Rough Finish

[pyrobrewer]

I am facing a large piece of mild steel flat bar that is just under maximum size for the swing of my lathe (SC4). It is a heavy but balanced and have set the speed for 400 rpm

I am getting a good finish on the outer portion of the piece but quality goes to crap toward the centre.
I guessing the cutting rate is changing as it nears the centre and I am reluctant to increase speed as the piece is quite heavy. Any tips

 

[chrsbrbnk]

try an oil based cutting fluid , change cutter tip radius , change cutter rake the cutting speed goes to zero as it gets to the middle but the feed rate remains the the same per rev, of the spindle. check cutter center height

 

[Forestgnome]

Looks to me like maybe the tool isn't on center. You didn't give enough info, but I'd use an HSS tool and use a finishing cut of 10-20 thousandths after sharpening the tool. You might be dulling the tool on the interrupted cuts. A lighter cut might extend the life of the tool through the duration of the cut.

 

[dave-in-england]

Facing off common or garden Mild steel never gives a good finish, even when turned under ideal constant surface speed conditions on a CNC lathe.
The leaded EN8a variety gives a slightly better cut and finish.

The intermittent cut here on a small and not too rigid lathe, it is going to be futile trying any more.

Your best option is to take the plate to a engineering company and have it cleaned up on a surface grinder.

You will then have a smooth finish which will also be perfectly flat.

dave's $0.2 opinion

 

[jwcnc1911]

as you go in, surface footage becomes "smaller" so the closer to center you need to increase rmp to match that of the outer where the finish is good.

Google "surface footage explanation" to see more

 

[Arcane]

This might be completely off base but you might check the cutting edge of your bit, especially if it's an indexable carbide bit and check that your bit holder is locking down tight. I had a similar experience once making an almost identical cut. I was in a hurry and was taking a too heavy cut and didn't lock down the tool holder (KDK) tight enough in my haste. The bit biting into the two metal edges every revolution eventually took it's toll, chipped the cutting edge on a carbide insert and combined with a loose tool holder... well, everything went to hell in a hurry. And cheap inserts sometime seem to chip as if by magic. I don't know what setup you have so maybe this is not relevant...

Best of luck,

billy

 

[Swifty]

As others have stated, it's all to do with the decrease in surface speed the nearer you get to the centre. Instead of cutting, the material is being ripped off leaving a rough finish. Try to make sure that your tool is sharp to promote cutting.

Paul.

 

[Tin Falcon]

The likely causes have been mentioned several times so will not beat on that.

This is one time where a fly cutter on a mill will do a better job than a lathe.
IMHO having it surface ground while will solve the problem is overkill and counter intuitive the point here is home shop machining.
I am guessing you do not have a mill or a large enough one for this piece.
Personally I would take the item to the belt sander and smooth down the ridges.
But you may not have a belt sander with metal cutting belt. so silicon carbide paper and oil should smooth out the ridges . A little flat filing to start may speed things up.
Tool makers and die repairmen have used files and abrasives for years to smooth and correct a lot worse. Your shop your dime you are the QC guy that needs to be made happy.
Tin

 

[goldstar31]

I have to disagree with most of the comments.
There are basically only two ways to fine finish metal apart from burnishing it and these are by cutting with a finely honed tool and secondly with a succession of abrasive materials. In professional practice both methods will be adopted. In other words, a highly finished part will be machined almost to size and then finished on a grinder. We are a long way from that and in all probability we haven't got a grinder. What we seem to have is ill set or ill ground and finished tools- or shudder- both.

Normally, a home lathe is set to first remove excess metal and then reset to remove the last bits to size and to a high finish. That means honing the finishing tool and then setting it to rub or to shave the last few 'tenths'.

What the expert will always repeat is that the finish reflects to finish on the cutting tool.

I've read a great deal about this and that here and elsewhere but the tool which does the work is the one which is so sharp that it will not only pare one's finger nail but reflect the dirt in it.

If it doesn't then one has to resort to poorer expedients. 'Nuff said

 

[WOB]

What you are witnessing is what my old college machine shop instructor call cutting below "critical speed". At the first lab class, he turned a piece of steel about 6" in dia. The rpm was set to show that the outer parts of the piece were smoothly turned and how the finish deteriorated when cutting speed dropped off as the center of the piece was reached by the tool. His suggestion was to speed up the lathe as needed to maintain cutting speed above critical as much as possible. This is less of a problem with carbide tooling but is a problematic with HSS tools due to their cutting speed limitations.

CNC lathes are often programmed to speed up as needed to maintain surface finish.

