Lathe size

[Sandiapaul]

I too missed you had a CNC mill, and now with your explanation the CNC lathe sounds appropriate! Keep us updated with progress please!

[Gra2472]

Evening guys

I have a G0704 that I converted a few years ago. It definitely has it’s limitations, but since I am learning as I go, it does what i need it to do. So far it has done everything that I have asked it to do, but I have to go slow. Small cuts and slow speeds work best. So far it has been quite accurate.

Back to my original question. If I bought an 11” lathe, what diameter wheel (for arguments sake) could I realistically turn?

[Harold_V]

Always use "climb" milling wherever possible, the finish is better and the cutters stay sharp longer.

Ret has provided some outstanding guidance here. That said, I'd like to make one small comment to avoid potential disaster in your future.

Climb milling is correct when operating a CNC, assuming it has no backlash, but that is usually not the case with manual machines. To be clear, the admonition to use climb milling is restricted to CNC operations, rarely to manual operations, where the backlash of the nut/screw interface can lead to self feeding, broken cutters and destroyed parts. It IS common practice (on manual machines) to take finish climb milling cuts, where only a few thou of material is removed.

H

[Harold_V]

Back to my original question. If I bought an 11” lathe, what diameter wheel (for arguments sake) could I realistically turn?

That would vary from one builder to another. I own a Sag12 Graziano, which has a natural gap by its design. While it is a 12" machine, it is limited to about 6" over the cross slide, but I can turn 17½" over the carriage wings.

I suggest you turn to the maker of the lathe you choose to purchase to determine the largest diameter that can be turned over the saddle. I would think that it would be no smaller than the advertised size of the lathe swing, and possibly slightly larger.

H

[Gra2472]

Thanks for the great advice guys. That sure is helpful for a greenhorn like me.

[RET]

Hi,

As Harold pointed out (another one of the many on this website worth listening to), I forgot to mention about the climb milling advice only applying to CNC, not conventional milling because of the backlash in standard lead screws.

I looked up your mill and I found out that its a Grizzly with about 17" X travel, about 6" Y travel and at least 9" vertical travel. Since it is a machine with conventional dovetail ways, quite a bit of the stepping motor power has to be applied to move the machine parts, so larger steppers are necessary to do this. You also have to worry about alignment between the ball screws and the dovetail ways as well as setting up the dovetails so there is no play in each dovetail.

Since normal wear on a machine tool slide tends to make the slide hourglassed, you also have to worry about this. All of this is why when I built my little mill, I went with 1" dia. Thomson Ball shafts mounted on support rails for support and 5/8 machine tool quality double nut preloaded Warner ball screws to drive the axes instead of a conventional dovetail setup. With this design, everything is preloaded so there is absolutely no play of any kind. There is a thread on chaski that shows how I made the mill and you might have a look at it. I used the other machine tools I have including a Bridgeport for this project.

Above are a couple of pictures showing a few of the parts I have since made with the little CNC mill. The little shield is a bit less than 1" tall by just a bit more than 1" wide and goes on the front of the 3 1/2" gauge Big Boy which is just one of the many projects I'm working on. The other picture is of the partly built power reverse for the same locomotive. The power reverse parts are not castings, I wanted to see if I could machine them from solid so the end result looked like a casting.

There are a number of things you need to do. First, put way covers of some sort on both the X and Y axes especially to keep the dirt and chips out. Next, you need to run some accuracy tests, like putting a dial indicator near one extreme of the X carriage travel but touching the vise and write a small program that runs the X axis from one end to the other (near, but not all the way) repeatedly at varying speeds and distances to see if it always comes back to the same reading on the indicator time after time after time. You also need to measure the X axis stop readings to see if 10" of travel in the program really is 10" on the machine.

That will give you an idea of what you need to do to check out your machine and I'm sure you can think up more tests yourself. You need to do this for each axis of the machine. On my machine, the stepper motors are double ended and I have graduated hand cranks on each axis. I know the machine is accurate, because the numbers on the cranks always come back to the same position, time after time for each pass the program runs.

Perhaps this will be enough for the moment. I'm not trying to discourage you, this is just part of what I've learned over a lifetime.

Richard Trounce.

[rmac]

I think this is the link to the link to the thread Richard mentioned describing his homebrew CNC mill project:

http://www.chaski.org/homemachinist/vie ... 8&t=109499

[RET]

Hi rmac,

Yes thanks, that's the right thread. There are two more threads that show some of the things I've made with the machine. One is the one for Big Boy and the other is the one for the 3 1/2" gauge couplers.

Thanks for looking it up.

Richard Trounce.

[LIALLEGHENY]

Gra2472,
Looking at your part holding for the journal boxes, I would advise that you limit to holding 2 parts at a time. The clamping forces from the vise will be on the 2 parts in the center, the outer 2 will have far less clamping force being applied on them, this is especially true if the castings aren't all the same size. You would be amazed out how much the aluminum jaws can deflect with pressure on them. If you are dead set on holding all 4, put a c-clamp at ends of the jaws, it will help stiffen things up and give you better clamping.

As for your lathe, I concur with others who have said, look for a used American made machine, in the range of 14-15" swing. Your not going to like turning a 11" wheel on a 11 or 12" swing machine. If your intention is to eventually convert it to CNC, wait and buy a machine that is already CNC, ideally something that can be used manually and CNC.

Nyle

[Berkman]

any suggestions on reputable used machinery dealers ?

[Gra2472]

HI Nyle. Oh that was just a test fit. The holes on the end are fitted for 1/4-20 bolts just to solve the exact problem that I expected, and that you pointed out. It actually holds 4 boxes no problem. But thanks for the advise, I appreciate it.

[Gra2472]

Hi Richard

Wow those are nice number plates. I built the mill a few years ago so I don’t remember the exact components i used, but they are zero back lash ball screws and nuts, Nema 23 and 34 motors, etc. I did my research before buying anything, It actually is very accurate. I’ve done several tests and it returns to the original (test) position to within .001 every time. I think that is as accurate as I need it to be. I’m not building anything for NASA. I have the ways and slides “tight” if you will. Everything moves with minimal effort, and there isn’t any slop in ways. I am a novice, but but I did do my homework. I actually have some machine shop experience, but no formal training.

HA! I should have known that someone would have noticed the missing covers over the column and bed ways! Your guys never miss a thing. I had torn those off not five minutes before the photo. The original covers were crap and were always getting in the way of the machine. They also didn’t do a very good job of keeping the ways clean. I still had to get under them to clean out the grit and chips. It was more annoying than anything, but I have some leather that I plan on using to make way covers out of. I oil and clean my machine every time I use it.