Boring Problem

prasad

prasad

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Jul 4, 2014
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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
 
Make sure your bar sticks out to a minimum from the holder. Just enough to reach the end of your bore. Too much overhang will produce unecessary flex
 
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
 
+1 to what Toolmaster said. Make sure that the tool is not rubbing on the work below the cutting edge, or anywhere else besides the cutting edge for that matter. Sometimes those tools are ground poorly, and if you have the cutting edge set level or with a positive rake, the tool can sometimes rub on the work. You can grind away any part that is rubbing, but if it is the lower portion of the brazed carbide then you will need diamond tooling to cut the carbide. 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.
 
Prasad, are your boring bars brazed or inserted tip carbide? I assume the shank is steel, right? Maybe post a pic of your bars?

Your tip needs to be about 0.005-0.010" above center to allow for tangential forces. I suggest zero axial rake (top of insert horizontal) to reduce tangential forces further. Align the center of the bar with the spindle axis and take a decent cut, at least deeper than the nose radius of the cutter to rough. Do not dwell or stop on entrance to the bore; start your feed and maintain it as you enter and proceed down the bore.

Since you must be very accurate, you need to know how your bar behaves with a given depth of cut. For a finish pass, you might try a depth of cut between 1/3-1/2 the nose radius. Check the ID, dial in your cut, make a pass and check to see how much that DOC increased the ID. You can then rough until you can precisely dial in that finish depth of cut and come in on size.

You should be able to run the lathe at max speed and take much heavier cuts. Watch your chips and adjust your feed to produce tightly coiled chips; should look like a coiled spring when you get the feed right. Use WD-40 with every pass.
 
Hi Mikey

The boring bars are made up of "special heat treated steel" (LMS description) and the carbide tips are brazed. I selected a bar with just the right length for the work. Here is a photo of the bars copied from their web page.

Boring bar set.jpg

I have the work mounted in the heavy 6 inch 4-jaw chuck. I calculated the spindle speed and I get a figure of 3000 rpm. My lathe can run at 2000 rpm max and I am scared to run this heavy chuck at that speed.

I am measuring the bore diameter only at the front end using my caliper. I can not measure diameter at the deep end of the work. I have sprayed WD40 before start of each cut. I will next try tool settings per your advice and see how it goes.

Thanks
Prasad
 
I have a boring bar similar that did not have a sharp edge on it, it was not dull mind you, when I put a very keen edge on it, all my issues went away and the surface finish is now amazing. It took me forever to get the edge right on the carbide as I don't have a diamond stone but the ceramic stone I have finally did the job. You can shave with it now. Make sure you keep after that WD-40 as aluminum will build up on the cutting edge and the surface finish might go away.
 
prasad said:
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
Click to expand...

You can turn (as in twist) the boring bar. This will effectively raise or lower the cutting edge of the bar. It won't take a lot of movement to change the finish.

"Billy G"
 
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