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Taking it to the final size

Discussion in 'QUESTIONS & ANSWERS (Get Help Fast Here!)' started by 8ntsane, Aug 13, 2012.

  1. 8ntsane

    8ntsane Active User Active Member

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    Ok Guys, I think we have all been here before.
    I was over at a friends place, and he hasn,t been machining very long. He was not a happy camper that day, and swore his lathe was junk. I thought I would start this topic because we have all been in this bad spot before.

    The target of the part is say, 2.750
    At 2.780
    creeping up on the final size
    2.770
    2.760
    Then your allmost there, should one start sneaking up on the final size .002 at a time, or take the final cut?
    I think we all have taken that final cut, and found out 2.750 ended up a 2.748, and damn near had a fit as the part did take some time to make.

    Ive found over the yrs, many things can screw you over just when you think your mins from being done, now your starting again.

    Here are just a few things Ive seen over the yrs.
    1 Made the measurement while the part was still hot.
    2 Used a caliper instead of a mic

    3 trying to hit the final measurement without leaving any room for expansion of the work piece, and not leaving a few thou extra for the final fitting

    4 Trying to hit your final size to a few tenths, and trying to do a fuzz cut or three to get there.

    Now these are just a few ways to add another piece to the scrap bin.

    Tell us how you get that final size, fit and finish you want.

    Hand lap
    Hone
    Abraisives/ sand paper
    tool post grinding

    We all have our method to the madness, so lets hear how you do them. This thread could help many newbies from the frustration of trying to get the parts to size, and have it go bad at the end of the job.
     
  2. jumps4

    jumps4 Global Moderator Staff Member Active Member

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    my small chinese lathe will flex so .005 may be .003 or .004
    I try to make the final pass with tool steel not carbide
    and if fitting to a bearing or needing exact fit i have a fine stone tool post grinder I made from a hf saw blade sharpener that takes very fine cuts.
    I have tried the files and sandpaper and I prefer to put on the grinder
    steve
     
  3. Bill Gruby

    Bill Gruby United States Global Moderator Staff Member H-M Supporter-Premium

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    There is a tad more involved here. Tha main one, for me anyway, heat. If you take your part to final cut and it is hot, when it cools it won't be finish size anymore. It will probably be undersize. Also when I get close, say .010, I will take more than one pass at each setting. You would be surprized at how much the second pass takes off. Final cuts for me are .002. Yes, I like all of you have missed the mark before.

    Please remember this is the way I do it, that doesn't make it the right way, it does make it my right way.

    "Billy G" :whistle:
     
  4. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    Well, since this is lathe work we're focusing on, I'll go....

    I usually run flood coolant, especially if the material requires high spindle speeds/feeds....I know it will get warm. That minimizes the shrinkage effect.

    I take the last .100 or so in even DoC. This gives me the time and information I need to take the final cut, as well as time for the part to cool back to ambient if needed. If I have 0.100, I take 0.050 and see if it really did, and make an adjustment if needed in the DRO, dial, or indicator (whatever I am using). If it takes 0.048, or 0.051, I mentally note that and create an offset in the tool nose. The next cut will be 0.025, based on that offset. Hopefully it will be much closer to "as dialed". If not, I repeat the process. Rarely do I need to make more than a few tenths correction at that point. If I do, the insert is dull, not on center, etc. That's another problem. If the correction is on the order of a couple of tenths, I make the correction and do a pass I've learned to call the "mic check" pass. This assumes of course that the insert will be repeatable. If I have 0.015 (or 0.010, depending on what kind of material it is) and that agrees with the dial, DRO, or indicator, I dial it off and cut. I just finished with some parts that have a bearing fit specified to be 0.4996 +/- 0.0002. Worked every time. No files, paper, or fuzz cuts.
     
  5. 8ntsane

    8ntsane Active User Active Member

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    There we go, thats a good start to this thread.
    Tips from our more experianced Good Ole Boys, allways good!

    Keep em coming
     
  6. swatson144

    swatson144 Global Moderator Staff Member Active Member

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    I'm with Tony on this, right down to the inserts. Start taking cuts to get the finish you need and that normally means some consistency. I just find out what it is consistently cutting. Tony calls it an offset but I think of it as a multiple. IE if I take .015 this cut then .051 should have me there in 4 passes. Check/measure each time and adjust as needed (hmm Tony;s offset?)but stay in the same ballpark. It usually works well but sometimes it seems the decreasing surface speed, gravitational forces, or something makes the final cut not as pretty as the previous cuts and at that moment I just start hoping I missed on the large side. It worked a lot better on the Navy's heavy lathes than my current lathe but it's the method I know.

