bought the brass hex bar from aluminium_online via ebay. They say it is grade CZ121. The speed I was using was about 300rpm.

I just tried tightening up the headstock bearings a bit (since I could see nowhere else that any play was getting in), and it seems quite a bit better. So maybe this is the root cause of the problem… If I need to replace the headstock bearing shells can I buy them, or do I need to make them? If I need to make them, what metal do I use? Do I need to do anything to the headstock shaft surfaces when I replace the bearing shells? Anything else I need to know to make good replacement bearings? Or should I be fitting roller bearings instead?

Thank you – Rowan

[old martParticipant @oldmart]

You will almost definitely have to make new bearings yourself, the picture does not show whether the bearing clamp is single sided or not. There might be shims in the gap. The best metals to make bearings from are the phosphor bronzes or leaded gunmetal, not brass.

[Rowan Sylvester-BradleyParticipant @rowansylvester-bradley37244]

Thank you for that. Yes, the bearing clamp is single sided.

One related question: if I fit drip oilers to the headstock bearings, do I need to fit them also to the counter shaft? Or does this just not matter because wear in the counter shaft bearings is a lot less critical?

Thank you – Rowan

[old martParticipant @oldmart]

Counter shafts are usually oiled manually, but you certainly can fit drip feed oilers if you want to and there is room.

Edited By old mart on 04/07/2022 18:21:10

[noel shelleyParticipant @noelshelley55608]

ML 7 headstock bearings are of white metal, Alloy E ! Noel.

[John HaineParticipant @johnhaine32865]

CZ121 is nice to machine. But…that looks like an AWFULLY wide tool! If you're really turning with what looks like a 1/4" wide edge it's not surprising you get chatter. If you are aiming to reduce the diameter of the hex down to 15mm you need a knife tool which you feed in the X direction (i.e. parallel to the lathe axis).

[old martParticipant @oldmart]

Where would you obtain white metal, and is it ready to machine into bearings or is there the added complexity of making moulds and casting it?

[Michael GilliganParticipant @michaelgilligan61133]

**LINK**

https://www.tiranti.co.uk/Products/white-metal–tin-alloys

… and, Yes

MichaelG.

Edited By Michael Gilligan on 04/07/2022 18:45:30

[DiogenesIIParticipant @diogenesii]

Rowan, the tool in your picture looks like it has a very wide, 'flat' cutting edge – is this for parting off?

Are you using the same, or a different tool, to reduce the diameter?

[IanTParticipant @iant]

Rowan,

That is a parting tool, which if fed straight into the work (on a small/old lathe like yours) will almost certainly chatter.

A knife tool would be better but you can test this with your existing tool by angling it slightly (so that there is a small angle between the end of the tool and the work piece) and then feeding the tool towards the headstock. Even though it's not the right tool for the job, it should still cut OK, abeit it will be noisy because of the interupted cut.

Take small cuts (small in-feeds) and traverse slowly – all being well it should cut your brass…

Regards,

IanT

[noel shelleyParticipant @noelshelley55608]

Old Mart. I bought an ingot of the correct alloy, as stated by Myford with the intention of making new ML7 bearings. I also bought a spare ML7 headstock to help in this project but things have conspired to stop me as of now. Yes it would need molds and casting then machining. One day I will get round to it ! Noel.

[HopperParticipant @hopper]

+1 on that toolbit is all wrong for this job. You need to grind the other end up to make what is known as a knife tool. See details of the angles here LINK

You dont need to make the radius on the tip as large as shown. Grind the flats on the grinder then put a small radius on the corner by rubbing it on an oil stone.

Some videos on YOutube too giving a run through on tool bit grinding.

[Author]

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