Re: [PSUBS-MAILIST] Lathe Problems

[<james@guernseysubmarine.com>]

hi Jim K\Dan.

Many thanks for the advice here.  I'll try to answer both in the same email here.

> It sounds and looks like chatter.

Yes.  I agree.  Thats what it feels like.

> the chuck and the tool extends just a small amount from the tool post.

The test job has been sticking out about 2 inches.  I havent actually tried it really close to the chuck.  I'll try that later.

Ive tried various different tools.  The best results were with brand new changeable carbide tips.  You know those triangular ones that you can spin round.  If i was REALLY careful, the result was just about bearable.  But only just.

> possible that some gibs need to be adjusted up also.

Not quite sure what you mean here.

> 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.

Yes, its a sleeve bearing with oil drip feeds.  All looks in good condition.  Ive checked lift and float with a needle gauge and there is none (now).

That bit actually seems in really good condition.

> 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.

I hadnt thought of that.  Ive been using it on full speed.  640 rpm.   Will try slower speed later.

> 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.

sob, i can see a new expensive machine coming up.

I need it to make the through hulls.  also the through hull bars and various bits that fit to the valves through the hull.  You know all critical stuff that has to be perfect!

These are going to be made from 316L stainless or A4.  Doesnt look like it will cope with that.

Also i thought it might be ok for the viewport retaining rings.  Again, i am going to use 316L stainless for these.

> Another reason could be the tooling your using, the way your sharpening it,

I think this was ok.  My sharpening has always worked on any other bigger machine ive used.  Plus i tried loads of different tips to varying degrees of success.

> 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.

Ok.  The test bar i turned was 20mm OD.
So, circumferance is 62.8mm
62.8mm = 0.2 feet (approx)
0.2 x 640 (My speed) = 128 Ft/min.

is that right?  i think so.

hmm, this might be part of the problem.  I'll try slowing it down.

> Also the material your cutting is a big factor.  Standard hot rolled steel
> bar is tough to get a good surface finish on.

Ive got to machine stainless.  Thats the jobs i need to do.

> 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.

I dont think that method is going to be acceptable for the through hulls in stainless.

> om your description I suspect your using HS steel cutters sharpened to a
> arp point and your part is turning to fast.

I used both carbide and hss tools.  Pretty sharp point.  But i did grind a radius on one and it didnt really make muh difference.

>  your tool isn't dulling it's getting a "built up edge."

Not sure about this.  I will have a look later.

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

Ok. I will try all the above.

I have begged one of the engineers from the college who i know here to come round later and have a look for me.  We'll see what he says as well.

Thanks again guys for the excellent advice, its really appreciated.   I'll let you know how i get on.

Cheers
James



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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