Parting off brass

Now the key question, is the disk of metal shown in your photograph concave on its other face or still flat as you machined it?

If its concave and the disk has ended up approximately 0.8mm thick, then one scenario I can imagine is that the right hand corner of your parting tool is not sharp so is deforming the metal instead of cutting. It could also be caused by the RH side of the blade having negative clearance (so the bottom of the blade is pushing the brass after its been cut.

If you have a parting tool that takes the tapered blades its is essential to check the the blade is held with its centreline vertical so there is equal clearance on both its sides. In my experience the blade holders are often far from perfect and I have fitted shims to the two I use.

Ian P

Ian the metal disk is oddly convexed on the inside of the cup but concaved on the outside as seen in the photo like a convexo-concave lens.You would have thought that the inside of the cup would have been flat as it was after I bored out the center to a 4mm depth to leave a ,8mm thick rim.

Thomas as I have said the cutting action is bending the thin metal away from the cut, this can be a problem with things like cooling fins on model aero engines if they are made too thin or the parting tool not sharp enough. The plug and tailstock support that has been suggested will stop the metal moving away from the cut.

J

Thomas,

[Aside from the trivial point that the terms convex and concave are typically used the other way around]: Your description does seem to indicate that job is being somehow 'pushed aside'.

As Jason and others have mentioned; the bottom of your cup [at a nominal 0.8mm] is rather thin ... but I would still think that all you need is a decent tool, set at the right height, and square to the lathe's axis.

As John pointed out, you may find that your cross-slide is set to face concave, but this should only be very slight [almost immeasurable on a 28mm diameter], and certainly not what we are seeing. .... What does the face of the 'parent' material look like ? [sorry, already answered 'flat']

To help us solve your puzzle; could you please post a picture of the tool that you are using.

Thanks

MichaelG.

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P.S. ... My 'workshop' is currently in utter dis-array, so I can't offer any sort of demonstration; but [from the safety of my 'armchair'] I feel sure that one of our better organised members could easily part-off a 0.8mm thick disc of brass, as proof-of-concept.

Edited By Michael Gilligan on 12/08/2015 08:07:48

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Thomas,

Ref. the image which David re-posted ^^^

I note that the 'pip' appears to be stepped ... Is the length of that pip the width of the parting tool, or did you take more than one cut ?

MichaelG.

My guess is bluntness on the right hand corner or the tool.

A tool used on steel is can be ruined for brass even if it's still cutting steel well. Hone the tip to a fresh edge before using on brass, paying attention to keeping the right hand corner square. And make sure it's dead on centre height.

Neil

Thomas

Sorry to be pedantic but that is a picture of the blade not the tool!

A 3/32" wide blade should have no difficulty parting off brass to 0.8mm. I don't gamble but a pound to a penny says that your blade is not set correctly in its holder.

I assume the blade is wedge shaped, also that the narrow edges top and bottom edges are not square (in section its a modified trapezoid). The centreline of the 3/32" thickness must be held dead vertical so that the side relief is equal on both side.

There are some other subtle factors that normally are not an issue but may come into play when you are parting off something so thin.

Because the top edge of the blade is not horizontal, if you grind it square only one point on the cutting edge can actually be at the right height and not the whole width of the cut. The front RH corner is the highest point and if it is set dead on centreheight it will be the first point to make contact when you start the cut. If you were able to part off all the way to the centre of the material, the pip left on the chucked stock would be conical (by a gnats wotsit). One way of getting the bottom of the groove dead square is to grind the top edge of the blade so it is horizontal. I would not suggest doing that here because it very slightly make the cut narrower so the RH edge will then rub and push the thin metal away creating the very effect you are trying to get rid of.

Sometimes its a good idea to stop, stand back, and have a think about what you are trying to do. Rather than spend time trying to solve a parting problem, approach making the component by a different method. Simply part off roughly leaving the disk much thicker and then turn it round and machine the outer flat face as a second operation using a wax chuck or mandrel.

Ian P

Anyone mentioned if the tool is parting off above centre? That would also break off the final pip.

Could also the the reason for the tool deflecting and cutting an arc and not straight.

Edited By jason udall on 21/08/2015 22:29:09