Hitting That Finish Size

[rake60]

Tin Falcon's recent post on Basic Skills set me to thinking about a few of them.

#1 on my list would be hitting the finish size on a part.

That should be simple.
Rough it to within .020", take a cut of .010 per side and it's perfect.

Probably NOT!

Say you are finishing a fit that is 2" long and the machine tapered .001" big from
the outboard end to the chuck at an ambient temperature of 62 F.

Today it is only 50 F ambient temperature in the shop.
Will that machine taper the same?

Probably NOT!

There are far too many other variables that will effect that finish cut.
When I ran a manual machine at work, I'd go like the wind roughing to leave
a little extra time for finishing leaving .060" on the fit, then mic the taper of that
last rough cut. Then I'd take a "pre-finish" cut of .015" per side, adjusting for the
machines tapering mood of the day, to be sure it would cut straight at that.
If that cut proved to take the taper out the finish cut was made in the same fashion.

Now in my home hobby shop the scale changes and there's no pressure to make
a quoted time.

For example, when machining a cylinder bore, I will rough it to within .030".
Then I take a spring cut through it at the same reading as the last rough cut to
eliminate any false taper caused by the "pushing off" effect of a boring bar.

Instead of one "pre-finish" cut, there are two. Both of them are .005" per side.
If the results of both of those cuts are identical, the final cut is going to finish size.
If they do not repeat the final cut is set to leave .0005 to .001" in the bore to be
polished to the finish size to eliminate any irregularities.

One thing I can guarantee is the are no guarantees...
Ambient temperature, grain of the metal, wear of the tool or mood of the machine will
cause a different result every time.

That's what makes it feel so good when you mic that fit and it's DEAD NUTS on size.

Rick

 

[Bogstandard]

Rick,

Your post brings up a very good point about temperature and accuracy.

I noticed that when I changed my old Atlas lathe from a Babbit head to a Timken tapered bearing head, if I wanted to face to length, I just couldn't keep accuracy at all.
Then I read that preloaded heads should be allowed to warm up before use because the spindle can 'stretch' by up to 0.003" between cold and warm.
Because of this I did a bit of further research, and basically it said that all machines should really be warmed up for a few minutes before use to allow for this machine expansion, and this should be repeated if the machine has been standing for a while as well.
I always now turn on the machine that I am about to use as I prepare the job for machining, and the results when doing precision stuff shows what I have said to be the case. Just a few minutes makes the difference between struggling to hit size and getting it spot on time after time.

John

 

[Cedge]

BINGO!!!
You've both just answered a curiosity that I'd noticed sometime in the past. I'd turn on the lathe and pick where I'd previously left things the night before and the machine behaved like a total stranger. I'd wind up fighting with the bloody thing through the first few cuts and then the gremlins would get bored and disappear. Now it perfect makes sense. Once everything was nice and warm and the machine was happy, it produced anything I asked of it. Thanks for the insight. New procedure now in effect here.

Steve

 

[Tin Falcon]

Rick brings up a good point here. Sometimes it seems like the metal and/or machine has a life or personality to it.
so here are some rules of thumb to follow
1)if you are working on anything longer than about 3 times the diameter of the piece you should support it with the center and check for taper.
2) always sneak up on finish size take off a little less than your calculated cut.
3) Know when to take a break when doing close tolerance work. If you heated up the part while roughing in the size you may want to take a break and let it cool down before going for the finished dimension. On the other hand taking a break when you only have a couple thousands to go is probably not a good idea because the gremlins will sneak in an move something as soon as you walk away from the machine.
Some other basics to keep in mind here are:
a good setup. If the work can be pushed away from the tool or visa vera you may get erratic cuts and therefore erratic dimensions. So change the setup or take spring cuts if needed.
Keep tool at center height.
Remember to take out back lash.
Keep tools sharp. If you need to, install a finish cutter before going for final dimension. Or you can touch up /hone the tool if using a gp bit.
You may need to de-bur before taking a dimension. If you measure on a bur it will give a false reading and you may dial in more than really needed.
It is the little things that get us. Developing good habits can take out a lot of the aggravation of "mistakes"
Tin

 

[BobWarfield]

The biggest thing I take away from Rick's post is to know your machine and know what it's doing at the moment on the job at hand. I would add to that, know yourself and your techniques.

When I first started out on the lathe, I kept a steno pad and sharpie handy. I measured what was happening on every single pass, wrote down what I wanted to see measuring the next pass, how much to turn the dial, and so forth. Believe me, this was not some systematic effort to study the machine--I was afraid I might forget where I was and do the wrong thing!

