Irregular or non-uniform surface finish on manual lathe
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  1. #1
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    Default Irregular or non-uniform surface finish on manual lathe

    We have a small 12 x 36 manual lathe that refuses to deliver a smooth, uniform surface finish. The photo shown is a piece of 1.5" OD x 0.125W DOM mild steel tubing. Spindle speed was 288 rpm. HSS tool lubed with oil. This is after polishing with 400 grit sandpaper.
    img_5980.jpg
    The striations appear regardless of spindle speed.

    I thought I had the problem fixed a couple of months ago. I discovered that the Feed Rod was bent. While I was straightening the Feed Rod one of my business partners remembered that the movers had strapped the lathe into the truck by wrapping a strap around the feed rod. Aaarrrrgggghhhh.
    Anyway, the problem still exists.

    Any speculation as to a solution to this problem? Something else messed up inside the carriage? Any help is appreciated.

    Signed,
    Distraught in Dallas.

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    With tubing tool geometry can be a factor. Cutting pressure can try to push the part away and cause such lines.
    Sharpness (does the bit shave your fingernail?, Did the edge get too hot at sharpening? the angle of back and side rake, nose radius too large(too small would make smaller lines(perhaps) the side cutting angle can tend to push harder to the chuck or harder away. Think I would try a solid to rule out the tubing deflecting. Then concentrate on the bit.Tool bit set to center correctly
    Yes higher RPM and a fine feed rate might work if that did not cause a small lathe to shake and shimmy.
    Tail support and less out of chuck might help. Notice to see how well your chuck jaws are touching the part.
    For abrasive polishing try going one way like threading with the paper for a little faster and nicer looking finish. Sanding is just like grinding as abrasive falls into the works.It is also dangerous so don't get caught or bump into the chuck.Check to see gears are not clashing too tight, or full of chips..How far out your tool holder or the tool bit, best both short.

    How does the finish feel? 400 might polish the top and make a fair finish look poor.
    Had an old Jamieson that made a poor finish because the gear for that speed was going out, other gears were ok.

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    Speed is a bit low. that being said, is it harmonics from too much stick-out?

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    Does the same thing with solid bar. Even bar supported with a live center in the tail stock.

    On my "things to do" list for tomorrow is to see if there is any correlation between the pitch of the "stripes" and the feed rate. I'll also try faster spindle speeds (I'll note any changes in the pitch of the "stripes").

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    You might try a side cutting angle of perhaps 30* off the lead side (that will look like less than a 45 looking down at the edge that does the cutting. Yes even 45* would be OK. Cutting side clearance perhaps 5 to 8, Front clearance perhaps 6 to 10*. back rake perhaps 10* Perhaps a .015 radius- that at about 8 to 10* clearance or just a little less than the intersect of two angles side cutting and front.. Try setting bit a little above center line perhaps .o30 or.050. Increase speed a slight. Try 350 then 400 if the lathe still runs smooth at better speed. Take about .010 per rev. Use the tail center.. but first be sure it does not have a wobble, check with an indicator as a bad live tail can wobble the work. A wobble center would likely make one side and the other different so with not seeing that the center may be OK.

    *Use a straight tool holder, set straight so any chatter will not increase or vary the depth of cut.

    Yes you should look at the way the tool holder holds the bit because it's angle and set might change the attitude of the cutting edge rake and clearances.

    Hold the tool holder and the tool bit very short.

    Increasing the side cutting angle(the angle and edge that cuts) changes the thickness of the chip going from same as the feed rate total with no angle(straight side), to half of that with a 45* side cutting edge angle.

    Good to sharpen with light grinds and wetting dipping often.Any heat felt can prove the very edge got very hot.
    Most bench grinders come with way too hard and too fine wheels and so can over heat the bit cutting edge. Even with cool grinding a smooth side hone can make the edge better.

    Be sure the lathe is level and solid set to the floor or bench.

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    Did the lathe ever cut this material with a smooth finish? Is the material C1020? That's pretty common for DOM tube. It should machine well. The picture looks like the problem is harmonic. Is the tool rigid? It also looks like its scraping metal rather than shearing it off. I'd try a carbide insert, 700 rpm and adjust feed to suit. A dull tool and too big a nose radius could be giving you a built up edge that breaks off at regular intervals. Since you say the problem is at all speeds and feeds, I definitely would try carbide insert tooling at higher speed. This would eliminate most of the tool geometry questions. Then start to look at the whole machine for rigidity. Start at the tool post, all the way thru the saddle to the spindle.

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    Could be the rack and pinion feed. If the gear and/or rack are not meshing properly or poorly matched, it may be printing the results on the work piece.

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    Use different inserts to see what may change. Live center being used and still having problems is disappointing because simply because it tends to work. Rotating the material one quarter and cutting again could be tried to see the effect.

    At this point I am wondering what the original ID to od trueness is to start. Is the ID off somewhat to the od. Is the machine balanced well or needing a adjustment? Have you tried setting the center of your cutting tool dead center or slightly below dead center?

    All this comes to mind. I am curious as to what will cure it. Does the tubing have a seam on the inside? Could it be the material composition is not quite consistent? Soft and hard mixed into different places. Turn a round surface to chuck on and spin it around then try a cut. Alter your depth of cut speeds and feeds. Are you getting chatter?

