Taking it to the final size

what I don't see in your post is how the sizes act each time if you are at 1.20 and dial in .010 do you get1.270 or something different. then you dial .n .010 again do you get 1.260 if you do then don't change anything and take the final cut it should come out on size if none of the other sizes came out then start figuring out if yo are going over and compensate for the change learn your lathe sometimes they will stick and then jump. put a dial indicator on the carriage and watch it move. buy the way in the machinist world tenths is short for ten thousandths .1 tens, .01 hundredths .001 thousandths .0001 ten thousandths abbreviated tenths so we don't have to say so much each time bill
 
In these parts, the "basic" unit is 0.001, so if I say 1 inch 100 (or "an inch 100"), I'm saying 1.100" or I say "one half" I'm saying 0.0005. Or if I simply say "that bore is 10 over!" That means it has exceeded tolerance by 0.010". Otherwise, it's directly indicated to be in ten-thousandths, as in "2 tenths", meaning 0.0002".
 
I have an old 1950's heavy iron workhorse 12x48" Utilathe. Kept the Canadian Navy steaming full speed ahead for 30 years. So it's a bit worn and loose. The only way I can sneak up on anything with this lathe is to use a very sharp, pointy, right hand HSS tool - ground and lapped before my finish cut, and up the RPM. Then If I really need it, I get usually one shot with a tiny skimming cut .001 or .002", usually. Usually afterwards, if the bit rubbed to much, Dialing another .002" will often rip up to .010" if the bit grabs and digs in - ripping the finish and ruining the part.

So , I've found, much better to use consistent deep cuts for my final roughing passes, say .015 to .025" (or more- anywhere up to .250) per pass. Then when I get down to the final finishing cut of 12 to 15 thou-subtract the full amount I want to take off, and go for it with a smaller feed rate -e.g. .003" maybe. A sharp, well prepared finishing tool gets me much more consistent results with deeper depth of cut, than sneaking up a few thou at a time.

I think The big thing is to discover your lathe's largest, not least, repeatable rough and finish depth of cut and stick with that.

Glenn
 
I think the tendency to use a tool fresh off of the grinder, or worse yet, an insert tool to do fine finishing is a bunch of the problem. A little radius on the cutting edge will allow a wedge of swarf to build on the tool, protruding into the workpiece and creating a false tool depth. With an insert tool, you need to go for it with enough depth and feed to wipe the accreted wedge off of the tool, or use a cutting material or edge profile that continuously wipes clean. The other choice is as Glenn speaks of above is a very sharp tool with a honed edge. Often we use the corner of the grinding wheel to put our back rake or chip break on the tool. This creates grind lines perpendicular to the flow of swarf over the top of the tool. The metal will accumulate on the tool quickly, and intrude into our finished part. Stoning the top of the tool in the direction of the chip flow helps a lot to counter this. If the tool edge gets chipped or deformed by heat, same story. Stacking is our enemy.

My tricks:
Incompatible cutting tools so that there is no frictional welding of material onto the tool. IE ceramic tools, coated carbide inserts of the correct type, cutting fluid that penetrates to the tool interface
Honed edges and minimal radius on ground tools.
Leave a bit and use a lathe file, emery cloth, finish grind, burnish.
If the machine is rigid and precise, finish turns and bores at no less than the radius of the tool
 
As a follow up to my earlier post about taking the max possible finish cut, not the least..., I spent today making test cuts on my new-to-me SB Fourteen. Wanted to find out exactly what the optimum, MAXIMUM, finish depth of cut I can make and still hit my target OD.

After many cuts, Long story short, I found this lathe will turn between .005" to .012" finish cut (with fine feed=.003") dead nuts on. Easy to cut .005" to .010" exactly.

Over .012" DOC and tool pressure on the work starts resulting in cuts that are slightly oversize. So a .015" DOC leaves .002" excess. I discovered that if I leave the cutter set at say .020" DOC, and run the compound back out to the beginning of the cut, I can make a second skimming pass and the cutter, if sharp and pointy, will skim off the excess .002". Leaving a nearly dead on OD.

Here are my notes from one test cut I made on a piece of junk yard shafting measuring 3.2" OD:

Tooling was an unused NOS 1/2" brazed carbide cutter.

Set my OD target at 2.500" .

Rough Turned it down to 2.585". With a feed around .016"

Dialed in around .050, rough turned to 2.535"

For finish cut, Set up .002 feed. Dialed in .020" and took first finish cut.

Measured OD at 2.517. So it actually cut 2 1/2 thou under my DOC - I think due to tool pressure against the work.

Took another finish cut of .012, then skimmed back through with same setting- OD was 2.505

Dialed in .005" and cut it exactly to 2.500".

Boy was I happy!

So I have concluded I can predictably make .010 to .012" depth of cuts for finish cuts on this lathe, any time I like.

Cheers!

Glenn
 
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