Parting off using a replaceable blade parting tool.

I have a parting off tool with a replaceable blade (similar idea to a snap-off knife!).

It seems to cut off the part a bit con-cave rather than straight,

is there a way around this? It's quite a big one, 30mm high x 15mm wide at a guess.

I normally use the carbide parting off tools but they've all broken because I get lazy and use the power cross-feed to part off!

Any ideas?

Edited By Chris Pocock on 13/06/2013 20:14:59

If the end is ground off at an angle as quite a few people do to make sure the "pip" is left on the stock rather than the work this can tend to push the blade off line. Also make sure it is perfectly at right angles to the lathe axis

Thanks.

I'm pretty sure it's at 90 degrees.

I have ground the tip to an angle though, so that there's no burr left on the part.

I will have to experiment with a lesser angle and see what happens. 

Edited By Chris Pocock on 13/06/2013 20:54:48

Hi Chris

I use a blade type parting tool, It is important to set it at exactly 90 degrees to the spindle axis.
to set mine I wind the cross slide back and move the parting blade in its holder (just gently held in the tool post) to the edge and face of the chuck. then after making sure it is exactly parallel to the face of the chuck I tighten it up.

This is one tool that it pays to set right on centre, and certainly not under.

I grind the blade straight across on the end only just a couple of degrees of front relief. and never power feed with steel, you have to feel the tool cut. and be ready to whip it out as soon as it starts to growl. Blade type tools do not have side relief at the top, You can grind it in but then you waste a heap of cobalt tool steel when the entire end has to be reground costing you 10 to 20mm of tool steel.

If the steel you are parting is not free cutting steel and you want to part deeply in stock over 25mm diameter It will help to part in stages widening the cut a little to the left and right as you go, this means that you have to face the end after parting but it avoids that loud bang! as the end of your blade leaves the building hopefully missing you (Wear eye protection).

Often when you part you will not get a good finish. Particularly on the unknown grades of steel from the scrap bin, leave an allowance for facing off later.

You may find "peck parting" that is cut 5mm, withdraw, more oil then another 5mm makes the job easier. It assists clearing the swarf.

Lots of cutting oil and listen to the cut.

Cheers
John

Carriage lock is a must too.

Thanks for that, I had better success today!

John

I'm not sure I understand the bit about not having side relief. The parting blades I have 3/8" Eclipse and a 5/8" high one, and both are wedge section so whilst parallel in plan view do effectively have relief. I dont have any problems with jamming in the cut groove.

Whilst I use both sizes frequently, both are flawed designs. The 5/8" one has the top face at about 10 degrees to the edge presented to the job is only at the right height 'at one point'. If I grind the top face so it level/horizontal then the cut width is slighly narrower than the width of the blade so it does increase the likelyhood of the blade being trapped by the slot side.

In practice it works ground flat or not, so I just carry on regardless.

One technique I do use is to mimic the type of cut that the carbide parting inserts achieve so that the cut ribbon of swarf does not roll itself into tight clocksprings. I use a diamond burr in a Dremel type tool and make a small 'Vee' shape in the top face of the cutting edge. Even if its not scientifically shaped to create narrow V shaped swarf it still does the job.

Ian P

Hi Ian

I also have a 5/8" inch parting tool Armstrong type, yes there is the small bevel on the top edge of the blade, I do sometimes put a small flat on top, quite a small distance back onto the top of the blade say 3mm. to straighten the line across the cutting edge. Doing this creates what is effectively a negative top rake on the cutting edge, and yes it very slightly narrows the tool.

You are right a small v in the centre of the cutting line does break up the spiral chip a bit.

As do you most of the time I don't bother doing the above. If I was in production it would be a different matter, presuming I planned to use HSS, not likely carbide is better in production. but HSS works fine on a one off basis. If you take your time and listen, if it starts complaining do something and you better be quick!

The lack of clarence i was referring to is the two parallel sides along the tool at the top edge, I leave them alone to save wasting tool steel. however that means there is no Clarence along the sides of the slot when the tool is cutting. some sticky grainy steels do not like that at all tending to work harden and gall, sometimes breaking the tool with a jam. Hence the need to widen the slot at the beginning by moving the tool laterally side to side as mentioned previously.

A final point for newcomers to lathe work is the parting blade should not project more than is needed to make the cut plus maybe a few mm to allow for over travel, the setup must be rigid.

Regards
John

Edited By John McNamara on 19/06/2013 01:41:44

1.5mm wide, surely?

Neil

What is Neil?

Ian P

> It's quite a big one, 30mm high x 15mm wide at a guess.

That would need several horsepower to drive it!

I use a 3/8" x 1/16" (10mm x 1.5mm) toolbit which is rectangular in cross section and works fine despite the lack of relief at the sides.

Neil

Sorry, that is the size of the holder! The blade is about 5mm accross.

Althogh the lathe has a 3hp main motor and a 0.75hp(?) motor for the hydraulics!

Yes, a "v" ground on the to of the tool helps but is difficult to acheive accurately without the aid of a Quorn or similar. I grind a convex "v" on the front of the tool, it only needs to be about 10 or 20 deg and makes the swarf turn away from the sides of the cut, reducing the likelihood of the dreaded jam.

Russell.

Russell

Its not essential to create a perfect V shape. I do it by hand and sometimes just put a crude central 'nick' in the blade.

It seems to work OK as all it has to do is break up or deform the swarf.

Ian P