Surface finish problems

[Carlos SA]

Hi guys,

I have been having some trouble turning a decent surface finish on mild steel. I've tried a variety of speeds and feeds, with and without cutting oil, freshly sharp knife tool and rounded nose. I've kept my feed consistent and made sure I wasn't leaning onto the machine but still I have these 'interruptions' in my finish, like the tool was tearing instead of cutting.

I tried with a piece of scrap brass and got an excellent surface finish.



Where am I going wrong here?

Regards,
Carlos.

 

[DMS]

You're doing just fine. Brass is known for being almost impossible to get a bad finish on, and mild steel is known for being almost impossible to get a good finish on. The only thinh I have found that will give a nice finish on mild steel is a vertical sheer tool. Take a look at Gadgetbuilders site

http://www.gadgetbuilder.com/VerticalShearBit.html

This is a tool for finishing pass only. Don't push it more than 0.004". It will leave a very good finish even on mild steel. Another bonus is they are easy to grind.

 

[Ray C]

This can happen with mild steel especially materials intended for construction use. These metals have very low carbon content and are very "gummy". You should always use auto-feed instead of moving the carriage by hand. For a shaft that diameter (it looks to be about 1" diameter or 2.5cm) using high speed steel cutter, I would spin about 600 RPM and set the feed rate to about 0.003 to 0.006 IPR and set the depth of cut at about 0.005 to 0.010". If the diameter is less than 1", you'll need to spin at higher RPM but the feed rate and depth of cut can remain the same.

Of course, you need a properly ground bit and the piece must be supported at the end with either a live or dead center. If you don't support a long piece at the end, there is no way it will ever cut cleanly as it vibrates while being cut.

Once again, with mild and "gummy" steel, sometimes you never get a great finish and the best you can hope for is an even finish that might not be very smooth but is consistent.

Ray

 

[pdentrem]

If that piece is cold rolled steel than I am not surprised about the finish. Cold rolled tends to gummy as stated before. Better quality of steels will give a much better finish. Barrel steel in rifles and the like usually are 41XX steels and they give an excellent finish even on light mini lathes. Properly supported of course!
Pierre

 

[Rbeckett]

I was going to suggest that you get to within a thou of your finished size and use a roll of crocus or machinists cloth. It is available in various grits and will shine and polish that shaft to a mirror finish pretty quickly. I too have just about given up on getting a high quality surface finish unless I polish it with crocus paper when I am done. I have tried, raising, lowering, adjusting the relief and cutting angles as well as the angle the tool is presented to the work and any other parameter I could think of and mild steel like 1045, and 1018 is just so low on carbon that they cut like Gumbo....


Bob

 

[markknx]

Carlos,
I was just trying to ajust my grinds on my tools for the same reason. I also tried faster slower and all it just seems that mild steels will act that way. I turn AL, or SS and they sine like chrome. If you do use sand paper be sure to cover the bed of the lathe. In the event you forget clean the ways right away of all the abrasive.
Mark

 

[drs23]

I've been experiencing the same thing. Thanks for posting the pictures and the thorough explanations. I'm relieved it's not just me.

Thanks again,

Dale

 

[pdentrem]

When I did a few rifle barrels, I used a belt sander to get the final high gloss. Used a worn out 600 grit to try, it worked so that was what I continued to use.
Pierre

 

[Privateer]

Thank you for enlightening the masses with the problem of a good finish on crs. I was trying everything, and could only get a marginal finish. And I'll have to look into getting some crocus cloth.

Terry

 

[Ray C]

Hey Bob, I'm pretty sure you got a typo down below... 1045 is the first of all basic metals where you can get a chrome finish pretty easily. In general, any metal with a four digit number where the last two digits are smaller numbers like 1018, 1020, 1117 etc. usually give dull grey finishes. As you get closer to numbers like 1045, 1050 and certainly 1090 and 1095, it's pretty easy to make it look like a mirror.

Also, sometimes if you have a piece of the low carbon material and if it's been heated-up or, if it is fresh stock cold rolled, the outer edge is a little harder and it will give a little shine. Once you dig down about 30 thou or so, the hardness is gone and it cuts dull grey.

Here are some pieces of 1045 cut fast and hard. It's my favorite non-alloy and I go through it like water. I stock the shop with many sizes of 1045, 4140 and a little bit of 4340 and 316 stainless.



Ray

I was going to suggest that you get to within a thou of your finished size and use a roll of crocus or machinists cloth. It is available in various grits and will shine and polish that shaft to a mirror finish pretty quickly. I too have just about given up on getting a high quality surface finish unless I polish it with crocus paper when I am done. I have tried, raising, lowering, adjusting the relief and cutting angles as well as the angle the tool is presented to the work and any other parameter I could think of and mild steel like 1045, and 1018 is just so low on carbon that they cut like Gumbo....


Bob