Advice sought, again (sorry)
|Terry Kirkup||17/01/2020 17:17:21|
60 forum posts
That's a good point (sorry!) Neil, thanks for adding to the conversation. I'm definitely going to at least try what I created and cry afterwards!
Edited By Terry Kirkup on 17/01/2020 17:17:32
Edited By Terry Kirkup on 17/01/2020 17:32:57
|5138 forum posts|
That's my issue really; a single point cutter can be set to whatever tpi is wanted, which can be extremely fine. To get a good finish, the feed-rate has to be reduced, which means less metal is removed in a given time. Adding a second cutter allows the feed rate to be doubled. Metal is removed faster but it doesn't alter the quality of the finish.
The tpi analogy doesn't quite hold water because it assume the tool cuts a clean groove. I'm not sure it does, rather metal is sheared off in front of the point, and - maybe - the tool smooths the finish by polishing the rough surface left behind:
The picture illustrates 'Built Up Edge' causing rings on the the work. Could this be what's causing Andrew Johnston's recent woe in EN1A?
Lathe tools intended for finishing are often shaped with rounded rather than sharp edges, isn't that because they polish rather than groove the work? And as a final obsevation, lathes produce excellent finishes with single point cutters: they don't need multiple teeth to get top results.
Got to admit I'm hypothesising. I think some of the facts support what I'm saying but I wouldn't bet the farm on any of it! But my proposition remains that multiple-teeth are there only to increase metal removal rates, and they don't produce a better finish than a single point tool at the same effective feed rate.
My case rests, and I ask for 27 other offences to be taken into consideration!
17086 forum posts
The metal does not know if you are using a single point fly cutter or a multi pointed tool.
Simple way to see the difference would be to run an indexable tool with all it's inserts and do the same pass with just one. If DOC and feed rate stay the same you will get a rougher finish. If DOC stays the same but feed reduced to keep chip load the same then time taken will be n times longer where n is the number of inserts.
Might try it over the weekend but off to Ally pally tomorrow so don't hold your breath.
I doubt Andrew is suffering BUE, but would be interesting to see what finish he gets using same feed, speed and DOC on a straight cut of the same bar but even then if the marks disappear will it be due to some issue with the copy setup or because the cut is not going through concentric ring in the actual metal a bit like growth rings on a tree.
As for lathes giving good finishes with a single tool try putting your 250 into the most course feed you have and see if the finish is rougher than when using the finest feed. it's exactly the same to get the finer finish you use a finer feed or cut per rev which is the same as chipload on a mill and the finer the feed the longer it takes.
Edited By JasonB on 17/01/2020 18:39:51
|Andrew Johnston||18/01/2020 11:00:23|
5115 forum posts
Absolutely, the cutting process is shearing, not slicing. I think the tool bit may well burnish the material as it passes, particularly for insert tooling. That may explain why insert tooling sometimes works better at higher speeds. The shear zone is much hotter at higher speeds, so the material is softer and more easily burnished.
Interesting idea about BUE on my EN1A. But I'm not convinced. The tool on the hydraulic copy unit is actually upside down. During roughing the swarf fell away in a steady stream of small blue chips. When finishing the swarf had more of a tendency to produce strings, but again no evidence of BUE. This morning I've used the same holder and insert in the normal toolpost to clean up a nut I welded on to my M16 drawbar yesterday. The insert looks clean and in remarkably good shape given the duty of producing a bucket load of swarf. I've also just cleaned the lathe and it really was a bucket full of swarf!
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.