    The cuts I really hate are the ones where the final size is so close to the stock size it's pretty much skim, measure, skim and measure, much the same as above but with HSS and less feed and much lighter cuts.

    I use shear type tools to get me out of a bind when I'm like say .001 off. With that tool you can literally make swarf that floats down like snow flakes.

    Steve
     
  7. george wilson

    george wilson United States Global Moderator Staff Member H-M Supporter-Premium

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    My first lathe was a 12" Craftsman Atlas. That blasted thing was too flexible and would lie to me all the time!! Drove me nuts. 2nd. lathe was a Jet Taiwan 1024. INFINITELY BETTER. Did not bend under a cut and give false readings.
     
  8. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    Steve, I think I picked up the term "offest" from the CNC days where the controls would allow you to add or subtract a little to compensate for either pushoff, tool wear, erroneous zero setting, etc.. If you expected the part to measure 1.0000 and it didn't, you could enter an "offset" factor on the control to adjust it's theoretical position with the actual position. It's basically the same as cutter comp on the mill. Most carbide end mills actually measure a couple under nominal, and cut small as well. So....you "lie" to the control and tell it the end mill is smaller than it is. For circular interpolation, you can dial in a close size with this method without actually having to change the program. That way, next time you run the program, with different tools, you don't get bit by a wrong tool setup.
     
    Last edited: Aug 14, 2012
  9. 8ntsane

    8ntsane Active User Active Member

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    All good infro guys.
    Hitting the target size can be a challenge for many less experianced members here. For the experianced members, getting to final size comes much easyer than a fellow thats startin out.

    As allready mentioned, heat plays a big factor in measurement, and final size. What you use for a cutting tool will affect the quality and surface finish of a final cut. I see some like to grind to the finished diamentions, and yet others still use inserts. Interesting that many different methods are used.

    I have found that heat has screwed me over enough times, that I will shut the machine down and wait till the part cools when Im getting close.
    Depending on what Im working with material wise, I will set the compound over to 45 degree,s and feed into the cut with the compound rather than the crosslide for the last few thou.

    I know once I do start feeding in with the compound, rather than the crosslide, Im without my DRO, and have to take a measurement after each pass. When Im getting down to the final passes, I will usually switch over to HSS cutting tool. The HSS cutter will also has enough nose radi to produce a smooth finish.

    Allso, if switching over the tooling, you should do this well ahead of time, and not on the last .002 of the cut. You,ll need to make a few cuts, measure and check your surface finish prior to getting to final size. Once getting close, I also like to make a spring pass too, and check measurement, and see if Im getting possibly a better surface finish to boot. I perfer using a Mic to verify my cuts, and spring passes. Calipers are fine for reading the roughing in, but its to easy to apply to much pressure when using a caliper, and can easyly fudge your reading. Yes, us experianced guys can use either, but for the newb, he may not have the feel yet.

    Tony,s method using flood coolant is a good idea, and I do it sometimes myself. It can shorten the cooldown period, and waiting time sometimes required to accuratley measure your work piece.
    The flood coolant can also help with surface finish as well. But not all home shop machines have this.

    As George has mentioned about his machines not being the most ridgid, and I have used machines that would not be reliable to get with in .002 for final size. I see no problem with using sand paper of various grits to get you to the final size either. I have used some lathes that I would not trust to take me all the way, and the paper of the right grit will get you there, and clean up the surface finish at the same time.

    The tool post grinder isnt something I like to use, but at times it is a life saver. Ive had some times when I can,t get the smooth finish I want, and need. The material is hard, and what ever I try isnt working out to plan, out comes the tool post grinder. It seems when ive only got a few thou left to go, and I know nothing else will do, the TP grinder has saved the day, and the job more than a few times for me.

    So, as one can see from the replys in this post, there is many things that effect your final size, fit and finish. And many things that can bite your ass in the process. Hopefully, the replys from all that have posted will help the newbie from making his scrap bin larger.

    For the newb, getting a decent fit is what most are striving for, and getting there is part of the learning process.

