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Fine cuts on Brass

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Old Fart UK22/09/2011 17:21:33
9 forum posts
Finding myself unemployed for the first time in 42 years, ive bought a 7 x 12 minilathe to amuse myself with (I mean, whos going to give a 57 year old IT engineer whos had bowel cancer and a heart attack a job, dream on).
Ive started making reproduction meccano brass parts, plus a few parts meccano never made but should have done.
The problem I have is how to achieve a really fine polished cut. Im using HSS 8mm tool steel, ground on a fine grey grindstone, then polished nominally with a whetstone to get a fine edge. I turn these small parts with a bit only 1 mm wide (because then I can use it as a parting tool as well), and i turn with the lathe flat out - 2500 RPM
The finish cut isnt bad, but its not as good as just polishing the unworked drawn bar. How can I get a mirror finish?

Edited By Old Fart UK on 22/09/2011 17:22:49

JasonB22/09/2011 17:31:39
23022 forum posts
2763 photos
1 articles
What top rake and back clearance are you putting onto the tool.
Brass works well with zero or slightly negative top rake and the minimum of back clearance.
There is also the likelyhood that your thin tool if flexing or chattering which won't help with the finish, better to have a good solid tool and try to rig up a rear mounted parting tool.
Chris Trice22/09/2011 22:54:32
1375 forum posts
10 photos
You might want to try different grades of brass. Some of the softer yellow ones often give a better (more shiny) finish straight from the tool. You tend to get a longer swarf chip than the free machining brasses. The latter tend to cover you with very fine splinters of brass. HSS tooling honed with one of those very fine diamond thingies is more than sharp enough.
Old Fart UK23/09/2011 22:23:06
9 forum posts
In using 5 degrees negative back rake on the top of the tip and 5 degrees end relief on the tool nose, and the tip is set exactly dead centre.
ive been honing on a whetstone, ill try a diamond lap
I dont get long swarf spirals, just a fine cloud of tiny chippings

Edited By Old Fart UK on 23/09/2011 22:25:39

Springbok25/09/2011 04:28:30
879 forum posts
34 photos
Firstly are you useing a chinese mini lathe. If so check the jib strips that there is no play with the revs you mention it certainly sounds like one. The next Q is what type of cutting tool are you useing.
Sorry to hear about your problems and hope all will be well soon.
Old Fart UK25/09/2011 20:57:06
9 forum posts
I dont know if its a chinese one - how do you tell?
HSS 8mm tool steel, ground on a fine grey grindstone, then polished nominally with a whetstone to get a fine edge. I turn these small parts with a bit only 1 mm wide (because then I can use it as a parting tool as well), and i turn with the lathe flat out - 2500 RPM

I cant detect any play in the cross slide, the carriage is fairly stiff and tight.
This one is pretty close, the tailstock on mine is slightly different
The speed control box on mine looks like the above, but the tailstock looks like this on below


Edited By Old Fart UK on 25/09/2011 21:04:14

Stub Mandrel25/09/2011 21:47:15
4315 forum posts
291 photos
1 articles
If you're parting and turning with the same tool, you won't get the best results.
Old Fart UK26/09/2011 09:34:26
9 forum posts
im turning reproduction meccano parts, so even if i were to make separate tools, there wouldnt be much difference, you still only need a very small tip.
Ive honed it with a whetstone, but ill try a diamond lap next.
blowlamp26/09/2011 10:06:53
1632 forum posts
105 photos
Neil is quite right.
A 1mm wide parting-off tool isn't going to be rigid enough or be of the right geometry when used for conventional turning.
I don't know why you would need a mirror finish on Meccano parts, but you won't get it straight from the tool anyway, no matter what lathe you have.
To get that kind of finish, I usually use a very fine file (No 6 cut) to remove the ridged surface left by turning, followed by the careful use of a Water of Ayr stone and Brasso.
PS. Have you got another name - I can't bring myself to refer to you as 'Old Fart UK' all the time.

Edited By blowlamp on 26/09/2011 10:14:19

Old Fart UK26/09/2011 20:38:15
9 forum posts
AHHHHHH so thats what this is.
i got a box of interesting junk at an auction, all sorts of stuff - blank tool bits, chucks, arbours, dead centres, all sorts, and this:

This is about 3/4" x 3/4" x 6" long. Its very very fine whetstone - it must be a Water of Ayre stone.
Presumably you lube it with water? Wont using it on brass knacker it ?

Edited By Old Fart UK on 26/09/2011 20:41:27

Ian S C27/09/2011 11:39:01
7468 forum posts
230 photos
A bit of 1/4" 6 mm HSS would be adequate for what you need, if all you turn is brass, and the funds are a bit tight, an old file can be converted into a good tool that will , if looked after retain a good shape edge.
I'v made a bit of Meccano, including 1" pullies, made a little die to make thetwo discsthat when assembled back to back form the pully. Ian S C
Old Fart UK27/09/2011 17:56:18
9 forum posts
Ah you mean this part?

Its usually made of three pieces, pressed together, how did you press the boss on tight ?

I wouldnt have thought these were worth making, theres millions in circulation still.

Edited By Old Fart UK on 27/09/2011 17:56:54

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