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  1. #1
    Join Date
    May 2021
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    australia
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    Default scratches in my finished part ? any tips or advice on how to remove them ?

    Good evening everyone. So i've made a thing and wondering what's causing the part to have all these scratches in it, even after polishing :'(

    i went up to 1200 grit sand paper and i have these scratches that won't come out :'(

    Any tips or advice on what's causing it and how to avoid or fix it would be awesome. thank you

    183395023_3970631796316331_6345197793851947023_n.jpg
    video-1620555826.mp4

  2. #2
    Join Date
    Oct 2004
    Location
    Southern Highlands NSW
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    1,630

    Default

    If you're using carbide toolbits, try HSS for better finish. Have a keen edge on a rounded point, slow speed, fine feed. Maybe use coolant.
    A single cut lathe file (I bought one from USA) is good for improving surface finish, but accuracy is destroyed.
    Then use some abrasive paper glued or stapled to flat wood wood strip.
    Then finer abrasive, buffing mop, elbow grease, metal polish.
    At any step, the abrasive should remove all scratches of previous treatment, before progressing to the next finer.
    Easler to aim for a good finish straight off the lathe.

  3. #3
    Join Date
    Aug 2010
    Location
    Melbourne
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    36
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    Fixing that one is going to be time consuming via sanding. You've moved up in grades of sandpaper way too early, there is no point in moving up a grade until the bottom of the scratches from your coarsest paper are level with the bottom of the deepest scratch you're trying to remove. It'll take you a long time to remove enough material via sandpaper to get that top one out.

    The cause is lack of experience.

    Bit hard to tell exactly whats gone on there, since you've sanded it - I'm assuming it's steel though. If it's carbide tooling you've used, I'd suggest spindle speed way too slow, giving a tearing finish, and maybe too small a depth of cut on finish pass. Possibly compounded by horrible soft steel, has a bit of that look to it, but may just be from the finish. If HSS, maybe not enough of a nose radius, maybe could have done with some cutting oil, edges not sharp enough, also possibly off on the spindle speed - not really a HSS guy myself.

    Also seems to be a bit of chatter in the grooves, which could come from a number of sources. Tool stickout, basic rigidity of machine not sufficient for cutting edge engagement, loose gibs on one or more slides, loose spindle bearings, probably a few more I can't think of right now. There are a lot of variables when you take a new machinist and an old machine with an unknown history, it's all part of the fun!

    If you've got some more steel, chuck some up and experiment. Have a look at the turning speeds here: https://littlemachineshop.com/refere...tingspeeds.php and bang your diameter (in inches) into the calculator. If you're using HSS, I'd pick 180 for the SFM (surface feet per minute), it should cover almost anything you're likely to be using, if carbide I'd completely ignore the SFM they have there at this stage and use 500sfm. That should at least get you into the ballpark for that aspect, over time you'll learn to tweak this part to suit the task at hand. You may well find the carbide speed is faster than your machine will go, in which case just use the highest you've got.

    Once you're getting a nice finish on your test pieces, you could chuck up a piece of thread that matches the thread I think I see in the bottom of your (I assume) gear knob, then screw the knob onto that, and skim the outside of it to clean it up. Then sand as required. Although by the time you've done enough experimenting to get to nice finishes, you may well decide to just make a whole new one anyway

    If you can be more specific with your particular geographical region, you may well find a local member who offers to give you a hand getting the basics down and checking the setup of your machine...

  4. #4
    Join Date
    Jun 2007
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    sydney ( st marys )
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    To help with people's guesses, you should give as much information as possible,as to material, speed, feed,diameter and tool type.

  5. #5
    Join Date
    Nov 2007
    Location
    melbourne australia
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    Without knowing the type and sharpness of your tool as well as your machine setup, I'd guess you've got some really crappy steel there. Some grades of steel just do not want to be machined. They tear rather than cut.

    As to why your sanding was unsuccessful, I agree with this:

    Quote Originally Posted by Jekyll and Hyde View Post
    You've moved up in grades of sandpaper way too early, there is no point in moving up a grade until the bottom of the scratches from your coarsest paper are level with the bottom of the deepest scratch you're trying to remove.
    Chris

  6. #6
    Join Date
    Oct 2011
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    Norwood-ish, Adelaide
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    If you sand on the lathe, you also need to move the emery around. Leaving it in the same spot just allows grooves to form. Moving it (especially along the length of the part) allows the grit to clear as well as not letting the scratches get established on the circumference.

