Model Forum / General / Models / July 2004

All.

I don't know if you remember but I originally posted a question about a 20mm
ball nosed slot drill and its suitability for cutting a 10mm semi-circular
radius in some aluminium.  Its been a while! but I did promise an update. As
before, bear in mind I have only been at this for a few months so there is
much "keystone coppery" going on.  A bit of a long post but it does cover
some weeks . Also  figured it may help other beginners, help others remember
when they were clueless, or just give you a laugh ;)

You all persuaded me away from the 20mm slot drill idea. by coincidence I
came across a 19.5 mm drill bit with a 19.5mm shank. Its huge! I thought of
it in the 3 jaw 7 x12 lathe chuck and small milling attachment and instantly
realised why you all were nervous about a 20mm slot drill.  I'm glad I
didn't go that route.It seems silly I even thought of it now. cheers.

I then tried a between centres boring bar.... I duly got some 1/2 mild steel
and some round HSS, ground the HSS as best I could and made up the bar. I
tried for about a week on this but I really lacked sufficient packing to
raise to centre height and it was making some horrible noises.  I did get
some profiles into the aluminium bar but not anything like what I wanted. I
decided, in a kind of running away manner, to try another way. Besides which
I literally got my fingers burnt. because I realised in a trying to get a
job done, letting go of hot metal too late kind of way.. grinding makes
things very hot. OUCH!

a classic example of why reading and thinking should precede doing. I made a
serious note to gain patience and have some thought for my own welfare. I
was being a berk there.enough said.

I was balking at the price of a 4 jaw, im in no way rich, but I spotted a
cheapish 80mm 4 jaw at chronos and after a call discovered it should fit
with a bit of machining. it was around £40. I was dithering still, until my
great uncle came around to see my little lathe.  I was looking 4wd to his
visit ( he was a toolmaker all his life). It took him all of 3 minutes to
convince me of the benefits of a 4 jaw and, oh, coolant.

Feeling even more of a novice now I decided to do as I was told. Although
machining the back recess of a new chuck to be true was really worrying me.
( the mini  7 x12 lathe has a recessed faceplate on the end of the spindle
for mounting chucks as opposed to a screw thread.)

4 jaw arrived. I put off fitting it. for several days. the first
dimensionally critical work I had ever had to do. hmm. Bit the bullet.
Mounted it and measured runout.  whilst doing this I noticed that there was
a significant amount of axial play. About 4 thou either way (more on this
later). radial runout about 1.5 thou. unmounted chuck. started thinking
about best way to machine backplate of chuck when it occurred to me to check
the spindle without the chuck.....

Axial play 4 thou either way. radial under 0.001.  I mounted and unmounted
the chuck several times. measured several times. Finally decided that  the
chuck needs a bit of machining and if the spindle is moving in an out by
that much under finger pressure something is definitely wrong. Its difficult
to trust your own measuring and judgement until you get some experience
under you belt. Deciding I had to trust my machining thoughts sometimes I
went for investigating the axial play....

Headstock off. Now im in over my head. Strip off counter shaft, gears
(plastic !) and spindle. off with the bearings. they are of the deep grove
ball bearing type. front and back and horribly horribly sloppy.  I decide
they need replacing. also decide after looking at prices why not tapered
roller bearings seen as my headstock is in bits anyway? Much sturdier and
fitted on the bigger lathes.

However looking at my non working lathe and headstock in lots of bits i am a
bit disheartened and wondering if i did the right thing and wether i'll ever
get it back together again...... Still no choice now!

Tapered roller bearing arrive (TRB). open packet. rollers all over the
bench. find a wall and bang my head against it. I then spend several hours
trying to get the rollers to stay in the bearing. I keep getting to the last
one or so and then... rollers all over the bench. arrgh.  I am about to give
up. then my dad pops round.

Im 32 my dad is 62. He not a machinist but he has fitted tapered roller
bearings before on a land rover. hearing my frustrations he picks up a tub
of grease in his hand. Not a word he said. Wipes clean one roller, gobs it
with grease, and places it in the bearing and then turns the bearing upside
down. No roller on the bench. He looks at me, grins and wanders off to make
a brew. There's that novice feeling again! enough said.

fully assembled TRB. now to get them on the shaft. gentle tapping with a
wooden mallet wont move them along the shaft. they are very very tight. This
time rather than upping the hammer force as I would have previously done I
remember some burnt fingers and decide to think.

