Phonograph turning finish.

[dennisa49]

Hello, a question re a poor finish when turning.
Small lathe (9 X 20),
Mild steel, 24mm diam.
Cemented carbide tool, .08mm radius tip,
900 rpm, feed rate of .005". Lowest available rate.
Cut .15mm, then .20mm and then .25mm.
Oil or Rocol lubricant.
Poor finish, distinct phonograph effect finish.
I have tried 500 rpm also.
Is someone able to help please?
Thanks and regards,
Dennis

 

[Wizard69]

My first thought is that your RPMs are too slow for mild steel and carbide tooling.

Also how the tool is sharpened has a big impact.

However is this true phonograph spiraling? I ask because a less than rigid tool can create grooving that while not spirals can look like spirals.

 

[Blogwitch]

You mention small lathe and ccmt tips, not really the ideal solution.

Cemented tips work on a different principle to HSS, sort of delaminating the material, cutting and and finishing with it requires good rigidity, fast speed and power, which a small lathe rarely possesses.
In all honesty, I think you would get much better finishes if you used standard HSS tooling, and for a very good finish, have a nice radius on the cutting tip.
If you really want to use carbide, then get the cheapo brazed tip variety and grind them up like you would a normal HSS.

You don't say whether you are trying to cut using the leadscrew as a feed, if you are, and you can't get a slow enough feed, then that will give you a corkscrew effect on the material.

John

 

[Niels Abildgaard]

Hello Dennis

Henry Nielsen has a 9 20 lathe and nowaday uses a tangential carbide tool of 3 mm diameter for most machining.
I know because I made the holder.

http://www.navnligthy.dk/oversigt/henrynielsen

It will be helpfull to se Your setup.
I guess Your lathe and tool is to flexible.

 

[dennisa49]

Thanks you for the help and advice.
Couple of things I have been able to do.
Reversed the final 2 gears.
This reduced the feed rate to 0.002" per rev.
Also tried 900 rpm. Fair result.
1,800 rpm - not enough grunt or mass to give a fair result.

Possibly I need to buy some bright bar or free machining steel.
I do have some brazed tip tools @ 16 mm height.
I will machine the shanks down to 12 X 12 then sharpen the tips as per HSS.
Looking at the tangential tools, I reckon the mass should be the same.
I also need to bolt the whole set up down more securely.
The next plan would be to make a tangential holder.

I will post the results, they may be helpful to other machine owners.
Regards, Dennis

 

[jayville]

Dennis.you answered the question I was going to ask..get some free machining steel...also I have a sc4 lathe and have no problem with carbide tips.even on stainless..comes up like a mirror..depends on the dia but I don't machine on very high revs and get excellent results and I,m no expert by any means ...clem

 

[purpleknif]

Welcome to the world of machining. I've been doing this stuff for over 30 years and I'm still looking for the tool that will work on every job. I work in a job shop so every job is different. You need to experiment to find what works for that particular material. There are a on of variables involved from tool geometry, surface feed rate, material and on and on. Hopefully you'll experiment and find a combination that works for that job. As far as a tangential holder, its relatively easy to grind tools to suit and easier to adjust them to suit. Just takes a little practice. Good luck.

 

[Niels Abildgaard]

Hello, a question re a poor finish when turning.

Small lathe (9 X 20),

Thanks and regards,
Dennis

Have You modified the compoundslide to crosslide system?
The original is known to be not very good and web is full of better arrangements.
http://bedair.org/clamp/9x20clamp.html
Again it will help if You put in a photo of Your present set up..

 

[Jasonb]

Can't see where CCMT type tips were mentioned Bogs?

Are you sure of that tip radius? 0.08mm is almost a point, most insert tips tend to have 0.2, 0.4 or 0.8mm tip radius.

On a small lathe a large 0.8mm tip radius gives quite awide cutting surface and can start rubbing on shallow depths of cut. Try 0.2mm tip radius or go for one of the polished tips $$GT ####02

 

[dennisa49]

Hello Jason,
I checked out the geometry as you suggested.
All measurements below are in mm.
It seems with the .8 radius unless an aggressive cut is being taken (.8 +) the cutting is not being completed by the tip. Rather by front left portion of the tip.
Now I see the logic of using a smaller radius.
Using an indexed .4 tip at 900 rpm, low feed rate .15 cut the result was a smooth finish, requires polishing for a top job.
Next stop HSS.
I am not familiar with the polished tips you mentioned, however I reckon Google can work a late shift.
Many thanks and regards,
Dennis

 

[Jasonb]

Dennis if you can let us know the code of your existing tips I'll come up with the ones you want.

 

[dennisa49]

Jason, bit more thinking required on my part.
The code is TCGT110204, any amount available including .2 radius.
Shown as aluminium tools, I am guessing they work OK on steel.
Thanks for your help,
Regards,
Dennis

 

[westgippy]

Hi

Yes the carbide inserts meant for Aluminium are the best for a small lathe , they have a nice sharp edge and a positive rake .

 

[Jasonb]

Jason, bit more thinking required on my part.
The code is TCGT110204, any amount available including .2 radius.

Thats ther right code for a triangular insert with 0.4mm corner radius, the code for 0.2mm radius would be TCGT 110202.

They are very good on Aluminium, brass and Bronze but will also work well on steel and stainless steel. As the cutting edge is more pointed they are a little less durable so save them for finishing cuts when using steel and stainless.