Surface finish

[Kevinb71]

A while back while turning some hard chrome shaft I posted about the bore finish being rough. That project didn't require a smooth finish so I chalked it up to the material being very hard. This project reuires some 2" OD aluminum with an internal thread of 27mm. I chucked up my 2.100 stock and centered it within .001. Then proceeded to take some off the outside to true it up. About .050 with the intent of leaving it at 2.100 finish dimension. i had taken off .025 or so and noticed that my finish was far from smooth. Tried light cuts down to .001 per pass and it just isn't smoothing. Being new I tried some faster and slower speeds and using the auto feed vs hand feed. All leave about the same results. I am using a carbide cutter and did try a different one of those as well. Checked my gib adjustments on both the saddle and compound slides. Both are on the verge of being overtight. I have attached a picture though it may not make it clear the roughness that is there. What am I missing?

 

[kd4gij]

looks like your feed rate is to fast. A higher RPM and slower feed shuold clean it up. Also you might try using a live center.

 

[Tony Wells]

Could be many things. Try reversing the feed at the shoulder and letting it back off under power. Not using a center is normal for work that short, but if you ease up to it with a little pressure from the center and it changes it, that would suggest bearings in the headstock. This could also be in the tool grind. Aluminum can take a high positive rake and a generous nose radius to get a good finish.

 

[george wilson]

It looks like some cyclic finish caused by gears in the apron of the lathe,possibly a bent leadscrew or something like that. What lathe are you using?

Is it possible that any of the gears associated with feeding the apron along have a missing or damaged tooth? Is the carriage slightly lifting off the ways with each revolution of the leadscrew or the feedscrew from something being slightly bent?

I do not think this looks like a problem caused by the feed rate,or the geometry of the cutting edge,though I am not there to see what is going on.

 

[Tom Griffin]

I'm with George, it looks like a missing or damaged tooth somewhere in the feed gear train. Finding that and switching to high speed tooling should take care of the problem.

Tom

 

[llarson]

Measure the "pitch" of the lines on the part and compare it with the pitch of the rack under the front of the bed; assuming your machine has such a rack. I've seen machines establish a rack pattern on parts a couple times, caused by the gear being too tight on the rack, or crud in the rack causing a bit of binding.

 

[Philco]

I haven't had any luck getting a good finish on sticky metals like aluminum and zinc with any carbide tools except micro grain at heavy feed rates and high RPM
A HSS bit with a radius of 1/32 should greatly improve your finish. The top of the cutting tool held flat on some crocus cloth on a piece of 1/2 glass will polish the tool until almost nothing will stick to it.

I learned a good trick from a armory machinists just a few days ago.
He said that he used Milnot milk mixed half and half with water for cutting fluid.
I tried this on some stainless today and I swear it worked better than Tap Magic.

Found some oddball ideas along 40 plus years of making chips that turned out to be great tips.

Another one is freezing rubber overnight before machining. Often you can get a glossy finish on it.

Papaseven what is "MILNOT" milk? Philco.

 

[Tom Griffin]

Hi Philco

It's just a brand of evaporated milk. View attachment 36700
I know the fat content is high and I can't for the life of me explain it but when tapping some 12x1.25 threads I tried it and it was instantly easier cutting vs Tap Magic.

It may not be available everywhere but even the convenience stores carry it in OK.
As far as I know Carnation might be just as good but the old guy that told me about it insists on Milnot.
It didn't work for me mixed with water but full strength it worked great.

Thanks
Geno:thinking:

Geno,

That stuff must get downright rude after sitting on the bench for a few weeks. :yikes:

For cutting and tapping aluminum, nothing beats good old WD-40. For tapping stainless, I was lucky to have had a gallon of the old formula Tap Magic stashed away before the greenies saved us all and managed to get the 1,1,1-Trichloroethane in it banned. The new Tap Magic is pretty much useless and I have yet to find a good replacement.

Tom

 

[Tony Wells]

I'm with you, Tom. I have a stash of the old stuff. I've tried just about all of the commercial tapping fluids, but haven't found anything I like as well. I'm not so sure about the evaporated milk though. I do have a refrigerator in the shop, so when I get a tough job, I may have to try it. Maybe Eagle Brand.....it's so sweet it HAS to work.

 

[Kevinb71]

Thanks for all the replies. I will check into the rack and I have wanted to try an HSS bit anyway. I need a different grinder wheel and then I can try one.
George next time I get to the lathe I will watch for that type of movement.