Turning a Small Diameter

[Antman]

Hi Guys,
I know this must be really basic and second nature to most of you, but my slow progress has hit the wall with my present workpiece. I’m trying to make a 5mm diameter cotter pin about 25mm long, using 8mm mild steel stock (The manager at the agri co-op told me it is EN2B), lathe running at 1000rpm, HSS and soluble oil coolant. The smallest I have turned before this is 6mm and that not too successfully. I’m quite wary of running the lathe so fast, but it sounds good and is not vibrating, so I think I’m ok on that score. I read somewhere that small diameters give you a steeper front clearance, but I don’t think I understand the implications. Should I grind a tool with near vertical front clearance? What about top rake?
5mm is gross big compared to some of the parts you guys are making, I’m only starting to work small. What I do instead of roughing it to near finish size is to try with many finish cuts as practice but I don’t get nearly as good a surface as if I were turning about 15mm. What am I doing wrong, or is it my tooling?
Thanks,
Ant

 

[n4zou]

You make one heavy cut leaving your pin the size you need. Take a short cut and measure it. Make your fine adjustment and then make the cut. If required file off the short section you used to make sure your cut was going leave the pin the correct size after the cut.

 

[Antman]

Thanks n4zou for your input, but it's not hitting size that's my problem. I'm trying for a decent finish. I can get a finish that is well acceptable to me on larger stuff, down to 10mm. I have difficulty as soon as I try to turn smaller than 7mm.

Ant

 

[MachineTom]

Can I assume that the work is being used with a live center on the end. As 5X D is too long for a small D material. Your speed is slow, 1500 rpm would be closer, on small D work on center is absolute, what are you using to set cutter height. A shallow DOC, a nose radius, oil and a feed of .005-008 per rev. should give the right results.

 

[smfr]

Also, there's nothing wrong with using a bit of emery backed by something flat like a parallel to get it down to final diameter, with a nice finish.

 

[lensman57]

Also, there's nothing wrong with using a bit of emery backed by something flat like a parallel to get it down to final diameter, with a nice finish.

I agree with you there, I normally use a 600 wet and dry followed by 1500 and its almost a mirror finish. I have not been able to come even close to that kind of finish no matter how I have tried.

regards,

A.G

 

[tel]

Allow me to remind all yawl of my little gizmo. Here it is taking 1/8" manganese bronze down to 1/16"

 

[petertha]

Allow me to remind all yawl of my little gizmo. Here it is taking 1/8" manganese bronze down to 1/16"

Thats neat. Almost like a travelling steady rest with nil side force on the material. So I guess for a specific OD of stock (before reduction) your bushing has a matching ID sliding fit ?

 

[Ken I]

A bit off topic but follows from prior rig.

If you've never seen a sliding head or "Swiss" type automatic lathe - the tools are arrayed about a guide bush and the headstock rotates the stock and the headstock is driven forward on a slide, the rotating stock is thus fed through the bush while the tools work on it.

That's how you turn those rediculously thin / long aspect ratio shafts used in clockwork mechanisms.

More to the point a travelling steady (follow steady) will work for long slender aspect parts.

Or make up a fixture like Tel's (nice one Tel).

Ken

 

[tattoomike68]

Tels box tool is sweet, thats the ticket. :bow:

 

[Antman]

Thanks Tom. Even 1000 rpm is scary for me. Why don’t I see much difference in surface finish at 300 or 600 or 1000 rpm? Is it the shape of my bits that they cut eqaully badly at any rpm? I still don’t get radiused tools. Most of my tools are like 3 planes intersecting. What means “shallow DOC “ ? I took my new live centre for a spin for the first time today. I’ve been doing stuff on the shaper and short things in the 4Jaw.
Nice little boxtool, Tel, I reckon that comes long after a follow rest which is still somewhere on my to-do list.
And like I’ve said before, I’m of the opinion that I won’t be using abrasives on my lathe until I think I know what I’m doing. I’m working on the very basics here, having never even watched this stuff being done before and plodding along by my own trial and error only. Thanks Ken, I think you must be an awesome engineer. I’m very hung up about turret lathes if you like box tools and such, and all sizes of them (mostly over 2 tons) go for like scrap value.
Ant

 

[AussieJimG]

Nice tool Tel. Now the question is whether I stop what I am doing to make one for when I need it,
or hope I remember it when later when I need it. :-\

It's a worry

Jim

 

[miner49r]

Tels tool looks like it could clamped in the DP vice and the work turned in the DP's chuck. I nice option for someone with a tight equipment budget.

