How to accurately bore on the lathe?

I've settled on a few tricks that might help.

- try to converge on the target bore by the same increment, say 0.005" per pass. Some materials are not as forgiving for sneaking up on a dimension, especially if they work harden or don't play as well with the cutter type/nose geometry/rigidity.... So for example 0.005 + 0.005 + 0.005" (measuring in between of course) might get you to closer to the target than 0.010 + 0.003 + 0.002

- make the boring bar spring passes at different feed rates on your lathe. It does make a difference because you are changing the traversing pitch, so stand a better chance of cutting off the hilltops vs. the tool following along in the same path like a record player. So my ritual is 3 identical spring passes with carriage locked at the 3 finest feeds on my lathe.

- use the heaviest most rigid boring bar with shortest stick-out. Listen/feel for any harmonics, adjust rpm/feed accordingly

- make sure your carriage is locked every pass & ideally using something like a DRO. Even the act of locking the carriage can be enough to move it slightly (+/- 0.002" is not uncommon) depending on your lathe wear & how the clamping mechanism imparts itself against the dovetails or whatever. A DRO sees this because its measuring independently whereas a dial machine will not.

- if you can find a reamer to match your bore, it can save a lot of work. Now boring means just getting it appropriately undersize so the reamer can do its job with more consistency. Ensure your tailstock is very well centered.

- assuming you are not into toolpost grinding, that then leaves lapping. Its best to use the correct tools & the don't have to be expensive. These brass laps work well & are reasonable. Ideally you want a softer material (brass or some folks use copper for home made ones) which will embed the grit. This is somewhat messy & time consuming work. I would recommend not being much more that 0.002" undersize because it takes a lot of lapping to get that final amount down. http://www.acrolaps.com/
 
If you take your time you can achieve any size you want, smaller holes are more difficult because boring bars have more deflection, i usually finish smaller bores with DIY flapper wheel to take the last few thousands off
 
P. Waller said:
.004 MM is 0.00000159", how did you measure this?
Click to expand...

0.004mm is 0.000159”. Measured with a telescoping bore gage and mitutoyo half tenths mic. Admittedly this tolerance is rather unnecessary, but the exercise in hitting repeatable tight tolerance dimensions on a lathe is not.
 
Your tool geometry is everything when it comes to boring. It s not only the insert but also the insert holder that gives you your tool geometry. A small radius and rake on your tool helps offset deflection. Brazed Carbide can be used for these tight tolerances if ground with the right geometry. 6061 loves rake and a sharp angle of keenes. Plenty relief up front and your side clearance. Myself when i am in to the tenths its hss all the way for boring. Carbide especially inserts don t like to sneek up on a dimension. Brazed Carbide if ground with the right geometry but I still prefer hss when I need to be that precise. I'd go for a 001" if it were me. It s 6061 and a metal shaft. Heat your pulley up if you don t have a press or can t use your lathe (tailstock) or a vise to help press it on. Finger press may not hold your parts. Disimilar metals and your pulley may heat up from your belt friction.
 
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MarkM said it as well as I could- with a sharp tool you should be able to shave the eyelash off a gnat
MarkS
 
If you need a reamer check Amazon I got a Morse 3/4 carbide tipped chucking reamer for 29 bucks
 
Thanks again all!

I just finished another half dozen test pieces and two of the pulleys. Here is what I found really helped!

I did my roughing passes at 20 thou until i was 40 thou from the final diameter. Then I switched the power feed to the finest setting and took a finishing cut at 10 thou. I measured this and took a second 10 thou cut. Measured again and took a third. At this point I knew how the final cut would work and I adjusted the final depth as needed (roughly 2 thou deeper). I liked this because it tended to risk an undersized bore rather than an over. My first piece wasn’t great but after that I got the 4 remaining test pieces and two of the pulleys within +/- 0.0004”. I’m happy with this! It’s amazing how much of a difference in fit there is between a .6295” and .6303” bore on a 16mm shaft. One pulley is a little to tight to hand press so I might lap the bore or heat it up before putting it on.

My finish pass was made at around 12 thou off the diameter and produced a nice finish and consistent diameter.

Still have a few more to go so we will see how it finishes up.
 
Nothing like a 350 degree oven and air in a can to freeze the shaft. a 400 degree differential in parts , with a16mm bore will add an extra .0016" or .04mm.Once it cools off it may be difficult to remove!
 
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