[4]

Unable to get a smooth finish in turning mild steel

[3]
[10] Like what you see?
Click here to donate to this forum and upgrade your account!

compact8

Active Member
Registered
Joined
Oct 13, 2012
Messages
34
Likes
7
#1
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.
 

Attachments

Latinrascalrg1

Active Member
Registered
Joined
Dec 9, 2016
Messages
145
Likes
69
#2
In all honesty i am probably much less knowledgeable about this stuff so take what i say more like a question then an answer please.....

Could it be possible that your cutting tool is set a bit to high? From your picture it looks alot like that metal is Gauling and tearing caused by the tool rubbing more then cutting.
 

compact8

Active Member
Registered
Joined
Oct 13, 2012
Messages
34
Likes
7
#3
In all honesty i am probably much less knowledgeable about this stuff so take what i say more like a question then an answer please.....

Could it be possible that your cutting tool is set a bit to high? From your picture it looks alot like that metal is Gauling and tearing caused by the tool rubbing more then cutting.
No. As spoken the same cutting tool used on stainless steel produces mirror-like finish. I believe the insert is designed for high-load cutting as the tip feels a bit blunt. I have tried using inserts with tips that feel sharper and the result is better but still I cannot get shiny finish.
 

4ssss

Active Member
Registered
Joined
Apr 25, 2017
Messages
492
Likes
280
#4
Stone a little radius on the tip, just enough to take the sharpness out of the tool. You'll need a diamond hone if your using carbide.
 

benmychree

John York
H-M Supporter - Gold Member ($25)
Joined
Jun 7, 2013
Messages
2,582
Likes
1,894
#5
Are you using a cutting oil? I like Tap Magic, a little dab on finish cuts makes a big difference. Mild steel does not finish as nicely as we would like, likely you will never see tha hoped for mirror finish without some filing and polishing, cold rolled is bad enough, hot rolled much worse.
 

compact8

Active Member
Registered
Joined
Oct 13, 2012
Messages
34
Likes
7
#6
Are you using a cutting oil? I like Tap Magic, a little dab on finish cuts makes a big difference. Mild steel does not finish as nicely as we would like, likely you will never see tha hoped for mirror finish without some filing and polishing, cold rolled is bad enough, hot rolled much worse.
Yes, high-quality cutting oil is used. I have the feeling that this is just the characteristics of mild steel but I have never get any experienced machinist to confirm it.
 

benmychree

John York
H-M Supporter - Gold Member ($25)
Joined
Jun 7, 2013
Messages
2,582
Likes
1,894
#7
Your suspicions are confirmed, I can boast of about 55 years of dealing with it, not counting school days.
 

Latinrascalrg1

Active Member
Registered
Joined
Dec 9, 2016
Messages
145
Likes
69
#8
Your suspicions are confirmed, I can boast of about 55 years of dealing with it, not counting school days.
Is this to be expected regardless of the type of cutting tool?
 

markba633csi

H-M Supporter - Gold Member
H-M Supporter - Gold Member ($25)
Joined
Apr 30, 2015
Messages
2,980
Likes
1,545
#9
I would try a HSS bit instead of carbide- you should see a better result
mark
 

mikey

Active User
H-M Supporter - Gold Member ($25)
Joined
Dec 20, 2012
Messages
4,501
Likes
4,870
#10
There are many kinds of mild steel. One of them, 1018, finishes poorly like the pic. Your best bet with that material is HSS tooling.
 

Norseman C.B.

Active Member
Registered
Joined
Dec 21, 2013
Messages
189
Likes
88
#11
Mild steel, decent finish, best shot is with with HSS tooling with positive rake............ My $.02
 

benmychree

John York
H-M Supporter - Gold Member ($25)
Joined
Jun 7, 2013
Messages
2,582
Likes
1,894
#12
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

H-M Supporter - Gold Member
H-M Supporter - Gold Member ($25)
Joined
Jun 16, 2016
Messages
118
Likes
91
#13
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

Active Member
Registered
Joined
May 31, 2018
Messages
81
Likes
66
#14
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

Active Member
Registered
Joined
Oct 13, 2012
Messages
34
Likes
7
#15
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

Active User
H-M Supporter - Gold Member ($25)
Joined
Dec 20, 2012
Messages
4,501
Likes
4,870
#16
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

Brass
Registered
Joined
Mar 12, 2018
Messages
526
Likes
553
#17
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

H-M Supporter - Sustaining Member
H-M Platinum Supporter ($50)
Joined
Mar 10, 2018
Messages
658
Likes
439
#18
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

Brass
Registered
Joined
Aug 7, 2018
Messages
660
Likes
334
#19
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

Brass
Registered
Joined
Mar 12, 2018
Messages
526
Likes
553
#20
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.
 

