Lathe Speed – What am I missing out on?

[Chris BradburyParticipant @chrisbradbury76252]

I got a mirror finish once saw a old man looking back at me scared the life out of me

[Anonymous]

Posted by Chris Bradbury on 19/06/2019 11:14:38:

I got a mirror finish once saw a old man looking back at me……….

That's why I haven't got round to fitting a mirror in the newly refurbished main bathroom yet!

Andrew

[Michael GilliganParticipant @michaelgilligan61133]

Posted by SillyOldDuffer on 19/06/2019 09:11:41:

A metric rule of thumb for HSS cutting speed in mild-steel is 10000 / diameter in mm.

The most common need for high-speed on my lathe is drilling. Applying the formula suggests:

1000 rpm for a 10mm drill,
2500 rpm for a 4mm drill,
5000 rpm for a 2mm drill,
10000 rpm for a 1mm drill,
20000 rpm for a 0.5mm drill

Not essential to cut at high-speeds, but the benefit of speed becomes more obvious with small drills. At high-speed they are less likely to break and stay sharp longer.

[ … ]

Similar advantages turning small diameter work in a lathe; high rpm makes the job a bit easier.

[ … ]

For ordinary workshop use I guess most of us work below optimum speed and it doesn't matter. The best speed is the one that suits you!

.

Sorry to labour my earlier point, Dave [and I note your remark] … But I think it fair to say that your 'rule of thumb' gives maximum, rather than necessary, speeds [optimum being a term with too many factors for my brain to compute]

Watchmakers habitually use drills, and turn work, smaller than 0.5mm diameter; sometimes at very low speeds comared to those listed, and have done so for generations.

What matters most is getting an appropriate balance of speed and feed … Small drills work well at low speeds, provided that you can feed them at appropriately microscopic rates.

MichaelG.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Michael Gilligan on 19/06/2019 13:25:05:

Posted by SillyOldDuffer on 19/06/2019 09:11:41:

A metric rule of thumb for HSS cutting speed in mild-steel is 10000 / diameter in mm.

The most common need for high-speed on my lathe is drilling. Applying the formula suggests:

1000 rpm for a 10mm drill,
2500 rpm for a 4mm drill,
5000 rpm for a 2mm drill,
10000 rpm for a 1mm drill,
20000 rpm for a 0.5mm drill

Not essential to cut at high-speeds, but the benefit of speed becomes more obvious with small drills. At high-speed they are less likely to break and stay sharp longer.

[ … ]

Similar advantages turning small diameter work in a lathe; high rpm makes the job a bit easier.

[ … ]

For ordinary workshop use I guess most of us work below optimum speed and it doesn't matter. The best speed is the one that suits you!

.

Sorry to labour my earlier point, Dave [and I note your remark] … But I think it fair to say that your 'rule of thumb' gives maximum, rather than necessary, speeds [optimum being a term with too many factors for my brain to compute]

Watchmakers habitually use drills, and turn work, smaller than 0.5mm diameter; sometimes at very low speeds comared to those listed, and have done so for generations.

What matters most is getting an appropriate balance of speed and feed … Small drills work well at low speeds, provided that you can feed them at appropriately microscopic rates.

MichaelG.

Thanks for the clarification Michael – worth emphasising again that the published figures and calculators are aimed at industrial users whose notion of 'optimum' is tuned to the economics of large scale machining. The main cost items are metal-removal rate, tool-life and power consumption. In a large production run it's well worth balancing these. Also other factors unlikely to apply at home like overtime!

I don't bother with optimising. Rule of thumb cutting speeds indicate a starting point, they're not essential. Mostly I work slower than recommended. Carbide is perhaps an exception; I've found good finish with carbide is most easily achieved by violent cutting at recommended rates. HSS seems less fussy.

Dave

[ChrisBParticipant @chrisb35596]

Posted by SillyOldDuffer on 19/06/2019 16:26:23:

I've found good finish with carbide is most easily achieved by violent cutting at recommended rates. HSS seems less fussy.

