What causes this type surface finish

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CMV
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What causes this type surface finish

Post by CMV » Sat Apr 05, 2014 6:49 pm

Best I can describe it is "banding". Sometimes when taking a light cut instead of a nice even cut I get these little bands. Hard to see sometimes but you can feel them. I never see it in a heavy or aggressive cut. Pic below is exaggerated, but there's high bands. Like the toolpost or carriage is rocking - but I'd think that would go away with a finish pass & be worse a heavier cut? Is this a common problem & something easy to correct?

If I were to put something like 416 SS in & take a .060 deep cut with a moderate feed, I'd get a a nice flat cut. If I were to then put a piece of 2024 AL tubing in and take a .004 pass to clean it up, it would most likely band across the surface. Perplexing....

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Re: What causes this type surface finish

Post by doubloon » Sat Apr 05, 2014 10:41 pm

http://www.youtube.com/watch?v=BDtd2jNIwAU MUSAFAR!
http://www.youtube.com/watch?v=8CrOL-ydFMI This is Water DavidW
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Re: What causes this type surface finish

Post by CMV » Sat Apr 05, 2014 10:59 pm

Pretty much that's it!
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Re: What causes this type surface finish

Post by doubloon » Sat Apr 05, 2014 11:29 pm

Don't know anything about it and not sure if there was an answer in that thread just ran across it with google. :(
http://www.youtube.com/watch?v=BDtd2jNIwAU MUSAFAR!
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Re: What causes this type surface finish

Post by zach h » Sun Apr 06, 2014 1:27 am

I think its a chip welding issue . Where the chip sticks to the cutting tool smearing the finish then breaks free then builds up again .

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Re: What causes this type surface finish

Post by Historian » Sun Apr 06, 2014 8:17 am

I had a similar issue. My solution was to use a HSS tool and under a loop rounding the
tip to produce a small radius.

The carbide triangular tool left such a noticeable '128 TPI thread' pattern -
my gear set up on my Atlas 618's finest feed rate.

A 10+ loop and a diamond file are most useful.

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Re: What causes this type surface finish

Post by doubloon » Sun Apr 06, 2014 8:22 am

zach h wrote:I think its a chip welding issue . Where the chip sticks to the cutting tool smearing the finish then breaks free then builds up again .
Interesting ...

http://mmu.ic.polyu.edu.hk/handout/0102/0102.htm

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Re: What causes this type surface finish

Post by ChimeraPrecision » Sun Apr 06, 2014 9:29 am

Try to grind a chip breaker into the tool. You may need to take a lighter cut
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Re: What causes this type surface finish

Post by Baffled » Sun Apr 06, 2014 10:05 am

That type of finish usually happens in steels, especially those that can work harden, like stainless, and the descriptions already given of the phenomenon are accurate. It IS more common on light, finishing cuts, which is annoying because you want that last cut to be perfect.

Carbide tooling with zero or even worse, negative rake, and a largish tip radius, which rock for production on high HP machines, is often the culprit. Carbide is way overused IMO on home machines, many of which are not rigid or powerful enough to use them to best effect.

HSS is the way to go. The really aren't hard to grind, and you can make any shape to deal with any situation. For a finishing pass on stainless, I'm going to go with a high positive rake, small tip radius, feed as slow as my machine can do. A good HSS tool can produce a single, continuous cut just a thousandth or less deep into stainless with no skip or rub. Try that with a TPG222 insert. :)

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Re: What causes this type surface finish

Post by Capt. Link. » Sun Apr 06, 2014 10:50 am

Its vibration see the harmonic pattern.Could be from one of many sources including tool.
Chip welding gives the surface a torn appearance.Using a cutting fluid is recommended with aluminum to prevent the build up on the nose of the tool.WD-40 works well as do CLP type oils and other cutting oils for aluminum.
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Re: What causes this type surface finish

Post by CMV » Sun Apr 06, 2014 2:11 pm

Here's an example. A friend came over a couple days ago and we used a small leftover piece of 416 to make a muzzle brake for his 300 BLK. Everything was fine until I made a finish pass. It's blasted ready for cerakote, but even so you can see what I'm talking about a little. This little piece was sticking out of the 3 jaw & I was pretty much much right up against the chuck so the work wasn't extended far at all. The bands almost look intentional - they're perfectly spaced and the same width. Each band is .100 wide, spaced .150 apart, .0025 high.

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The tool I was using.

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Re: What causes this type surface finish

Post by doubloon » Sun Apr 06, 2014 4:24 pm

Looks nearly identical to the picture from the other thread.
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Re: What causes this type surface finish

Post by CMV » Sun Apr 06, 2014 7:48 pm

Did some playing around with it and trying some suggestions from CPT Link. It seems if I feed with the the leadscrew the issue does not show up so it must have something to do with the feed rod or gears the feed rod uses. I set the gearbox to 112-116 tpi (forget which, but the highest tpi setting it has) and fed with the leadscrew/halfnut (like I was threading). Not a hint of the problem. Same setting went back over it but with the feed rod driving it and a cut just deep enough to clean up the last pass & the bands returned. Tried multiple spindle speeds & feed rates, all had the banding on light cuts. .070 cut at a moderate feed (gearbox set for 28 tpi, feed rod driving) and nice smooth finish with no banding. Some faint lines I could see but not really get to show up in a picture and could not feel, but no banding 'problem' with the heavier cut. Different tool, same insert shown below.