Tel,
I gotta know. Is that a wheel keeper you are using for a tool post?
Alan

 

[terryd]

I still don’t get radiused tools. Most of my tools are like 3 planes intersecting. What means “shallow DOC “ ? ............
Ant

Hi Ant, for a finishing tool you should put a radius on the cutting tip. Just a small one with a slipstone or a diamond file/slipstone will do although I use a fine wheel on my grinder. The radius will mean you get overlapping cuts which smooth out the finish whereas a sharp point can leave a rough looking finish as under magnification you get what appears as a series of 'grooves' (actually one long helical groove rather like a thread).

Also make sure that your tool grinding angles are correct especially the top rake and that the tool is sharp, again hone the faces with a slipstone to ensure the best edge possible with no grinding marks.

DOC is an abbreviation for 'Depth Of Cut' so a 'shallow DOC' is another way of saying a 'finishing cut'.


Terry

 

[tel]

Thanks blokes, it's a handy gadget, even if it doesn't get used a lot. And yes - you need a different bush for each size of stock.

Tel,
I gotta know. Is that a wheel keeper you are using for a tool post?
Alan

Nope, that's just the standard ML7 toolpost clamp.

 

[picclock]

Hi Antman
Just my two cents, but the tool height is very critical when you get to small diameters. I was turning some silver steel rod from 6 to 2 mm and the adjustment had to be precise (araond a thou or two). Using a fixed centre may be better as you have no bearing play (just centre drill and load with Molybdenum grease). Other point is what lubrication and tooling are you using ? For the best finish (IMHO) HSS tooling and I use lard as a cutting lubricant. Picked up the habit from an old book and it has got me the best finishes. Use a fine stone (india stone ?) on the tool edge and a small nose radius around 0.4mm. I made an electronic leadscrew control so that I can accurately set the feed rate - your mileage may vary ;D. The other point is that as stuff gets small the rigidity of everything really matters more so nice tight gibs will prevent any other untoward movement. Depending on how fine you want the finish, fine emery then toothpaste or similar will do the job.

Good luck and don't give up. It took the members of this forum a long time to acquire the expertise available, freely given, so make the most of it. No doubt when everyone here started they were in the same boat, so if in doubt persevere.

Best Regards

picclock

 

[mzetati]

Tel,

that little gizmo of Yours is neat! Thank You for the idea.

Marcello

 

[Antman]

Thanks Terry and Picclock. Well, finally I got some acceptable results. The centre height was not spot on and also I was taking too deep cuts. I ended up using a sharp squareish tool with a chipbreaker and I found my best results came at about 400 rpm. Recommendations for a 5mm diameter are about 3 times faster. I still don’t get decent results at the higher recommended cutting speeds and I still get lousy results with radiused tools. Another thing I had to come to terms with on this little job was clearance using a 5” 3Jaw and the bearing housing of my new live centre is all of 45mm, so I had to overhang the tool out of the toolholder by nearly 20mm. I must modify a toolholder or make another to give me better clearance on the tailstock end. Thanks again guys for all your help.
Ant

 

[JohnLanark]

I have a commercially made tool on the same lines as Tel's one. I think it came from Chronos. They are in the catalogue near the bottom of this page but shown as out of stock.
http://www.chronos.ltd.uk/acatalog/Myford_Lathe_Compatible_Acessories.html
John

 

[zeeprogrammer]

Very useful thread. Thanks for asking the question Ant.