Machining what am I doing wrong?

[Stephen RowleyParticipant @stephenrowley13300]

I have a Chester 920 lathe and as you can see from the pictures I can not get a good finish on any metal apart from brass or bronzes. I have included a picture of the tools I have. The tool have all had new bit.

 

When I am machining I can only cut down to 20thou of the finished size as I then have to finish with a fill and then polish to get a good finish.

 

I know I am doing something wrong but what?

I have tried different tools, depth of cut, speed and am now at a loss can anyone help.

 

James.

[Stephen RowleyParticipant @stephenrowley13300]

[The Merry MillerParticipant @themerrymiller]

It would be interesting to know what the specs. are of the materials you are having trouble with.

I suggest that most of us have had experience with crap steels in the past.

 

I recently picked up a bit of scrap mild steel that had been the holder for a diamond wheel dresser and could only get a rubbish finish on it no matter how I sharpened and honed the cutting tool. The fact that the tool came from the far east say’s it all.

To prove to myself I wasn’t dreaming I chucked up a piece of 303, the same diameter and got an almost mirror finish on it.

[Martin WParticipant @martinw]

James

 

Some simple questions, other than the brass/bronze materials do you know the type of steel, assuming that is a piece of steel in the picture, you are trying to turn. Are you turning the material at the right speed with the tool height set at the centre line of the workpiece.

 

If you haven’t checked these adjustments then put a centre, either fixed or revolving, in the tailstock and lock it down. Move the tool to the centre and set the cutting edge to the same height as the centre point. If you don’t know the type of steel being turned then buy some EN1 or similar then turn at the correct speed, see tables for turning speeds, often quoted in fpm, and see if this improves the situation. Secondly I would recommend the use of HSS, high speed steel, tools rather than tipped items as a starting point, once you get proficient with them then go over to tipped tools.

 

Hope the above helps and is not teaching ‘my granny how to suck eggs’ as they say.

 

Good luck

 

Martin

[JasonBModerator @jasonb]

Have you checked to see if the tools have their tips perfectly on centre height, I seem to recall some of the sets of tools supplied with chester lathes are too large to fit below ctr height.

 

J

[Tony Pratt 1Participant @tonypratt1]

Check centre height of tool, you are using carbide tools so faster rpm is usually better. Try cutting oil and slower feed. What diameter steel are you using?

Tony

[adyParticipant @ady]

Looks like pretty bad chatter to me.

 

The most important thing with carbide is stiffness, you can see the grooves as the cut jumps in and out of the workpiece.

Power is less important until you suss things out.

Are those rough sharp bits of surface swarf when you stroke it with your finger?

Good tailstock support for the workpiece is critical, if you don’t have a good running centre then make or buy one.

 

The other thing is centre height.

You only need to be a teeny bit too high and carbide squeaks and rubs badly, it’s so hard.

Don’t shove it in, present it to the workpiece and the hardness of the tool, combined with the stiffness of the workpiece, do all the work for you.

 

Once it’s cutting properly, carbide is almost silent, a quiet hiss as the metal falls away.

Then you can crank up the power and speed.

 

You can see what a carbide tool can do in my album piccies, they are almost diamond hard and do amazing amounts of work once you suss them out.

 

[adyParticipant @ady]

Once it’s sorted don’t cut too slowly with carbide, it’s different from hss and most of the heat gets transferred to the chips, making them go blue.

 

You can race along a workpiece about twice as fast with carbide, and with a generous cut as well, if you go too slowly you will dull the tool prematurely.

 

If you don’t have a green grit grinding wheel then get one, carbide only needs a tiny amount of redressing when it goes off and then you’re back to making vast amounts of chips in next to no time.

 

Even with the intermittent cuts on that reinforcing bar it made a kind of purring noise as it cut, like a smoothly running motor boat engine.

It’s totally amazing stuff once you get it sussed.

Edited By ady on 22/05/2011 22:02:04

[Stephen RowleyParticipant @stephenrowley13300]

Thank you everyone for the information.

It seem as if it must be the centre hight that is out but as it is now 23.00 I will try it in the morning.

 

A one or two of thing I should have mentioned at the start, I have tried all sorts of metal and the peace in the picture is supposed to be mild steel. I used the slowest feed speed, chuck speed of about 600 RPM and a cut depth of 15 thou on the bit in the picture and yes sometimes it makes a lot of noise and if I was to run my finger along the work it would be ripped to shreds.

 

Once again thank you all for your invaluable help.

