$#!&%# Boring Issues!

[clevinski]

It's official; boring is now my least favorite machining activity, supplanting even the dreaded parting off!

Most of the projects I've worked on to date haven't really required boring. I've done a few; a 1/4 inch deep, three-inch hole in a Lexan disk, a 3/4 inch hole in an aluminum rod about one inch deep, and now I need a 5/8 inch through-hole in a 7/8 inch deep piece of 1.25 inch drill rod. I first drilled it to 1/2 inch diameter, and then started boring using (inexpensive) carbide boring bars, though I don't know if the carbide makes any difference. I have very little experience with carbide, but I do know that it likes to run somewhat faster than HSS, so I've tried speeds from 250 up to 500 RPM.

My biggest problem has been adjusting the tool to avoid rubbing. The cutter is located slightly above center. I'm taking very light cuts (.005 inches per pass) but I seem to need to reset the tool every few passes because it's rubbing again and you can feel resistance while moving the carriage. I had expected the opposite; that the rubbing would be less of an issue as the hole opened up. Though, in fairness, the 1/2 inch hole is now about .560 inches, so there hasn't been very much opening up! I stopped because I wanted to get as good a finish as possible and didn't want to ruin the part.

My second problem is the finish, which is unpredictable. I'm cutting as I enter the part, and then I cut again (without changing the depth) on the way out. I've read that this will prevent or minimize bell-mouth. Sometimes the finish looks fine. Other times, it looks like grooves have formed at certain points. Sometimes I can hear something odd going on while cutting; it almost sounds like something's bouncing around randomly inside the part. I'm using the shortest boring bar that will reach all the way through the part with the smallest cutting surface due to the small initial diameter of the hole.

Since boring is considered a rudimentary lathe skill, I'd like to really learn what I'm doing wrong here. Can anyone shed some light or provide some suggestions? I appreciate any input!

 

[Ray C]

Hold on here... what's rubbing? Are you saying the carbide insert is slipping and the boring shaft is making contact with the piece?

Once we get that resolved, I'll go on to mention that my boring DoCs are only about 20% less than normal surface DoCs. I use the suggested/recommended SFMs and feed rates. Also, I only cut on the way in, stop the carriage when reaching the desired depth, move the bit away from the cut (noting the DoC on the dial), retract the bar, dial in the next amount of DoC then take the next pass. Depending on how stout your boring bar is, you might want to take a spring pass using the same dialed-in number from the previous pass.

Use coolant/oil as you normally would.

Maybe show some pictures of your setup with details on the tip and boring bar. That would help a lot.

Ray

 

[clevinski]

Re: $#!&%# Boring Issues!

Hold on here... what's rubbing? Are you saying the carbide insert is slipping and the boring shaft is making contact with the piece?

Once we get that resolved, I'll go on to mention that my boring DoCs are only about 20% less than normal surface DoCs. I use the suggested/recommended SFMs and feed rates. Also, I only cut on the way in, stop the carriage when reaching the desired depth, move the bit away from the cut (noting the DoC on the dial), retract the bar, dial in the next amount of DoC then take the next pass. Depending on how stout your boring bar is, you might want to take a spring pass using the same dialed-in number from the previous pass.

Use coolant/oil as you normally would.

Maybe show some pictures of your setup with details on the tip and boring bar. That would help a lot.

Ray

Hi, Ray,

Sorry, I should have been more clear. It seems like the front face of the carbide tool is rubbing; i.e., the front relief is inadequate and the cutting edge is not making contact. I adjust it (in particular, raise it a little more) and it will cut OK for a few more passes.

Here are some pics... In some you can see the inside surface of the part.



- - - Updated - - -

Hi, Ray,

Sorry, I should have been more clear. It seems like the front face of the carbide tool is rubbing; i.e., the front relief is inadequate and the cutting edge is not making contact.

Just to clarify, by "front face", I'm referring to the relief below the edge facing to the side of the part. In other words, the relief that prevents the tool from rubbing on the rotating, cut surface.

 

[Richard King]

I think you have the wrong type of boring bar. I use that type of boring bar when I am using a boring head on a vertical mill.

The link below shows those types of bars in the box with the boring heads. 2nd one down

http://www.ebay.com/sch/i.html?_trk...nkw=Boring+head+boring+bar&_sacat=0&_from=R40

You are probably getting lots of vibration using that type in a lathe.

I found the types of boring bars I use on a lathe on Ebay
.
http://www.ebay.com/bhp/boring-bar-holder

You can get different sizes for small or large holes.
Rich

 

[blaser.306]

I too am very new to machining anything!!! , and am interested in any advice given on subjects such as these. I do have one question, When you are setting up your carbide boring bar . Are you setting the carbide @ 90deg ( horizontal ) to the surface to be cut? I have done a little bit of boring with a HSS tool bit that I ground myself and found it no better or worse that cutting on the outer diameter. It may just be an optical illusion but in your picture , it looks as though the cutting edge is facing down ( Positive / negative rake angle ) and thus would be acting as a scraper and I would imagine that the swarf would just " ball up" on the leading edge instead of cutting a curl and clearing the chip? Like I said I am as new as new can be, and wanting to learn as I go as well. Watching with interest!

 

[Ray C]

OK, I see...

