View Full Version : What am I doing wrong?

03-12-2010, 03:20 PM
Here is a piece that I just turned down. I tried slow feed and several different turn speeds. The cut pictured was at 1200 RPM, the slowest possible feed setting on my lathe and a .010" pass. This one turned out the best of them all, but can't figure out how to get a glass smooth finish. I tried to keep the insert clean of all chips, but for some reason I couldn't get it any better than this. Was using a coated carbide indexable insert. The piece is supposed to be cold rolled steel. Any ideas?


03-12-2010, 03:24 PM
you getting any harmonics? the repeating pattern seems to indicate so.

it's hard to chase that out of a setup. you need to minimize too flex and take a good, hard look at every possible variable in your machine which may contribute to vibration...

03-12-2010, 03:24 PM
I would try going full speed at the spindal and maybe try a .020 pass if she has the power. What color are the chips coming off and after cooling? Might need to snug up the gibs, might be flexing. What kinda lathe? try turning the carriage back slowly without retracting, does it cut again and does that cut look better? If going back over it with the tool set at the same depth, it's likely flex.

Glenn Wegman
03-12-2010, 03:29 PM

Is it for rouging, medium roughing, finishing, fine finishing, positive, neutral, negative?

Lots of variables.......

1018 steel can be difficult as it likes to tear rather than cut.

There was a discussion recently on alternative tool bits for finishing.

03-12-2010, 03:35 PM
OK, here we go.

The lathe is a LeBlond Regal 13x18.
I don't know if it has harmonics or how to check for them.
I don't know what the insert is meant for finish wise, it is a coated insert TiNi or something like that.
Most of the passes I made, I could run the carriage back without retracting and no cut was made, but on one of the higher speed passes, it did cut on the way back. I didn't do it slow enough to take a good cut.

I have another piece of stock that I am going to try on again tonight, I will try a faster spindle speed, and a heavier cut, see what happens.

Thanks, you guys are the best

03-12-2010, 03:53 PM
You might get better results with a sharp high speed steel tool with a small tip radius (like the inserts) 10 degrees back clearance, 5 degrees top rake. The inserts don't have a knife sharp cutting edge. Probably 1 or 2 thou. finish pass.
For softer materials like cold rolled steel, alum., plastic - a really sharp edge usually gives a better finish.

03-12-2010, 03:56 PM
Here is a piece that I just turned down. The cut pictured was
at 1200 RPM, the slowest possible feed setting on my lathe and
a .010" pass. Was using a coated carbide indexable insert. I
don't know what the insert is meant for finish wise, it is a
coated insert TiNi or something like that. The piece is supposed
to be cold rolled steel.

http://i40.tinypic.com/ycjyp.jpgWhat diameter is the material? It looks small, 0.500" or less.

Using the formula RPM = (CS x 4) D, where D is diameter, and where
CS is Cutting Speed in feet/min, I believe that insufficient RPM may be
a factor.

A CS value recommended as a starting point for coated carbide on 1012
steel is 500-900. If the material is 0.500, then this suggests the spindle
needs to be running at 4000-7200 RPM.

For comparison, the CS value for HSS on 1012 is 90-100. For 0.500" dia
material this suggests spindle speeds of 720-800 RPM.


03-12-2010, 03:56 PM
I have some high speed cutters too, so I will try one of those this next time and take a small cut and see if that works better.

Thanks again!

03-12-2010, 04:04 PM
I have had some material that just doesnt wish to machine nice. Rather than beat my already beaten head against the wall i just leave it a couple of thou oversize cover my ways up and use strip of emery cloth. It isnt a nice way to do it but it does work well, hi rpm and sand er./

03-12-2010, 04:16 PM
Welcome to the real world DebosDave. It's next to imposible to get a glass smooth finish on most steels. If you want a glass smooth finish stick with using brass and aluminum.

You can try using a shear tool that was discussed here to great extent or you can learn how to machine to .003-.004" oversize and filing to fit and finish.

Your going to get some tell you they get a glass finish on everything they do but they are lying. Learn to deal with what you get from a turned finish.

A CNC is probably the only machine that will get close to a glass smooth finish.

The only thing that can get real close to a glass smooth finish is grinding and/or burnishing.

03-12-2010, 04:23 PM
is the incert in tight, also how much of the tool is over hang on the holder, is the holder tight on its mount, might check the gibs and also make sure to lock the compound as well that will make a difference, also yes also you wont get a glass finish to easy on cold or hot rolled , regual steel, i have better luck with 303 316 316L for nice fininsh also on a finsh pass i take the last cut setting and leave it alone and then take several passes, then a bit of emory cloth 150 gritt helps a bit to then take it to the polisher and your good to go, lots of sanding involved in cold rolled with sand paper..

just some of my experiences

03-12-2010, 04:33 PM
you'll know when you have harmonics going on. the machine will sing

03-12-2010, 04:37 PM
Try sharp HSS or carbide, either one with a reasonable nose radius. Also, brush some cutting oil on it. I'm also assuming it is supported by a center ... otherwise is could be a lot worse ;) Be sure the tool is on center and you are producing clean chips or turnings.