BaronJ

Brass
Registered
Joined
Aug 7, 2018
Messages
660
Likes
334
#21
Hi Cadillac,

Ah, you are referring to a carbide insert ! I hadn't considered using one that way, I'll have to try that.

This is what I was thinking,
shear.jpg
 

Doubleeboy

Active User
Registered
Joined
Dec 3, 2014
Messages
769
Likes
439
#22
If you insist on carbide inserts and want a fine finish in junk steel like 1018 or A36 your best bet would be a negative rake tool like TNMG or CNMG, crank up the speed to at least double what would be reasonable for high speed steel take a deep enough cut to get thru the mill scale and feed at .003 to .005" for finish cut, maybe twice that for rough cut. Dial it in so your chips are tan to just turning blue, make sure you ready for the flesh melting chips, but with right speed and feed you can easily get a near mirror finish when you get just short of blue chip zone. No oil, and if you use coolant you need to consistently apply it with flood or mist, no periodic spraying as that will cause havoc with the carbide heating and cooling, so either run dry or use lots of mist or flood.

This procedure works for me. The other method that works is to use a vertical shear tool, very light feed, depth of .001" and standard HSS speeds., The shear tool will give acceptable finish but usually will not give you that burnished, almost mirror finish the above negative rake carbide method can.

michael
 

Cadillac

Brass
Registered
Joined
Mar 12, 2018
Messages
526
Likes
553
#23
Hi Cadillac,

Ah, you are referring to a carbide insert ! I hadn't considered using one that way, I'll have to try that.

This is what I was thinking,
View attachment 277277

That's one works good but like said it just cuts OD work. The one I'm speaking of is like this. You just grind your hss at the end of the blank with a touch of positive rake. I believe theirs a company out of Australia that makes the tool holder but they are easy to make. They come in left and right.
image.png
 

savarin

Active User
H-M Supporter - Gold Member ($25)
Joined
Aug 22, 2012
Messages
1,974
Likes
3,005
#24
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.
Have a read of this
http://www.conradhoffman.com/advancedsharp.htm
and try out the shear tool.
Thats my goto bit when I cant achieve the finish I want.
 

homebrewed

Active Member
Registered
Joined
Jul 28, 2017
Messages
181
Likes
88
#25
If you want to experiment with a vertical shear bit but find the grinding profile(s) a bit challenging, you might want to try a variation as shown in the photos below. I came across this during an online search for shear bits so it is not original to me.

Prototype VSB_scaled.JPG

This version only has one ground face, produced on the radius of a grinding wheel. Relief is provided by rotating the tool in the holder, as shown below:

Top View._scaled.JPG

The finish is better, probably could be improved if I had used a slow power feed.

It's my understanding that, due to wear during a cut, there can be some "drift" of the OD when using a shear bit. I bought some cobalt tool blanks to (hopefully) reduce this effect, but have not had a chance to try one to see how it performs in this regard.
 

tq60

Brass
Registered
Joined
Jan 11, 2014
Messages
672
Likes
394
#26
Socket crews are hard things and difficult sometimes.

Some places look to be cutting and others look like tearing.

Try altering cutter height to see has it changes.

A bit high can cause the edge of the cutter to almost touch and as it lowers it alters the cutting angle.

If you do not already know this trick do this.

Take a picket rule or other small strip of flat steel and hold it verticle between cutter and stock with machine off.

Carefully bring the cutter to touch the work and release the strip.

If the tip of the cutter is exactly on center the strip will be vertical.

Any angle indicates not at center.

Sent from my SAMSUNG-SM-G930A using Tapatalk
 

tcarrington

Making miscellaneous parts for years now
H-M Platinum Supporter ($50)
Joined
Dec 27, 2013
Messages
60
Likes
47
#27
A plan B or whatever letter you are up to would be to finish it out with a file or abrasive. Depending on how scored or "torn up" your surface is, you will need to leave some extra material. A good file properly applied to that work piece might finish it out much better. The strip of emery or a carbide abrasive will also give a good finish if it can be had at all.
 

stupoty

Active User
H-M Supporter - Gold Member ($25)
Joined
Dec 2, 2012
Messages
983
Likes
232
#28
Hi Cadillac,

Ah, you are referring to a carbide insert ! I hadn't considered using one that way, I'll have to try that.

This is what I was thinking,
View attachment 277277

that kinda tool will give an amazing finish on mild steel.

Stu
 

Kroll

Active User
H-M Supporter - Gold Member ($25)
Joined
Dec 23, 2012
Messages
1,038
Likes
242
#29
Guys since I never heard of Shear Cutting I did google it and came across this.
Seeing is a big help for me to understand how it works.Good for only .001 to .002 but that is all that is needed for a good smooth finish and its also good for SS listening to Tubalcains Youtube.Good subject
 
[6]
[5] [7]
Top