Re: [PSUBS-MAILIST] Lathe Problems

[" Dan. H." <jumachine@comcast.net>]

James,

It's not real easy to pinpoint the problem without seeing and feeling first 
hand, but I suspect a few things.

First of all, it looks to be an old machine with a poured Babbitt or bronze 
sleeve spindle bearing. Not a taper bearing or ball bearing.  When this 
lathe was built, cuts were done at low speeds with only carbon and high 
speed steel tooling available.  It's not designed for the speed and heavy 
cuts of Carbide tooling.

Being a small, light work vintage lathe, it's spindle is small and not as 
rigid as a lathe designed for today's tooling.  When you take a heavier cut 
the spindle will actual deflect and setup vibrations.   Sometimes just the 
fact that a part is heavier then the lathe is designed for will make it 
chatter even on small cuts because of the small spindle.

It's a small bench lathe and designed for small parts.  It may have a gap 
bed and let you chuck larger diameters but don't be fooled into thinking it 
will do it with any efficiency.  The lathe just doesn't have that much mass 
to machine larger parts well. Bolting it down to something with more mass, 
like the floor will help, but it's still not a substitute for a heavy 
machine.

Another reason could be the tooling your using, the way your sharpening it, 
or the speed your turning at.  I assume you know some about tool geometry, 
i.e. relief angles and tool clearances.  If your way out of the ball park 
with the geometry, it won't cut properly.  If you have too much tip radius a 
tool is more likely to cause chatter.  To little and it will cut minute 
grooves and dull quickly.

Speed may also be your problem.  Carbon steel tooling will work up to around 
50 Surface Feet per minute.  High speed tooling up to about 100 SF/min. 
Simple carbide tooling up to 400SF/min. and more specific carbide tooling 
far more then that.  Figure the circumference your cutting, convert it into 
feet, and figure the RPM you can turn at to not exceed the tool rating.

Also the material your cutting is a big factor.  Standard hot rolled steel 
bar is tough to get a good surface finish on.  Grades of steel made for 
machining will machine easier and give a better finish.  Free machining 
steels have a bit of lead in them that make them even easier to machine but 
they aren't recommended for welded parts.

Sometimes if you can't get a good finish no matter how you try, a way to get 
what you need is to machine your part almost to size, leave about three 
thousants of an inch extra diameter, then use a flat file to finish to size. 
It's not what a professional on a top notch machine would do but it can get 
a job done when you don't have any other way.  Play around with the 
technique and you'll soon learn how much to leave for a file clean up by the 
condition of your machined finish.  Mike you part as you go to monitor your 
filing so you get what you need and your continuous along the cut.  Remember 
you file is a carbon steel cutting tool so don't have the lathe turning fast 
when filing and clean it often.

Parts that you see done on other machines with a smooth and shiny finish are 
done with carbide tooling at high speeds on heavy machines.  Good finishes 
are best gotten at high speeds but if your machine isn't capable you have to 
work around it.

> From your description I suspect your using HS steel cutters sharpened to a 
sharp point and your part is turning to fast.  It sounds like your burning 
off the nose of your tool.  Grind a very small nose radius on the tool and 
it will cut flatter grooves and not dull so fast.  On heavier machines 
coolant is used to keep the tool tip cool but it would be tough on that 
lathe.

If your tool isn't dulling it's getting a "built up edge."  Do you notice a 
sharp piece of steel stuck to the top of the tool when it starts making the 
noise?  If so, it's a built up edge.  Again, poor quality steel will cause 
that to happen more.  The tool geometry can add to it also.  Mostly it's the 
material.  Coolant or lubricant can help. Carbide tooling doesn't do this as 
much as carbon steel and high speed steel tooling.  On your lathe the best 
cure may be plow through it the best you can and then file it.

Try more nose radius, check your speed, use material that is easier to 
machine, file to a smooth finish, and use emery cloth to polish if you need 
to.  One or all of the above should help you.

Thanks for listening, Dan H.


----- Original Message ----- 
From: <james@guernseysubmarine.com>
To: <personal_submersibles@psubs.org>
Sent: Tuesday, March 25, 2008 6:24 AM
Subject: [PSUBS-MAILIST] Lathe Problems


Hello All.

I have yet another problem someone could maybe help me with.
I really need to get my Lathe working properly.  You can see it here.

http://www.guernseysubmarine.com/index_files/Page10613.htm

Its an old one but apparently should be ok.  It has all the features i need 
and to by a new one that does the same things would be about $5000.
So ive really got to get it working.  I can get all the spares for it from 
the original factory.

Here is the problem.

Shoddy finish and dreadfull bad cutting.
Have a look at this picture and you can see what i mean.   My camera didnt 
like zooming in that close so its the best pic i could get.

http://guernseysubmarine.wordpress.com/

If i make a VERY shallow cut and go VERY slowly, it cuts ok ish, but still 
leaves a rough finish.  (like a very fine screw thread with burred edges).

If i make a deeper cut or slightly faster, it cuts a bit and then makes a 
hideous squeaking noise and produces the shown poor cut.

This is what ive done so far.

Eliminated all traces of splidle lift and end float on the main spindle.

Tightened up cross slide and sadle play.

Securly bolted machine to the floor and shimmed up the bed to flat.

Ive tried various different cutting tools of all shapes and sizes including 
brand new ones.

Tried different 4 jaw independant chuck.

Tried different speeds.

Speed is 640 RPM, tool brand new, centred and job material is low carbon 
steel.

Im almost sure the main spindle and chuck assembly is ok.  There is no play 
at all and it seems very good.

I believe the problem must be on the saddle/cross slide/tool post somewhere. 
I cant detect any vibration/movement, but it must be there somewhere.  There 
does seem to be backlash on the cross slide hand wheel, but then there 
always is.

I know these machines have to be reground sometimes and i suspect that may 
be needed.  There must have been a reason the college i got it from were 
getting rid of it....

I've done loads of lathe work before and im pretty sure its not my 
technique.  I've never had this problem before.

Am i just asking to much of a small machine?  Maybe its just "general 
vibration" over the whole machine.  It is 50 years old.

I could try and get a part exchange for a reconditioned one. 
http://www.myford.com/html/186801.pdf

Any ideas anyone before i go and turf it over the cliffs...?

Thanks
James









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