RPMs for Lathe Work

[jdrichard]

What is the best speed to cut Blue Steel on a lathe? In RPMs. I am measuring the speed by using an iphone app that strobes the light: then i watch a point on the belt to determine when the point matches the strobe. Like the old turntables.

 

 

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[Marc]

I don't think that there is an absolute "best" speed as such as it comes down to individual preference. There are factors to consider though.

Essentially, the faster the work rotates the faster you can cut, and the faster you can totally screw things up. If the lathe is spinning fast and you choose to progress slowly by taking finer cuts you can end up work hardening the piece, you can also get a build up of heat. Work at to high a feed rate and the graver will dull too quickly. Fail to notice in time that it needs touching up and you end up burnishing the surface of the work. If the graver should dig in, the faster the work is turning, the greater the damage. However, get it right and higher RPM means that you can turn out wore work in a given time.

The flip side though is that if you have the luxury of being able to take your time, slower RPM affords you much more control, it's much easier to take finer cuts and to get a better finish. Don't forget that a lot of the very best work is done on a turns powered by a bow, which, as it's working between centres, has the added advantage of allowing the work to be taken off for measuring or fitting and then be returned without any loss of concentricity.

[vinn3]

the ideal speed (and feed) is what makes the best chips.  vin

[oldhippy]

My advice to you is, whatever you are comfortable with when you use your lathe and you graver that you are using. I had a foot control to operate my watchmakers lathe so I never had a clue the speed it was working at, all I can say I never had it going too fast as you can lose control. Any work on a lathe you need to be in control.

[praezis]

Hi,

I underline what was said above. To give you figures: I do not go beyond 2000 - 2500 rpm, mostly between 1k - 2k, depending on the diameter of the piece. Bigger diameters (and steel in contrast to brass) need lower rpm.

Frank

[jdrichard]

Hi,
I underline what was said above. To give you figures: I do not go beyond 2000 - 2500 rpm, mostly between 1k - 2k, depending on the diameter of the piece. Bigger diameters (and steel in contrast to brass) need lower rpm.
Frank

2000 seems a bit fast. I clocked my lathe at average cut speed and:


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[jdrichard]

the ideal speed (and feed) is what makes the best chips.  vin

So the are the best chips long swirls for blue steel or granular


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[jdrichard]

I don't think that there is an absolute "best" speed as such as it comes down to individual preference. There are factors to consider though.
Essentially, the faster the work rotates the faster you can cut, and the faster you can totally screw things up. If the lathe is spinning fast and you choose to progress slowly by taking finer cuts you can end up work hardening the piece, you can also get a build up of heat. Work at to high a feed rate and the graver will dull too quickly. Fail to notice in time that it needs touching up and you end up burnishing the surface of the work. If the graver should dig in, the faster the work is turning, the greater the damage. However, get it right and higher RPM means that you can turn out wore work in a given time.
The flip side though is that if you have the luxury of being able to take your time, slower RPM affords you much more control, it's much easier to take finer cuts and to get a better finish. Don't forget that a lot of the very best work is done on a turns powered by a bow, which, as it's working between centres, has the added advantage of allowing the work to be taken off for measuring or fitting and then be returned without any loss of concentricity.

Thanks Mark. I did purchase a foot petal from china and it seems to have 5 rheostat steps, each providing more speed.


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[jdrichard]

Here is my work


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[vinn3]

if you are turning steel and the chips are blue, its too hot and may harden.  vin

[transporter]

Nice work JD ive yet to switch my lathe on, still waiting on the motor to turn up, then to mount it all on a nice e board, whats the app you use on the iPhone?  

Also can't quite believe the price of gravers , pretty pricey but I suppose they last awhile if looked after, time for another bhi watch fair to pick up some more second hand tools methinks 

[jdrichard]

Nice work JD ive yet to switch my lathe on, still waiting on the motor to turn up, then to mount it all on a nice e board, whats the app you use on the iPhone?  
Also can't quite believe the price of gravers , pretty pricey but I suppose they last awhile if looked after, time for another bhi watch fair to pick up some more second hand tools methinks 

Carbide gravers are about 15 to 19 bucks each and i bought a couple of credit card size diamond grit sharpeners off Amazon. I also bought a variable speed foot controller from the US. All good. Hopefully thats it for now. My belt is a leather shoe lace sewed together with 5 lb test fishing line. Works great .


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[jdrichard]

Nice work JD ive yet to switch my lathe on, still waiting on the motor to turn up, then to mount it all on a nice e board, whats the app you use on the iPhone?  
Also can't quite believe the price of gravers , pretty pricey but I suppose they last awhile if looked after, time for another bhi watch fair to pick up some more second hand tools methinks 

The app is called RPM Strobe.