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    Quote Originally Posted by nealbert View Post
    On my "things to do" list for tomorrow is to see if there is any correlation between the pitch of the "stripes" and the feed rate. I'll also try faster spindle speeds (I'll note any changes in the pitch of the "stripes").
    oh there will be.......

    More rake, less feed, more speed.
    Last edited by Trboatworks; 07-24-2015 at 10:48 AM.

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    First pass with a lot of out of round and perhaps hard scale outer surface can cause problems, and perhap ruin the tool bit edge. How deep is the first cut? Does a light second pass with a fresh resharpened bit make an improvement?

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    Check your spindle bearings. The affect you are seeing can be caused by many of the reasons given above, but in my experience, the problem is usually to much clearance in the spindle bearings, which is an easy fix. If the bed ways are so worn that the saddle doesn't track straight but rather kinda waddles down the bed, you may see that affect as well. OR, you said the machine was recently moved... Was it leveled properly after the move? If the whole machine can rock, or for that matter if there is a good twist in the ways and the saddle can rock on two points opposite? Start with basics, level machine Properly! Check gib adjustment, spindle bearing clearance, lubrication, ect.. Give the machine a careful going over. You may or may not find one dramatic problem which is definite cause, but at least you will be sure of what it is not and will no doubt improve machine all around performance.

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    The number one problem in cutting anything is chip clearance. I think those scores are created by trapped chips being dragged against the finished work by the tool. The cutting tool radius should be at least two times the feed rate with adequate back rake to allow the cuttings to fall free and not get wedged between the work and tool. There of course could be other reasons, but that's my bet.

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    Simple 2x4 light pry with an indicator over chuck might prove what Derek is suggesting about spindle. Perhaps looking for .oo2 lift or so. With a ball or roller bearing spindle none.

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    Quote Originally Posted by michiganbuck View Post
    Simple 2x4 light pry with an indicator over chuck might prove what Derek is suggesting about spindle. Perhaps looking for .oo2 lift or so. With a ball or roller bearing spindle none.
    Yeah, I probably should have included a way to check it! Thank you Michiganbuck for clearing things up. My bet is that's where the OP's problem is. I've had that pattern, or at least what it seems to be in the pics, a few times and it's been loose bearings. Like I said though, any time a machine is moved it should be thoroughly checked and leveled. A lot of weird things can happen when a machine is not properly set up, and a lot of time can be wasted chasing one's tail to correct a simple issue.

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    I know this machine.
    Well.

    BTDT, and it could be almost anything.

    bearings,gibs,bent shaft, crap in the gearbox,etc,etc... Hell, even a mouse turd stuck in the endgearing is enough to move that spindle.

    man, I have become jaded since I ran that damn 13" 61 series.

    Can you get a better result feeding by hand? That can at least help narrow it down.

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    Quote Originally Posted by nealbert View Post

    Any speculation as to a solution to this problem? Something else messed up inside the carriage? Any help is appreciated.

    Signed,
    Distraught in Dallas.
    Don't know what make/model but:

    If there is a motor drive belt, is it hard with a set. Replace.
    Motor mount bushings/rubber mounts are worn. Replace.
    Cross slide gibs are loose or cracked. Adjust or replace.
    compound slide gibs are loose or cracked. Adjust or replace.

    No guess work, my experiences.

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    Wow big list we all came up with. Check them off and let us know how the problem was solved.

    Looks are deceiving and a good top finish as from fine paper can increase the poor looks of a fairly good surface. *Close your eyes and feel the finish with your finger nail and then with your finger.

    Polished is best done with rough then finer abrasive in steps.
    My method is going one way not back and forth with crocus cloth..
    I have used cut strips of a plastic milk bottle on occasion for a backer and the back side a thin honing stone. One shop where I worked used a clamp over device with abrasive roll paper, Yes they had give so not to lock up on the job (crankshafts). I was a tool grinder at that shop. *Again some have lost fingers and lives with polishing.

    HSS should be Ok for mild steel but with the scale possibility and with one pass, trying carbide is worth the effort. Yes you should have carbide in house for harder work.
    Yes carbide is a bit expensive when HSS will do just as good.

    Treat turning harder materials like grinding. Clean the machine after and try to keep chips out of the works as best you can.

    $10 for an uncoated bargain brand TPG perhaps great where needed but I have some HSS bits that I have used for years, and when I need a special it is only a grind away.

    Not knowing how to grind a tool bit limits one to doing only standard work. (yes I made special one-up cutters on manuals and CNC, and the customers paid like they were made of gold)

    Not knocking carbide, I love it to.

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    Ditch the HSS.

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    Lathe was NOT leveled after delivery. I'll start there.

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    After leveling the lathe, I'll check the bearing play. This lathe had at least 2 previous owners that we know of. Maintenance history is unknown, but judging by the shape it was in when we got it, maintenance was marginal at best.

    I did double check the stripes this morning and they are indeed bands around the piece. The stripes are not in a helix or threading pattern.

    The fact that the stripes are uniform bands (about .050" pitch) makes me think the problem is a mechanical one, not a tool/cutter problem.


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