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Thread: Realistic tolerance when building a rifle?

  1. #1
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    Realistic tolerance when building a rifle?

    I try to dial everything in to where my .0001 indicator doesn’t move, or as close as possible. When mentioning this to real machinists, they said it isn’t possible to get .0001” tolerances on a shop lathe without temp control, etc. So the question to the machinists that do gun plumbing, what tolerance should I really expect?

  2. #2
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    Quote Originally Posted by Mram10 View Post
    I try to dial everything in to where my .0001 indicator doesn’t move, or as close as possible. When mentioning this to real machinists, they said it isn’t possible to get .0001” tolerances on a shop lathe without temp control, etc. So the question to the machinists that do gun plumbing, what tolerance should I really expect?
    Well, there's a difference between dialing in to <0.0001" and holding a 0.0001" tolerance. The former can be done, with patience. But holding a tolerance (e.g. machining a diameter to 1.0000" +/- 0.0001" is hard. Very hard. You need to start with climate control, you need to control heat in the workpiece and the tooling, and yeah, it's just not very practical. Stick a bar out a couple diameters from your chuck, turn some wide grooves (say 1/2 a diameter each). Try to get a fine surface finish. Now go back and test each groove with your tenths indicator. Note both the runout and the absolute measurement. *That's* about as good as _you_ can do on _your_ machine.

    GsT

  3. #3
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    Any final sizing on parts that are sized to within .001 should be done at room temperature, which we call 78 degrees F.

    As an example, when we Machine large bearings, they pick up enough heat to expand .002 to .003 inch.
    You simply cool the bearing down before final sizing.

    A heavy mist cooler usually works fine.

    As was stated, there is a difference between indicating something and sizing something.

  4. #4
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    Keep doing what you're doing.

    They are technically pretty much correct, but that doesn't change your work.

  5. #5
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    Gene, I check my lathe about once a month with a test bar. I’m not a real machinist so I lack the knowledge to know what to realistically expect.

    Jackie, I had no idea bearings would vary that much with temperature! I’m working in a shop with temps from 55-70 degrees depending on when my shop heater decides to kick on...

    Rubicon, I definitely will. Thanks. Never ending learning process....

  6. #6
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    Quote Originally Posted by Mram10 View Post
    Gene, I check my lathe about once a month with a test bar. I’m not a real machinist so I lack the knowledge to know what to realistically expect.

    Jackie, I had no idea bearings would vary that much with temperature! I’m working in a shop with temps from 55-70 degrees depending on when my shop heater decides to kick on...

    Rubicon, I definitely will. Thanks. Never ending learning process....
    I am talking about large bearings, often made of some type of bronze allow and larger than 10 inches. If you have 1/4 inch or more to come off, it can easily get to 150+ degrees. That’s enough to make it grow .002 to .003 inch.

    Often I will put a mist cooler at the cut, keeping the temp down.

    By the way, we have no coolant pumps on any of our lathes. In the end, they tend to be more trouble than they are worth on the type of work we do.

    The point is, you have to insure your piece is at room temperatures if you want exact measurements.

  7. #7
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    Quote Originally Posted by jackie schmidt View Post
    I am talking about large bearings, often made of some type of bronze allow and larger than 10 inches. If you have 1/4 inch or more to come off, it can easily get to 150+ degrees. That’s enough to make it grow .002 to .003 inch.

    Often I will put a mist cooler at the cut, keeping the temp down.

    By the way, we have no coolant pumps on any of our lathes. In the end, they tend to be more trouble than they are worth on the type of work we do.

    The point is, you have to insure your piece is at room temperatures if you want exact measurements.
    Copy. I’ll keep it a big oversized till it cools

  8. #8
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    Quote Originally Posted by Mram10 View Post
    Copy. I’ll keep it a big oversized till it cools
    So how will you shrink the bore before final cut?

  9. #9
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    [QUOTE=Mram10;831437]I try to dial everything in to where my .0001 indicator doesn’t move, or as close as possible. When mentioning this to real machinists, they said it isn’t possible to get .0001” tolerances on a shop lathe without temp control, etc. So the question to the machinists that do gun plumbing, what tolerance should I really expect?[/QUOTE
    Perhaps a better way is to determine what degree of precision is needed to get a great result. Surface imperfections are in the .0001 area. It's actually in the od grinding arena. I've been a toolmaker for over 40 yrs and have taught metrology in jr college. In most instances you can work to a .002 or so. Perpendicularity and center line integrity is getting as good as your tools and machines will allow. Good luck!