Sand the insides of the bearing or the shaft? no because I could sand it out
of alignment? make a press of some sort? No would take forever? Or would it?
aha. I order two steel shaft collars and some M6 nuts and bolts. I affix
shaft collars, they are the two pieces type. I screw one fast to the spindle
and the other just touching the spindle and the TRB behind that. I use the
M6 bolts screwed through the first collar pushing on the second and the TRB
to edge it along the shaft. 30 mins later a fully assembled spindle. 30 mins
after that a fully assembled and refitted headstock. TRB and headstock all
nicely greased.

Arrgh. All I want to do is cut a profile! ( patience grasshopper  )

Anyway having messed about with the headstock so much and realising that
this is really key to the alignment of the lathe. I know i need to measure.
Measure lots.

I measure the axial play less than 0.01mm (dti is metric) hurray! radial
play (about 0.01mm) hurray! so far so good. I mount the 3 jaw and a 4inch
piece of tool steel. DTI wobbles about (0.01mm) at the end of the tool
steel. that's good. I check again. same thing. hurray! hmm I mount a 1/2
round, 6 inch long piece of ally supported by tailstock live centre. take a
cut a long the whole length and measure with my callipers. I  detect about a
0.03mm ,around a thou taper. the callipers are cheapish and over 6 inches I
am happy with that. feeling very happy but not believing I could have
dismantled a headstock and upgraded to TRB and the whole thing be pretty
well aligned. I check it all again. same result. Hurray!

I think im now going back up the problem tree towards what I actually wanted
to do in the first place .But now for the 4 jaw....

I mount the 4 jaw. dti on outside edge of 4 jaw. I spin it. dti wobble about
0.02mm under a thou. I do it again. same result. I haven't machined the back
of the 4 jaw???? I accept my luck. now for that tool steel again. I spend an
inordinate amount of time  centring the tool steel in the 4 jaw using the
dti. only to realise that im not going to measure run out this way! then
quickly realising an advantage of the 4 jaw. you can correct run out? I put
the dti on the front face of the 4 jaw. spin it. less than 0.01mm towards
and away from the headstock. hurrah. I decide to count my blessings and
accept the lathe appears to be pretty much aligned within a thou all ways
round. I play about with the 4 jaw.

What a cracking bit of kit it is! you can hold and face square stock. ok,
you know that, but. What an addition. After a few goes even centring the
work using a DTI is very quick. why would I use a 3 jaw now? I chuck some
ally off centre on purpose and have a test of  that. looks like some sort of
cam. very pleasing. beginners note. get a 4 jaw independent chuck!

Oh dear. I notice the headstock is warm. tight bearings? is it going to
fail? Eeek. i have a very bad feeling. I read some 7 x 12 groups. its warm
but its NOT hot. I hope that this is just the bearings settling in a bit and
that with more cutting etc is will not get any hotter. A week of nervousness
follows. a week later after much turning its not getting noticably warm any
more. I relax. phew.

back at the top of the problem tree again. ( problem tree - as a senior data
network engineer for over 10yrs I use this phrase and technique for teaching
my staff problem resolution. its just a buzz word for explaining that
problems can explode, branch like, from the original starting point etc)

Now how to cut a semi circular profile of  10mm radius in flat bar stock? I
start thinking.

Well I am ok at drilling and boring so why not drill/bore a 20mm hole and
then cut the bar in half? I do this and discover I cant cut 5cm of flat bar
straight enough with my hacksaw. Ok why not mount 2 pieces of bar together
and drill/bore a 20mm hole so that when I separate the two pieces of bar I
have a semi circle in each? I try it.

SUCCESS!!!!!

two lovely profiles exactly how i want them that fit snugly around my 20mm
OD tube. you know that feeling when you finally get a part the way you want
it. i do now ;)

Thanks for all your recommendations. Mr sparey's book is excellent. cheers.

Some one said about that crispy bacon sound... now i know what that is ;)

yes with the added sturdiness of the TRB in the headstock and a self-made
(grin) front and rear carriage lock  i get that sound even taking .5mm cuts
on stainless on my small 7 x 12 lathe. You see everbody talks about chatter
but i think with the old bearings, lack of carriage lock, and trying to take
too big cuts I was always getting chatter. Now i've heard the crispy bacon
sound you just know don't you! but i hadn't really a clue until then. When i
hear , or more accurately feel the chatter now, it makes me wince a little
and the stop button gets hit and things get looked at.

If  any one out there have a chester or machine mart 7 x 12 and want to talk
about the TRB upgrade feel free to get in touch.

Im still no where near any sort of machinist but i feel I've moved a little
in the right direction. Thanks for the pointers

Best Regards,

SImon