Poor quality finish

[SpannaParticipant @spanna]

Take a facing cut to the centre of the bar, or a shorter one thats close to the chuck and center the tool until. no pip is left.

Remember, its a lathe and not a cylindrical grinder. Experience will improve your work finish. You have a lot to learn, so go and play for a while.

You need an experience person to be with you for a while. A lathe can be a dangerous place. Hope you are wearing safety specs all the time you are beside a running machine.

[John McNamaraParticipant @johnmcnamara74883]

Hi Myles

That looks like a rather fine feedrate to me?

What is the feed set at? A very fine feed and can cause rings like that with carbide particularly if the tool is a little blunt. Even a new tip can be blunt. Depending on its intended use.

You may find increasing the feed will give a better finish

Or maybe the tool is set a little high (Above centre?) causing rubbing

It looks like the tool is rubbing rather than cutting.

Cheers

John

[Clive HartlandParticipant @clivehartland94829]

I am in agreement with John, but I also think you might have some saddle play and it may be worth checking the saddle gib strip to see if the play is excessive. Check it up by the chuck and then check back by the tailstock and see if there is any difference in play. Most wear is up under the chuck or thereabouts. With a broader cutting edge on the tool it may give you a better finish and also hone the cutting edge, practice makes pefect. Are you turning using the top slide or traversing using the saddle, it makes a big difference.

Clive

Edited By Clive Hartland on 22/02/2013 17:24:14

[Myles Douglas-WithersParticipant @mylesdouglas-withers29517]

It is a very fine feed rate, one off the slowest I think so I will try speeding it up. I am wearing my glasses and I have done some reading and watched some video about safety so I hope I'm not too great a risk.

I am traversing using the saddle, the top slide I have locked. I have made cuts with the cross slide locked also but it does not make much difference. I set the tool until it would face without leaving a pip and I have also tried it set a fraction low.

Clive, what method should I use to check saddle play? I have DTI and magnetic base but what would I measure against what?

Thanks to all

Myles

[Myles Douglas-WithersParticipant @mylesdouglas-withers29517]

I have tried making cuts with both the three jaw and four jaw chucks – again not much difference.

[Anonymous]

I rarely use less than 4 thou/rev for finishing, if that's any help.

Andrew

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

When using inserted tip tooling with the normal small radius at the cutting edge something some people find a little bit counter intuitive happens when you try taking very fine cuts .

With deep cuts a large part of the tip radius engages the work and any one bit of work surface gets cut progressively by different parts of the cutter tip as it passes along . In this way the screw surface that is really being cut by the tool gets successively finer and finer cleaning up cuts as well and the finished surface is very smooth .

When the cut is reduced below a critical level only one small area of the tip is effective and the successive cutting and cleaning up action stops working and you start just cutting a screw thread .

Apart from simply cutting a rather rough screw the actual cutting action is harsher in this light cutting mode and surface tearing becomes more likely .

For fine finishing cuts the remedies are either :

Use much finer feed – will sometimes work but not guaranteed and anyway a bit tedious .

Use a different tool tip – one with a much larger radius or one set to a more glancing angle relative to the work surface . If you get it right the feed can be quite coarse .

When using tipped tooling there are lessons to be learned from simple HSS tooling shapes – study roughing and finishing tools as used by professional turners and in particular the now almost forgotten ' broad finishing tool ' and the way that it was used to get superb finishes at very high production feed rates .

Michael Williams .

[Myles Douglas-WithersParticipant @mylesdouglas-withers29517]

A quick update (I've been away on hols). I followed some advice from above and tried a deeper cut at a faster feed and got a better result. It was still a rough surface but much more even, before you could easily feel ridges in the surface but now it is smoother. I will try the same thing with a larger diameter insert that I have.

[I.M. OUTAHEREParticipant @i-m-outahere]

Myles , looking at your photo it seems as there is a black finish to the piece of steel you are machining ,i would suggest this is not a free machining steel but could be anything and more than likely black mild steel or black round bar .

This stuff is usually crap to machine at the best of times as it can have tougher layers and found getting a decent finish difficult with TC tooling so i did the bulk of the material removal wth this then switched to a very sharp tool steel bit for finishing and i found it a lot better .

Keep tool overhang to a minimum also .

Wait until you cop some work hardening steel then you're in for some fun !

Ian

[michael howarth 1Participant @michaelhowarth1]

After reading the excellent advice tendered here by experienced machinists, as a novice, I hesitate to speak on this subject but I can recall the times when I could not get a decent finish for love nor money until I built a tangential tool. That piece of kit must be very forgiving of inexperience and lathe set up etc because it invariably gives a decent result on all materials.

Mick

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