Cutting an exact diameter using a carbide tip.

[Rod RenshawParticipant @rodrenshaw28584]

[Rod RenshawParticipant @rodrenshaw28584]

Hi all

Any fans of Joe Pie on the forum?

I was watching one of his youtube lectures a few weeks ago and now I can't find it again. Joe was explaining how to use a "3 near identical cuts" method to turn stock to an exact diameter using a carbide tip, and get a good finish at the same time.

It's well known that a carbide tip will only cut well if it has a decent depth of cut. So the usual amateur method, when using HSS, of taking finer and finer cuts as one approaches the final size does not work at all well when using a carbide tip.

I was impressed by his method but now I can't find the video again to check I am doing it correctly! Is there a list or even an index to these things?

Any suggestions of how to find the one mentioned, I can't remember the title.

Thanks

Rod

[Martin ConnellyParticipant @martinconnelly55370]

On the Youtube homepage you should have a history option that will list what you have been watching. It may still be listed there.

Martin C

[old martParticipant @oldmart]

If you don't have the nerve to make the last cut a big one to get the better finish, but prefer to creep up the last bit, I suggest you try using a carbide insert intended for aluminium on steel. You can take a 0.0005" deep cut with one of these on steel and get away with it.

[Tony Pratt 1Participant @tonypratt1]

What do you mean by 'exact' diameter?

Tony

[old martParticipant @oldmart]

My take on exact diameter is when cutting with carbide inserts, a fairly deep cut is found to give a good finish. For instance, if a depth of cut of 0.020" is giving a good finish, and the penultimate cut gives a diameter of plus 0.040", then you will have the right size and a good finish with a final 0.020" cut. If, however, you take smaller cuts as you get near to size, the finish gets poor as the tool is not cutting at its best.

[Anonymous]

Posted by Rod Renshaw on 12/06/2021 19:00:16:finish at the same time.

It's well known that a carbide tip will only cut well if it has a decent depth of cut.

Dunno where you got that from, but it's wrong, at least as a universal statement. For sure, in some materials, especially low carbon steels, the finish can be poor with small depths of cut. But in other materials, such as brass and austenitic stainless steel, the finish is much less dependent on depth of cut.

Being lazy If I need to take 20 thou off the diameter I dial in 20 thou on the cross slide, whether I'm using carbide or HSS. No point in taking multiple cuts when one will do.

Andrew

[old martParticipant @oldmart]

It is a good idea to take time to experiment with cut depths on different materials to find what combinations work best.

[Tim HammondParticipant @timhammond72264]

Are you sure it was a Joe Pie video? I seem to remember Stefan Goetteswinter uploading a similar topic video some weeks ago.

[nigel jones 5Participant @nigeljones5]

Agreeing with Andrew, we use nothing but carbide tips – you need the correct tip for the job but anyone who telly you they have to be deep fast cuts clearly doesnt know what they are talking about! No problem taking cuts of 0.0002"

Andrew – have you spotted your deliberate error? LOL

[Rod RenshawParticipant @rodrenshaw28584]

Thanks for the input everyone.

Martin, thanks for that, I will check my history.

Old Mart, yes that is the idea that Joe was advocating, with measurement of actual diameter ( using a tenths reading mechanical mic) at several stages to avoid the long creep up to "damn, it's under-size, and the finish is not good!" Thanks for the idea of using inserts intended for aluminium to take small cuts.

Andrew, yes I am sure you are right but I tend to use low carbon mild steel more than most other materials.

Tony, I can't remember exactly what Joe said but he normally works to sub one thou sort of accuracy, seems good enough for most model work. When I said exactly, I should perhaps have said near enough but that might have started a different debate.

Tim , I am fairly sure it was Joe, I am not familiar with Stefan's work but I will try to find the video you refer to.

Rod

[old martParticipant @oldmart]

It's well worth watching all of Joe Pi's YouTube videos, he is a most talented engineer.

[EmgeeParticipant @emgee]

Rod

If you Subscribe to Jo Pi you can check all of his videos on his Home page.

Emgee

[Mark RandParticipant @markrand96270]

Posted by fizzy on 12/06/2021 21:25:13:

Andrew – have you spotted your deliberate error? LOL

 

It might be that he's got a lathe like my Hardinge that is calibrated in diameter rather than depth of cut.

 

The optimum depth of cut of a CCMT06040X insert is just under 6mm/.25" otherwise you're paying for a lot of insert that you aren't using…

 

Edit:- deleted inappropriate comment .

