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

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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

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