 

James

[Ian S CParticipant @iansc]

jsk,Ithink I would use a bit deeper cut, try .030″ and speed up the feed rate, one or two notches. Butas others have said get going well with HSS before worring about Carbide tooling.

ady, is’nt the quiet hiss the sound of burning flesh as the hot blue chips land on the back of your hand, soon to be followed by “Oh Bother” or some thing similar. Ian S C

[mickParticipant @mick65121]

I assume that the work piece is supported by a back centre or pressure pad? Carbide isn’t always the answer to every maiden’s prayer, you need the right conditions regarding speed, which needs to be top range and feed, which also needs to be quick. Mild steel comes in many grades, I would hazzard a guess and say the steel you are machining is comercial BMS, which can be a problem, where inserted tooling needs high revs and feed to produce any kind of finish. Try Cobolt HSS tools, quite often the old tried and proven route is best.

[Gordon WParticipant @gordonw]

I was /am a novice at this game, the pic. looks like my early efforts. I see you have insert tips, these can chip very easily ,especially when we are learning. Worth looking at the tips, even use a magnifying glass.

[mgjParticipant @mgj]

I beg to differ about carbide and feeds and speeds. Use it all the tipped tools at exactly the same speeds that I use with HSS – works perfectly, with several different makes of tips in both positive and negative rake configurations. For those who don’t believe, go try it.(Yes I know carbide will go a lot faster, but its nonsense to suggest it HAS to – especially on a lathe where coolant is being used, for the very good reason that the coolant can go everywhere which is messy and uncomfortable)

 

As for all this advice to use HSS, well that’s fine if he has a means of grinding it and the skill to grind it, at least adequately. If not, he’s much better off with indexable carbide tips because he should have a tool which will cut and is of known quantity – and dare I reiterate, doesn’t need to be run at warp speed to cut effectively.

 

So as with all things, let us start at the beginning.

Head drive belts are properly tight and not permitting skidding?

Are the lathe bearings good and not allowing deflection.

Is the chuck in good nick and not permitting movement (try the 4 and 3 jaws)(you are giving the chuck jaws something to grip ie your test bar extends right to the back of the chuck jaws?)

Is the tool at centre height.

Is the tool holder holding the tip securely and correctly – we had a case where the tip holder was causing rubbing. Look at it carefuly when iti s in place and you should see only line contact – if it is above centre usually you get rubbing. If below centre you can get tearing. check against a point cnetre held i nhte tialstock.

If you have a 1″ bar are you cutting at about 4-500 rpm with a feed rate of about .005″ per rev. (or metric equivalent) Because that will give you a good finish especially in 220M07 or free cutting mild..

Are the saddle and cross-slide gibs properly adjusted after having been degreased from new.

Is the feedscrew nut properly secured and is not permitting the x slide to bounce back and forth. And is the other end of the feedscrew properly adjusted by the handle.

When testing is the tailstock engaged to provide support.

Tool overhang?

Toolholder securely locked in place?

Ccorrect tips – you are not using some funny tip designed for some strange metal – ie you have a steel cutting general purpose grade?

You are using coolant – yes?(At which point a lot of people say they cut dry. Good luck to them, a dose of coolant produces a finer finish and tools last longer. My definition of a good finish is =/-.002″ on diameter, and a mirror shine. )

 

If you have all those addressed, and it is still doing this, then you have a problem, but not until..

[JasonBModerator @jasonb]

Can I ask which of the 4 tools shown you are using, for turning along the length of a bar the only tool with the right shape is the one on the right, if you click on the picture to enlarge the image it looks as blunt as a very blunt thing. This seems to be a brazed carbide tool which you will not be able to sharpen on a standard bench grinder, it needs one with a “gree grit” wheel

 

The middle lefthand tool could be used but you need to mount this with the shank along the lateh axis and its really best used for facing cuts.

 

The far L/H one is best kept for getting into tight spots or a bit of rough whitworth threading.

 

The round profiling tool is best kept for freehand profiling.

 

Jason

Edited By JasonB on 23/05/2011 19:04:03

[Stephen RowleyParticipant @stephenrowley13300]

Well today I went though everything.

I found play in the cross slide then I put a 2′ long bar in the chuck and found that there was movement in the spindle, the tool was 20 thou to low and one needed a new tip.

Now with all that fixed I get a much better finish but the tools some time screech like hell.

 

JSK

[Clive HartlandParticipant @clivehartland94829]

I will tell you a tale of a man who came to us and proffessed to know all about maching being skilled on Mills and lathes.

He went to the lathe and all I could hear was swear words and I could see clouds of cigarette smoke as he puffed vigorously, I leaned over his shoulder and could see he was running the lathe backwards.

I said nothing and walked away and vowed not to ask him to do any work for me.

 

Clive

[mgjParticipant @mgj]

If they screech = tailstock in for support? Thats the common one.

 

Technically you should use the tailstock and a running centre all the time. None of us do but any appreciable overhang out of the chuck and its a good idea.

