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8th Oct 2020, 10:06 AM #16Senior Member
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Thanks, I just checked the max speed at it taps out at 1400rpm or there abouts. I do have another step on the motor pulley but my belt isnt long enough to use it and I am maxed out on the adjustment so Ill grab a longer belt.
According to the fswizard I pretty much always need to be running at max rpm and just the feed rate changes, so Ill work on that in future. No wonder i am breaking tips off my inserts, I have been machining way too slow and no doubt a little too aggressively!
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8th Oct 2020, 12:42 PM #17Most Valued Member
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Speed is only one part of the equation when turning, you also need the horsepower to back it up. When you put your parameters into Fs Wizard, you will notice that it gives a horsepower (or kilowatt) figure required, if that is more than your lathe has to offer (don't forget that available power will be less than motor power due to gear train losses etc), then either speed, depth of cut or feed need to reduce to bring you back within your available power capacity. It's not good for any tool, but carbide positively hates running out of power and stalling in a cut and will reward you with a chipped or broken insert every time.
I ran some hypothetical numbers in Fs Wizard. Material 4140, Turning diameter 38mm, RPM 869, feed per revolution .007" (I'm guessing your South Bend is Imperial), depth of cut .5mm/.020" which requires .49 Hp. If we jump up to .015" feed per revolution, our Hp figure also jumps to 1.07 Hp which may still be achievable if you have at least a 1.5 Hp motor. A carbide insert can be pushed far harder than these numbers, but as you can see, the horsepower is not there to allow that on most small lathes.
Carbide is still well worth the effort though.
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8th Oct 2020, 01:17 PM #18Gear expert in training
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It's also worth noting that the fswizard feeds are for roughing, not finishing; basically how hard you can push it, not necessarily how hard you should push it.
If you need to reduce the load, reduce the feed, not depth of cut, as carbide requires a depth at least as big as the corner radius to properly form a chip; our Colchester (2hp I think) will happily do 4140 with a 1mm depth and .001" feed and the finish looks like it came off a grinder.
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8th Oct 2020, 02:19 PM #19Senior Member
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Thanks guys
My little lathe lacks the hp and the rpm really to use carbide at its optimum I think. But I am still happier with the results I get from the carbide inserts vs the hss bits even though I am obviously not using them as they would prefer to be used.
At the end of the day I will only ever be a hobbiest in anything to do with machining but for the most part I am still making everything I need to make. Albeit a lot slower than most and no doubt I am going about it all wrong so I have a lot to learn yet.
I am struggling to find a SCRCR 100 deg holder for general turning?? I can find a SCKCR for facing but none of the others.
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8th Oct 2020, 02:38 PM #20Gear expert in training
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SCRCR is going to be hard to find I suspect, you can't turn to a shoulder with it so it has limited use. Seco makes one but not cheap https://www.aimsindustrial.com.au/se...ew-lock-89-x-1
Dorian Tool in the US does a SCYCN holder, again probably not cheap and the only one I could find.
The "odd" angle holders are much easier to find for CNMG inserts. If you have a mill you could probably make your own
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8th Oct 2020, 03:35 PM #21I break stuff...
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SCDCR is fairly common, at least allows you to use the 100 degree corner for general turning. SCBCR is also pretty common, which is a 75 degree approach - I started with one of those to use up my 100 degree corners.
SCSCR is harder to find - I wanted this because it's a 45 degree approach angle, convenient for chamfering the end of things without rotating the toolpost. Think I found two suppliers, both well in excess of $100, so I made my own, which faces (with the 80 degree tip), turns (100 degree tip) and chamfers at a 45.
I did it this way because I normally booger up the 80 degree tip using the various tools, and can't use up my stock of broken inserts fast enough in the SCDCR. This way, I figure I'm going to kill the 100 degree corner first as most operations are mainly external turning, and if I bugger up that corner, the insert is still usable in boring bars or the ubiquitous SCLCR, but I can use this toolholder for 95% of my work (other than turning to shoulders).
Also, if you happen to have the "Engineers Black Book", pages 128-141 cover insert designation, toolholders and applications.
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8th Oct 2020, 07:25 PM #22Most Valued Member
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8th Oct 2020, 07:33 PM #23Pink 10EE owner
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Negative inserts have the advantage they are double sided. So a WNMG has six cutting edges to destroy.
Gold, the colour of choice for the discerning person.
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9th Oct 2020, 10:02 AM #24Senior Member
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22nd Oct 2020, 08:20 PM #25Diamond Member
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[QUOTE=elanjacobs;19757401mm depth and .001" feed and the finish looks like it came off a grinder.[/QUOTE]
The carbide setup that I bought from H&F with my lathe had a chart on the back that advised that rough and finish speeds were 50 and 80m/min respectively. Naturally I followed the instructions and have always had fairly ordinary surface finish, resorting to HSS tooling, a file & emery paper to get anything decent.
Just looked at the box of spare inserts (Sumitomo TCMT110204) and it specifies a Vc of 200 (150-240m/min range) and a feed of 0.15 (0.08-0.3mm/rev range).
Loaded some 1" BMS, wound up the RPM to 1500 (a bit gun shy to go any faster at the moment) and the feed to 0.15mm/rev, took a 1mm cut and by gosh the finish was excellent.
Just need to work out the best technique to get to a finished dimension if I need to be taking a minimum 0.4mm cut...
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22nd Oct 2020, 08:43 PM #26Gear expert in training
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What I find works (I didn't discover it, but it does work) is to rough turn to finished size + 1mm on radius, take a 0.5mm finish cut, then measure it and take the rest. Taking 2 equal(ish) finish cuts means that the tool deflection will be the same on both passes, so you can be confident that what you set on your final cut will be what it actually does.
The other option is to get some 0.2 radius inserts for finishing.
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22nd Oct 2020, 09:52 PM #27Pink 10EE owner
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From what I understand tungsten carbide does not cut, but rather tears the metal off.
Gold, the colour of choice for the discerning person.
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22nd Oct 2020, 10:16 PM #28Gear expert in training
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Depends on the insert; pressed inserts could be thought of as more of a tearing/shearing action, but ground and polished inserts definitely do cut
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23rd Oct 2020, 07:00 PM #29Diamond Member
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24th Oct 2020, 08:37 AM #30Golden Member
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Hi danshell, i know nothing about inserts (but sure would like to) but i do have some experience with a south bend 10. Not an expert, just an owner.
Im pretty sure you will have plain bearings in that lathe head. 2000rpm is likely a bit if a stretch unless you set the bearings and/oil selection up for it. Maybe momentarily getting there might be alright, but, me, id not run a south bend that fast.
The spindle oil and clearances / shimming recommendations are for lower speeds than that.
But, im no expert. Just sayin'.
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