How to accurately bore on the lathe?

macardoso

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I am finishing a number of timing pulley bores on the lathe and want to achieve a finger press onto a ground 16mm motor shaft. The pulleys are aluminum and currently have a 12mm bore. Before starting the actual pieces I ran half a dozen test pieces in 6061-T6 bar stock. I attempted to use a very cheap (I mean ~$4 cheap) 16mm reamer but not surprisingly it cut like crap (it also has a significant amount of taper along it's length (~0.001-0.002")). My next attempts were fine feeding an indexable boring bar (TCMT21.51 insert) working my way up from the 12mm to 16mm and cutting the last pass at 10 thou off the diameter. It gave an amazing finish and was on size easily within +/- .001 (.0254mm) or less.

My issue is that 15.98mm is quite a bit tighter than a finger press and 16.02mm is a sliding fit. I did hit a few of my test pieces around 16.004mm which went on very nicely, but I'm struggling to get a reliable process to cut them like that every time. Here's my thoughts which I hope someone can chime in on!

1) My roughing cuts going from 12mm to 16mm are not always the same and might be leaving variable amounts of stock for the two finishing passes. Not surprisingly this would change the cutter deflection and change the finished diameter. Is there some magic way to approach a bore size so you don't have to do anything crazy to hit a nice tolerance?

2) If a part came out undersized and I tried to just kiss the diameter with a few tenths, my cutter would rub rather than cut. Eventually I would bump it enough that it would "bite" and blow the hole way over sized. I think a aluminum specific sharp insert (like a TCGX) would avoid the rubbing? Would a smaller nose radius improve this (like a TCMT21.50)? What about those cheap brazed carbide boring bars?

3) Suck it up and buy a better reamer...


I know there are many tricks of the trade for more precision operations, so I'd love to hear anything you guys can share!

Thanks!
 
What is your definition of a "Finger Press"? You'll need to define your required tolerances in terms of decimal places. Basically, there are press fits and sliding fits and you'll have to determine what it is you want.

And with the tolerances you are talking about, I am thinking you are in the realm of grinding and not boring.
 
I would not make the final pass 10thou, too much spring in the boring bar. I would want to sneak up on this and make the final pass perhaps 1 thou then measure and if needed make a final spring pass without changing the carriage setting.

The tool should be able to cut without needing to be bumped. Perhaps a dull cutter.
 
Fair enough. I'll be honest, I'm not great at relating shaft fit identifications (eg. ANSI LT3) to how they actually "feel" but lets say i'm aiming for 16mm (0.6299") +/- .013mm (0.0005"). I got nice fitting bores when they were in the 16.000 to 16.010 range.
 
I would not make the final pass 10thou, too much spring in the boring bar. I would want to sneak up on this and make the final pass perhaps 1 thou then measure and if needed make a final spring pass without changing the carriage setting.

The tool should be able to cut without needing to be bumped. Perhaps a dull cutter.

That sounds reasonable! The cutter is a new TMCT insert, but some grades of inserts sacrifice sharpness for strength. I'm thinking a different insert or a sharper corner radius would help the rubbing.


EDIT: I also have a pile of brazed carbide boring bars which I could try honing to a razor edge. I've never used them as I've been spoiled from day one with indexable cutting tools.
 
I lucked out and picked up a set of Metric Chucking reamers at an auction from 8mm to 30mm, Over and under sized by .02mm
They are a mixed brands of Cleveland, Hertel and Made in USA.
Had to do the same thing on a 10mm motor shaft. Used a 10.02mm reamer on pulley and it was perfect finger press fit.
Reamers are costly but just buy the one size you need for the job and before you know it you'll have a set.
 
If you are rubbing you need to fix that , you might try setting the bar at above center . One thing I used to do was once I was happy with how the bar was cutting I would feed in and then feed out check the size then dial in and do it again , now you have established how much the machine is taking . Also you could wrap some fine sand paper around a precision diameter and "hone" in your bore . Hope that helps .
 
Dont forget thermal expansion of steel: if you measure at 140degrees(right after you finish boring) and ambient temp is 70, the piece will be about 5 ten thousandths difference per inch. expansion.

If you use a reamer under power then you have to factor in runout from your machine also.
At .02mm you probably want to ream by hand.
 
What Mr. Paine said and may I suggest hss instead of inserts. It’s not that difficult to grind tools and in MHO they provide a much better surface on aluminum.


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I have heard about setting your boring bar above center to increase the effective rake and make it "sharper" although I haven't tried this. That might be my first line of attack. I don't have any HSS boring tools, but I will also try a sharp brazed carbide bar.

Lapping or honing would be a good way to tackle this I think. Maybe some fine lapping compound and a 5/8" carbide endmill shank would do it.
 
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