Dave

Agreed, had to machine the thread off a 50mm dia high tensile bolt. Going at about 700rpm with carbide the interrupted cuts until the thread cleared were going to shear the tool post right off the crosslide. Switched to a CBN insert and cranked up the speed to 2100rpm and the process was way smoother. The only downside it the burning chips flying all over the place!

Edited By ChrisB on 19/06/2019 21:20:50

Edited By ChrisB on 19/06/2019 21:21:12

[old martParticipant @oldmart]

Sometimes just changing the speed a little will improve the cutting, reduce chatter or change the surface finish.

My car has a six speed box, I don't stay in third all the time.

 With carbide inserts, you have to take deep cuts with inserts designed for steel or the surface finish suffers. Using inserts designed for aluminium to finish steel allows very small depth of cut, useful when you are creeping up on the required size. They are sharp like hss.

Edited By old mart on 19/06/2019 21:44:54

[Del GrecoParticipant @delgreco45469]

Hi Gents,

So, This is a total newbie follow-up question I guess…

I am turning around 10mm Brass. (I'm trying to make a small whistle, spinning top, etc etc.) all with some small stock to get in some practice.

From the posts above, and from what I can find online, I should be turning at >2000+ rpm?

Is that correct? I'm still a little nervous about spinning my lathe that fast, and have mostly been using it at around 200-300 rpm for similar work, with horrible results. I get a crappy finish, often the thinner the brass stock can bend quite visibly instead of being cut, and it can make a nasty grinding noise.

I have carbide tools, and have the extra sharp mirror looking inserts, which work great on alu at around 300-400 rpm. (Although that was thicker ~30mm alu stock!)

All this cutting speed stuff is very new to me, and I'm starting to think that my previous efforts were a lot more luck than skill. ha!

Thanks in advance.

Del

[Mick B1Participant @mickb1]

Posted by Del Greco on 15/08/2019 14:23:07:

Hi Gents,

…have mostly been using it at around 200-300 rpm for similar work, with horrible results. I get a crappy finish, often the thinner the brass stock can bend quite visibly instead of being cut, and it can make a nasty grinding noise.

I have carbide tools, and have the extra sharp mirror looking inserts, which work great on alu at around 300-400 rpm. (Although that was thicker ~30mm alu stock!)

…

Del

If those inserts really are sharp, I'd guess you're off centre height, probably high. I think I could get a pretty nice finish on any kind of 10mm brass I know of with sharp HSS tools on centre at those speeds, although I'd normally run about twice as fast at that diameter, even with HSS. If you get rubbing and deflection, there's usually something more than just speed that's wrong.

[JasonBModerator @jasonb]

As Mick says it sounds like your tool height is wrong, any error will show up more as the work diameter gets smaller.

Fine to run at 2000rpm but you won't see a lot of difference running at say 1000rpm while you build up confidence. 2-300 is too slow.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Del Greco on 15/08/2019 14:23:07:

Hi Gents,

 

So, This is a total newbie follow-up question I guess…

I am turning around 10mm Brass. (I'm trying to make a small whistle, spinning top, etc etc.) all with some small stock to get in some practice.

From the posts above, and from what I can find online, I should be turning at >2000+ rpm?

Is that correct? I'm still a little nervous about spinning my lathe that fast, and have mostly been using it at around 200-300 rpm for similar work, with horrible results. I get a crappy finish, often the thinner the brass stock can bend quite visibly instead of being cut, and it can make a nasty grinding noise.

I have carbide tools, and have the extra sharp mirror looking inserts, which work great on alu at around 300-400 rpm. (Although that was thicker ~30mm alu stock!)

All this cutting speed stuff is very new to me, and I'm starting to think that my previous efforts were a lot more luck than skill. ha!

Thanks in advance.

Del

LittleToolShop's calculator suggests 3000rpm for HSS on 3/8" brass and 10500 rpm with carbide. Although those speeds are for economic industrial production and needn't be applied religiously in a home-workshop, they're a strong hint that 200-300rpm is too slow!