But it had a side benefit. It made me really familiar with how accurately the machine performed on various cuts. It got me to trust my dials ( and tinker a bit with the machine until I could trust them). It got me to adopt a consistent technique that was the same each time so that every cutting pass was not some new adventure. It taught me facility with my measuring tools, because if I'm trying to make sense out of what's happening on every pass until I got predictable results, I have to be able to measure accurately and repeatably. All that gets you to understand what a particular cut sounds like, feels like, and looks like. Don't like the way those chips are unfurling? Well hopefully you have enough experience to change the right parameter and fix it.

I'm happy to say that on my lathe, cutting to a thousandth has become automatic and routine. I'm anxious to learn to do better, but I haven't yet really embarked on that path. It isn't yet routine on the mill, which I regard as a harder machine to master, but it can be done with concentration. LOL!

One other thing I've noticed about my operations is temperature is thankfully not terribly relevant so far. I've gotten the speeds and feeds for my carbide tooling down such that most of the heat goes out with the chips. I know some folks here have disputed that, but let me assure you it can be done. I can be flinging out blue meanies, nice little 6's and 9's, and when I get done, the part is hardly warm to the touch. For an accuracy of a thousandth, that seems to be fine. I'll need to think about it harder to get to tenths, and parting off seems to generate a fair amount of heat that isn't taken away in the chips.

The taper issue is more problematic. Getting a long shaft turned to decent accuracy requires some real fuss. I have to check things at several points along the shaft and compensate. Frankly, I'm not happy with the lathe for long work. I need to spend a day or two really fussing with the headstock and tailstock alignment and clean that up. Eventually I will get to it. Fortunately, my needs have been for short work most all the time accuracy has been needed. That will change if I wanted to build a spindle or some such.

Cheers!

BW

 

[rickharris]

The school lathes I learned on were of some vintage and occasionally abused - the first lesson I got was about backlash!

Approach the work from one direction only and stop at the right place - If you go past your point then backing off is going to mess up all the dial readings, (no DRO then), big time.

Oh how long it took me to turn a bit of steel to an exact measurement - and my mentor didn't even ask for a particular measurement just for it to be exactly a multiple of 10mm. I started at 50mm and got it at 30 mm

Took all afternoon, most of the time trying to figure out what I was doing wrong because it looked easy when he did it.

 

[Loose nut]

What if your turning something like 4140, I can take a cut of .003 or .004" and if I crank up the speed it will give a nice mirror finish but if you have to take a single thou. off to get to an actual finish dimension it leaves a horrible finish. Many recommend filing or using emery paper to take off the last bit, lapping would be a better idea but a lot of work if it's not necessary.

 

[BobWarfield]

I know what you mean about the 4140. It can work harden, BTW.

I like to try to take a bigger finish cut than a thouandth. I shoot for anything under 10 thousandths and bigger than say 3 thousands and work the roughing so I wind up there.

Cheers,

BW

 

[Bogstandard]

When doing one offs in the lathe, you can't beat bringing down to size using emery. Not only for finish but also for accuracy.
If done correctly and safely, even a crappy old (or even new) lathe can produce results that are astounding.

Tapers can be washed away, or introduced (if needed), and the parts can be brought to size for exact fitting. We are not talking thous here, but almost undetectable dimensions.

What it is nowadays, people want the machine to do everything for them and strive to use the latest gadgetry to obtain it, and spend large amounts of cash trying to achieve the impossible.

The little bit of emery cloth can do the same thing for pennies, what it is people don't want to get their hands dirty by using the traditional methods, or have never been shown the correct way to do it. Not a wrap it round and run it up attitude, but a touch here, a wipe there.

Money isn't the answer, feel and experience is, and that only comes with time.

In our workshops, we are not on production bonuses, time is not an issue, but quality is.

Bring it down to near enough, then hand finish it for that something extra.

John

 

[Mcgyver]

I try to take a pass or two of about what i want for taking off the final cut and us those to learn just how much its taking off for this particular set up. I also don't have a problem taking off less than a thou if need be, a good sharp tool should let you do so easily.

My German tool and die maker HS teacher would start yelling and screaming and smacking metal benches with 3' flats of steel and raising all hell when he saw anyone using a file or emery in the lathe. He was to smart I think to throw things in the shop, but with his feigned or real rage we thought it a strong possibility. I think in hindsight the reaction to the emery was protecting the machines and i can see how a little applied judiciously makes sense - even with finishing tools the magnified surface is hills and valleys so this is just knocking off some hill tops. I use emery occasionally to polish and like John says sometimes to size - but try to make that an exception rather than norm. One should protect the lathe especially if polishing however DO NOT put rags down. A rag anywhere near something rotating is a big hazard. instead use paper towels and hold in place with pot magnets.