Making parting off tools

Duncan,

Read my posts - I find RDG ' s flat top and bottom blades perfectly satisfactory.

John

5/16 - is that depth, width, or typo?

ARC does similar to eclipse type, but the smallest width is 10mm rather than 5/16. Here is the link.

Ketan at ARC

won't fit my existing holder unfortunately. I've doctored an RDG blade using a diamond file, but I'd prefer an original

The very slightly concave top on the Eccentric Engineering T-section blades gives a beautiful long curl of narrow swarf that falls out of the groove without jamming. Definitely superior to any other parting blade I have ever used, although they were all flat on top, not angled like Gray describes in his "dovetail" post above.

Unfortunately all eccentric blades are out of stock

Tony

Edited By Tony Pratt 1 on 09/07/2022 07:14:03

Very eccentric! Supply chain issues to Australia still have not recovered from Covid. I believe the blades are made in the USA and there have been massive problems with shipping from there. I had several bike parts returned to US sellers as undeliverable. But as posted above, there is a UK supplier of similar T section blades with the concave top.

This is what steel swarf looks like when using an insert parting tool:

The swarf is slightly narrower than the insert and considerably thicker than the selected feedrate.

See this discussion: Parting Off

Andrew

Edit: Messed up the link

Edited By Andrew Johnston on 09/07/2022 11:59:16

Nice pic there Andrew. You can really see the curve in the cross-section there. No wonder it does not jam in the groove!

The T section blades perform the same function as taper sided blades, providing side clearance.

The flat top provides no Top Rake, although a holder with the groove machined at an angle (a la GHT ) will set the blade with top rake.

As Graham warns, grinding top rake onto a tapered blade reduces its width, and the depth to which it can enter a workpiece. So that doesn't sound to be a good idea..

A parting tool,or insert with a groove in the top will produce U section swarf. which should be narrower than the groove and so be less likely to block with swarf.

Mounting a parting blade inverted in a rear toolpost uses gravity to encourage swarf to fall out of the groove, again, reducing the risk of blockage and jamming.

The advice to maintain a steady feed is good, it is never good to let a tool rub and generate heat. I have now become so overconfident that I often use power cross feed with the rear toolpost mounted parting tool.Seems to produce a better and flatter surface than manual feeding.

Howard

The Multifix tooling on the Hardinge I worked during the 70's came with a parting blade which had more of a "V" shape ground in the top face for the length of the blade, so these types of tools have been around for sometime. I have been trying to remember the make of the blade but with no success. I do remember it was only about 11 mm high.

GHT did try a parting tool ground like this as well as one ground with a shallow inverted "V" on the Front Rake. The latter is something I have tried, but found a straight face to be just as good when using a dovetail shaped blade. The end goal is trying to get the width of the swarf narrower than the cut width. The inverted "V" might seem the wrong way to go, as the cutting edge is longer than the width of the tool, but the secret is in the fact that the bottom of the "V" hits the work first before the outer edges. This forces the centre of the swarf to rise up from the tool and thereby reduce its width.

The problem with power feeding is the change in the peripheral speed as the tool nears the centre, with a constant feed rate. On the Hardinge this is easy to overcome as it has a variable feed. On larger lathes like the DSG, VDF and Colchester this does no seem to matter so much, as on these machines you do have the facility to change feeds on the move as it were.

Regards

Gray,