Unable to get a smooth finish in turning mild steel

[Norseman C.B.]

Mild steel, decent finish, best shot is with with HSS tooling with positive rake............ My $.02

 

[benmychree]

Is this to be expected regardless of the type of cutting tool?

I think the other guys are pretty much on the level with the recommendation towards HSS; Hobbyists seem to gravitate towards carbide inserts to avoid learning how to grind a HSS tool, this situation is not so conducive to fine work, especially on lightly built machine tools, and cheaply bought inserts, especially negative rake styles. As I stated, I use MagicTap for nearly all work requiring a cutting fluid, it does make a difference, as opposed to cutting oil, to say nothing of "motor oil" that some folks swear by.

 

[Briney Eye]

I have tried many kinds of inserts and different cutting speed but still unable to get a good finish on mild steel. Achieving it on stainless steel , brass and aluminium is super easy. I can get a better finish on mild steel by using sharper inserts designed for fine cutting but the result is still poorer than that obtained on other metals. May I know if there are things that I have missed ? The picture below shows the typical finish I get . With the same insert, the finish on stainless steel is close to mirror like.

Search for "HSS shear tool" and it might give you some ideas.

 

[SamI]

Just a thought - it looks like you're turning down an old bolt. This is likely to be a higher tensile steel. I have often struggled to turn down threads compared with a piece of round bar. Possibly because the round bar clamps better in the chuck and reduces chatter.

It could also be that the cutting forces in the mild steel are higher than the likes of brass and aluminium and on small diameters the bar is deflecting more. As has been said a sharper cutting tool or a smaller depth of cut may help.

A mirror like finish in mild steel is probably a bit ambitious but you should be getting a smooth and consistent finish in either carbide or HSS.

 

[compact8]

Just a thought - it looks like you're turning down an old bolt. This is likely to be a higher tensile steel. I have often struggled to turn down threads compared with a piece of round bar. Possibly because the round bar clamps better in the chuck and reduces chatter.

It could also be that the cutting forces in the mild steel are higher than the likes of brass and aluminium and on small diameters the bar is deflecting more. As has been said a sharper cutting tool or a smaller depth of cut may help.

A mirror like finish in mild steel is probably a bit ambitious but you should be getting a smooth and consistent finish in either carbide or HSS.

Yes it's an old socket-head bolt with high tensile strength. It has an unthreaded segment on which the clamping is done so the amount of chattering should be the minimum possible . The bolt was sticking out very little from the chuck so the amount of deflecting should be insignificant. The cutting depth was very small , within 0.1 ~ 0.15 mm so that could not be the reason either. Yes a sharper ceramic insert with positive rake did produce better finish although it is still far from mirror-like but just a more even dull grey. I think I will just accept that this is the characteristics of mild steel and select the cutting tool appropriately.

Thanks to all for the replies which are all very useful.

 

[mikey]

A hardened bolt is not mild steel; it is made from medium carbon steel and hardened to varying degrees depending on the grade. For example, a grade 8 bolt is hardened to a Rockwell 33-39C.

A cvd carbide insert will cut a hardened bolt like this readily provided you take a deep enough cut. Your depth of cut is only 0.004 - 0.006" deep and depending on the nose radius, this may not be deep enough for the insert to even cut. Instead, it is likely deflecting or skating on the surface, which sort of explains your finish issues.

Determine your nose radius. Take a depth of cut of at least 1/2 the nose radius and it might cut. An even better option is to go with the nose radius + 0.003 - 0.005" deep and that will definitely cut without chatter if you get the speed up and keep the feed conservative. You insert manufacturer will be able to give you your cutting conditions.

If you must cut that bolt then try the above. If you're just messing around, try something softer, like real mild steel. 12L14, 1215 is easy to turn and will give you a much nicer finish.

 

[Cadillac]

A diamond tool or shear tool bit would probably give you the best finish on mild steels. The tool is a hss tool stood vertical with slight reliefs built into the tool holder. Very easy to sharpen tool. Only take .001-.005 cuts works good on all types of materials.

 

[P. Waller]

Use a carbide insert tool.
Increase the spindle speed until the finish improves, this may be faster then your machine will spin and likely far faster then you are comfortable with but this is what is required. For a 1" diameter 1018 round I would turn it at 2000 RPM's, this is 500 SFM.

 

[BaronJ]

Search for "HSS shear tool" and it might give you some ideas.

A shear tool will work but you wont be able to cut right up to a shoulder. The closest you will get is about half the tool width.

 

[Cadillac]

A shear tool will work but you wont be able to cut right up to a shoulder. The closest you will get is about half the tool width.

The ones I’ve seen do OD and facing. Now if use it in the standard horizontal position and grind a shear yes you won’t be able to go to a shoulder. If you use the tool in the vertical position pitched about 7* in both direction the diamond shape with the included angle allows use in both directions. I don’t have the tool holder it is on my todo list of projects.