Lathe tool shank size (for engine lathe): 3/8" or 1/2"?

[spudboy]

I need to buy some lathe tool bits for my eBay chinese "engine" lathe (36" between centres max).

The tool shanks seem to most commonly be sold in 3/8" or 1/2" sizes. My machine will easily take 1/2" shanks( 3/4" is prob the biggest that will fit), so wondering what size is best, and perhaps the reasons why.

I only have a simple tool head and have to shim the cutters to bring them up the the centreline (i.e. I don't have a Quick Adjust tool head thingy).

Thanks
David

[paulthepilot_5]

Hi Spudboy,
The 1/2 would be the better choice as it would be more rigid, but it would not really matter unless you are doing heavy cuts in steel. You would also have less space needing to be shimmed as the tool tip would sit a little higher to the center or the workpiece. Have you tried making any tooling from blank tool steel? you can save a lot of money buy making your own ;)
Cheers Paul

[Bigbjorn]

Measure the distance from the floor of the tool post to the lathe centre line, that is the line between the head stock and tail stock centres. A steel rule is adequate. This determines the maximum tool size you can use.

That size Chinese lathe will have a centre height of 1/2" or 5/8".

Are you using HSS tool bits held direct in the tool post or in a tool holder? Tool holders to suit this size lathe are usually for 1/4" HSS tool bits.

Have you considered getting a set of 1/2" or 5/8" shank tool holders that use tungsten carbide inserts? Should cost in the vicinity of $80-$90 for five tool holders with five triangular inserts, usually TCMT16.

[Bigbjorn]

Attached are photos relevant to my earlier post. The inserted tool holders are 5/8" shank. The HSS toolbits are 1/4".

[Bigbjorn]

Hi Spudboy,
The 1/2 would be the better choice as it would be more rigid, but it would not really matter unless you are doing heavy cuts in steel. You would also have less space needing to be shimmed as the tool tip would sit a little higher to the center or the workpiece. Have you tried making any tooling from blank tool steel? you can save a lot of money buy making your own ;)
Cheers Paul

Those little Chinese lathes are not capable of taking heavy cuts, not enough power. A 1/2" tool bit would be a waste of money as well as the time taken grinding one to shape..

[spudboy]

...Have you tried making any tooling from blank tool steel? you can save a lot of money buy making your own ;)
Cheers Paul

Didn't think of making my own! Guess you'd need special steel.

The ones I initially got to test things out were carbide tips, but on slow/small work I keep chipping them, so from what I've read it seems that HSS is better at slower speeds.

Sounds like 1/2" is the way to go - thanks for that.

[spudboy]

Those little Chinese lathes are not capable of taking heavy cuts, not enough power. A 1/2" tool bit would be a waste of money as well as the time taken grinding one to shape..


Uh-oh.... conflicting points of view :D

The 1/2" stuff is definitely a bit more than the 3/8".

[spudboy]

BTW, this is the lathe: 36"X12" Geared Head Metal Lathe w/ Coolant +Light+Stand (eBay item 350441206295 end time 21-Mar-11 15:36:44 AEDST) : Industrial (http://cgi.ebay.com.au/36-X12-Geared-Head-Metal-Lathe-w-Coolant-Light-Stand-/350441206295?pt=AU_Hardware&hash=item5197ec7217)

https://www.aulro.com/afvb/images/imported/2011/02/19.jpg

[Bigbjorn]

Didn't think of making my own! Guess you'd need special steel.

The ones I initially got to test things out were carbide tips, but on slow/small work I keep chipping them, so from what I've read it seems that HSS is better at slower speeds.

Sounds like 1/2" is the way to go - thanks for that.

Workshop formula for lathe rpm on mild steel workpieces using high speed steel tools is 9000/workpiece diameter in millimetres. So a 25mm workpiece should be turned at 360 rpm or nearest setting available.

When using tungsten carbide tools at least double your speed. Say 900 rpm for 25 mm mild steel. If you are running carbide tools slow you may damage them as you have found out. They need speed to cut efficiently.

Did your carbide tools have a sharp point or a small nose radius? If they have a sharp point then you will chip this off first time you use them. You need to put an appropriate nose radius on them using either a silicon carbide wheel or a diamond file. Roughing tools need a greater nose radius than finishing tools.

Didn't the merchant supply HSS tool holders with that lathe?

[spudboy]

The carbide tips have rounded edges, and yes, I was going too slow on a small piece when I chipped them.

No tool bits came with the lathe unfortunately, so I just grabbed a cheap basic left/right/straight set to get me started.

Now, after a bit of research, I have read the HSS can give better results especially at lower speeds, so I want to buy some proper tools.

Thanks for the tip about doubling the RPM speed with carbides too.