Boring Problem

[master53yoda]

Guys,

I am trying to bore a 1.5 inch deep hole in 6061 aluminum of 1 inch OD using my 9x19 Grizzly G4000 lathe. Hole has to be quite precise to 0.708 inch (18 mm) diameter to mate a shaft. I am using 1/2 inch shank carbide tip boring bars I bought from Little Machine Shop. Lathe runs at 1000 RPM setting. The piece is held in my 6" 4 jaw chuck and 1000 rpm is the highest speed I am willing to run it. next speed setting available is 2000 rpm and I am too scared to try it.

I wasted three aluminum pieces till now. Each I got very poor finish. I have made sure that every thing - all gibs, tools and everything is tight. No play of any kind. The boring bar is quite thick and should not flex (I hope). I take very tiny cuts, never more than 0.002 inch at a time. I use plenty of WD40. The finish I get is terrible, almost like a sand paper to touch. The boring bar was never used before this and therefore new as shipped by LMS. It is set at precise axis height.

Where am I going wrong? What will get me better or smooth finish?

Thank you
Prasad
Eastern PA

as far as speed is concerned the 1000 is close, the chart shows 1200 for 1" aluminum.

turning the cutter up or down will move off of center, I had to sharpen the boring set that looks like yours as the cutters were not very sharp. on a 1" bore you may also need to relieve the steel portion of the botton of the cutter as well.


Art B

 

[JimDawson]

Try slowing down to about 100 RPM, and and increase the feed a bit.

It is also OK to set the tool at a slightly negative rake to stop the rubbing (cutting edge pointing slightly downward.) That actually works pretty well, and usually leaves a good finish, which is hard to believe at first.

And what Bob said.

 

[Tony Wells]

Might want to look at the corner radius also. A sharp corner requires less pressure to cut, so is easier on the material and the machine, but isn't always optimal. Even as small as a 0.002-0.005 radius will change your finish. Really, it shouldn't be smaller than whatever feedrate you are using. If it is, you will see a more pronounced toolmark, almost like you were threading at a very very fine pitch.

 

[Andre]

Make sure no chips are falling between the nearby cutting edges (not in contact with the work) and the bore. This can happen when there is too little point relief. Hard to explain though

 

[stupoty]

I can not raise the boring bar. It is already as high as it can fit in the 4-way tool post. I can lower it. I will try that and see if it helped. The boring bar is shortest it. If I shorten it further I will not be able to plunge to 1.6" deep I want it to go.

I will try all this tomorrow and post my result,

Thank you
Prasad

if it is way to low you can always shim up the whole tool post, I do that quite a lot with booring tools that have a very low cutting edge compared to my other turning tools.

You can do the rule method of height setting on the OD of the part then turn the tool round. For booring it's the reverse of OD turning, that is to say the tool will cut when too high but just rub if it's too low.

Stuart

edit (their may be cases when you can get it to cut when too low but the whole clerence thing is going to be more problematic)

 

[mikey]

I have one of those sets. They are marginal as delivered but mainly because of two things: first, the area under the carbide tip is ground round but still rather thick so clearance can be an issue and second, as the others pointed out, the edges are not that sharp. You can gently grind the area under the carbide for more clearance. Keep the rounded shape but take off some of the material at the bottom of the tool. Use a diamond stone to sharpen the edges; it won't take much to do this. Then form a tiny radius at the tip and then hone the top. The tool should cut pretty well after that.

I suggest using the biggest bar that will fit in the hole and still reach the depth you need. The supposed hardness of the bar, even if it was true, is immaterial. Deflection is influenced by bar diameter and the biggest bar that will fit will deflect less.

As for speeds, go with what you are comfortable with. Try different speeds on some scrap and see what works best. On my lathe, I bore aluminum at the max speed my lathe can generate, about 2800 rpm, and get a nice finish.

 

[4GSR]

With the rough finish you are getting sounds like the corner of the carbide is chipped. As others have said, you need some kind of corner radius on the tip of the boring tool. And definitely slow your spindle down to about 600-700 RPM. Once you get it cutting like you want then start playing with increasing your RPM's. Feed rate should be around .005-.010" per revolution. Ken

 

[prasad]

Guys

Woohooo... I made it. Thanks for all your advice and I did every trick you gave. I raised the boring bar, tilted it up, shimmed the entire tool post and finally took it out a ground the tip. In the end I was able to get a decent smooth finish. Main contributing factor was the carbide tool tip sharpness. I don't have a diamond tool grinder wheel. I ground carbide tip on my ordinary 6 inch bench grinder and it worked. I also ground the bottom of the steel part under the carbide tip.

I learned many things today. Here is a short summary.
1. Carbide tip of Chinese origin may not be sharp and may need some sharpening.
2. Carbide tip of Chinese origin can be sharpened using an ordinary grinder wheel. (Obviously poor quality carbide tip)
3. Boring bar height can also be adjusted by tilting the tip up or down (this came in handy when 4-way tool post has been shimmed up (under boring bar) to its full capacity.
4. Additional height adjustment can be done by placing additional shims under 4-way.
5. Take very small cuts to minimize flexing of boring bar.

Thank you all, your help is immensely appreciated

Cheers
Prasad
Eastern PA