How do I fet a better finish on lathe turned mild steel???

[brian Rupnow]

My machining skills are progressing as I get more practice on my 10" x 18" Craftex lathe. One of the things making me a bit crazy right now, is that I am unable to get a nice, smooth finish on the parts I turn. My cutting tools are the type with 3 cornered carbide inserts, held in place with a small screw. The carbides have a very small radius on the tips, they do not come to a sharp point. I have tried slow speed, small cuts, slow feed, everything the book tells me to do, but my surface finishes just plain SUCK!!! The attached picture is a tailstock mounted die holder that I made. It works fine, but the finish is horrible!!! How do I improve this????

[bobw53]

Mild steels tear, alloy steels in an annealed condition tear. You have 3 options.

1st) keep your exhisting tooling and up your surface speed, to 500-1000+sfm for your finish and leave a decent amount of stock.

2nd) very very sharp ground inserts, aluminum finishing inserts will usually give you an acceptable finish, a finishing insert for exotics, Ti, Inconel, will give acceptable results.

3rd) HSS.

[b2u44]

Like bob said, increase the speed. When using carbide cutters in steel, the speed should be high enough that the chips turn blue as you cut. Also, depending on the insert that you are using, you may need to increase the depth of cut and feed-rate. Carbide needs to be used much more aggressively than HSS in order to get a good surface finish.

[lane]

For small light duty lathes for get carbide insert bits . machine does not have enough mass to run with enough speed and feed for carbide to work like it is suppose to.
Learn to grind high speed tool steel bits . they will cut just about any thing carbide will just a lot slower ,but you can get better finish easier.
Just a reminder all these text books give feed and speed for full size commercial machinery. For small machines you Must slow it down.

[dp]

That's what 1018 steel looks like if I turn it too slow. I have a very small lathe too. When I want a nice finish on that stuff I run it up to 850 rpm and .005" feed (2" dia or less). I use a carbide chip to rough and HSS for finish. It never looks great but it definitely can look worse.

I bought some 2" 12L14 a couple weeks ago and that turns like butter. I don't know all the downsides of it but for a small lathe is nice stuff.

[JRouche]

Well, carbide does work well with our slower HSM machines. The nose radious can be changed on them inserts. Sure, not what they were designed for. They were designed to take a nice heavy cut at high speed, thats how they were formed. But nuthing wrong with re-shaping the nose with yer grinder to suit yer needs.

Put a nice fat, almost round, nose on the tip with your bench grinder. If you leave the top surface intact and just work on the face you should be able to get a nice shear zone. Follow up with a hand held stone, I use diamond flats.

I use nothing but carbide and have "similar" lathes. I shape most of the inserts to my needs.

Do be aware that you will bring the cutter hight down by going in on the face with positive inserts. JR

[Paul Alciatore]

As others have stated you can do some things to get a better finish while turning. I may add using a good cutting fluid with HSS can also help. But fluid is not recommended with TC tools.

I also make some things from mild steel. Cold rolled, hot rolled, 1018, etc, It is cheaper and easier to come by. I keep some rolls of emery cloth or wet/dry paper strips at the lathe. Tear off a 6-9 inch length and put a few drops of oil or cutting fluid on the work and apply the emory cloth like a shoe shine cloth. About a half wrap around the work and work it back and forth for several inches along it's length. Work it back and forth along a section of the workpiece. Start with about a 150 or 180 grit for a half minute or so. Then a finer grade like 320. Then 500 or 600 and finish off with a 1000 or 2000 grit. Be sure to clean the work off between grits. Use a speed that is one or two steps higher than the turning speed for the metal and diameter. You should be able to get close to a mirror finish on the part in a minute or so.

I have finished a lot of parts this way. It removes only about 0.001 to 0.0015 inches on the diameter for mild steel.

I do not use this technique on sliding or bearing surfaces as some grit may remain embedded in the finish and cause accelerated wear. For that kind of work, I just bite the bullet and purchase a better grade of steel.

[macona]

Machining 1018 is like machining taffy.

Take heavier cuts. Learn not to sneak up on you final dimension. With mild steel it is when you try to take off that last .002 that it starts tearing. If you have .030 to go take .015 and see where that puts you. Use the information you learned from that to adjust your next cut to hit the diameter you want.

If you do need to get that last .003 off use some good sulfur cutting oil and a real sharp HSS bit or a sharp carbide. High rpm for carbide and as high of a feed as you can while maintaing the surface finish you want.

[BadDog]

As said, any soft non-leaded (or otherwise highly machinable) steel is going to be a problem when it comes to finish. This is what I've found.

1) Carbide has it's place. But for most HSM machines, mild steel is not it.

2) A traditional HSS bit grind honed very sharp with positive rake sometimes works with light cuts (if you MUST take them, but better to take a larger cut as stated), fast speed, and dark sulfur cutting oil. Often still looks "fuzzy" though. Play with speed, feed, DoC, and rake (with minimal clearance) can net considerable results, which will vary with your machine/setup/material. Likewise, point radius can be increased and minimal trailing clearance can help (or hurt). You have to get it all working together, but for me, it has proven to be hit-n-miss (often more the latter). Trying to do it like this, before I found the answer in #4 below, was very frustrating at times...

3) If it's just aesthetic (+/-0.001 or more), a file followed by a strip of emery cloth (or similar - applied safely disclaimer) will get a nice looking finish.

4) Use special ground HSS finishing bit (for me, this is the single best answer for accuracy and finish). It has a honed edge oriented 30*/210* (assumes 0* is vertical-top) is traversed in the normal way, but the edge slices rather than shears (tears) the metal away. The cutting edge looks like "/" from the operators perspective, and "center height" is a total non-issue, so it's prefect on the back side of the holder from a facing bit. Max DoC is around 0.003, and swarf comes off like tiny hairs, but the finish is often VERY nice. Also nice for sneaking up on bearing/press fits which need accuracy AND good finish. I saw this on someones site, or maybe a post here, but don't recall who. I've been using it for months and absolutely love it. Also gives a near mirror finish in aluminum when run "wide open".

[Norman Atkinson]

In no way do I want to detract from earlier suggestions but I use almost exclusively HSS lathe tools. I also use what many would describe as a small and flimsy lathe which was not designed for carbides.

What most of us tend to forget is that the finish on a lathe tool determines what appears on the work. Fine- include a lubricant to assist. I use lard oil- I like the smell of frying.

I use an eye glass to look at the tiny bit which is actually doing the cutting.
The rest of all this grinding lark is wasted unless that tiny bit is so relective that you can see your dirty finger nail.

One book that I read showed a lathe tool that had been rough ground, then the angle changed to do a medium grind, then a fine grind and onto a finer one still and then honed- like a razor.

Speaking of dirty thumb nails your lathe tool should be sharp enough to pair your nail-- and that is after it is has already blunted.

I use a diamond paste.

You now have a start--Good Luck!

Norm