Which gives best finish out of the stainless steels?

[Chris DentonParticipant @chrisdenton53037]

I need to make a couple of parts on the lathe and wondering whether to use 303 or 304 stainless steel.

The part needs to have a smooth finish so it can be polished well.

It's a fair size lathe so I'm happy using 304.

Thanks.

[Chris DentonParticipant @chrisdenton53037]

[Anonymous]

In my experience freecutting 303 is a joy to use, and it requires real ability not to get a good finish. On the other hand I find 304 to be a real pig.

Andrew

[AjohnwParticipant @ajohnw51620]

303 is fine. You might find that as with all extruded materials that the skin is a bit dodgy but not much needs turning off to fix that.

There are better grades. If you google StainlessSteelsforMachining_9011_.pdf you should find a run down of many different types by the Nickel Institute

I just looked and it is still about.

John

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[Anonymous]

I'm with Andrew, prefer 303 every time for a decent finish and it polishes up to mirror easily..

[Ian S CParticipant @iansc]

I wanted 303 to carve out a hot cap for a Stirling Engine, The steel merchant only had 316 in the size I wanted, so when I said I'd take it, they gave me a discount of 50% with the words, your going to have a hell of a job with that stuff, hope this is a bit of compensation. Apart from streams of swafe, it machined to a good finish. Definately not free cutting.

Ian S C

[Chris DentonParticipant @chrisdenton53037]

Thanks. It's looking like 303.

interesting link, I'll read it later!

[Anonymous]

I've never had a problem with 316 either, turns beautifully and seems happy with fine cuts. It's only 304 that seems to be a pig.

Andrew

[Carl Wilson 4Participant @carlwilson4]

How strange. I've never had any trouble with either 304 or 316. I haven't ever used 303 so no idea. I always get a brilliant finish on either 304 or 316.

Carl.

[Brian WoodParticipant @brianwood45127]

Like Carl, I have had excellent results with 304. This is just one example

Brian

[Carl Wilson 4Participant @carlwilson4]

That is nice Brian. The British Stainless Steel Association give guidelines for machining all types. For the 300 series the main thing is a correct grade of carbide tip, deep enough cuts to prevent work hardening and plentiful lubricant.

Carl.

[AjohnwParticipant @ajohnw51620]

I wonder if this is a case of all will give a wonderful finish if emery cloth etc is used on it? I assumed a good straight turned finish is required. Just about anything can be polished up.

John

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Edited By John W1 on 15/06/2015 09:15:32

Edited By John W1 on 15/06/2015 09:16:15

[John Stevenson 1Participant @johnstevenson1]

One note on these materials.

316 which is commonly referred to as food grade is available in many forms, round, flats, hex etc

303's most common form is rounds and hex, flats are very hard to source.

304 which is the original hardaswitchestits, is also available in many forms but in flat form this now tends to be in sheared plate as opposed to rolled.

Probably to do with stock levels.

So if you want flats then please ask your supplier if it's rolled or sheared and if sheared then add 2mm onto the width to clean up if you want square edges.

[Anonymous]

Here are some actual results, and measurements, for turning 303 stainless. Unusually for insert tooling a shallow depth of cut still gave an excellent finish.

 

Material: Stainless steel 1.25" diameter, grade 303 free machining:

 

Trial 1: 180rpm (59fpm), no coolant

Trial 2: 180rpm (59fpm), coolant

Trial 3: 800rpm (262fpm), coolant

Trial 4: 800rpm (262fpm), no coolant

 

General Appearance:

 

Trial 1: Surface is rough and torn, picks up fibres when wiped with a paper towel

Trial 2: Smooth finish and uniform

Trial 3: Smooth finish and uniform

Trial 4: Smooth finish, but slightly less uniform that 3

 

Under a magnifying glass:

 

Trial 1: Surface is dull, torn and uneven

Trial 2: Surface is smooth, little or no tearing

Trial 3: Surface is smooth, no tearing

Trial 4: Surface is smooth, no tearing, but slightly uneven axially

 

Swarf:

 

Trial 1: Long tight coils, several feet long

Trial 2: Short coils, about 1"

Trial 3: Short pieces, not coiled

Trial 4: Short pieces, not coiled, light yellow colour

 

Surface roughness (Ra µm):

 

Trial 1: 5.07 3.52 3.84 Average=4.14

Trial 2: 1.83 1.80 1.72 Average=1.78

Trial 3: 1.97 1.86 2.12 Average=1.98

Trial 4: 2.06 2.06 2.30 Average=2.14

 

Small depth of cut test 2 thou off diameter, 800rpm, 4 thou per rev, with coolant:

 

Difference before and after, 2.1 thou, excellent finish, as good as the best of four trials above.

 

Conclusion: Best is slow speed with coolant, although high speed with coolant isn't far behind. It is possible to take fine cuts and still retain a good finish.

Andrew

[Brian WoodParticipant @brianwood45127]

John W1

This example was a case of hewing out the metal, in this case with a small carbide button tool, and then smoothing the shape with various files before using a worn Scotchbrite pad to finish the job. It was a satisfying piece of freehand turning as well..

I've had equally good results making taper turned furniture knobs in 304 which were then finished with Scotchbrite to leave a satin finish. Good support to the work is I think very important so that flexing is kept to a minimum. Work speed is modest at around 300-400 rpm

Brian.

[Chris DentonParticipant @chrisdenton53037]

I went with 303 in the end.

I didn't realise it was a pain to find in odd numbered metric sizes though. Found some at mkmetals eventually!

[Ian S CParticipant @iansc]

I'v kept clear of using emery cloth on stainless, the finish is usually good enough off the tool, 90% of the work is done with HSS tools. The hot caps I used to make from solid, first bore out the inside, then Loktite it on a steel mandrel and take the outside down to size. Now days, I use thin walled SS tube, with an end TIG welded in place, might have to go back to the old method, my tame TIGger has been having eye problems.

Ian S C

[Chris DentonParticipant @chrisdenton53037]

I did have to make something last week and used a large A2 (304) stainless bolt. It wasn't too bad to cut, although I didn't try and get a mirror finish. I also threaded it (original threads were machined off) with one of those cheap packs of dies that you get in pound shops, no problems. Thread was very good!

[Chris DentonParticipant @chrisdenton53037]

I had a hour spare today but unfortunately the stainless 303 hadn't arrived (only ordered it yesterday so not moaning) so I used some 6/4 titanium!

It's not really a mirror finish but it's smooth and shiney!

[John Stevenson 1Participant @johnstevenson1]

Did a long pump shaft yesterday in stainless, 50mm in diameter and a couple of foot long.

It had worn quite badly where the seal track ran and so needed building up and turning back to size with a good finish for the seal.

Shaft after welding with 316 wire.

Completed shaft.

This was turned at 1200 rpm feed of 0.07mm per rev and no coolant.

I did take a video of it but You Tube will not upload it although it plays on my computer.

[Ian S CParticipant @iansc]

A bit that I missed, 316 is the choice for marine work, prop shafts etc, made lots of fittings for my Brother in law's 50' power cat.

Ian S C

[AjohnwParticipant @ajohnw51620]

I found a pdf by the Nickel institute mentioned earlier. It's comments on 316 are odd. It's machined a lot and they mention excellent machining and finish properties and also list it in the free machining list but point out it isn't classed as a free machining stainless. They also list 304 in the same sort of way but suggest that 316 is better for machining in places. Maybe semi free cutting.

Was that pump shaft polished in any way or is it straight turning?

John

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