 

Screeching (or chatter?) is a function of the undamped natural frequency of the set up, so you can change Fn, and remove the chatter.- generally by stiffening the setup and thereby increasing Fn. It’s caused by rapid (high frequency) small movements, and because the work is moving away from the tool and springing back, the cut is inconsistent and the finish usually lousy.

 

Another way of getting rid of it is often to shorten the overhang of the tool from the toolpost. That overhang should always be the minimum possible for the job in hand.

 

Oh and reduce revs and (often) increase feed a bit.

 

Glad things are getting better.

[Anonymous]

Posted by mgj on 23/05/2011 18:33:13:

I beg to differ about carbide and feeds and speeds. Use it all the tipped tools at exactly the same speeds that I use with HSS – works perfectly, with several different makes of tips in both positive and negative rake configurations. For those who don’t believe, go try it.(Yes I know carbide will go a lot faster, but its nonsense to suggest it HAS to – especially on a lathe where coolant is being used, for the very good reason that the coolant can go everywhere which is messy and uncomfortable)

 

You are using coolant – yes?(At which point a lot of people say they cut dry. Good luck to them, a dose of coolant produces a finer finish and tools last longer. My definition of a good finish is =/-.002″ on diameter, and a mirror shine.

 

(Selective quote to save space)

 

Hmmmmm, time for an experiment. I did a series of four trials. Common features were:

 

Lathe: Harrison M300, running from a true three phase supply

Material: 1.375″ diameter steel EN3B

Depth of Cut: 40 thou, ie, 80 thou off the diameter

Feed: 4 thou per rev

Coolant: Biokool14 from Hallett Oils, or whatever they’re called these days

Tool Tip: Korloy – CCMT postive rake insert 0.4mm nose radius, grade NC3120, general purpose turning of steel

 

The four trials were:

 

Trial 1: 260rpm (94fpm), no coolant

Trial 2: 260rpm (94fpm), coolant

Trial 3: 1200rpm (432fpm), coolant

Trial 4: 1200rpm (432fpm), no coolant

 

Pictures of the four turned surfaces are in one of my photo albums, although my photography is not really good enough to be able to discern the differences.

 

General appearance:

 

Trial 1: Surface is rough and inconsistent axially, if wiped with a paper towel it picks up fibres

Trial 2: Looks and feels slightly smoother and is more even axially than 1

Trial 3: Smoother and more shiny than 1 & 2, but is inconsistent axially

Trial 4: Surface feels smoother than 3, is very uniform axially and shiny

 

Under a magnifying glass:

 

Trial 1: Surface is dull, torn and uneven

Trial 2: Surface is dull and torn, but not as uneven as 1

Trial 3: Surface is slightly torn, but is also slightly burnished

Trial 4: Surface is much more highly burnished than 3

 

Swarf:

 

Trial 1: Came off in small, cold, uneven pieces, generally a few mm long

Trial 2: Came off in small, cold, uneven pieces, generally a few mm long

Trial 3: Came off in long cold curls

Trial 4: Came off in pretty dark blue long curls; longest piece was over a metre

Measurement of Surface Roughness (Ra), taken axially at three points around the periphery (all in micrometres):

 

Trial 1: 6.01 6.75 6.30 Average=6.35

Trial 2: 4.97 4.52 4.16 Average=4.55

Trial 3: 2.72 2.62 3.05 Average=2.80

Trial 4: 2.12 2.45 2.76 Average=2.44

 

So, for me, using carbide at high speed without coolant, gives the best finish and appearance. Given what I understand about the cutting process that makes sense to me. It is interesting to note that at slow speeds coolant helps, at higher speeds it doesn’t. I can think of reasons why that might be, but have no definitive proof.

 

Disclaimer: The above doesn’t mean mgj is wrong, just that it doesn’t work for me!

 

Regards,

 

Andrew

Edited By Andrew Johnston on 25/05/2011 21:53:48

[mgjParticipant @mgj]

Andrew – thats putting it very politely.

 

I can get a mirror finish at 4-500 rpm (use for almost everything because I like it on the big lathe. 600 on the Myford), and Morrison Edgeplus. Feed for finishing will be .005 per rev.

 

Tips are either Sandvik or SECO with .4mm radius. I did try some other ones and I didn’t like them – bought them recently from Chronos. I normally take 2 final cuts of .010 to finish Thats .020 on dia. And genuinely I don’t do better with HSS, either in a tangential holder or a conventional knife shape with more conservative rakes. I don’t have the ability to measure surface finish to CLA.

 

However I have found that HSS at higher rake angles is more flexible in its use, in that it will tolerate higher feeds and cuts. Sandvik tips particularly don’t like being pushed outside their recommended feed rate, SECO don’t mind, and will take a bigger cut and greater feed than the Sandviks. And the chipbreaker is better too – I don’t like long curling chips!! (Get them when I’m taking 250 thou on radius on the Myford with HSS and high rake angles – PITA) the only trouble with a conventioanlly ground HSS tool is hte trouble regrinding, compared with just changing a tip.