    Keep the infro coming guys
     
  10. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    On some lathes, in certain situations, when you feed up to a shoulder, or through a bore, simply shut down and reverse the feed (or hand feed slowly back off the cut if it's short), and you won't have disturbed the actual position of the tool. Often you'll see a bit of material coming off the part during this process. This is pushoff being cut, either from the part deflecting, or the tool being literally "pushed off" the part. It's the same as a spring pass, except the back of the tool is usually sharper than the front that just cut off all the material, especially if you have taken several passes. When you get close to finish size, doing this will allow you to take moves only in one direction, allowing the carriage and cross-slide assembly to remain in the "stressed" position they get in during a cut. You'll get more predictable cuts this way. If you are really sneaking up on it, do this with 0.005 left. Dial in 0.003 and "drag" the tool back (with the spindle running) and then stop for a measurement. Dial what is left and poof, you're there. Feed on.......feed off. Presto! (and no, it's not like Wax on!....Wax off!)
     
  11. 8ntsane

    8ntsane Active User Active Member

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    Tony
    It should be like wax on/ wax off!

    The Master would say
    Pay close attention to details Grass hopper, When you can cut to the tenth,, You have learned ;)
     
  12. george wilson

    george wilson United States Global Moderator Staff Member H-M Supporter-Premium

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    I cut to tenths now that I have had decent lathes!!(for the past 35 years).
     
  13. Bill Gruby

    Bill Gruby United States Global Moderator Staff Member H-M Supporter-Premium

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    Why should you get branded? 0.1 is 1/10th of 1 inch. You are correct.

    "Billy G" :lmao:
     
  14. 8ntsane

    8ntsane Active User Active Member

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    I have no problem cutting to a tenth either Frank.
    Do you get branded now? Ok, I will brand you as a
    OLDIE, and not a NEWBIE, as this thread was intended to help.:rofl::rofl:
     
  15. jumps4

    jumps4 Global Moderator Staff Member Active Member

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    this thread is helpful to me thanks guys
    steve
     
  16. 8ntsane

    8ntsane Active User Active Member

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    :rofl::rofl: Frank, as a Ole Fart myself, I thought when we graduated to old farts, we gave up shaving, and wearing pajamas too.:lmao::whistle:
     
  17. PurpLev

    PurpLev United States Active User Active Member

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    good stuff so far.

    I also use the compound for the last couple of paths and run each setting twice to remove material that was left from tool deflection.

    never thought about heat though - thanks for bringing this up.
     
  18. November X-ray

    November X-ray United States Active User Active Member

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    and all this time I thought they said I'm a "Pretty Smart Feller, not pretty fart smeller"!
     
  19. 7HC

    7HC Active User Active Member

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    Have you got a pic or a link to such a tool?

    Thanks,

    M
     
  20. 8ntsane

    8ntsane Active User Active Member

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    Sharon

    Thats a good habit in my book. Ive gotton into the habit of using the compound for the last few hairs of the job. My old lathe has a compound dial of 100 grads per rev, and have adapted a 200 grads per rev dial to the compound so Its easyer for me to dial in the fine cuts.

    Next time you think of it, mic a work piece while its still warm, then re mic it a room temp, might surprize you. Some materials have a greater expansion rate than others.

    We all have a scrap bin, but small details can keep it from growing to enormous size :whistle:
     
  21. Metalmann

    Metalmann Active User Active Member

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    "Depending on what Im working with material wise, I will set the compound over to 45 degree,s and feed into the cut with the compound rather than the crosslide for the last few thousandths."




    I usually do it this way, but feeding straight in with the cross slide does as good a job.....on a lathe that you are comfortable with. Experimenting with different tool angles will get you there fast.

    Chuck up a piece, and start experimenting with the best feeds and speeds for the kind of tolerances, and finishes; you want.

    Seems I always remembered more, due to my tests; instead of what I was being told. wink wink..nudge nudge.
    :whistle:

     
  22. 4gsr

    4gsr Global Moderator Staff Member H-M Supporter-Premium

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    I do use the Tony Well's aproach most of the time, gets me "right on" or most of the time "OH Shoot"...
    Wish I had flood coolant but I usually coat the surface with a flim of cutting oil for that "last past", most of the time I get it right on.

    On smaller lathes like most of us use, and using carbide tooling, keep the edge lightly honed for finish cuts. On carbide tooling honed edge means "break the cutting edge by x amount", you don't usually cut with a razor sharp edge in carbide. The more the edge is honed, the more you have to take in depth of cut to get it to actually cut. If you are using insert tooling use a lightly honed positive edge for finish cutting. If high speed steel, hone that edge to near razor sharp edge, totally the opposite of that of carbide.

    On more ridigit heavier machines, you can generally hold size without difficulty with any carbide tooling provided the lathe is in good shape. Most of us are using vintage lathes in varying degrees of conditions so we will encounter problems holding tolerances. If youre like me you have various grits of emery cloth and a collection of files that aid us in holding tolerances as needed.

