Just noticed the boring primer, should have read that first.

J

John TV

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So full disclosure, I've had my mill (Van Norman # 12) for more than a year now but I'm just getting to use it more in the last few months and with my starting from zero knowledge base I feel like I'm probably at a senior high metal shop level of skill. But I'm getting better. Thank god for this group and YouTube!

Now my situation. First time using my brand new Shars boring head with my new LMS carbide braised boring bars.

I am making the knurling tool shown on toms techniques channel. He has vids showing the entire project so good project for us beginners.

Boring a 2.06 inch hole from a 1 inch starter hole in one inch thick 6061 aluminum. I chose a bar that was the shortest and stoutest to get through the one inch thick material. Set the cutting edge on outside perimeter with cutting edge on center (I know it creates a small negative rake when set this way but seemed like the most frequent way to solve problem in my reading about boring).

Had a lot of stock to remove so I started taking 30 thou on radius cuts, seemed smooth with no chatter, then went to 40 thou and all still good. Used WD40 for lube.

Cuts seemed to agree with occasional measurement and this is not a critically precise hole so I decided to take the last 24 thou as a final boring. Significantly less than the 30 and 40 thou cuts up till now. Same set up only not as big a bite and almost instant chatter.

Tried more lube, slower feed rate and still chatter. Cleared bottom of hole and came up slowly hoping to clean up some of the chatter but no luck. Hole measures out ok but significant chatter marks.

Help me understand what went wrong? Any ideas?

Started out at around 340 rpm the moved down to around 200 as hole got bigger, mill didn't seem to strain at all on any size cut.

Thanks for your thoughts.

John in Minnesota

Just saw the boring bar primer,
Haven't read it yet. Sorry if the info I'm asking about is in the primer. I will read it soon.

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Not knowing or seeing your set-up it would be hard to guess why , but I would assume you had a radius on your bar . On the deeper cuts the radius on the tool didn't matter , on your finish pass it most likely rubbed . Go back in with a dead sharp bar and rebore it at a slow speed to remove the chatter marks if you can . New bars aren't correctly ground from the factory , that's rule number 1 .

A good rule # 2 . When boring or turning for that matter , it's a good process to sneak up on your final dimension by taking 2 or 3 successive cuts at the same depth . Just .005 or .010 for example . You're letting the bar spring and getting good dimensions at this point . Your final cut would be the same DOC . No surprises . If that process was followed , you would have picked up on your chatter problems long before your finish cut .

Machinists in the trade refer your finish cut as the " $hithouse or the Penthouse cut " . You'll be in one or the other when you're done . :)

I loved your original thread title . " Boring Bar Blues " . Kinda reminds me of an old country song . :big grin:
 
John TV said:
... new LMS carbide braised boring bars.

... 2.06 inch hole from a 1 inch starter hole in one inch thick 6061 aluminum.

... started taking 30 thou on radius cuts, seemed smooth with no chatter, then went to 40 thou and all still good. Used WD40 for lube. Cuts seemed to agree with occasional measurement and this is not a critically precise hole so I decided to take the last 24 thou as a final boring. Significantly less than the 30 and 40 thou cuts up till now. Same set up only not as big a bite and almost instant chatter. Tried more lube, slower feed rate and still chatter. Cleared bottom of hole and came up slowly hoping to clean up some of the chatter but no luck. Hole measures out ok but significant chatter marks.

Help me understand what went wrong? Any ideas?

Started out at around 340 rpm the moved down to around 200 as hole got bigger, mill didn't seem to strain at all on any size cut.
Click to expand...

Hey John, not sure what went on here but I have some thoughts:
  • Was the boring bar sharpened or as it came from the supplier? The reason I ask is that most of these brazed tools are not sharp to begin with and not all of them have enough relief to provide adequate clearance. Most do not come with a nose radius; while they cut okay like that, they do better with a miniscule radius at the cutting tip. Best to sharpen the edges of that brazed cutter and don't forget to hone the top. Then radius the nose. Make sure there is enough clearance under the cutting edge so it doesn't rub.
  • Was the gib on the boring head adjusted correctly? I assume you have a clone of a Criterion DBL head. If so, on the side of the gib will be three screws. The center one is to lock your depth of cut setting ONLY. It is not used to adjust gib tension; the two on the ends are. The procedure is to back off of all three screws, then bring the two on the ends just into contact with the gib. Tighten one screw while moving the feed leadscrew back and forth. You are looking for just enough tension on the screw to feel a whisper of drag on the leadscrew, then repeat for the other end screw and you're good. The moving part of the head should move smoothly and without much effort but there should be no play when you move that lower part around by hand. When setting your depth of cut, loosen the center screw only and set the next cut depth, then snug that center screw and make your cut. The only time you ever touch the two end screws is when adjusting gib tension.
  • Your speed was a bit slow for aluminum. Using brazed carbide, you can go into the 800-1,000 rpm range and when boring under 2" ID you should do okay as long as there is no chatter or vibration from the head. If there is, slow down.
  • 0.030" depth of cut is a healthy cut for a boring bar in a boring head, while 0.045" is ambitious. My personal approach is to find a depth of cut that works and stick with it rather than trying to take it all off ASAP. It doesn't pay to rush a bore. What works best for me is to find a roughing depth of cut that works, then find a finishing depth of cut that I can predict. For example, say a 0.020" DOC works well. I will make however many passes at that DOC as needed until I can set my finishing DOC. Say your bar can take a 0.005" DOC and reproduce it, pass after pass. I would rough at 0.020" until I am about 0.015" away from final ID. Then I'll take 0.005" cuts until I come in on size. This works better than trying to be a Youtube Manimal. You will find that taking huge cuts is ego-boosting but coming in on size is actually more important.
  • You should not drag any cutter out of any cut at any time. Your tools are meant to cut in one direction, the direction in which it cuts; dragging it out while running will lead to premature flank wear and dulling of the cutter.
  • Boring bars need adequate feeds to cut well. Most of us feed too slow. The cutter needs to cut continuously and when it does, your finish actually improves. Try increasing feed a bit more.
So, what went wrong? I have no idea but take a look at the above and try again, even if its only to see if one of these things might fix it. I might have missed something but I can't think of anything else.
 
Thanks for the feedback. I think several of the suggestions will help a great deal.

Also really wish I had noticed the awesome boring primer. I am in awe of your knowledge Mikey. On the other hand I didn't break anything and the confusion led to good lessons learned.

Now to start saving for yet more tools....

I am making mistakes left and right but learning all along the way. Now if this old brain can just try to hold on to all those lessons.

The members of this forum are awesome and so appreciated.

John from Minnesota


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Thanks for the kind words, John. I hope the primer helps you.

Most of what I know came from digging out from under mistakes so you aren't alone. That's how we learn, right? We try it, screw it up, figure out how to do it right and then we never forget it. That's what I love about this hobby ... skills are hard earned but they come honestly.

Let us know if you figure out what went wrong with your set up. The followup will help those that come behind us.
 
As I've always said Mike , sometimes knowing what won't work is half the battle ! :grin:
 
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