Guidelines for Boring Tool Grinding

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mcman56
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Guidelines for Boring Tool Grinding

Post by mcman56 »

I see a fair number of articles on grinding lathe tools for OD cutting but nothing for a boring bit. I'm planning to use a 3/4" bar with 5/16 HSS bit and would like to cut to a flat bottom so am going to use the 45 degree mount. See picture. Material is steel and ID will be a bit greater that 2 3/4". These will be fine cuts looking for precision. Are there any references to look at or recommendations for grinding bits for boring? The 45 degree angle makes it difficult for me to visualize. Alternately, is this a bad idea?

Bore Bar.JPG
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Harold_V
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Re: Guidelines for Boring Tool Grinding

Post by Harold_V »

No, it's not a bad idea, but it will be somewhat challenging to get the flat bottom truly flat and without steps. A facing head would be so much better, but they are not commonly found in the home shop. They are not cheap.

To rough the needed tool, leave it in the boring bar. That will help you determine the angles required. Once roughed, you most likely can remove it from the bar and do the finish grinding. By doing so, you'll have much better flexibility in shaping the tool as desired (like adding a chip breaker, mentioned below)
.
One thing to remember is that you'll be working in a hole, so the amount of relief you'd use for an external tool won't be great enough. The end won't make any difference, but the portion that contacts the hole you'll bore does.

Once you have fashioned the tool, you'd be well served to provide some positive rake, which you can do by grinding a chip breaker. In this case, it would be on the end portion, the part that generates the flat bottom. The increased rake angle will lower cutting pressure, plus lend some degree of chip control. If your cuts will be shallow in depth and a fine feed used, the chip breaker should be relatively narrow, so it curls the generated chip. It most likely won't break the chip, due in part to fine feed and shallow depth of cut, but performance will be improved markedly over a neutral rake.

Depending on the bar, the tool may already be above center. If it is, clearance won't be as much of an issue as the lead of the tool creates additional clearance.

H
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mcman56
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Re: Guidelines for Boring Tool Grinding

Post by mcman56 »

Thanks for the reply. I may not have described this well. A flat bottom is not needed so some steps are OK. I was planning to use the 45 degree tool mount vs the 90 degree tool mount because the 90 degree mounting position needs to stay 3/8" or 1/2" off the bottom. (Clearance for end end of the bar.) I would like to get as much depth as feasible. Would this change the advice?
boring bar 2.png
Mr Ron
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Re: Guidelines for Boring Tool Grinding

Post by Mr Ron »

How about a boring head in a vertical mill?
Mr.Ron from South Mississippi
pete
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Re: Guidelines for Boring Tool Grinding

Post by pete »

Ok, "as much depth as possible". But you need to define that for just how deep is this hole going to be you need to bore? It's generally accepted that somewhere between 3-5 times the bar diameter is the maximum you can reliably bore with steel boring bars. That moves up to roughly 7-8 times the bar diameter for solid carbide bars. Those are the general maximums and material type, hardness, tool sharpness, tip radius, depth of cut etc also play a part in what will work. Softer material, lighter cuts and ultra sharp tools can allow you to push those limits a bit and still get away with it. It's also helpful to set the cutting tool tip a few thou above the lathe C/L as that increases the clearance between the bore wall and what I'd call the side rake on the cutting tool. That also compensates a bit for that bar deflection. And the deeper the hole/smaller the bar is, the more you might have to set the bar above the lathe C/L.

And on deeper holes, tool chatter could become a real issue. Probably 90 + percent of the time, dropping the part rotational speed and/or changing the depth or feed rate of the cut helps because the tool is then loaded differently and that changes the harmonic range it vibrates at. It can sometimes work going the other way with lighter cuts or more rpm, but that seems to work much less often for me. Tool or part harmonics are about impossible to predict. A few tricks like tightly wrapping the tip of the boring bar with soldering wire or even sticking a lump of window glazing putty to the bar can change those harmonics enough to stop that tool chatter. You also haven't said how thin the part walls are going to end up at. Those same harmonics can also happen in the part if it is thin walled enough. Stretched and tied on surgical tubing, large elastic bands, or even multiple wraps of a rubber bungee strap on the part O.D. can sometimes work to deaden those harmonics.
mcman56
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Re: Guidelines for Boring Tool Grinding

Post by mcman56 »

I'm using a 3/4" bar to go 2.5 " deep but only need maybe 2". I have it set up and cutting but am not getting a nice finish. The hole is 2 3/4" diameter and I'm getting a minor ringing sound when running at 190 rpm on the slowest feed of an Enco 14-40 lathe. I believe the tool is on center. I could not figure out how to put in a chip breaker but am getting tightly curled chips. The material is 8620 and I was able to get a nice shiny finish when facing off the bar so suspect that it is my bit grinding. Is there a way to diagnose the issue? I used a skinny carbide boring bar made for a mill boring head to hog it out and it rang also with visible deflection when it started to cut.
Bore 1.JPG
Bore 2.JPG
John Hasler
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Re: Guidelines for Boring Tool Grinding

Post by John Hasler »

Is it the bar ringing, or the part?

