Poor quality finish

Myles,

A good finish that turns (oops, there's a pun) bad is exactly what I experienced using poor grade steel.

Some metals, like aluminium and free cutting stainless, produce good finishes with carbide tooling, even at small depths of cut. This doesn't seem to be the case for low carbon steels. However, the finish is not badly degraded with depths of cut in the order of 0.25-0.5mm. I would regard anything up to about 1mm depth of cut as a finishing cut. Roughing cuts are up to 6mm, depending on the material.

If I put on a depth of cut of 0.5mm I expect the lathe to take 1mm off the diameter, so I try not to sneak up on dimensions.

Regards,

Andrew

Hi Myles, I live in Darfield, so also just a bit out of Christchurch, I get my carbide tips at Trade Tools, if you tell them what you are using them for they will advise which is the grade etc, they can also supply tool holders, although I make my own. Another place I'v just found when looking for BA taps and dies is Drills Taps &Dies at Hornby, they have an amazing amount of stuff (so are the prices). Ian S C

I'm new to this lath and milling thing, but I had a smiler problem with my Bantam lathe and it turned out to be the chuck not holding tight enough do you have a different chuck to try.

Al.

Here's a picture of my work so far.

Thanks for all the replies folks. Please bear with me while I work through these and another excellent list of things to try that I got from Tony at the www.lathes.co.uk site : a very helpful chap. I have a wife and teenage daughters so I don't get to spend as much time in the workshop as I would like, that and a host of other projects...

Edited By Myles Douglas-Withers on 21/02/2013 20:25:42

Edited By Myles Douglas-Withers on 21/02/2013 20:39:56

Edited By Myles Douglas-Withers on 21/02/2013 20:40:55

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.

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

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

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

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

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

Andrew

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.

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

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