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[dwhite]

On 10/26/2017 at 7:09 PM, jdrichard said:

Carbide gravers are about 15 to 19 bucks each and i bought a couple of credit card size diamond grit sharpeners off Amazon. I also bought a variable speed foot controller from the US. All good. Hopefully thats it for now. My belt is a leather shoe lace sewed together with 5 lb test fishing line. Works great .


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Do those grit sharpeners actually work to sharpen carbide?  I sort of wanted to get carbide but had read you had to use an expensive electric hone to sharpen them.  Thanks!  Dan.

[measuretwice]

you pretty much have to have diamond to cut carbide, especially to a fine finish like you need for a graver.  Silicon carbide can also grind it, but the finish is poor.  Not sure what an electric hone is, but I sharpen all manner of carbide tools - whether lapping paste and bit of cast iron, flat stone style or various wheels....it needs to be diamond to do a good job with carbide.

imo you do want carbide for hardened steel.  They just last so much longer.  I've made gravers by taking a small blank of carbide and silver soldering it to a length of steel....but I'm super cheap

Edited January 19, 2018 by measuretwice

[dwhite]

measure twice:  an electric hone is also called a power hone.  It's like a record player except it has a diamond coated disk instead of a record.  They are very expensive.  I'm wondering if there is a hand held diamond sharpener that would sharpen a carbide graver.  jdrichard mentioned a credit card sized sharpener and I was wondering if it worked well.  I spent a couple of hours regrinding the tip of a steel graver, mostly on a diamond sharpener.  Mine has elliptical holes on the surface so I had to work between the holes.  It was an unmarked steel graver and it was much harder than my mueller gravers.

[measuretwice]

I made a rotary diamond lap for sharpening carbide scrapers (used in machine tool reconditioning), perhaps similar to thing called an electric hone?  Doesn't matter, was just curious as i'd not heard of an electric hone.

As for hand held, you can get the diamond "stones" as shown below where the sell stuff to woodworkers, i.e. Lee Valley.  You need a few grits likely and for you need the finest.  They come in many sizes, but these standard stone sized one's feel about right imo.

For an even better finish (the edge you achieve is the product of two finely finished surfaces), a bit of diamond past on a piece of cast iron or AL makes an excellent lap.  10-12 micron paste (by it where toolmakers stuff is sold, ie KBC tools) it will put a mirror finish on.

 

Photos to make it more interesting  - the first is the rotary lap I made, it could be used for the ends of gravers, I made it for machine tool scrapers which need constant resharpening...scaping is close to impossible without one.  The disks are cast iron and you make them a lap by charging with diamond paste, the tool never touches the CI and is cut by the damond.  Next is diamond paste lapping and last, are diamond "stones"  which, in its finest grit, is probably the most practical and easy to use and what I'd recommend and found quite up to task of graver sharpening.

PS,   bit of sharpening theory.  An edge is where two planes meet.  The better the finish on those planes, the better the edge.  you also want the grain the sharpeing 90 degrees to edge, i.e. not parallel.  With most tools, be it a wood plane iron, machine tool scraper or graver, you want finish the long surfaces leading to the end once and thereafter sharpen by removing material from the end.   With a graver, in my experience you do have to properly finish those surfaces leading to the end....if the  thing is mounted on a AL handle this can be awkward, a small diamond slip stone or lap would be helpful.

 

 

 

 

 

Edited January 19, 2018 by measuretwice

[jdrichard]

Do those grit sharpeners actually work to sharpen carbide?  I sort of wanted to get carbide but had read you had to use an expensive electric hone to sharpen them.  Thanks!  Dan.

They work really well. I have a 400g and a 1000g. No issue at all and cheap.


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[dwhite]

Thanks you jdrichard!  Measuretwice, that is a beautiful machine you have made.  You must have access to a machinists lathe and milling machine.  Did you ever consider copper for a flat lap?  Your post made me remember something from a GIA diamond grading course I took 30 years ago.  They actually cut diamonds with a copper saw blade that has a smooth rim.  They bring the blade into contact with the diamond and apply a slurry of diamond grit and olive oil.  The grit easily embeds itself into the soft copper and begins to cut.  As it does so, it generates more grit which keeps embedding into the copper so they can cut all the way through the diamond with little if any loss to the copper blade.  I don't know if they have to anneal the copper blade, I haven't worked with copper much.  Silver quickly work hardens and will break so it has to be repeatedly annealed while you're shaping something.

[dwhite]

On second thought, I withdraw my recommendation for using a copper plate for a lap.  I went online to see if I could buy one and was horrified at the price.  One square foot of 1/8" copper was about two hundred bucks!  And I checked several sites.  What's up with that?  I looked up the scrap price for copper and it's in the 2.75 range.  This plate weighed a little over 5 pounds so it shouldn't cost more than 20 bucks.