Looks like quite a bit of stickout there. I try to minimize the radial stickout of the cutter so as to minimize the torque on the bar.
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GlennW
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Re: Guidelines for Boring Tool Grinding

Post by GlennW »

Is the tip of the tool bit a sharp point or a radius?

A small radius for a finish cut might work better.

Also look below the cutting edge and make sure no part of the tool bit is rubbing the stock you are cutting.
Glenn

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pete
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Re: Guidelines for Boring Tool Grinding

Post by pete »

Then your hole depth is well within the recommended allowable.Your pictures seems to show a couple of score marks and a change in the finish towards the outer end. I'd guess that's where you heard that ringing sound. And using the slowest feed rate isn't always the best for surface finish. Sometimes a faster feed rate does give a much better surface finish. That's something I had to learn the hard way. Those already created chips or long stringy swarf can easily get between the cutting edge and bore wall and either help to push the tool away from the cut a bit or create those score marks if you get any chip welding to the tip. Are you using any brushed on cutting fluid or cutting dry? It doesn't always work, but running a shop vac while your boring to suck out the chips or swarf as it's being generated can sometimes help to prevent that scoring. Like John said and I completely agree, run the absolute minimum tool extension out from the tool holder to maximize rigidity. Sometimes you have no choice but to run more than the ideal, but if you can choke up on the tool a bit more it might help.

Are you locking down both the top & cross slide before starting each cut? I'm also not adverse to purposely using the stock 4 tool turret my lathe came with over any qctp set up since there's a lot less deflection and the tool shank is held much more rigidly. The inevitable over hang with no support below a qctp tool holder works against us in some situations. Glenn also has an excellent point of having a small radius on the tool point. I can't really help with the tool shape and grinding since my boring bars are of the more conventional type. The South Bend How to run a Lathe book might have some finished tool tip geometry recommendations for that type of tool. There's easy to find online PDF's of it. And if I had any doubts there might not be quite enough clearance with a boring tool, I'd probably coat the tools cutting edges with a Sharpie or Dykem and then closely check it after making a cutting pass. If it's rubbing at all there should be at least a thin line showing it and probably on the tools lower clearance face or side. For light finish passes, it also helps sometimes to bore while cutting from the bottom of the hole to the outside if your lathe allows setting it up for power feed in the reverse direction.
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Bill Shields
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Re: Guidelines for Boring Tool Grinding

Post by Bill Shields »

Wrap a big rubber band around the cylinder. Think bungee cord
.do similar on the boring bar. Anything to dampen vibration can help. You have pretty good advice so far
Too many things going on to bother listing them.
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Harold_V
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Re: Guidelines for Boring Tool Grinding

Post by Harold_V »

It's clear that I misunderstood your post. I interpreted your comments as an attempt to bore the hole in a mill, not with a lathe. For that reason, the comment about difficulty in achieving a step free bottom isn't true.

Rule of thumb---Keep cutting tools as short as you can. That goes a long way towards eliminating chatter and adds greatly to rigidity of the setup.

Rule of thumb---Increasing feed rate and/or lowering spindle speed tends to limit chatter.

Rule of thumb---Fine feed rate and shallow depth of cut tends to yield a good finish.

It is common for thin walls to resonate, yielding a singing sound--which usually manifests as chatter in the cut, reducing the quality of the surface finish. Bill's advice to add snubbers is a good one. You can also entertain the idea of simply running an acid brush against the revolving piece, or a finger on the boring bar (or both). Anything to discourage resonance.

In lieu of an insert type bar that you're using, I usually use a hand ground boring bar. It's easier to mount and takes up less space when using an indexing block instead of a quick change. I favor the indexing block, especially for small work. The boring bar would look something like this:
Boring tool1.jpg
Boring tool2.jpg
Boring tool3.jpg
H
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mcman56
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Re: Guidelines for Boring Tool Grinding

Post by mcman56 »

I stepped through most of these ideas with the same result. Finally, I tried an outside cut and got the same result. Basically, I have machined a bell and it rings like a bell. (I did not try any added damping material due to safety concerns and not having anything appropriate.) Is there a better way to go about machining a large thin ring from bar? Can you buy soft jaws for all chucks or is there DIY method for soft jaws?

Or.....can you buy high precision steel rings? The closest thing I could find was the inner race for a needle bearing but they only went up to about 50 mm. I'm looking for a 72 x 80 x 19mm ring.... although should be able to marrow a wider one.
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