    Michael

  7. #7
    BobL is offline Member: Blue and white apron brigade
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    Perth
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    Default

    I'll add my name to those saying you shouldn't move up to the next grit if you still see scratches that are not a result of the grit being used.

    But I would not call those scratches but gouges and others have already provided plenty of info about there possible causes and as they say it will take forever to remove those with paper.

    Even when there are no gouges the time to move up to the next grit is when you cannot see any scratches from the previous grit.

    eg lets say you start with a a 180 grit and reckon you have it smooth enough so you then try 240 and then 320.
    If you haven't removed all the 180 scratches with the 240 then trying to do that with 320 is a waste of time and its far quicker to go back to using the 240 again.
    And same again all the way up the grits.

    This is what I use for sanding on a lathe
    https://metalworkforums.com/f307/t20...-2#post1954969

    I need to make a set of these so I don't have to keep changing the grits..

  8. #8
    Join Date
    May 2021
    Location
    australia
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    Default

    wow lots of information here. Thank you guys.


    I'm on the north side of Brisbane ( Australia ), and only got the lathe a few days ago.


    The lathe is a southbend 9 " ( we think ) and it uses V belts, has 16 different speeds, but none of them are labelled. so all i have is different rates of slowish to fast haha. I need an RPM counter i guess and then make a chart to stick with the machine for future use. Any ideas on getting the rpm ?

    The material i used was steel, from a longer length and as 38mm diameter when i got it and it's about 35-36 now I wasn't going for any particular dimension, just getting a nice round part with all the mill scale and surface rust ect off of it and i continued until it looked pretty cleanish cut. Oh, the guy at the steel place said it was 360 grade or something like that, whatever that means, i don't know.


    With the feeding, i've been doing it by hand, turning the saddle wheel, perhaps i'm going too fast, and yeah i have had the tool dig into the piece and the machine freeze up, so i guess it was gouging and got stuck a few times. I don't/didn't have any carbide bits/tools, but i got a few tools from gumtree yesturday to use I believe they are carbide tipped on steel and i'll give them a whirl today after i get a few other things done.

    Using a new piece of steel, i'll see how one of the big tools on the left side of the container goes as they seem to be the perfect tool size so that i don't need to shim it in the tool post ( Wish i had a quick change post )

    new tools off gumtree.PNG

  9. #9
    Join Date
    Oct 2004
    Location
    Southern Highlands NSW
    Posts
    1,630

    Default

    Could it be 316 you bought - a type of stainless steel?
    It's more difficult to machine than mild steel or leaded steel.

    Some info: https://geospacemfg.com/blog/machini...ainless-steel/

    Your brazed carbide tools need to be sharp.
    Used to be that these type needed a special green grinding stone, but maybe a diamond wheel would work.

    Do you have a copy of "How to Run a Lathe" published by South Bend?
    https://www.practicalmachinist.com/v...dition-344476/

  10. #10
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by nadroj View Post
    Could it be 316 you bought - a type of stainless steel?
    It's more difficult to machine than mild steel or leaded steel.
    The OP said it had mill scale and rust so it was unlikely to be 316.

    Heres the RPM chart from the Text book of turning.
    Screen Shot 2021-05-10 at 10.01.16 am.png

  11. #11
    Join Date
    Aug 2010
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    Melbourne
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    36
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    Default

    Quote Originally Posted by mathuisella View Post

    With the feeding, i've been doing it by hand, turning the saddle wheel, perhaps i'm going too fast, and yeah i have had the tool dig into the piece and the machine freeze up, so i guess it was gouging and got stuck a few times. I don't/didn't have any carbide bits/tools, but i got a few tools from gumtree yesturday to use I believe they are carbide tipped on steel and i'll give them a whirl today after i get a few other things done.
    If the tool is digging in and jamming up, my first suspects given your experience would be either tool above center height, excessive depth of cut for the machine (probably unlikely at this point), or failing to account for backlash in the cross slide screw. If you're turning the OD, never wind the tool in towards the part, then back it out a bit and go. If you overshoot the setting you wanted on your handwheel, back out a bit past it, then wind back in to sneak up on it. Failing to do this means the cross slide can pull into the work under cutting load by the amount of backlash you've left in it, especially if your cutting parameters or tool aren't great. Obviously the directions would be reversed for boring. The tool can still pull into the work from a combination of backlash and excessively loose gibs, but having the slack against the screw in the right direction seems to make it far less likely IME, especially if you have a few degrees of lead angle on the tool. Either way it's still best practice when you start trying to hit specific diameters.