Korloy I haven’t used, but clearly tips differ in their performance. However for JSKs purposes, I would still argue that he’s better off getting things sorted with carbide and amateurs/beginners rates than with possibly blunt HSS.

 

The interesting thing you didn’t perhaps mention was thermal stability. You start turning metal at the speeds you mentioned, and it gets pretty hot – enough to make a difference on diameter? Certainly if you have a few passes to make to form a deepish shoulder?

 

There can also be other factors too.

The chuck – mine happens to be a brand new Pratt in the big lathe, and the Myford also has P&B chucks.

 

Headstock bearings. On the Chinaman at least I have some brand new (and epically expensive) precision taper rollers from Germany. Long sad story but they made a difference. The Myford is still on its old bronze cone, but the tail end bearings are new and very smart, and it is very well adjusted.

 

So lets just say then that carbide can perform very well in “amateur” conditions, depending on the circumstances. That avoids an argument

Edited By mgj on 25/05/2011 22:34:08

[Anonymous]

Hi mgj,

 

Errr, I’m not looking for an argument!

 

Here are a few random notes:

 

I have very little experience of using HSS tooling on the lathe, mainly specials, screw cutting and form tools, so I don’t really have any comparative experience. I’ve never used a tangential holder.

 

The trial turning was done using a Burnerd collet chuck. I’ve no idea about the spindle bearings, I assume that they are the original Gamet bearings as fitted by Harrison.

 

I’ve no doubt that different brands of tip produce varying results but, like most of us, I don’t have the luxury of doing comparative trials. So far the Korloy ones seem pretty good and very resistant to impact damage, and believe me I’ve done some pretty heavy interrupted cutting. In general I buy recognised makes of tip from professional tool suppliers. It’s a problem faced by the professionals too, judging by some of the posts on the practicalmachinist forum.

 

In theory most of the heat from the cutting process should be in the chip, which is why it is blue. However, you are right that the workpiece does get warm. I’ve never measured the temperature, but I’ve always been able to hold it for at least a few seconds. That means less than 100°C, probably about 80°C? So from 20°C that’s a 60°C rise. The linear thermal coefficient of expansion for carbon steel is about 11×10-6. So for a 2″ workpiece that’s an increase of about 1.32 thou on the diameter. If dimensions are critical (less than a thou) then I’ll go and make a cup of tea or do check of the drawing etc while the workpiece cools down.

 

I deliberately chose EN3B because I find it, and hot rolled steel, a pig to machine while getting a good finish.

 

I rather suspect that my conclusions would not apply to other materials. The weather over the Bank holiday weekend isn’t looking great, so I probably won’t be flying. If that proves to be the case I might find time to do so tests on other materials. I have in mind free machining EN1A, aluminium 6082, brass CZ121 and stainless steel grade 303.

 

If I do so I’ll post in a new thread, rather than hijack this one again.

 

Regards,

 

Andrew

[JasonBModerator @jasonb]

I think the M300 (and big chinaman)will have very different properties to a 920 lathe which makes your tests somewhat irrelevant to the OP as the amount of flex in a 920 and the other small hobby lathes means that the carbide tooling is just pushed off the surface.

 

THey would be pushed to take a 40thou cut, and trying to sneak up on a finished size they will just not take that last 1-2though off, certainly not with the commonly used inserts that most seem to use.

 

Jason

Edited By JasonB on 26/05/2011 07:44:47

[Stephen RowleyParticipant @stephenrowley13300]

Oops have I started something!

 

All is running well now thanks to all the good advice but, would it be a good idea to get a quick change tool post?

 

James

[NJHParticipant @njh]

Hi James

 

In my experience – yes a very good idea. Not cheap but very useful.

 

regards

 

Norman

[The Merry MillerParticipant @themerrymiller]

If your getting on a bit and are running out of time then yes but if you are a youngster then whats the rush.

[mgjParticipant @mgj]

My experience is mostly in 220M07/ EN1A and with that you can take shaving cuts with carbide as easily as with HSS. So you can on the Myford.I don’t like to, but you can.

 

Commonly used inserts – no idea. Chronos were supplying tools with SECO inserts, which is how I met them. Greenwoods were supplying Sandvik tips, and DHS my local toolshop is a Sandvik agent, but gets me SECO. I also have Stellram non ISO tips. you can’t tell hte difference between any of htem in free cutting mild, apart from hte Sanviks which don’t like so much feed. (Taht doesn’t apply to .8mmradius tips) Take your pick.

 

At the end of the day, the material has no idea wheter its being cut with carbide, HSS or a hacksaw. The difference is to do with rake angles.

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