[How-To] Hitting a measurement

DavidR8

DavidR8

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When I was making the plugs for my alignment guide I tried very hard to match the diameter to that of the pipe.
I made five pieces and none are exactly the same nor do they match the outside diameter of the pipe :(

Here's the scenario:
OD = 1.000"
Target OD = .875"
Amount to be removed = .125"

Am I correct in thinking that if I remove .025 per pass, .125" should be removed in five passes?
I verified that my lathe has direct reading dials by putting an indicator on the carriage and measuring the travel. .020 on the dial results in .010 in movement.

What I found was that I could get to say, .0877, then I'd dial in an additional .002 expecting to hit .0875". Instead I'd get .874 or .873.
Stick out was about an 1.5" total on 1" CRS.
 
I'm embarrassed to say that it was carbide CCMT.
(on retrospect I think that's the problem...)
 
yendor said:
At .877 did you take a 2nd pass without any movement to see if there was any deflection in your 1st pass that resulted in the .877
Click to expand...
Hmm no I didn't actually. That sounds like it would have been a good idea.
 
Yeah, when you get close, it's time for a sharp cutting edge.
I know Mikey knows his stuff.
yendor, said it. How many times have we gone too far just trying for those last few thousandths and oops.
 
Janderso said:
Yeah, when you get close, it's time for a sharp cutting edge.
I know Mikey knows his stuff.
yendor, said it. How many times have we gone too far just trying for those last few thousandths and oops.
Click to expand...
Indeed, thank goodness they were completely non-critical parts. My fledgling TIG skills covered up a multitude of sins.
 
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