  10. #10
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    [QUOTE=WallyDog;831465]
    Quote Originally Posted by Mram10 View Post
    I try to dial everything in to where my .0001 indicator doesn’t move, or as close as possible. When mentioning this to real machinists, they said it isn’t possible to get .0001” tolerances on a shop lathe without temp control, etc. So the question to the machinists that do gun plumbing, what tolerance should I really expect?[/QUOTE
    Perhaps a better way is to determine what degree of precision is needed to get a great result. Surface imperfections are in the .0001 area. It's actually in the od grinding arena. I've been a toolmaker for over 40 yrs and have taught metrology in jr college. In most instances you can work to a .002 or so. Perpendicularity and center line integrity is getting as good as your tools and machines will allow. Good luck!
    Good to know. Thanks

  11. #11
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    I have a buddy that runs a shop machine shop. He said they will see a difference in their work if someone opens a door on the other side of the shop. I'm guessing their tolerances are way tighter than I could even measure. Heck, my shop is an uninsulated pole barn. It's 23 outside right now so it's 23 in my shop. LOL

  12. #12
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    [QUOTE=WallyDog;831465]
    Quote Originally Posted by Mram10 View Post
    I try to dial everything in to where my .0001 indicator doesn’t move, or as close as possible. When mentioning this to real machinists, they said it isn’t possible to get .0001” tolerances on a shop lathe without temp control, etc. So the question to the machinists that do gun plumbing, what tolerance should I really expect?[/QUOTE
    Perhaps a better way is to determine what degree of precision is needed to get a great result. Surface imperfections are in the .0001 area. It's actually in the od grinding arena. I've been a toolmaker for over 40 yrs and have taught metrology in jr college. In most instances you can work to a .002 or so. Perpendicularity and center line integrity is getting as good as your tools and machines will allow. Good luck!
    “In most instances you can work to a .002 or so”

    With all due respect, that’s a rather large tolerance for most general machine shop work.

    Also, surface finish and size are two different things. You can have two shafts that mic exactly the same diameter while one has a 32 finish and the other a 16.

    I do agree that holding tolerances that are not required is a waste of time.
    Last edited by jackie schmidt; 02-09-2020 at 10:43 PM.

  13. #13
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    Tolerancing

    [QUOTE=jackie schmidt;831480]
    Quote Originally Posted by WallyDog View Post

    “In most instances you can work to a .002 or so”

    With all due respect, that’s a rather large tolerance for most general machine shop work.

    Also, surface finish and size are two different things. You can have two shafts that mic exactly the same diameter while one has a 32 finish and the other a 16.

    I do agree that holding tolerances that are not required is a waste of time.
    He asked what tolerances HE could expect too hold. I assume he is asking as a hobbyist with a lathe and regular non modular tooling etc. The surface finish example is valid as removing the tops off a lathe tool finish with crocus would easily remove .0001. Its all in your reference point. Surface grinders hold .0001, not lathe tools in a home shop imo. He could expect to hold closer than .002 once he gets a feel for his tooling I suppose or if he was doing a slip fit he would have to do better. He should be able to indicate closer than .002 for sure. When I say .002 we mean plus or minus .001 right? That's .002 total in practice.

  14. #14
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    I think the problem is that everyone buys electronic calipers or DRO's that have readouts to .0001" inches. They then expect the machines to actually do that level of precision. I have had young engineers bring drawings with tolerances of .00001" on them for drilled holes. Part of this is no experience in actually making things, only theoretical work. They have never used old school measuring devices like a vernier caliper. There is also no understanding of how parts need to fit together with clearance for dirt, heat expansion, etc. How many galled barrel threads have we all seen from trying to make the fit too close?

  15. #15
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    Can one honestly qualify a +/-.0001” tolerance in a home garage shop with a mic?

    I think the .002” window (+/-.001”) stated above is more realistic.

    Other than certain chamber diameters, is there a feature you’d toss a barre in the scrap pile if you were at the top/bottom end of that +/-.001” window?

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