HSS cutting speed calculation

[GoCreateParticipant @gocreate]

From Practical Engineer part 1

[GoCreateParticipant @gocreate]

I was reading through Practical Engineer part 1 article on this forum  regarding cutting speeds but I am a bit confused.

The article states:

1" x 375 = 375 RPM for steel

1/2" x 375 = 188 RPM for steel

1/8" x 375 = 47 RPM for steel

What am I missing?

Nigel

 

Edited By tractionengine42 on 10/08/2014 07:16:13

[JasonBModerator @jasonb]

Seems it should say divide 375 by cutter dia

375 / 1 = 375rpm

375 / 0.5 = 750rpm

375 / 0.125 = 3000rpm

Link to article if anyone else can come up with an explanation

J

[GoCreateParticipant @gocreate]

Thanks Jason

That makes sense now and useful to know.

The article needs a revision.

Nigel

[Michael GilliganParticipant @michaelgilligan61133]

Jason,

You are obviously correct, regarding the error in the formula

Mistakes like this are easily made when an author is describing a "convenient rule of thumb" that has been stored away in his subconcious: The underlying theory gets skipped-over, and the error is not noticed.

I know that MTM runs as a "lean" operation, but this is a perfect example of material that needed "peer review" before publication,

There is an insight to the origins of David's rule, in the opening line: "Referring to a chart of cutting speeds, a 1in. HSS cutter in mild steel requires a speed of 382 RPM. For practical purposes, this can be taken as 375 RPM."

Why would we use 375 as a "practical" approximation for 382 ?

… Because 375 is the decimal for 3/8 … which will relate to some other convenient rule.

[Well, that's my explanation … others may differ]

MichaelG.

 

 

Edited By Michael Gilligan on 10/08/2014 08:32:44

[JasonBModerator @jasonb]

Yes I think you are right with the reason to use 375 Michael, maybe for the metrically minded beginner a figure of 400 would be easier to work with when using finger and thumb as most of the older MEs will know in their heads what the various figures in that decimal sequence are off the top of their heads.

Also agree that proof reading is not enough, you need someone who understands whats being written.

J

[jason udallParticipant @jasonudall57142]

I had cause to right a spreadsheet to calculate cycle times for estimating ( quoting) jobs for a machine shop…

Firstly my “estimate”..were within 2% of final optimised and leaned on cycle times some timemore sometimes high..
This took in ,material machinability, cermet ceramic, carbide, bcnt hss or what ever tooling.
Ok
So for a laff, I compared my childhood 10, 000 rpm / drill diameter (mm). Which I worked out for myself also worked for outside dia.

Again for hss and machinability 100% this matched to about 1/2 a percent.

Now to the real world…does YOUR lathe have constant surface speed?..variable to a tenth of a rpm?…
No , you will have a selection of speeds and at best steps of 100-200 rpm…

In short rules of thumb can be very helpful

I for one labelled my drill stand in rpm as well as diameter. .colour codeing sufficed. .

[JasonBModerator @jasonb]

I can get variable to within 2rpm if I'm bothered

[Neil WyattModerator @neilwyatt]

I think Jason was correct so I've changed 'multiply' to 'divide'.

I'm surprised it took five years for that to be spotted, the article has well over 8,000 views!

Neil

[JasonBModerator @jasonb]

Thats 7999 people having a job getting a good finish

[BazyleParticipant @bazyle]

Hopefully most people latched on to the one figure for 1" and work it out. They do say you should keep your brain active so chuck away the tables and graphs and work it out each time from that one figure.

Didn't we have another thread on this a few weeks ago (starting with milling cutter speed) pointing out that old and hobbyist values were way out compared with both theory and industrial production rate practice.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 14/08/2014 11:43:42:

I think Jason was correct so I've changed 'multiply' to 'divide'.

.

Nice to see that stated for the record, Neil

[ = version control ]

… I was beginning to think I was dreaming.

MichaelG.

[JasonBModerator @jasonb]

Neil, just had a look and it will require a bit more than changing multipy to divide, try this out for a 0.5" cutter which may be a bit slow

"First, divide the cutter diameter in inches by 375."

Needs to read

"First divide 375 by the cutter diameter in inches"

J

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