Achieving precision in machine work

[briankenyn]

Hi all,

I've been pondering this question more frequently as I make more mistakes and miscalculations. Despite acquiring a war-worn Atlas 6" Lathe in HS Many years ago, and after a 25 yr. lapse, my real experience with machining commenced two years ago with the acquisition of a HF 8x14 Lathe. It was good out of the crate, but then I've seen exceptional work done on treadle lathes. There is a great temptation to blame the equipment [which, in my case is limited] but my senses tell me that the outcome might not be very different even with the very best of facilities. This reasoning begs the question, and hopefully a discussion: "How does one approach learning to be precise in machine work"? This may seem simplistic, but its been annoyingly elusive.

Many thanks,
Brian

 

[Brian Rupnow]

Brian---You raise a very valid point. I strive for accuracy, but live with less than satisfactory results. Even if I had state of the art, majorly expensive equipment with digital readouts, I'm not sure how much it would improve the quality of my work. Perhaps the secret lies in knowing what specific operations are going to have the 'Highest probability of error" and finding 'Work arounds" so that the results can be corrected while there is still metal to be removed. Once you get beyond that point, then its too late, and you either have to start over or live with it.-----Brian

 

[Tin Falcon]

Getting expected results every time is the holy grail of machining.
Consistency is probably the key, but there are so many variables.
Tools wear and are sharpened. Parts heat up when machined. diameters change belts slip under load reducing speeds. .....
In the home shop you can take the time to see what the material and machine tool does as you rough out apart. typically dial in about half of what you think you need to remove, measure then repeat.
Tin

 

[rake60]

Accuracy comes from experience not the machine.

That's not to say experienced machinists always hit their sizes every time.
They may hit it more often, but none will ever be perfect.
There are simply too many variables in machining to ever expect a perfect
result every time.

If you ever meet a perfect machinist, congratulate him on his achievement
then get away from him before you get something nasty on your shoes....

Rick

 

[briankenyn]

Thanks, Brian and Tin;
It seems that the common thread is to provide an escape hatch whenever possible, to avoid a complete disaster. From time to time, I've made the mistake of not doing that and its terrible to be boxed in!

Thanks again for your kind response,
Brian

 

[Brian Rupnow]

Brian---If you want to see my theory in action, go over and look at the last 7 or 8 posts on my Kerzel engine build----

 

[compressor man]

I am glad to see that it is not only me that puts out parts that are "good enough sorta" instead of dead on. I am teaching myself the skills of a machinist and I do not have to tell you that when you measure in thousandths of an inch it is really easy to be off on that measurement! I come from a woodworking background where 1/16" is more than enough of an error margin!! How many thousandths is that???I have yet to make something that is dead on perfect but recently I had to make six of something...wow...some were a little big, some a little small, or a little long. They all looked the same but the calipers tell the real tale. I too like to blame my machines sometimes just to take a little of the self blame from myself! But I know that the overwhelming amount of the discrepencies come from my most used tools: the left and right hands

 

[Ned Ludd]

Hi Brian,
If there is a secret to achieve precision lathe machining, it is getting a good smooth cleanly cut finish. You can not measure a rough surface with any degree of accuracy. You will have to find your own best method of getting a good finish on your work but for a newbie I would strongly recommend a Diamond tool holder because of its amazing ease of sharpening and clean cutting ability. Without the ability to cut cleanly you will not able to take the cut you expect, a blunt tool will, on shallow cuts, just rub. When this happens until you wind on more depth of cut and then the tool starts to cut, but much more than you want.

The second secret would be practice and a dogged determination to do better.

Ned

 

[Mainer]

It is pretty elusive, isn't it? My thoughts....

It takes a long time to get enough experience to be "good."

Patience is absolutely required.

Near-daily work in the shop is required to maintain the proper mindset.

When you feel yourself start to get impatient, STOP! and take a break. The best I ever worked up to was about 4 hours at a stretch, but it's generally more like 2 hours before I feel myself pushing too hard.

Remember the quote in Guy Lautard's "Machinst's Bedside Reader: "The secret is knowing what good work is, and taking the time to do it that way."

 

[briankenyn]

Many thanks to all for your thoughtful comments. Some skills seem to come easier than others- turning a steel piston to +/- .0005 for example, while other skills such as parallelism and alignment require a good deal of attention and patience and experience. Its encouraging to share the struggle for perfection with others, and to know that others struggle as well. I've always loved metal work and its wonderful to have a group such as this from which to learn.

Best Regards,
Brian

 

[Maryak]

Small machines cannot take big cuts

Sneak up on the finished size. e.g. If you need to remove 0.200" on a lathe:
Cut one = 0.050" cut two = 0.030" cut 3 = 0.015" using a roughing tool, (adjusted after actual measurement), cuts three+ = finishing cuts with finishing tool.