Assuming a suitable tool-tip, the three factors that have to be balanced to get good finish are rpm, depth of cut and feed-rate. They vary with material and diameter of the work but there's usually a fairly broad band of conditions that will produce acceptable results.

HSS is more forgiving than carbide, but make sure it's sharp, especially if the depth of cut is low. Took me a long time to realise feed rate is important too. Feeding too slowly risks leaving a fine screw-thread effect, whilst feeding too fast can cause tearing. A shaky hand can produce both effects on the same pass.

Try taking the speed up to at least 1000 rpm and taking a few moderately deep cuts, say 0.3mm or 10thou whilst experimenting with the feed-rate. Best results from power feeding, but good results aren't hard to get by steady hand feeding, it takes a bit of practice though.

Carbide works best at speeds and cuts well beyond anything HSS can do. To get full benefits from carbide the cutters are distinctly blunt, and it pays to attack the job with vigour, provided the lathe can cope that is! One trick is to use sharp carbide cutters intended for Aluminium on other metals. The sharpness makes them behave more like HSS.

Don't be disheartened, it takes a while to learn how to get the best out of a lathe. I've been turning for several years now and still get it wrong now and again. Two common beginner mistakes, mine was pussy-footing, the other is angry gorilla with a headache. I use sound quite a lot to judge; seems to me cutting in the right zone has the lathe audibly working for a living without being flogged. Practice, practice and success will follow…

Dave

PS I failed to spot 'nasty grinding noise' = see Mick and Jason's identification of tool-height as the cause!!!

Edited By SillyOldDuffer on 15/08/2019 15:11:08

[peak4Participant @peak4]

Posted by ChrisB on 19/06/2019 21:20:15:

Agreed, had to machine the thread off a 50mm dia high tensile bolt. Going at about 700rpm with carbide the interrupted cuts until the thread cleared were going to shear the tool post right off the crosslide. Switched to a CBN insert and cranked up the speed to 2100rpm and the process was way smoother. The only downside it the burning chips flying all over the place!

 

There is another, longer winded solution, which might help those with less rigid machinery, or no suitable cutters.

Set the lathe up to screwcut the relevant pitch, and work with the thread rather than effectively intermittently across it.

Bill

Edited By peak4 on 15/08/2019 21:10:48

[Del GrecoParticipant @delgreco45469]

Good morning,

Ok, so last night i was turning ~15mm brass.

I upped the speed and I checked the center height of my single cutting tool I am using at the moment. It was very slightly high. Like <1mm high. I re-shimmed it and got it to where I think is bang on.

The result was much better, so thanks for the advice.

One thing I did notice, (I am trying to make a whistle btw, like this one Whistle pic) that once i'd bored out the tube section, it would ring like a bell when cutting, giving a very rippled effect of the cut. A bit like what you would see if you look really closely at a record. I did the remainder of the cuts with a finger pressed against the brass, dampening and reducing the ringing. The brass only protruded about 10cm from the 4jaw chuck face!

Thanks in advance.

Del

[KWILParticipant @kwil]

If it rings like a bell, this shows you are exciting it at a resonance and a sure indication you need to change speed. The advantage of a VFD (variable speed) is that it may only take a small change to go off resonance and hence improve surface finish.

I would hope that your <1mm would be considerably less, perhaps 40x less (I work in thous from time to time.)

[Del GrecoParticipant @delgreco45469]

Posted by KWIL on 16/08/2019 11:18:14:

If it rings like a bell, this shows you are exciting it at a resonance and a sure indication you need to change speed. The advantage of a VFD (variable speed) is that it may only take a small change to go off resonance and hence improve surface finish.

I would hope that your <1mm would be considerably less, perhaps 40x less (I work in thous from time to time.)

Changing speed is a good idea. I'll up the speed again to the next pulley.

Also, yes it was a fraction of a mm. I didn't really measure it, as I knew it was off. I've some new (to me) measuring tools arriving over the next few days, so I'll update you once they arrive.

[Author]

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