With a sharp, radiused nose you should be able to get a decent finish (with some dependance on the CRS) at just about any speed, as slow as you want. Coated inserts generally are not sharp due the coating and or edge treatment to reduce chipping. They need to be run fast enough so the material flows over the cutting point. On alloy steels, you will smile when you hit this point as the finish is bright and shiny.


03-12-2010, 04:38 PM
I think something is funny, maybe just the speed. I don't get a fantastic mirror like finish, but I get a much smoother cut than OP, with cold roll or even hot roll. I usually run that at estimated 2400 RPM, or full speed on low (half that?), on a 7 x with TCMT-21.51 or TCMT-21.52. I don't get the polished look at one end and rough on the other, or the ring like areas (where it dug in?).

I was thinking about the test on returning the carriage, if the tool is only getting pulled in part of the time, you might get normal depth some times, then deeper when it pulls in, then it would not cut on the return. On my inferior lathe that bows down and worships your lathe, it pulls in the whole cut, though consistently, and cut on the return.

Might also verify your tool tip is dead nutz on center of the work piece.

My lathe forgot to say, "Yes me Lord." So I will beat it's ass on your lathes behalf, as soon as I get home.

03-12-2010, 04:57 PM
you'll know when you have harmonics going on. the machine will sing

IOW (in other words) "It ain't over 'til the fat lathe sings!":p


03-12-2010, 04:59 PM
Here you go.



03-12-2010, 05:21 PM
Crappy steel- probably has toyota gas pedal metal in it :)

03-12-2010, 05:41 PM
Crappy steel- probably has toyota gas pedal metal in it :)

Touche` pu$$y cat :D

03-12-2010, 05:50 PM
If you want a really good finish Toolguy has it. When you have used your carbide to get within three to five thou change to a H.S.S. tool with lots of top rake and a radius tip. Then hone a little flat in line with the work maintaining clearance. You don't have to use a real fine feed as you are now overlapping flats. 100 or less rpm and keep wet with soluble oil water mix (50-1) That is if you are fussy about finish and not in a tearing hurry. That's how us old timers do it. Peter

03-12-2010, 06:02 PM
With inserts Ive found that your last roughing cut is your finishing cut .you cant take mincing cuts without them skipping.

You also need to make sure all your gibs are tightened.

Make sure its not riding on the way felts .

Quality of steel .basic mild steel will produce a finish like that no matter how hard you try to get a good finish.

Angle of attack.....if you are pushing the tool into the cut ..it will deflect .

Tool must hang out of the tool holder the minimum amount required to carry out the work.

Head bearings must be adjusted up true and free from slop.

Base of tool post must be true and flat ..so that contact area is fullest .

Way oil must be used .no substitute .

Cutting oil will help a better finish insuring tool glides smoothly and evenly over the work.

If the lathe has adjustment for holding the saddle down ..this also needs to be adjusted correctly

Workpiece must also be supported at both ends.....centres should be good ones and not worn out crap

all the best.markj

03-12-2010, 06:21 PM

Cold rolled is kind of junky stuff to turn. You might try grinding a HSS bit like Miker suggested, there was a fairly long thread, with pictures, about that bit in the last few days.
Or the next time try some some stress proof steel, if it will work for your project.


03-12-2010, 06:26 PM
are you usuing a follwer rest at all in order to keep everything steady as your cutting, if not it could explain some of the dig in , when i turn stuff thats more then 4 inches out of the chuck i use a follower rest this way no flex issuses or dig in problems, i also us a heavey duty live center as well..

just some ideas

J. R. Williams
03-12-2010, 06:30 PM
Look up the feed/speed table for the insert being used. All the carbide inserts have a "window" of operation for feed and speed. When you are outside the window, the finish will be poor. Cold rolled steel only adds to the finish problem.


03-12-2010, 06:37 PM
Lots of revs, big DOC. If the chips aren't blue and then some you are p*ssy footing around. Carbide loves it.

You will get the finish but the problem is hitting + or - 0.0005 at the same time, it gets difficult on a light-weight lathe. Still if you want it all you'll have to pay for it.


03-12-2010, 09:12 PM
Like philbur and others said, you will get a good finish with carbide at the right SFM with about .030" DOC and then when you go for the finish size with a DOC of a few thousandths the surface finish looks like crap.

I have been trying to find the best way to a great surface finish when taking .001-.002" since I started machining in about 1962. The only thing that worked until lately was a file and emery cloth but when I saw the shear tool and experimented with it I found my solution. Do a search for Shear tool.

It's not a glass smooth finish and turning process will give a glass smooth finish but it's as near a polished finish as you can get with a cutter tool.

Dave, it's best to forget the glass smooth idea and I hope you were exaggerating when you said you wanted a glass smooth finish. The only way I know to get glass smooth is grinding and polishing or burnishing.

03-12-2010, 09:44 PM
What is the nose radius of your carbide insert? You need to cut at least 1/2 to 3/4 depth or greater of the nose radius or you get rub of the tool, because the part is riding on the end of the radius, not on the cutting edge/rake angle edge. This is very common for "2" type nose radius (1/32 nose radius) inserts on almost any material. Even a 1/64th insert will have "rub" if cutting only .005 depth. This is also common on CNC's taking a very small cut with an oversize NR tool for the depth of cut.