    Could also be caused by slop in the machine (loose/worn bearings, gibs needing adjustment type things). Sometimes slow rpm means the tip tends to drag and dig rather than shear the metal, depending on what it is...

    Speaking of which, A36 is an American grade for fairly generic mild steel, which is apparently a touch softer than 1018. If that is what he meant, might be a little tricky to get a decent finish on it. Flip side is even if it's difficult, once you've done it other steels will seem easy!

  12. #12
    Join Date
    Sep 2012
    Location
    York, North Yorkshire UK
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    4,774

    Default

    Quote Originally Posted by mathuisella View Post
    wow lots of information here. Thank you guys.


    I'm on the north side of Brisbane ( Australia ), and only got the lathe a few days ago.


    The lathe is a southbend 9 " ( we think ) and it uses V belts, has 16 different speeds, but none of them are labelled. so all i have is different rates of slowish to fast haha. I need an RPM counter i guess and then make a chart to stick with the machine for future use. Any ideas on getting the rpm ?

    The material i used was steel, from a longer length and as 38mm diameter when i got it and it's about 35-36 now I wasn't going for any particular dimension, just getting a nice round part with all the mill scale and surface rust ect off of it and i continued until it looked pretty cleanish cut. Oh, the guy at the steel place said it was 360 grade or something like that, whatever that means, i don't know.


    With the feeding, i've been doing it by hand, turning the saddle wheel, perhaps i'm going too fast, and yeah i have had the tool dig into the piece and the machine freeze up, so i guess it was gouging and got stuck a few times. I don't/didn't have any carbide bits/tools, but i got a few tools from gumtree yesturday to use I believe they are carbide tipped on steel and i'll give them a whirl today after i get a few other things done.

    Using a new piece of steel, i'll see how one of the big tools on the left side of the container goes as they seem to be the perfect tool size so that i don't need to shim it in the tool post ( Wish i had a quick change post )

    new tools off gumtree.PNG
    Hi Mathu,

    Before rushing out and buying expensive tool posts, learn about your machine, what play you have in the various parts, are the gibs properly adjusted, is the spindle loose, does it have any end float, and things like this. Learn to grind your lathe tools, using carbide is OK but but insert ones are better, you still have to make sure that the tool cutting edge is on centre height. You need to measure that because it is important to know what it is.

    I primarily use HSS lathe tools preferring them to carbide ones ! Generally HSS produces sharper tool edges than carbide, though modern carbide inserts are far better than they used to be.

    Looking at your picture you seem to have a mixture of HSS and carbide in there. Brazed carbide needs a green grinding wheel for sharpening, whilst HSS can be sharpened on an ordinary grey or white wheel. Just because you got those tools with the lathe doesn't mean that they are any good ! Often tools like that just thrown into a box probably means that they need sharpening. Good tool bits need taking care of, looking after good tool bits saves a lot of effort.

    HTH.
    Best Regards:
    Baron J.

  13. #13
    Join Date
    May 2021
    Location
    australia
    Posts
    35

    Default

    Thank you everyone read all the posts and lots to digest.



    So yeah, i get extremely close to centre height with my tools, but i believe the tools are blunt or chipped and that's why they are grabbing on occasion.

    i saw this video

    https://www.youtube.com/watch?v=__A2xtLF0AU

    i think i'll give this a shot to always be able to have a sharp tool on hand or a few mins later.


    now to find some hss local to me haha.

  14. #14
    Join Date
    Jul 2008
    Location
    near Warragul, Victoria
    Posts
    3,477

    Default more

    Hi

    You will soon pick up the tips of the trade

    This is a good intro to grinding your tools https://www.youtube.com/watch?v=2OzRtY8SVcM&t=1193s

  15. #15
    Join Date
    Sep 2012
    Location
    York, North Yorkshire UK
    Posts
    4,774

    Default

    Quote Originally Posted by mathuisella View Post
    Thank you everyone read all the posts and lots to digest.


    So yeah, i get extremely close to centre height with my tools, but i believe the tools are blunt or chipped and that's why they are grabbing on occasion.

    i saw this video

    https://www.youtube.com/watch?v=__A2xtLF0AU

    i think i'll give this a shot to always be able to have a sharp tool on hand or a few mins later.


    now to find some HSS local to me haha.
    Hi Mathu,

    I have picked up most of my HSS tool steel in the local scrapyard ! Most firms throw it away when moving onto carbide insert tooling. Talking about carbide inserts/tooling my local scrappie has a small 8 ton skip well over three quarters full of them. Unfortunately they all are unusable for one reason or another, but the small HSS is worth picking up.
    Best Regards:
    Baron J.

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