|Martin of Wick||05/02/2021 20:13:36|
|249 forum posts|
A torrid afternoon trying to part some rounds of PB - at least that is what it was sold as.
No problems encountered turning and facing dry with tangential tool HSS so wasn't anticipating problems parting.... Oh dear, how wrong....
Using a GHT backpost and eclipse style taper blades (set at correct height) at speeds between 150 to 400 the blades made no impression on the bar and just bounced off, refusing to bite. When I eventually managed to force the tool in it generated so much noise with the blade twisting and bucking that I chickened out lest I break a blade.
Managed to half do one using a tiny MGEHR insert holder not big enough to cut through completely and had to finish with a hacksaw. It was a scary operation all round, needing about 400 rpm for a steady cut and the job became so hot that the cutting oil was burnt on to it!
Can anyone please suggest an approach that works for them when parting PB? what speed, oil/no oil, or different grinds for the blades etc. (currently ground flat, sloped down at 8 degrees giving circa 10 t0 12 degrees front rake).
Edited By Martin of Wick on 05/02/2021 20:27:55
|old mart||05/02/2021 21:32:19|
|3717 forum posts|
Depending on the size of your lathe, you could look at larger holders for the MGEHR type inserts. I have some of this type in 2 and 3mm for aluminium and would use them dry on bronze. ARC have small parting blade systems which are good for small machines. Try 200rpm, it might be better.
Edited By old mart on 05/02/2021 21:33:30
|Simon Williams 3||05/02/2021 22:06:55|
|652 forum posts|
Please forgive me if this sounds silly, but those circumstances say "tool above centre height" to me.
Probably need to use zero or very small top rake as well. If the tool grabs go slightly negative as for cutting brass. If it truly is phosphor bronze, and the set-up is stiff enough, you should be able to peel out a nice even spiral. I agree with the suggestion to go fairly slowly.
If things are getting hot that means there is rubbing not cutting going on. The majority of the heat should be carried away in the waste chip material.
What machine are you doing this on?
|Robert Butler||05/02/2021 22:30:42|
|382 forum posts|
I assume back post = rear tool post - silly question was the parting tool inverted? if not this would explain the problems experienced.
Edited By Robert Butler on 05/02/2021 22:31:32
|Andrew Johnston||05/02/2021 22:48:20|
6574 forum posts
It's been a while since I parted off bronze, but I don't remember it being a problem. I'm in the "part off under power feed" camp using a mid-price (Interstate?) parting blade and inserts from Cutwel. The main key to parting off with inserts is feedrate. I never use less than 4 thou per rev. Below that you're asking for chatter. I set my parting tool slightly above centre height, as per the professionals.
5065 forum posts
bandsaw? (if you have one)
|Martin of Wick||06/02/2021 09:26:36|
|249 forum posts|
Thanks to all,
should have said, the lathe is a Myford S7.
It just seems strange the HSS parting blade set up that was performing flawlessly with steel and silver steel moments before, completely balked at the bronze.
I usually set the blades about 5 thou above centre line using the rear tool post (and deal with the annoying little pip later) Perhaps I need to set the HSS a little higher as I couldn't seem to start the cut with that configuration. The blades project quite a way from the mount on the GHT toolpost, so perhaps that is contributing to the problem.
Using the MGEHR carbide tool I was eventually able to establish a consistent ribbon of cut material about as thick as aluminium foil, but is was clearly hard work for the lathe. On inspection the actual cut surface was slightly dished towards the headstock, as if the forces involved had tried to drag the spindle out of the bearings or twist the blade and this was probably contributing to the heat build up. The blade was set as true as was possible to measure using a parallel off the chuck face and the carriage locked, so I dont know what was happening there.
Plan is to resharpen the HSS blades and set 10 thou above centre and have another go, If that doesn't work, put a new blade in the MGEHR, if that fails, buy a bigger carbide parting tool and if that fails, give up and use something softer!
Will feed back any results good or bad!
|not done it yet||06/02/2021 09:33:59|
|6719 forum posts|
I just read the thread where Martin put up his first post. Started 2017 and been added to recently. Where he states ‘ I would agree there is nothing 'super' about this relic of 1930s technology’ - but also adds that it is a joy to use at times....
I reckon, if he is still using the same lathe, it is likely the lathe - the tool height going below where he thinks it is as soon as there is any load on it. Might be wrong as he says he is parting off with a rear tool post.
I don’t particularly like the term “eclipse style” as that likely means a cheaper one in reality.
Nor do we know if he is starting with an inch (he’s imperial), or more, of cutter overhang.
I part off using the rear tool post. Power feed and wait for the part to drop off. One hand with the oil brush and t’other on the power feed trip at times. Not done PB, but the silver steel (only 14 mm or less) the last three days has simply ‘grumbled’ momentarily at the start and then just got on with the job. The parting-off cutter may need a tickle on the linisher but it just works.
|Simon Williams 3||06/02/2021 10:09:02|
|652 forum posts|
My comments earlier were based on the supposition that the parting tool is the normal way up in the front tool post. Now we know it's upside-down in the rear post the logic is reversed. So the cutting edge is too low.