Need to know your NR. I see this quite often, the problem is the same almost every time.

Another idea/comment: 1018 cuts like this almost every time anyway, especially the uncontrolled China import stuff. For a smoother finish every time in steels, use an 1117 or 12L14 (resulphurized) steels.

Of course, I have to stick with Nose Radius, this would occur even in the 11's, and 12's.

03-12-2010, 10:00 PM
For cold rolled I think that is a mirror finish. But I can get better when using inserts if I turn it fast and don't try any "finish pass" stuff. It seems to like being gored and responds well to abuse. If I'm above or below centerline by so much as a neutrino's diameter it will spoil the cut. 1018 is a lot of things but fun to make pretty isn't on that list. 12L14 and similar on the other hand cuts so clean you need sunglasses when you're done.

But then I have a much smaller, lighter lathe. I keep thinking I need to move up.

03-13-2010, 12:10 AM
Your much better off with the tool on center or just a few thousandths low for a clean cut. If the tool tip is above center the front will rub on the work and for a light cut it will look like crap for sure.

A tool set low will be the same as a negative relief and you have to be careful with that.

Paul Alciatore
03-13-2010, 01:10 AM
CRS and "glass smooth" in the same sentence? In the same part?

First, you might consider a leaded alloy instead of CRS. CRS isn't really a definite alloy, so it is actually mystery metal.

A real sharp HSS tool will give a cleaner cut. Do use a good cutting fluid.

OTOH, I have gotten nice finishes on CRS or the equivalent, but I used abrasives. Turn it to about 0.001" oversized and then crank up the spindle speed toward the high end and use strips of emery cloth to finish it off. Start with about 150 or 180 grit and use some cutting oil on it. That will take out the tool marks in the picture.

Then clean it off and do the same thing with a 320 or 400 grit. If the diameter needs to have a tight tolerance, take frequent readings along the length with a mike and adjust the application of the abrasive strips accordingly.

If you still want a better finish, use progressively finer grades until you get what you want. The oil is an important ingredient here. Keep the strips wet. This also helps to prevent any abrasive dust from getting all over your machine. And water based cutting fluids do not seem to work as well for this. You can get a mirror finish on almost any steel or aluminum this way.

Another thing that can produce that kind of tool marks is an irregular feed when cutting. It is almost impossible to have a constant feed rate when manually cranking with the small cranks or knobs on most lathes. Power feed or CNC will help this. Also, a larger handle on the manual feed will help smooth things out. I'm talking about a six or eight inch radius crank, instead of the ususl one or two inch ones that the OEMs use.

03-13-2010, 03:50 AM
I have a 12x36 Clausing. I've found with carbide, when I take a rather deep cut I get a much nicer finish. A shallow DOC always gets me rankled due to a poor surface finish. Still learning how to use carbide and my machine. I need to learn how to hit the number I'm looking for with a deeper cut, I haven't been able to sneak up on it. The finish always sucks. To many variables and lack of experience for me. But, that's part of the fun! LEARNING!!!! Just my 2 cents.


03-13-2010, 01:42 PM
I cut hard and fast with carbide, at least hard and fast for a little 7 x! I think that or tool height might be part of the problem. If the part isn't flexing, I think something else is. It really shouldn't be almost polished at one end, rough at the other, and uneven all across. Especially with carbide, because we know it's not getting dull or destroyed by the cut. I didn't realize, more didn't think about the fact that cold rolled is a process and not a steel type (cranial rectal inversion attack :D). So, we can't compare what OP is getting to others' results. We can only compare the part to itself or compare it to what we see in other materials from a generic point of view.

If worst comes to worst, read my bloviating in the debunked carbid myth thread, and sand the snot out of it. Especially if you are getting good results with other materials.

I typed colled rolled. That's not a spelling, typing or grammar problem. That's a brain problem. I caught it after three proof reads, only because it was underlined by spell check. First two times through, I saw cold, and thunk spell check was just stupid. Reading disabilities suck. Take it easy on the guys that type bad, often not their fault. I don't feel sorry for me, just for those trying to make sense of my typing. But some guys get hurt. Like the song says, don't mess with cowboy burt, or he'll get his feeling hurt. :D

03-13-2010, 02:32 PM
Feed is too slow and not enough depth of cut...

You have to be making 6's and 9's with insert cutters to get any decent finish..

03-13-2010, 06:24 PM
You need a dead sharp cutting tool. I have had the same thing happen in the past. Cold rolled or 1018 is tough to cut. However with the right insert you can do a decent job. I think the reason your seeing a polished finish on one end is because that is most likly the supported end the rest (every thing inbetween centers) if your useing centers is due to deflection. My rule of thumb when selecting inserts is that if you cant shave your finger nail with the inset it's not going to give you a good finish.


03-13-2010, 09:21 PM
Feed is too slow and not enough depth of cut...

You have to be making 6's and 9's with insert cutters to get any decent finish..

Also should see straw that turn blue or black.