Need help with lathe dials

[MrWhoopee]

If ypu look carefully at the cross slide dial in the first photo, the metric scale is 2.54mm/rev. The lead screw pitch is .100"/2.54mm.

Ah, yes, I see it now that the second cup of coffee has kicked in.

 

[WobblyHand]

I was going to mention the 25mm vs 25.4mm thing, but that's been covered. Still might be worth checking carefully over an inch of travel with a known good dial indicator. But for anything important, use a micrometer, not your dials.

 

[RJSakowski]

Just started my second cup.

If it actually was a metric lead screw with a pitch of 2.5mm, the movement for a full turn of the dial would have been .0984" rather than the .095". Another possibility is a well worn lead screw but I don't believe that you would lose .005" in a single turn. This usually manifests itself when traveling over an extended distance, e.g moving 1" by the dial from an unworn section but actually only moving .995"

My Grizzly G0602 has a metric lead screw but the dial reads .001"/.025mm per division. The inch scale is actually .000984"/div. This is accurate enough for small movements but if you are moving an inch, the actual amount would be .984" or .006" short.

This is actually OK because I usually would measure the work piece after the movement and make adjustments accordingly. OTOH, if there would be overshoot as would occur with an Imperial lead screw but a dial marked as .025"/div. and I was moving by the dial, the work piece would have been ruined.

 

[matthewsx]

All of the above is good advice.

One thing I realized when I started trying to hit accurate dimensions on my lathe is metal expands when it gets hot.

This may not be a factor in what you’re seeing but I did find if I measured right after turning and made a final pass based on that I often overshot my desired diameter.

John

 

[RJSakowski]

I think the biggest factor is tool deflection. A light cleanup pass after a heavy cut is very likely to cut undersized due to deflection. Joe Pieczynski did a video regarding this subject. His recommendation was to make an oversize "final" pass, measure, and make a second pass with exactly the same depth of cut and feed rate. Repeat as necessary to hit your final number. The trick is to come close enough that you don't have many repetitions but your increment is small enough that you won't overshoot.

 

[matthewsx]

Also, know what your tolerance is. No point worrying about tenths when it’s +/- .002

 

[Pevehouse]

Thank you to everyone. I’m getting tons of feedback and help from everyone’s input. You guys blow my mind with how smart you are with this stuff. I called it a night pretty early last night but I’m dying to get back home and get back on my lathe to try some of the things suggested. I do have one thing that kind of confused me and maybe this is where I’m going wrong with some of this but if I want to take say.030 off of a piece of 1” stock, I should be dialing in .015 and not a straight.030 ?

 

[Larry$]

Take a cut and see whether your dials are for diameter or radius.
Some lathe tooling doesn't like really thin cuts and will just slide over the surface rather than taking a cut. Some carbide inserts fall into that category. Sharp HSS will work better for most very thin cuts.
See Steffan's video about determining the last two cuts.
Life gets easier with a DRO. The only time I try for .001± or better is for press fits. Then a good lathe file is your friend.

 

[Jake M]

... if I want to take say.030 off of a piece of 1” stock, I should be dialing in .015 and not a straight.030 ?

You need to prove that, but I believe that yes, this is the case. You said you measured with a micrometer, you moved 0.100 inches, and you measured (with some tolerance) pretty much that same amount. That means, with the evidence provided, it APPEARS that your dial reads the tool movement. That means that your dial measures the actual depth of cut, which is based on the radius. I call those a radius dial. (Just like any other dial in the shop, it measures pretty exactly just how much you actually move). But then YOU have to remember that the "cut" is on both sides of a rotating workpiece, so to reduce a diameter by a given amount, you must cut half of that amount off of the radius of the part.

The other option is a "diameter dial". In that case, when you measured the travel with the micrometer, 0.100 inches, the micrometer would have told the tale that the TOOL only moved 0.050 inches. That dial halves you input every time. (Every gosh darn stinkin time......).

Either one is fine, you've just got to know which it is. And you'll be confronted with math either way. Try that out and see where it gets you.

 

[SLK001]

In addition to the cross slide lead screw, you could have a worn lead screw nut. If the nut is worn, or not even the correct tightness, hitting your numbers is very difficult.