Another cause of the OP's symptoms could be that the tool is chipped, moving the actual cutting edge and making it less efficient. Either way all that heat is trying to tell us something.
|norm norton||06/02/2021 10:27:22|
|183 forum posts|
Obviously tool height must be right and a strong foundation for the tool, but it depends on what type of PB you have. PB102 flexes under cutting, hence can close in on the tool shank.
The corners of the tool must be absolutely sharp, best to hone an HSS one. I don't like the insert carbide cutters as they are not sharp enough.
Slow speed, with cutting oil dribbled on, it all gets very hot. Same issue with drills. Colphos and SAE660 don't suffer the same problem in my experience.
1345 forum posts
I have been parting off quite a bit of bronze of various types recently. For 1.5" diameter I would drop the speed considerably to less than 100rpm. For reference this is on a Myford S7 with rear tool post holding a correctly ground Eclipse HSS blade and using a neat cutting oil. Confidence is required, either the tool should be working hard or not at all, do not dither. Bronze work hardens.
I could go on but I have an important appointment at Bath Racecourse - ouch!
|Phil P||06/02/2021 11:25:00|
|802 forum posts|
Maybe you could start the operation with the parting blade only partially extended, then when it is half way through extend the blade for the last bit. You want the least amount of tool overhang you can arrange.
8469 forum posts
Probably not strange at all. We often generalise about alloys, perhaps assuming here that all Bronzes are the same. They're not! Alloys are formulated to emphasise a particular property such as toughness, malleability, ductility, hardness, springiness or - maybe - machinability.
Most Phosphor Bronzes are more than averagely difficult to machine, and some downright nasty; hard, tough and springy. A few Phosphor Bronzes are formulated to improve machinability, typically by adding a dash of Lead, but do you have one of these? What sort of PB is it?
My guess is the Super 7 is struggling to part an uncooperative bronze. All the usual parting problems are amplified by difficult materials. Insufficient rigidity and critical tool height. If the parting blade is too low, the work tends to climb and jamb the tool underneath, crunch, snap, gouge. If the parting blade is too high, it rubs, overheats, and work-hardens the job. Applying pressure to get a rubbing blade to cut is likely to end in tears too!
I'd hacksaw it.
Edited By SillyOldDuffer on 06/02/2021 11:47:05
|3549 forum posts|
|3549 forum posts|
Missed the text!! 3" dia PB tool overhang a bit long, Machine is M300
|Roger Best||06/02/2021 12:30:25|
369 forum posts
It does sound like the tool and material are very out of wack.
I think there is a lot to learn from this, I have some PB from the scrap bin of a previous employer who used it for plain journals on a BIG printing press, all chunky stuff about 2-3 inches in diameter.
I have had no problem turning it, even with tools designed for steel, and yet it was suitable for decades of use on a machine operating 24/7. I can't comment on its properties for soldering as it was selected for an interchangeable wear component, but whatever it was is top of my list for nice materials.
|Mike Hurley||06/02/2021 12:49:50|
|305 forum posts|
Allied to this issue, a while back I was working on some 1 1/2" Dia PB I'd bought at a ME exhibition, so unsure what type it was. I had no problems working it with normal tools, turned / bored 'a treat', however I needed later to split these so attacked it with a unused .5mm slitting saw in the mill. Started OK then jammed, freed up and retried a few times - different speeds/ feeds etc then tried bit of paraffin lubricant - seemed to make things worse. Pushed on though thinking there was perhaps a 'hard bit' to get past. Ended up totally jammed fast. when I extracted everything the saw blade was a total wreck about 1/3 teeth missing all the others blunt.
In frustration I manually split these with a fine saw blade (it cut dead easy!) and faced the cut sides up - job was OK in the end. Anybody any ideas why I might have had these problems (incase I need to do this process again in the future)?
|Martin of Wick||06/02/2021 12:53:31|
|249 forum posts|
Thanks to all responders, problem solved I think.....
A HSS blade tool 'good enough' to chew its way through bits o' steel is just not going to cut a chunky bit of bronze.
Using a 2mm quality cobalt HSS parting blade, straight off the grinder, the workpiece was sliced through it as if it was a piece of soft cheese, tool set slightly high, cut at about 100 rising to 150 rpm with neatcut. So I learn that a really, really, sharp tool is what is needed!
I take the point about blade overhang, but with the GHThomas / AKA Hemingway rear post you dont have an option to change projection easily as the tool height is defined by the tool angle which is designed in at 8 degrees. This results in a slightly scary projection of 1 1/8 inch on the large blade. Am considering making an alternate turret to take a horizontal blade - one day!
|Roger Best||06/02/2021 13:04:06|
369 forum posts
Mike - that's a thin saw - was it hollow ground. i am guessing that it would have run hot, and caused itself to get squeezed like a failing brake caliper.
2421 forum posts
The best part about parting PB, is you think you have won the lottery. The swarf looks like pure Gold.
Please login to post a reply.
Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!
You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.
Click THIS LINK for full contact details.
For subscription issues please see THIS LINK.