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Finally able to get good finish on mild steel

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Y C Lui03/06/2022 11:24:22
83 forum posts
35 photos

I have never been able to get good finish when turning mild steel ( the kind used for making bolts ) no matter what RPM or feeding speed I used. Using sharper inserts produced slight better results but still the surface was not shiny. Before long I accepted that the metal is to be blamed because there is no such issues on aluminium or stainless steel. Today, I did it again without doing any finishing pass and much to my surprise, the finish is very good. It turns out that the DOC selected in the past was just too small ( < 0.1 mm ) and today I set it to 0.5 mm. Just want to share this experience with other beginners like me.

0.1 vs 0.5 doc mild steel.jpg

Lee Rogers03/06/2022 11:37:32
176 forum posts

The industrial suppliers of inserts will often have a spec sheet giving a figure for minimum DOC. Sometimes that number is a pretty damn scary one for small lathe owners, you just haven't the power to get close to minimum let alone best DOC.

Hopper03/06/2022 11:38:07
6706 forum posts
347 photos

Yes often those carbide inserts like to do some heavy work, not light rubbing. But also I often find that commercial bolts don't machine very well. Their material is selected to roll well in their manufacturing process, not to cut well in a lathe. Also the threads and hex heads can be work hardened, adding to the challenges. But looks like you are getting a good result. Good stuff!

Thor 🇳🇴03/06/2022 12:56:06
1661 forum posts
46 photos

Carbide inserts wants enough Depth Of Cut to cut well. A small nose radius on the insert is best when DOC is shallow and generally also give better chip breaking. More about choosing inserts for the job at hand here, (yes, intended for industrial use).


old mart03/06/2022 13:38:26
3912 forum posts
268 photos

That is how carbide should be used, however, you need to be absolutely sure that the last pass will be exactly to final size. That is why creeping up on the finished size is popular. There is a great deal of difference between steel alloys in their machining qualities. Same with aluminium, the pure metal is horrible to machine, the stronger alloys are much better mostly. You can google metal alloy properties and some sites give relative machinability.

By experimentation, you will soon find the best ways to machine metals.

Edited By old mart on 03/06/2022 13:39:39

SillyOldDuffer03/06/2022 14:18:40
8906 forum posts
1999 photos

It was Andrew Johnston, he of this forum, who brought my attention to the importance of Depth of Cut. I was nearly there with RPM and feed-rate, but DOC makes a difference too, and not just with Carbide.

The shape of the cutter matters too: this thread discusses them.

Sparey recommends these HSS shapes. Note the rounded finishing tool 'C'.

Not sure how finishing tools work? Ordinary mild steel is often a bit smeary, and a sharp point tends to leave lines in it. Possibly rounded knives have a rubbing, polishing action that leaves a shiny finish, but isn't good for roughing out.

The more Model Engineering I do, the more I realise I don't understand it properly!


Nigel Graham 203/06/2022 16:19:42
2288 forum posts
33 photos

No tool should have a "rubbing, polishing" action.

Turning marks are really microscopic screw-threads, and the effect is the more evident the finer the tool point. The radius smooths the crests by planing them down as the cutting edge actually doing that is a lot wider than the groove pitch. It does not rub them out if it is working properly.

The "often a bit smeary" may point to often using different grades of steel as provided all the other conditions are constant, the finish should be consistent as long as the tool is sharp.

It can be affected by other factors such as very slight play in the slides, or very slight chatter, influenced by work radius and tool setting dictating where the slide is along its dovetails. This will also bear on depth of cut as it might make too shallow a cut more prone to breaking into a rubbing action.

If you use carbide inserts, these fussy little darlings can be more affected by steel grade than are HSS tools, and they don't really last long before starting to lose their sharpness. Their manufacturers' catalogues show a bewildering variety set as much by material as anything else - but they are designed for serious metal-munching at terrifying rates in hefty great, very rigid,machines.

If you use HSS tools, carefully honing the ground edge to a finder surface can help improve the finish.

bernard towers03/06/2022 19:24:29
691 forum posts
141 photos

The bolts in the picture do not look like mild steel as they are black which means they are likely to be HT of some strength or other from 8.8 upwards, that's maybe why you where having trouble getting a good finish. I have found that when turning these that a low mandrel speed helps.

Neil Lickfold03/06/2022 22:14:53
892 forum posts
195 photos

With tungsten carbide inserts, a great rule of thumb is for finishing using the radius of the tool as a basis for the feed rates and depth of cut. So in finishing, depth of cut from 1/2 the radius of the tool used, but is easier to say is the diameter of cut is the radius of the tool. So R0.2mm tool is Ø 0.2mm depth of cut up to the Radius of the tool deep. So on a R0.2 insert cuts of 0.4mm in diameter. For a fine finish, use 1/4 of the radius as a feed rate. So on a R0.2mm tool a feedrate of 0.05mm or 2 thou per rev, up to 1/3 of the radius of the tool. A 0.05mm feedrate on a R0.4mm insert will give an excellent finish if the material will still shear nicely with the power that you have. I find that the sharper the tools, often the better the surface finish, which is a reason that I use a lot of the ground carbide inserts for finishing. So like TNMG insert is not ground, but a TNGG is.

The work piece needs to be sufficiently supported as well, or else the work piece will just push away and be tapered etc. So planning the job is very important.I always like to take 2 finish passes after roughing out, and if a separate finish tool is used to the roughing tool, I plan on 3 finish passes of about equal depths of cut. With a R0.2 tool I like to leave about 0.6mm on the finishing diameters and 0.1mm on finishing faces. With a R0.4 tool I like to leave 0.75mm on diameter. Mostly I finish with R0.2 inserts unless the part needs the R0.4 from the tool in a corner some where. Often I will undercut corners where possible in the roughing stage and they become finished, with the part looking funny until it is finished.

Sometimes you get a better surface finish with a very thin coating of oil on the parts, other times it will stay dull until some surface treatment is done, like touching the surface with a light grey scotch brite pad. You won't be easily able to measure the difference in size, but the parts will look nice and shiny afterwards.

SillyOldDuffer06/06/2022 13:01:36
8906 forum posts
1999 photos
Posted by Nigel Graham 2 on 03/06/2022 16:19:42:

No tool should have a "rubbing, polishing" action.


What, not even a burnisher?

Sparey's HSS knives have a variety of fit for purpose shapes. His finishing cutter is slim and rounded; my question is why that shape? I suggest the profile burnishes as much as cuts, flattening the hills and valleys left by a roughing cutter by pushing them into each other.

Strikes me cutting tool designers can exploit a number of metal moving mechanisms to get a desired result. There's more to it than simply presenting a sharp edge to the work, which is why a multitude of different carbide inserts are available for different purposes and materials, and the Victorians had dozens of shapes to get the best out of Carbon Tool Steels.


JasonB06/06/2022 13:09:08
23076 forum posts
2771 photos
1 articles

The rounded nose does indeed flatten the hills and valleys but it should do it by CUTTING the high bits off not smearing them flat

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