JET ghb-1430A lathe... having trouble getting precise cuts. any advice?

[jonogt]

As some of you have seen in my other posts, my access to equipment is my school's student shop, and this http://tinyurl.com/d3dty7 is the lathe we have in there.  They just got it a couple months ago, so I guess you could still call it "new".  I'm working on my first steam engine right now (also in my other posts) and the results I got from turning my piston on this machine are fairly mediocre. 

The rod I was turning it from is stainless steel, and the finish after using a carbide tip was rough to say the least.  I got the part to what I'd call an "acceptable" fit in the cylinder, but only after extensive use of a file and sandpaper against the turning piece... not nearly as good a result as I'd expect from the time spent.

http://img.photobucket.com/albums/v171/Jonogt/p1010006.jpg
http://img.photobucket.com/albums/v171/Jonogt/P1010010.jpg

I also had quite a bit of trouble getting it center drilled (for the connecting rod) nicely... I probably cut off the end and tried it over atleast 10 times before getting it OK.

In my manufacturing lab we just started the machining unit and we are using Monarch engine lathes from the 50's and 60's, and the results are just uncomparable in every aspect.  I caught a few things from the instructor in there that I can improve on when using the JET, but not a lot.


Do you guys have any suggestions on how I can get better results?  I know that the JET isn't an actual "engine lathe" and the guy in charge of the student shop even regarded it as "pretty cheap", but I can't help thinknig it can do a little better.  Should I go and buy some better tooling to bring there and use?  Or should I just accept that it's not gonna be real great and that tedious finish work will be required when doing precise things like this?


Thanks,
-Jon

[bogstandard]

Jon,

The lathe you are using is basically nearly the same one that I use, but yours is much better made and a lot more expensive than mine.

I think your problem is that they haven't spent the time to set the machine up correctly on the gibs and also your centre height of the tooling might be out. Your centre drilling problem might be you have the ram out too far. Get the saddle closer to the chuck (make sure nothing hits the chuck when turning) and get the tailstock as close to the chuck as you can.
Remember also, high speed and slow feeds usually end up with a better finish.

I can take a 1/8" deep cut on stainless and end up with a finish like a mirror. For 1/2" diameter and below, I usually run at 1200 RPM, with the power feed going as slow as possible. But a lot of things like finish and holding size is more to do with experience with the machine, getting used to how it works.

John

[sbwhart]

John

John is spot on with his advice the only thing I can add is regarding the TC tip, TC tips are highly developed as to material and geometry some are specificaly design for just one application and material, if your using the wrong tip you can get bad results, its suprising how easily they can lose their edge if they are used wrong, either change the tip or grind your own HSS tool up. Make sure you've been using a general purpose tip that will meet the needs of most hobby machinist.


Love the JET machine you've got access to, I've got my eye on a Smaller Jet Lathe, I've only got to work out a way of convincing my boss that I realy nead one. 
   
Cheers

Stew

[malcolmt]

Hi Jon

Just one thought occurs to me here other than what John has said, that is do you know what grade of stainless you are using ??
Through reading on here i am aware that 303 is a nice steel to work with whereas 304 is an absolute B*TCH.
I may be completely wrong but thats my two pence worth.

Kind regards

Malcolm

[Bernd]

Malcolm,

You are right. I remember Marv on HMEM had a saying "303 she's for me, 304 she's ****". One way to remmebr it.

Bernd

[jonogt]

hey thanks a lot guys,

Your centre drilling problem might be you have the ram out too far. Get the saddle closer to the chuck

i'm not all there on terminology yet... is the ram the threaded piece with the morse taper that telescopes out of the tailstock that you stick centers, drill chucks, etc. in?  I make sure to keep that under an inch past the point where it breaks the taper's hold on the drill chuck, but I didn't do the same with the work piece.  It was about 10 inches of rod, and I think I had atleast 7 inches of that sticking out in front of the chuck  that would cause my problem, wouldn't it?  Cause more length = more gyration of the true center point on the end you're trying to drill?

TC tips are highly developed as to material and geometry some are specificaly design for just one application and material, if your using the wrong tip you can get bad results, its suprising how easily they can lose their edge if they are used wrong, either change the tip or grind your own HSS tool up. Make sure you've been using a general purpose tip that will meet the needs of most hobby machinist.

Most guys in there seem to just use whatever tip is on there when they need to turn something, so it makes more sense now why most of them are in crap condition.  How much should one expect to pay for "pretty nice" TC inserts?  Does price vary depending on the type of metal they are meant to turn?  If it isn't excessive, I might get a few to call my own that I can take with me only when I'm working.


One other question I had (right now) about turning:

I have a piece of scrap aluminum that I want to use to make the flywheel for my engine... it's square stock, about 3"x3" by 5" long.  Would it be acceptable to mill off the corners (making it into a hexagonal prism) and then use the lathe from there?  Is this dangerous/hard on the tooling?  If so, what is the correct procedure for preparing square stock for lathe working?


thanks again,
-Jon

[Darren]

HI Jon,

You got the Ram correctly identified 

But 10" out from the stock  That'll be where your centering problem is, not many lathes could cope with holding it central at this length without some sort of support.

What tips you want depends on a few things, but identifying the tool holder would be a good start. We can go from there.

Your square stock, do you need it 5" long, can you cut it shorter first. Rounding square stock esp ally in the lathe can be done. But very carefully and slowly. Feed the cutter in just a tiny amount at a time with each pass. It can take a while to do. What you really don't want is heavy interrupted cuts. That would most likely either jam the lathe, damage the work or break the tool tip off. But it can be done.
There are a few examples in the projects section of rounding square stock.

[sbwhart]

Jon

Your getting the right idea, when you try and get everything nice and short, ie work as close to the chuck as you can keep the tail stock ram as short as you can etc, same applies to your turning tool don't have too much sticking out of the tool post.

TC tip can work out expensive because you need to buy a holder as well I'm assuming your in the US in the uk a tip can work out as £3- £5 each an holder £10 + (for 1£ * 1.5 to get to $ Roughly). For home machining you can't beat HSS for economy lots of the chaps use them.
  

Cheers

Stew

[bogstandard]

Jon,

It is fairly easy to round off square stock, in fact I do all sorts of shapes to round very regularly. The secret is to only take very small cuts to begin with, and the rounder it gets you can start to take slightly larger cuts. The job isn't going to get up and run away, so take as much time as you can to get the job done, rushing it will only lead to you having to make a new one.
Tipped tooling doesn't like what is called interupted cuts very much, and does have a tendency to chip if you push them too much.

I will be doing a flywheel project in the next couple of days, and they will be made from thick sheet aluminium, so that might give you an idea how it is done.


John

[rleete]

jonogt,

First of all, welcome.  This is a good place for us newbies to learn all sorts of tips and tricks.

As to the flywheel, you have it right.  Bogstandard did a nice write up on it a while back, over on that other site.  Here's the link: http://www.homemodelenginemachinist.com/index.php?topic=228.0