    I know many will disagree with me on that, that's ok, this is how I learned. I would love to hear from others on this subject so we can have many thoughts on "taking it to final size".
     
  23. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    Ken, you won't get any disagreement from me. On the old Monarch, I can't quite use the procedure I outlines above, but then, that's one reason I have the two lathes. Don't get me wrong, I can hit tenths on the Monarch too, mostly the same method, but there isn't the level of confidence there that is on the newer lathe, which by the way is an Acer. Pretty fair lathe, I think.

    I try not to do the finicky work on the Monarch, unless the Acer has a setup I don't want to disturb. It's capable, and surprisingly repeatable for a lathe of its age. In reality, most of the work I do doesn't call for tenths tolerance performance. And the Monarch does just fine holding 0.002 for 'O' rings and the like. It's a pretty tight machine. One difference in the tow is probably the gear/bearing condition. The Acer will cut a finish that passes without much ir any polishing. I generally don't like papering on it anyway, but the Monarch just isn't as slick, so I do use paper. I prefer to use a flat file to back up the paper rather than just wrap it. In fact, this may be a good time to point out the danger of just throwing a strip of abrasive cloth around a spinning shaft. It will tend to be pulled in the direction of rotation, and given enough slack, can come all the way around and pinch itself up and snatch it right out of your hands, if your fortunate. Always be ready to let go of the paper if you insist on handling polishing like this. You can't win that wrestling match. I think most anyone who has spent time polishing shafting has had to give the paper to the lathe. I've been cut near to the bone by the edge of the abrasive cloth. Kind of painful, and makes you more cautious. And it should go without saying that wrapping paper around your finger to polish an ID is very, very risky. It can be very hard on the health of the knuckle joint of that finger. I keep a couple of "fingers" in the box for that when needed. One is simply a broomstick with a slot sawed into one end that I can wrap some cloth around and stick in the bore. Smaller than that, I have a couple of pieces of leftover Delrin set up the same way. For smaller yet ID's, where you can't get much pressure on the "stick", I use what most machinists around here call a butterfly. It's a 1/4" steel rod sawed to accept the cloth, and fit in an air motor. On this tool, the spindle doesn't even need to be spinning. Plus this can be used to deburr drilled/reamed/bored, etc. holes in nearly any part.
     
  24. 8ntsane

    8ntsane Active User Active Member

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    Tony

    Very good tips about the use of sand paper on both OD and ID. Allways good to point out the dangers as well. Using files have there own danger too.

    Lots of great infro guys
    Keep it coming
     
  25. GK1918

    GK1918 United States Active User Active Member

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    Pauls got it, use the compound, everyone has their own method mine is always using my thread stop,
    actually I never take it off. And as said compound swung over and it things are getting warm, go get
    a cold one, when cooled then take that final whisker cut.
     
  26. Tom Griffin

    Tom Griffin Active User Active Member

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    You'll normally get a better, more precise final cut by leaving more stock than by sneaking up on the final dimension a few thousandths at a time. I generally rough to within .010", move in .005" and take just enough of a cut to measure, then take the rest to full depth. The full .010" finish cut puts a little more load on the tool and produces better results. Other things to do that will help in holding tight tolerances is use high speed steel instead of carbide, select the best machining steel possible for the part (medium carbon or leaded steels), and use cutting fluid or oil.

    Tom
     
  27. 8ntsane

    8ntsane Active User Active Member

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    Well said Frank ;)
     
  28. 4gsr

    4gsr Global Moderator Staff Member H-M Supporter-Premium

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    I second that!!:drink:

    I will say, I have come up with a few more thoughts from postings to try next!
     
  29. 8ntsane

    8ntsane Active User Active Member

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    HSS, or Carbide?
    I know some of the members here will use Carbide. I tend to sneak up on the final cut. Carbide does like a fairly deep cut, and if you dont, it doesnt give a nice surface finish, for me anyway.

    Im used to using HSS for finishing up the job. Ive allways had better luck, and surface finishes by using HSS, with a sharpened and honed edge. I guess if I was willing to take a deeper cut on the final pass, carbide would work for me too.

    I have stayed with the HSS to do the last few passes because at least when Im taking a few fuzz cuts at the end, my errors are smaller than when I take a bigger bite at the end with carbide.

    Right, wrong , or other wise, thats just how I do it. I say, if what you do works, no sense in changing your methods.
     
  30. FarFar

    FarFar Denmark Active User Active Member

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    Hello Steve

    Can we have a picture of Your homemade tool post grinder?.I need one
     

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