[measuretwice]

16 hours ago, dwhite said:

Thanks you jdrichard!  Measuretwice, that is a beautiful machine you have made.  You must have access to a machinists lathe and milling machine.  Did you ever consider copper for a flat lap?  Your post made me remember something from a GIA diamond grading course I took 30 years ago.  They actually cut diamonds with a copper saw blade that has a smooth rim.  They bring the blade into contact with the diamond and apply a slurry of diamond grit and olive oil.  The grit easily embeds itself into the soft copper and begins to cut.  As it does so, it generates more grit which keeps embedding into the copper so they can cut all the way through the diamond with little if any loss to the copper blade.  I don't know if they have to anneal the copper blade, I haven't worked with copper much.  Silver quickly work hardens and will break so it has to be repeatedly annealed while you're shaping something.

Thank you for the compliment.  I come to horology (a beginner) with 25 years of a model engineering/machining experience, last count at home there are 7 mills/jig borers and 16 lathes (a few have to go).....so yeah, I'm seriously afflicted .  Better than a life spent in the pool halls I suppose (what I keep telling the wife). 

When it comes to making parts there is lots and lots of overlap and also lots new to learn (and tools/machines to acquire).  As well there are lots of tool making applications with horology.   I suppose all that is why horology appeals to me.  Anyway, the machine was specifically built for sharpening scraper blades (machine tool scraping) and its construction was part of a 12 parts series I did in Home Shop Machinists on scraping/machine tool reconditioning.  Scraping requires a very fine edge and they don't last long, so putting one on quickly matters

Copper makes a great lap, depending.  Mainly you want something softer than the work but imo something more substantial is preferred for a flat lap.   I use copper in internal laps (photo below).  Compared to other laps they are sacrificial in that there is lose abrasive present, when too worn a new layer of copper is soldered on.  Whereas the rotary lap above is charged by pressing the diamond into it, there is no loose abrasive.  With a flat lap properly charged, the surface of the lap is never touched and maintains its geometry.   Here's a couple of shots of internal copper laps I've made as well as one showing how I charge the wheels on the rotary lap - just a bearing held in a vise grip 

 

 

 

 

 

 

 

 

Edited January 21, 2018 by measuretwice

[measuretwice]

12 hours ago, dwhite said:

On second thought, I withdraw my recommendation for using a copper plate for a lap.  I went online to see if I could buy one and was horrified at the price.  One square foot of 1/8" copper was about two hundred bucks!  And I checked several sites.  What's up with that?  I looked up the scrap price for copper and it's in the 2.75 range.  This plate weighed a little over 5 pounds so it shouldn't cost more than 20 bucks.

lol, yes copper is crazy.  Start working out the amount of engravers brass require to make a clock and its equally disheartening.  One of the photos above shows a quick and dirty lapping of a piece of carbide - just an old piece of AL, some diamond embedded and a bit of water.  You can put a mirror finish on with 10 micro paste and needs about no tools....just saying there are lots of ways to come at it, and they don't have to be complicated.

 

Edited January 21, 2018 by measuretwice

[dwhite]

Thanks!  I admire your experience in machining.  I know nothing about it or watch repair but hope to learn.  I have some background in jewelry and silver/gold smithing but I stopped that over 20 years ago.  In the past few weeks, I have set up a room with two jewelers benches, one for jewelry and one for watch repair and all the related equipment.  This forum has been a great help because I can ask questions about things not in any books that I have.

[dwhite]

Measuretwice, how do you charge your steel lap?  You show a picture with a roller held in a visegrip?  Is this what you use?  I'm looking at one of those old waller graver sharpeners that go into an 8mm lathe and was wondering how best to work the diamond compound in.

[measuretwice]

lap charging.  In that photo I'm use green diamond paste (iirc green = 10 micron) and a cast iron lap (not steel).  It will put a mirror finish on the work.  An old deep groove bearing with the inner race held in a vise grip (so the outer race can rotate) is used to press the diamond in the the cast iron.  Put a pin head size bit of paste on the lap and roll it out with some downward pressure.

A point here is the relative hardness of the surfaces.  Cast iron is much softer than the hardened race so when the diamond is caught between them with a bit pressure, its embedded into the cast iron.  If the race was the same or softer, the diamond would embed in it.

Another thing that might not be appreciated is that the lap itself is not really touched by the work.  Its surface becomes a cutting tool, the work comes into contact with bits of diamond, not the lap.  That lap shown has been in use for 10 years and the cast iron surface is a flat as the day I made it - its not worn way by the work and the work doesn't really touch it. 

Hope that helps, ask away and i'll help if I can

 

 

Edited February 6, 2018 by measuretwice