Remember 0.01" off the radius is 0.02" off the diameter.

Allow cuts to wash out as you get close i.e. take at least another pass at the same setting. Especially when using a boring bar.

Try and ensure the work is well supported against flexing away from the cutter.

Leave around 0.010"-0.006" (diametrically) for finish cut(s).

Hope this helps.

Best Regards
Bob

 

[Blogwitch]

I am one of the lucky ones who can hit spot on size (within 0.0002") time after time on my lathe, mainly thru the use of technology (DRO's).

But before that, if I needed exact size, it would all be done with hand finishing. Which seems to be a bit of a dying art nowadays, where everyone relies on the machinery to do the job, rather than their hands.

With care, it is a simple matter to remove 0.0001" at a time, and to hit size spot on every time. Lollipop sticks are my preferred method, with a bit of Wet&Dry stuck to them.
For taking things down to nearly size, files work rather well. Make sure you have handles on them, or do as I do, remove the tang all together, I find that I can control them much better by doing that.

Be very wary of getting too close to the chuck.

Using hand filing, you can also easily take down high spots as well, where maybe you have a bit too much sticking out of the chuck and you have introduced a taper into the part. It is all to do with taking your time and having lots of patience, plus measuring often.

I also use this method if I want to make something to a shape that isn't normally a turning job, say fancy handles.

Just think about it, how did they hit exact size a century ago when they didn't have the technology to take such super fine cuts?

Imagine uncle Bogs standing on a soapbox (a very sturdy one), a shaking finger pointing to the skies, and shouting 'Look to the past my friends, the answers are all there'.


Bogs

 

[precisionmetal]

Brian,

The key word is: experience

Unfortunately, there really aren't many shortcuts to that.

It just comes from an awareness of all the variables, and a feel for what the machine and the cutting tool are doing.

Does size wander with temperature? (yes... it does on any machine). Do you know which way it wanders? It only comes from experience.

Does the lathe hit size easier if the weight of the cross-slide handle is at 3 o'clock or 9 o'clock, or maybe 6:30? Experience will tell you. How about how "quickly" you approach the number you want on the dial or the DRO? Approach slowly and the part will usually be just a touch fat... quickly and you might end up removing an extra tenth or two. How tight is the gib on the cross slide? Too tight and you'll usually be chasing size. Too loose and you'll usually be.... chasing size.

How about size control due to temperature? Won't even discuss how the part gets warm when you cut it... that's the easy one. How about the temperature in the shop? Or the change in temperature of the micrometer from handling it for 10 seconds or one minute? (can't cut to a tenth if you can't measure to that or better). Or how the temperature of a long part will vary from one end to the other... and varies differently depending on the type of material you're cutting and/or how long it took to feed from one end to the other?

It's endless.

I've been standing in front of an engine lathe for 41 years (and a mill, and a gear cutting machine, and a hone, etc.), and I can usually hit plus/minus a tenth on a lathe without too much trouble, but I'm still learning every day, and probably will 'til the day I can't stand at that machine any more.

A zillion variables... and the more you experience them, the easier it gets.

PM

 

[Captain Jerry]

I do some of my best thinking while I'm asleep, but sometimes it's just a dream. For example, last week I dreamed that I could use the "REVERSE" switch to put metal back on the part. ??? I guess I got thinking that "taking the cut off" could be done literally.

Jerry

 

[chillybilly]

Hitting sizes bang on LOLOLOLOLOL :big: Thats why they give you tolerences ,engineers best friend .

 

[kf2qd]

It is called polishing. That is what they make emery cloth for. Or 320 grit wet or dry paper. When you are making parts on relatively light machines it is harder to get exactly what you want. Heavier machinery is a bit more labor to run, but the mass of the machine does help with the results. Yes - the machine does affect your ability to get it perfect every time. BUT - tiem on the machine will let you learn what to expect from you equipment and you will get better results. REMEMBER - it is not a race to get things done when you are doing it for a hobby. Slow down and think a bit and the results will improve.

And get your self some emery cloth and learn to use it.

 

[Lakc]

Machine to print, then change the print.
Seriously, emery cloth is one of the first tricks I learned, right after I learned that some depths of cut will cut and some merely work harden the surface. It takes an extremely sharp tool, at the perfect height, on a very rigid lathe, to take tiny cuts. Ganging up and shooting for +.002 with deeper cuts, and finishing with emery cloth is perfectly acceptable.

 

[don-tucker]

Actual size in model engineering I find is not so impotant,but fits are,that's my twopenneth anyway.
Don

 

[Blogwitch]

You are quite right on that score Don, when you are making single items. I do that occasionally, make 'em fit.

When making multiples, it is always better to make parts to plan size and always the same, otherwise you can cause yourself some very awkward problems.


Bogs