Poor Surface finish

I posted a thread a while ago on my poor surface finish.

https://www.model-engineer.co.uk/forums/postings.asp?th=165810

Since then, I've been persevering and things have been getting better. I've discovered:

• Life seems easier with HSS than carbide - its also cheaper too
• I've tried to be more religious with using the dead centre to hold the other end of the stock still which I think reduces chatter
• I've tried to keep the tool closer to the toolpost so that less is sticking out, again, aiming to reduce chatter.
• I try and use the autofeed all the time as it gives a smoother pass over the surface

one odd thing I still can't track down is that facing cuts are way smoother thatn normal turning operations. I get that lovely crisp sound, no vibes through the machine and a nice clean trace of swarf. It could be that I'm not very good with sharpening HSS yet and somehow whatever point I'm acheiving is better for a facing cut than a regular turning operation.

Anyway - things are better now than they were, but still am finishing off the peices with some emery paper to get a reallly nice finish.

any other suggestions are very welcome.

/A

Not only cheaper than carbide, but HSS is more versatile in that you can shape the tool for best results with the material. Insert tolling will cover most tasks but is a fairly fussy system in some respects. I use both, depending much on what I am doing.

'

Also try to make the ground finish on the tool as smooth as possible.

You could do a lot worse than but a copy of Harold Hall's book on tool and cutter grinding. He tries to tempt you making simple, adjustable grinding-rests but the main thing is careful attention to tool geometry.

Nevertheless, his grinding-rests are intended to greatly enhance an ordinary bench-grinder and help you grind lathe tools to decent standard, including the accuracy needed for things like screw-cutting.

'

Dead-centre... Or the steady if you have one, where applicable. A piece of thin card well-oiled, wrapped round the bar once and its ends trapped in the steady clamp is an old dodge for avoiding the steady marking the work.

'

Emery paper? Well, yes, I use it occasionally but not on the lathe if I can possibly avoid it. It's better to achieve a good standard of tool-finish.

Sounds like a tangential tool holder could be for you, I just bought the 16mm left and right set from eccentric engineering, they weren't cheap, but very well made, not had a chance to try them yet, you could make smaller ones to experiment with quite easily.

Watch the videos on YouTube, it's quite impressive, it holds round HSS also, you can buy carbide of the correct dimensions very cheaply also to use in them, worth a look.

Ace

Make sure you have the HSS tool ground to give clearance from the work after the cutting edge, any drag there will cause poor finish, the term is Back Clearance and needs to be around 5-7 degrees.

Emgee

Edited By Emgee on 15/03/2021 22:23:44

Edited By Emgee on 15/03/2021 22:32:48

Here’s an idea for you to try Ace.

**LINK**

Ace, you don’t say how much material is sticking out of the chuck which can be a factor or if your chuck jaws are not bellmouthed, try reducing spindle speed or use a centre if the stock is long.

That's nothing to worry about. Standard procedure in plenty of professional machine shops. Good way to get the best finish and the last half thou of exact size without going under. Even the great GH Thomas in his books on Workshop Techniques etc recommends it -- but draws the line at using a file to get final finish and size. He recounts this would bring hoots of derision if seen in the machine shop at his "works", but emery cloth or paper was quite socially acceptable. Best to use a little oil on your emery paper or cloth for a good finish.

Some pics of your work and the set ups used to achieve it would help further analysis.

If you have a lathe with power feed for Cross Slide as well as the Saddle, the feed rate is often half for facing compared to that for sliding.

Assuming that your tooling is correctly ground, accurately set on centre height, and rigidly held, the surface speeds and feed rates need to be suitable for the material being machined.

If the work and / or tool are flexing you are unlikely to get a good finish or accuracy.

For HSS and mild steel, 100 feet per minute usually works pretty well. Some materials are ridiculously difficult to machine nicely. Rebar is terrible stuff, not meant for machining and behaves like it!

Tool angles, ideally, need to match the material being cut. A tool ground for mild steel will not function as well on a much harder grade of steel, or on brass.

For most of my finishing cuts, I use a Tangential Turning Tool, either shop made or the Eccentric Engineering version.

I always use a centre height gauge to set tool height, whether HSS or carbide.

Do not grind large clearance angles, since this weakens the tool. Also, the lack of metal behind the edge means that heat cannot be conducted away, so that tool life is shortened.