Edited By Mark Rand on 12/06/2021 23:00:27

[Howard LewisParticipant @howardlewis46836]

Obviously totally wrong, according to the lore, but I achieved a good finish on steel using a CCMTO6O4 insert with a 0.001" cut at low speed with plenty of neat cutting oil.

Maybe more burnishing than cutting?

The important thing thing is that it produced the required result..

Remember, according to an aerodynamicist, the Bumble Bee can't fly!

Howard

[BufferParticipant @buffer]

My Harrison will take off 20 thou if you dial on 20 thou. My myford won't

[CirclipParticipant @circlip]

As an inexperienced apprentice, shiny finishes were frowned on by the Toolroom foreman. "Shiny means the tool is rubbing. Smooth grey means it's cutting"

Regards Ian.

[not done it yetParticipant @notdoneityet]

Posted by Buffer on 13/06/2021 08:14:43:

My Harrison will take off 20 thou if you dial on 20 thou. My myford won't

I find this unbelievable. How many 20 thou cuts would you need to take to remove 200 thou with your myford?

[John ATTLEEParticipant @johnattlee20632]

Dear All,

I know that, in theory, my Denham DL6 with plain headstock bearings and very low RPM should not benefit from carbide insert tooling but it certainly does! I use a combined tool post and tool holder and this means that the cutting tool is always at the right height. Obviously, I run it in top gear unless the work is more than about 3" in dia.

I can turn or bore to about a thou. The machine is fitted with a three axis DRO system which I think is essential to make up for my limited skills. I use three passes to eliminate 'spring back' and obtain a consistent size to be measured. Thereafter, the DRO displays the correct dia, after three passes have been taken. I would not hesitate to advance the tool just a thou.

I certainly achieve good enough results for my automotive work. I very rarely use HSS cutting tools in the lathes now.

John

[BufferParticipant @buffer]

Posted by not done it yet on 13/06/2021 10:10:15:

Posted by Buffer on 13/06/2021 08:14:43:

My Harrison will take off 20 thou if you dial on 20 thou. My myford won't

I find this unbelievable. How many 20 thou cuts would you need to take to remove 200 thou with your myford?

I think your missing the point.

[Anonymous]

Posted by fizzy on 12/06/2021 21:25:13:

Andrew – have you spotted your deliberate error? LOL

I wondered when someone would fall for that one.

As Mark says, on my lathe setting 20 thou on the cross slide means 20 thou off the diameter, not the radius.

Andrew

[Rod RenshawParticipant @rodrenshaw28584]

Many thanks for the additional help on this.

Emgee, thanks for the pointer to Joe Pi's home page information, I will search through it.

As Buffer says, someone has missed the point which, as I understand it, is that though a rigid industrial lathe may take off 20 thou when you dial it in, a less rigid hobby lathe is more likely to take off 19 or 21 thou, and Joe's method of multiple cuts enables one to compensate for this so the final size is near enough.

Rod

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Rod Renshaw on 13/06/2021 16:49:29:

[…]

As Buffer says, someone has missed the point which, as I understand it, is that though a rigid industrial lathe may take off 20 thou when you dial it in, a less rigid hobby lathe is more likely to take off 19 or 21 thou, …

.

’fraid not, Rod … the point was that the calibration of the dials differs

[diameter vs radius]

MichaelG.

[Kiwi BlokeParticipant @kiwibloke62605]

Crikey! This question encompasses almost all there is to know about turning!

Stefan Gotteswinter produced a video on 'balanced cuts'. I don't think he explained the idea very well, however the video is well worth watching – as are all his others. He often re-grinds carbide tip[s so they are sharp, and will take very shallow cuts. If you look at tip manufacturers' literature, you'll rapidly discover that those designed even for finishing cuts are not designed for 'our' idea of finishing cuts. There are ground tips available, like those intended for alloy, and, as others have said, carbide tips can (sometimes) take very fine cuts, but do they provide consistency?

The idea underpinning 'balanced cuts' is to discover how your machine cuts in practice. It will be different from what the dials say. Then you can take deep cuts, and finish to size, rather than creeping up to size and wearing out the tip by more passes than neccessary. So, get close to finished size, leaving, say, an amount oversize equivalent to three passes at a depth of cut that is known to produce good results. Measure, calculate oversize amount. Divide that by three (in this example). Take a cut at this one-third depth. Measure. Calculate the error (there will be one!), and build this error-compensation into the remaining two cuts.

This all assumes that your machine is consistent, if not accurate. If it's not consistent, all bets are off!

[Author]

Posts