Parting off brass

[AjohnwParticipant @ajohnw51620]

There are trolls about that like to start off threads and then sit back and laugh as people reply. Silly people really because others read the posts and some probably gain something from the replies. Usually though they keep the thread going. Looks like I took part in one recently on an astro forum concerning lathes. That's one conclusion that could be drawn from an email I received from the person who started it. Silly again as lot read the posts and as a result are more aware of the things they need to think about before buying one. And more aware of the problems with some specific models. A moderator closed this one down. I wonder why. It's not a terribly good forum and has a few silly people about also a sort of counter culture rival forum.

John

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[David CambridgeParticipant @davidcambridge45658]

For what it’s worth, the image of the convex surface on the brass is exactly representative of one of the problems I’ve been having with parting. To the best of my ability the parting tool is square with the work piece (i.e. parallel with the chuck) and hence I have been meaning to ask for advice on this very forum. For the sake of clarity I’ve reproduced the relevant image below.

David

[AjohnwParticipant @ajohnw51620]

It looks to me like the sort of thing that happens if the tool is rubbing on the sides. Might this be down to the lathe facing too concave ? Say that is 2 – 3 in dia the amount of "concave" should be truly tiny.

I mentioned one way of widening the slot – go in so far, move cutter away from the wanted part and go in more on that then back to the original slot etc. Also waggling.

Another technique is to extend the parting of blade in stages. If that is done any rubbing will be obvious when the tool has been extended and engaged with the slot again so something can be done about it, It is possible to take cuts less than the width of a parting off blade but the blade may bend if too small.

Having seen this I have decided to give disposable tip parting of blades a try. The work is 125mm diameter, tool advanced in stages. Niels will be happy for people to see it.

**LINK**

John

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[KWILParticipant @kwil]

If the mount of concave working is truly tiny you would not get the "step" shown on the photo. Disposable tip or straight HSS blade (use both), if the cutting edge is square and the blade is fed straight in at the correct height they part cleanly, no problem.

Using the cross slide (not the top slide) the carriage would need to be a long way out to cause trouble, even not locking the carriage does not, but if the carriage is really loose, ie a sloppy fit, then pressure on the tool edge could tilt the approach and give a slanting cutoff.

[Neil WyattModerator @neilwyatt]

Tool may be a tad too high as well – the 'banded' appearance suggests a tool that isn't cutting, then suddenly makes progress as the pressure rises and stops again.

Neil

[AjohnwParticipant @ajohnw51620]

Recalling my ML7 loose headstock bearings cause this sort of problem as well – the work rises at an angle under cutting pressures. Add to that wobbly gibs on the sides of the bed and all sorts can happen, Prismatic V beds as are on many of the chinese lathes now shouldn't have the problem if the bed is the right width. Dovetail beds may if they are too loose.

John

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[Thomas Clarke 2Participant @thomasclarke2]

Thanks for all the replies considering there was no text with the images. I did add text but for some reason it wasn't added when I posted on the forum. I think most of you deduced what the problem was and as you can see it was a parting off problem causing an elliptical shape on the face which I did not want. I am thinking of getting a more rigid parting off tool that uses inserts. On one of the replies it suggest that I run the lathe fast but I have also been told I should run it slow. I am using a ML7 and the brass thickness is 28mm. The locomotive splasher is quite thin walled .8mm. The top of the splasher was shaped with a fly cutter to the diameter of the boiler the splasher sits into.

Edited By Thomas Clarke 2 on 11/08/2015 11:01:49

[Ian PParticipant @ianp]

Thomas

A 1.5mm width parting blade should easily cope with 28mm diameter brass. Looking at the pip left by your parting operation the blade used looks to have been much thicker than that so its not width that has caused the problem.

You need to ensure the blade is parallel to the face of the chuck, also the front of the blade must be dead square and have identical side clearance 'on both sides of the blade'. If you are using a blade that has a tapered section the blade holder must be one to suit that blade.

Out of interest, how thick is the 'disk' of brass that is being parted off?

Regarding speed, on brass it is not critical, if the chips are clearing well you could easily run at several hundred RPM or faster.

If all else fails you could always true up the parted face as a second operation by making a snug fitting mandrel and pressing the part on.

Ian P

[Michael GilliganParticipant @michaelgilligan61133]

Thomas,

Further to my first response [on page 1] … This is a pretty good demonstration of how brass should part-off with a sharp tool at the right height. It's obviously not an exact match for what you are doing; but near enough, I think.

You have machined the cut-out very successfully, so it seems unlikely that there is a problem with either the material or your lathe … therefore, on the balance of probability, it is most likely be a problem with your parting-tool.

MichaelG.

[JasonBModerator @jasonb]

Thomas are you getting a matching concave surface on the remaining brass bar which would indicate the tool flexing or is it only on the part you want as the 0.8mm thickness may be too flexible and the metal is being bent away from the tool

[Thomas Clarke 2Participant @thomasclarke2]

Out of interest, how thick is the 'disk' of brass that is being parted off? .8mm

Thomas are you getting a matching concave surface on the remaining brass bar. The remaining brass bar is flat were it has been parted off.