.070 depth of cut - nice finish with no bands
Image


.010 depth of cut, set for threading 112 (or so) TPI, driving with the leadscrew.
Image

.004 depth of cut going over the above with the feed rod, bands show up.
Image

I've noticed this before on aluminum but never in any harder material. Also seems somewhat intermittent - until it showed up on this piece I had kind of forgotten about it, but it was easy enough to reproduce today.
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Re: What causes this type surface finish

Post by zach h » Sun Apr 06, 2014 10:40 pm

Take heavier cuts . To get good finishes at .004 or so ( that is only .002 per side ) you need a really rigid machine , sharp tooling and some tool pressure . The harder the material the easier it is to take light cuts . On a decent machine I for important parts I leave 2 passes of .01 each . cut one , check it then dial it in for what you need .

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Re: What causes this type surface finish

Post by daviscustom » Sun Apr 06, 2014 11:14 pm

I am not familiar with how much experience you have on the lathe, so I hope this isn't telling you something you are already doing.....but maybe it will benefit someone if not you...I think this is what zach h is describing too....he posted while I was typing.

If you are happy with the finish you are getting with the more aggressive cuts and you can live with +/_ .001" for your final pass.......I normally take a current diameter measurement, subtract the desired final measurement, and divide what is left into equal passes (.050-.200" depending on how much you have to take off and how stout your set-up is) to take it right to the final dimension. Check along the way to be sure you are taking a consistent amount off with each pass and when you get down to the next to the last pass check how much is left to take off, divide that into two equal final passes and double check to see how much needs to come off when you go for the final pass ( you may need to adjust the cut up or down a thou or so to get the final pass to come out where you want it)

The carriage, tool holder, etc. all has a little flex in it so if you plan it out so each pass is about the same, your tool pressure remains pretty constant and your cuts become more predictable. Usually after the first pass everything should settle down and you should start taking a consistent amount off with each pass. If you don't have a lot to take off, you can still do the same thing....just try to make your passes in the ballpark of .010-.030" for each pass and that seems to give enough tool pressure in most materials (that I regularly work with) to still leave a consistent finish.

Where I normally get into trouble with finish is when I try to sneak up on the final dimension with real light cuts. If you use a micrometer to make your measurements on the last couple of passes you should be able to hit pretty close to right on the money....assuming you have good scales on your handwheels to accurately set the depth of cut. It all hinges on accurate measurement and being able to accurately set your depth of cut. Once you eliminate the inconsistent tool pressure issue by taking equal cuts, everything becomes much more predictable and you can take a heavier cut right to your final dimension.
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Re: What causes this type surface finish

Post by CMV » Mon Apr 07, 2014 7:04 pm

I can do that & get to ±.003 with the dials (no DRO) & heavier cuts. .003 is close enough for most of what I'm tinkering around with and doing it that way to avoid the bands is an acceptable work around. But it feels like a work around - like I'm compensating for something that isn't right with the machine. If I really wanted something dead nuts and with great surface finish from the machine, I'm afraid I'd be in trouble because sneaking up on that last few thou could make the bands show up.

I don't think my machine *should* do this and something is most likely wrong causing it. Very well could be operator error.

My machine isn't a heavyweight, but it's not a minilathe either. 14x40 & about 1300 lbs (incl stand). I've produced some really nice work on it. Also produced plenty of scrap. But it's done everything I've asked of it as long as I figured out how to do my part.
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Re: What causes this type surface finish

Post by 57fairlane » Mon Apr 07, 2014 10:45 pm

Everyone else pretty much touched on it . . . light cuts don't ever leave a great finish unless the material is very hard IMO, like above 30 rockwell.

The worst is 4000 series chromoly. Your finish pass pretty much needs to be in the .015-.020 DOC range and ~200sfm+ to get that nice mirror-like finish.

I typically set the finish feed rate to just under half of whatever the tool nose radius is.

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Re: What causes this type surface finish

Post by Historian » Tue Apr 08, 2014 12:45 pm

57fairlane wrote:Everyone else pretty much touched on it . . . light cuts don't ever leave a great finish unless the material is very hard IMO, like above 30 rockwell.

The worst is 4000 series chromoly. Your finish pass pretty much needs to be in the .015-.020 DOC range and ~200sfm+ to get that nice mirror-like finish.

I typically set the finish feed rate to just under half of whatever the tool nose radius is.
Excellent rule of thumb. I now remember hearing this in 1950's machine shop but forgot until you just brought it up.

Thank you.

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Re: What causes this type surface finish

Post by 57fairlane » Tue Apr 08, 2014 9:14 pm

Historian wrote:
57fairlane wrote:Everyone else pretty much touched on it . . . light cuts don't ever leave a great finish unless the material is very hard IMO, like above 30 rockwell.

The worst is 4000 series chromoly. Your finish pass pretty much needs to be in the .015-.020 DOC range and ~200sfm+ to get that nice mirror-like finish.

I typically set the finish feed rate to just under half of whatever the tool nose radius is.
Excellent rule of thumb. I now remember hearing this in 1950's machine shop but forgot until you just brought it up.

Thank you.
Ironically enough I heard that from an older guy at the first machine shop I worked at . . . :mrgreen:

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Re: What causes this type surface finish

Post by paul463 » Tue Apr 08, 2014 9:51 pm

Speaking of light cuts and crappy finishes......
Go here; http://www.conradhoffman.com/advancedsharp.htm
and read about the lathe shear tool.

I used it to finish the od of my Ti f1 build and it worked great for taking off a whisper thin amount and gave a real good finish. It has limitations, but if you want to take off .001" or even less it works pretty well.

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