The exact angle is probably not too critical, a degree or two either way is unlikely to be world stopping, but clearances are important, to ensure that the tool cuts at centre height, without rubbing anywhere else.. Personally, I tend to minimise top rake. (Parting off is often a problem for some folk My inverted parting tool is used with no top rake )

Usually, clearances are in the range of 5 to 10 degrees. Ian Bradley's "Amateurs Workshop", L H Sparey's "Amateurs Lathe" and Harold Halls books or one of the Workshop Practice Series will give advice on tool grinding.

Some times a lubricant or soluble oil, or neat oil will improve matters.

BUT not always, it can result in intermittent cutting! You can reach the situation where the lubricant prevents cutting until a lot more pressure builds up, and the tool then cuts with a vengeance. But having released the pressure, until the oil film is broken again, no cutting takes place! And so the cycles repeats itself.

Remember that even with a fine power feed you are effectively cutting a screw thread. It may only have a pitch of 0.002" (50 microns) and be 0.001" (25 microns ) deep but it is still a helix.

A small radius on the tool will disguise this and improve the finish

A large radius can cause chatter, especially with deeper cuts.

When stoning a rad onto the nose of the tool, be careful not to take more off the cutting point than below it, otherwise the actual cutting point will not be at centre height and the tool will rub rather than cutting as wanted.

Turning will not match a ground finish, although with care good finishes can be acheived.

You are gaining experience, keep working at it!

HTH

Howard

There is a good video on you tube by Oxtools. Where he shows finishing a hss tool on an oil stone. Each time he cuts some steel he says "yeah it's not bad but I think I can do better than that." So you get to see how finishing the hss improves the finish on the work. Worth watching in my opinion.

Indeed - my ability to grind tools well is definitley 'basic' at the moment, so that's the next skill to learn. At present, I'm a bit ham fisted with the bench grinder and so the faces of my tool look "not flat", they look more like geodesic domes at present

all grist to the mill though on a journey of learning.

Ace

I can definitely recommend a tangential turning tool form Eccentric Engineering then, the link is on this site. It only has one side to grind and that is flat. It can be done with a linisher, which is what I do or touched on the flat side of the grinding wheel. The jig that comes with it takes care of the angles. It is my most used tool and it leaves a great finish on Steel Ali and Brass with no messing about whatsoever. You just need to give the top and sides a light rub on an oil stone to get a superb finish. It seems pricey but it honestly works very well.

Apologies to Howard for adding my points (in bold) to his superb and fairly comprehensive listing above, but I think they do show that there are often more ways than one of ‘skinning that cat’.

Polishing with Emery is not a bad practice, if there is a need for a better finish or to lose that last few tenths as a really fine adjustment for size, finish or taper. Just make sure that no abrasive gets in between the lathe moving surfaces.

Hi Richard!

No apologies needed.

None of us are always right on every subject. Your points are good, and add to the information. There will always be an exception to some rule.

I was once told that according to aerodynamic theory a Bumble Bee should not be able to fly!

Tried a shear tool, with 0.001" DOC and feed of 0.0025"/rev. Purely a one off my results, with or without oi neat oil seemed to be the same as freshly ground Tangential tool. Quite impressed, must try it again.

Another case of Horses for Courses.

Whilst I try to avoid using abrasives on the lathe, Emery, used safely, is a form of free hand grinding, so will improve surface finish. My trouble is that my hands are not graduated. The important thing is to clean up afterwards.

Howard

Yes agreed Buffer. I have both bought and Home made Tangential tools and they work very well. I particularly appreciate the built in height adjustment so works great in a four way tool post. I’ve even turned HSS in my lathe with one using a carbide cutter.

Hi Howard if you use emery cloth on a lathe as I do sometimes, don't use a finger to hold it use a piece of stick or wooden part like a sanding block but thinner. I saw a work mate twist of his finger whilst polishing a champher on a piece of steel tapped 3/4 UNF. He had just turned a lead in champher and got a slight chatter and using his finger he pressed the small piece of emery on to the champher when it grabbed and screwed his finger into the hole and twisted off his finger at the second knuckle. Also if you wrap a piece of emery round the outside it can grab and pull in your hand. Use a piece of wood with the emery around to press on to the piece.

David

Edited By David George 1 on 16/03/2021 12:23:35

Hi David!

Quite correct.

I used the word "safely" implying wrapping the emery along a piece of wood, (Which Neil Wyatt pointed out some time ago, is the approved method )

Lollipop sticks are handy way of holding the emery.

If you don't, you could become a lot more wrapped up in your work than you want!

Howard