I viewed the video that Michael put a link to and interestingly an insert was used as apposed to a parting off blade. Would this method insure that you would have no flexing with the cutting tool. I suppose I could fabricate the splasher as you would in the larger scales buy working out the curvature for the top of the splasher were it sits into the boiler would be difficult.

Edited By Thomas Clarke 2 on 11/08/2015 14:09:20

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Thomas Clarke 2 on 11/08/2015 14:08:06:

I viewed the video that Michael put a link to and interestingly an insert was used as apposed to a parting off blade. Would this method insure that you would have no flexing with the cutting tool.

.

[in my opinion] The use of an insert is interesting, but not particularly significant … I use the Eclipse HSS blades on brass: The depth gives plenty of vertical stiffness, and the slightly dovetail section gives adequate side clearance.

MichaelG.

[JasonBModerator @jasonb]

Posted by Thomas Clarke 2 on 11/08/2015 14:08:06:

Thomas are you getting a matching concave surface on the remaining brass bar. The remaining brass bar is flat were it has been parted off.

So the problem is NOT really with the parting tool but as I suggested the thin metal is being deformed and moving away from the cut.

Make a simple plug as I suggested earlier to fit in the recess that can have pressure applied with the tailstock ctr while you do most of the cut, remove the tailstock support when there is say 5mm left and just part that off unsupported

Edited By JasonB on 11/08/2015 14:56:10

[JAParticipant @ja]

Just a thought: Glue, using Super Glue, what you have already on the end of a short length of bar turned and faced to suit the inside diameter, mount in lathe and face the parted off face to what you want. The part can then be separated from the bar by boiling the assembly in water. With care, and some thought, you should get a 0.8mm wall thickness.

JA

[AjohnwParticipant @ajohnw51620]

I suspect people are forgetting that lathes face concave by design so there is always a degree of rubbing. It should be a truly tiny amount concave but I have no idea how good some lathes are in this respect.

One of the advantages of the disposable tip blades is that the holder is thinner than the tip and the tip can be correctly shaped. To obtain a suitable holder I bought one complete with blade from RDG. The tip is tapered front to back and down the sides. I do expect to have to buy and industrial blade as the width of the RDG one seems to be uncommonly thin. The blades are 1in deep but the tip fits lower down and the holder has the same profile as the Myford QCTP's. The extra depth is available because the blade overhangs the end of the compound slide, actually by not much more than an ordinary parting off tool does. I did wonder about making one for a QCTP but lacking a universal mill for horizontal work that would be a pain. It would be a lot easier to make one to fit an ordinary n way tool post.

John

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[Russell EberhardtParticipant @russelleberhardt48058]

Posted by Michael Gilligan on 11/08/2015 14:37:54:I use the Eclipse HSS blades on brass: The depth gives plenty of vertical stiffness, and the slightly dovetail section gives adequate side clearance.

 

A problem with these is that they need an accurate holder. I bought a cheap Chinese holder from one of the usual suppliers and it cut very poorly, giving a domed finish and rough surface. On investigation I found that as I tightened the clamp for the blade the blade twisted so that the left hand side rake was negative. A quick clean up of the holder on the mill and all was well.

Russell.

Edited By Russell Eberhardt on 11/08/2015 16:14:32

[Ian PParticipant @ianp]

Posted by Thomas Clarke 2 on 11/08/2015 14:08:06:

Out of interest, how thick is the 'disk' of brass that is being parted off? .8mm

Thomas are you getting a matching concave surface on the remaining brass bar. The remaining brass bar is flat were it has been parted off.

I viewed the video that Michael put a link to and interestingly an insert was used as apposed to a parting off blade. Would this method insure that you would have no flexing with the cutting tool. I suppose I could fabricate the splasher as you would in the larger scales buy working out the curvature for the top of the splasher were it sits into the boiler would be difficult.

Edited By Thomas Clarke 2 on 11/08/2015 14:09:20

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

[Thomas Clarke 2Participant @thomasclarke2]

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.

[JasonBModerator @jasonb]

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

[Michael GilliganParticipant @michaelgilligan61133]

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

[Michael GilliganParticipant @michaelgilligan61133]

Posted by David Cambridge on 07/08/2015 12:32:38:

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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.

[Neil WyattModerator @neilwyatt]

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 Clarke 2Participant @thomasclarke2]

Posted by Michael Gilligan on 12/08/2015 08:01:33:

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.

.

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

Here is a picture of the parting off tool I use.

[Thomas Clarke 2Participant @thomasclarke2]

Posted by Michael Gilligan on 12/08/2015 08:50:01:

Posted by David Cambridge on 07/08/2015 12:32:38:

.

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.

I made more than one cut.

[Ian PParticipant @ianp]

Posted by Thomas Clarke 2 on 13/08/2015 12:40:04:

Here is a picture of the parting off tool I use.

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

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