WOB

 

[Philipintexas]

I've looked at your photo and read the replies and I'm still intrigued by your dilemma. I make a part of aluminum that requires a similar cut but I get no such problem. May I ask how you hold the material? is it in a 4 jaw chuck gripped at both ends and near center on the sides, or is there a protrusion on the back side at the center of rotation held in a 3-jaw? It's odd that when the cut goes bad it isn't the same across the part at a given diameter, but starts near the edges with some good results but goes bad part way across the material. Also, I assume you are cutting from outside toward the center ? My take is there is some resonance caused by poor support of the material that allows this to happen. Does it destroy the cutting edge of your tool? I've turned lots of solid steel to the center at many different surface speeds, certainly slower than you are using, but never saw a result like yours.
Again, is the material solidly held near the center or does it flex away from the cutter???

 

[goldstar31]

Somewhere about 1955 or so, an article was written up in Model Engineer. The writer was using a 7 x 19( a Myford ML7) and was producing swarf which was what he described as 'barely perceptible' and this was far finer than the finest steel wool.

I was writing about this condition which was made using hss tooling and ground up on a home made double ended grinder as these things were too expensive for home workers like him.

He hadn't a milling machine and his drilling machine was also home made. There is no mystery in all this- people are still doing this on home made jigs.

That is what I am writing about.

 

[rcfreak177]

I totally agree with the statements in regards to surface speed being too slow towards the center.

Yes surface grinder is an option povriding you have the access to a machine or have the funds to take it to a shop.

As Tin said, fly cutting on a milling machine will give you a good even finish, providing you have access to a milling machine.

Also a carefully honed HSS tool may work as well, although with the intermittent cut this will probably dull the edge quite quickly and there is still the surface speed issue. Done correctly will leave a nice even finish though.

There is another way that comes too mind, may be a bit crude and is not the most healthy option for you'r lathe. In the past I have made a bracket which holds a 4" angle grinder in the tool post with the disk roughly parallel with the job surface and mounted a flapper disk. Then used this to act as a surface grinder as such, obviously the part will need to be rotating, generally the opposite direction to the grinder wheel. This has given great results in the past providing you only apply light cuts with a fairly slow feed and spindle speed to match.

Cleaning all excess oil from the machine to stop grinding dust sticking and setting up the lathe with newspaper or similar across the bed-ways is crucial also thorough cleaning and oiling afterwords to prevent premature wear.

Baz.

 

[Swifty]

Philipintexas, you mention that you have no trouble finishing aluminium of this size in your lathe, but you must bear in mind that the original poster was machining mild steel. Some plate mild steel is fairly dubious when it comes to its properties and can be very hard to get a finished surface with a single point tool. On the other hand, aluminium is far more easy to machine and gentler on the tooling.

Paul.

 

[Forestgnome]

If you all aren't getting a better finish than that facing mild steel you're doing something wrong. I know it won't be as good as grinding or flycutting, but way better than the photo.

 

[goldstar31]

Sorry- no! One doesn't use a single point toil. To finish one 'rubs' the part with the side of a lathe tool-- not the tip.

It seems that I'm not really getting anywhere here. We have got the simplest problem of the simplest tool in the workshop. On another forum a couple of us were happily prattling about removing metal. He had trained as a fitter to do one of the first jobs of an apprentice and that was to make a set of three accurate reference plates by filing, scraping and finishing with abrasives.
I'd overhauled several machine tools and a lot of 'funny things' as well. The important thing was that we both knew how to scrape minute amounts of metal- with the slightly rounded edge of a scraper- not the tips.

Politely, can we re-run the problem- using traditionally accepted practices? All that a lathe or mill is doing is exactly the same as that by another generation who did it did it by hand.

 

[Philipintexas]

SWIFTY: Yes, I realize alum. is different, just saying I think this is a problem of lack of rigidity in the work as opposed to the cutting speed. As I said, I've cut to the center of junk steel at slow speed and never experienced this kind of finish.
I think the poster wants an answer to his particular problem not suggestions of different methods that might be better.
Given what he's trying to do, I think it can be done with decent results if there is adequate rigidity in the work piece and tool.

I just did a trial cut similar to the one originally posted. 2" X 4.5" X 3/8" thick hot-rolled steel (?) in a 4-jaw chuck, light cut (.025) with a HHS tool @ 425 RPM and a slow feed, no lubricant. My results are, the finish improves toward the center of rotation. As expected, the interrupted cut is slightly rough and finish improves when the cut is continuous even though it is painfully slow surface speed near the center.

To address the original question, Even though it's not necessarily the best tool for his job, what caused his finish to go to pot?

 

[pyrobrewer]

I think i'll just get a mill! Needed a good excuse
I'm thinking Seig X2 or X3 family (liking X3) Any one had both?

 

[Forestgnome]

I think i'll just get a mill! Needed a good excuse
I'm thinking Seig X2 or X3 family (liking X3) Any one had both?

Getting a mill doesn't fix the problem. Just like the lathe, you still need to learn how to use it. All of the same issues apply to both machines, speed, feed tool geometry, etc.

 

[danstir]

Not having done machining that long myself I created several pieces that looked like this. Eventually I seemed to get better by practicing sharpening my HSS tool and by getting over my fear of turning the work piece at a higher speed. Hope this helps.