Just put a few degrees more angle on the QCTP. The boring bar does not have to be perfectly straight. Make sure everything is tight. Do you have any relief on that carbide edge? If you can't grind it in (due to lack of diamond or SC wheel), try to raise the bit a little higher. On inside cuts with a tight diameter hole, the relief is important. That would help. Double check that your compound gibs are snugged-up -matter of fact, make sure the x-feed gibs are properly set too. Is the QCTP nut tight? You don't want it pivoting under pressure.

That should fix the problem. Let us know. You'll get through this... It may take a little adjustment of the carbide to get it to cut. Also, a 5 thou cut with carbide will get you nowhere fast. Carbide need a little pressure. Probably 8 thou is the minimum you'll get away with on that mild steel; 10 is better.

Ray

EDIT: Yes, you do need a little positive rake but not too much or, you'll lose all the bottom relief.

 

[clevinski]

Re: $#!&%# Boring Issues!

I think you have the wrong type of boring bar. I use that type of boring bar when I am using a boring head on a vertical mill.

The link below shows those types of bars in the box with the boring heads. 2nd one down

http://www.ebay.com/sch/i.html?_trk...nkw=Boring+head+boring+bar&_sacat=0&_from=R40

You are probably getting lots of vibration using that type in a lathe.

I found the types of boring bars I use on a lathe on Ebay
.
http://www.ebay.com/bhp/boring-bar-holder

You can get different sizes for small or large holes.
Rich

Hi, Rich,

These are standard boring bars for use with mini-lathes. You can find my particular set here. I see the ones you are referencing with the replaceable carbide inserts. Those are nicer than mine, and certainly more rigid, but mine are supposed to work with a lathe. They are even labeled so in the link.

FYI, I am not getting a lot of vibration, but I am taking a very light cut.

- - - Updated - - -

I too am very new to machining anything!!! , and am interested in any advice given on subjects such as these. I do have one question, When you are setting up your carbide boring bar . Are you setting the carbide @ 90deg ( horizontal ) to the surface to be cut? I have done a little bit of boring with a HSS tool bit that I ground myself and found it no better or worse that cutting on the outer diameter. It may just be an optical illusion but in your picture , it looks as though the cutting edge is facing down ( Positive / negative rake angle ) and thus would be acting as a scraper and I would imagine that the swarf would just " ball up" on the leading edge instead of cutting a curl and clearing the chip? Like I said I am as new as new can be, and wanting to learn as I go as well. Watching with interest!

Hi, Blaser,

No, the cutting edge is up, not down. I considered grinding an HSS cutter, but the initial hole is only 1/2 inch and the part is 7/8 inch deep. I was pretty sure it wouldn't be stiff enough with that much hanging out of the tool holder. I'd love to see a photo of yours. How deep did you bore with it?

- - - Updated - - -

OK, I see...

Just put a few degrees more angle on the QCTP. The boring bar does not have to be perfectly straight. Make sure everything is tight. Do you have any relief on that carbide edge? If you can't grind it in (due to lack of diamond or SC wheel), try to raise the bit a little higher. On inside cuts with a tight diameter hole, the relief is important. That would help. Double check that your compound gibs are snugged-up -matter of fact, make sure the x-feed gibs are properly set too. Is the QCTP nut tight? You don't want it pivoting under pressure.

That should fix the problem. Let us know. You'll get through this... It may take a little adjustment of the carbide to get it to cut. Also, a 5 thou cut with carbide will get you nowhere fast. Carbide need a little pressure. Probably 8 thou is the minimum you'll get away with on that mild steel; 10 is better.

Ray

EDIT: Yes, you do need a little positive rake but not too much or, you'll lose all the bottom relief.

Hi, Ray,

OK, that all makes sense. I tried to keep the tool in line with the hole, but that's probably a mistake as I may have too much edge and not enough point cutting. There IS relief on the cutter, but not all that much. I don't have a diamond or silicon carbide wheel, but I do have a set of diamond hones, so I'll give them a try. I don't think I need much more relief, so maybe the hones will work for me. I did make sure that the gibs and the QCTP nut were tight.

I was wondering about the cutting depth. I vaguely recalled reading somewhere that there is a minimum cut you can take reliably with carbide, but I didn't know what it was, as I normally never use/need carbide on a mini-lathe. I'll give it a try with .010"; the lathe should handle that easily. Thanks for the great advice!

 

[Ray C]

Dont thank us until you get the hole bored the way you want it.

 

[clevinski]

Dont thank us until you get the hole bored the way you want it.

That's a project for tomorrow night!

 

[Richard King]

I still believe you would get better results with the other style IMHO. I would make sure the top of the tool is hitting first and not dragging on the front and side. You may want to spray the top of the insert with bluing and don't turn on the power...slowly turn the spindle by hand and with a light shining in to the hole see what part of the tool hits first. I always have the tool on the center line. Use a shorter one. I think if you ever used the other style you would never use that type.

A lathe is a lathe weather it is a mini or a full sized. Rich

PS: Go to the Machine Scraping and Restoration forum and scroll back as some of the members have built inexpensive diamond wheel lapping / grinding machines. We use them on carbide scraper blades, but they would also work on boring bars or tool bits too.