Struggling with boring

raferguson

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I have a 7 x 12 mini-lathe. I am trying to bore a hole to match a shaft, which measures slightly over 0.750." I have an 11/16" drill, so after a pilot hole, and an 11/16" drill, I started to bore. It seems like I am not making much progress. 11/16" is .6875. 0.750 - 0.6875 = 0.0625. Looking at the dial on the cross slide, one revolution should be 0.040". I believe that the cross slide motion should produce a change in dimeter of twice the motion of the cross slide. (Am I wrong?). So a single revolution of the dial should change the diameter of the bore 0.080". However, I have turned the dial several revolutions, and the diameter is still less than 0.750".

I am using a boring bar that sticks 2" past the end of the holder. I can see the lack of rigidity in my lathe, of course. I can see some chips, so it is not like it is not cutting at all.

Am I just kidding to think that I can bore a hole with my mini-lathe? I could post a video if that would help. It looks like the site supports videos...... Do I need to tune up my mini-lathe.

Richard
 
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I have a 7 x 12 mini-lathe. I am trying to bore a hole to match a shaft, which measures slightly over 0.750." I have an 11/16" drill, so after a pilot hole, and an 11/16" drill, I started to bore. It seems like I am not making much progress. 11/16" is .6875. 0.750 - 0.6875 = 0.0625. Looking at the dial on the cross slide, one revolution should be 0.040". I believe that the cross slide motion should produce a change in dimeter of twice the motion of the cross slide. (Am I wrong?). So a single revolution of the dial should change the diameter of the bore 0.080". However, I have turned the dial several revolutions, and the diameter is still less than 0.750".

I am using a boring bar that sticks 2" past the end of the holder. I can see the lack of rigidity in my lathe, of course. I can see some chips, so it is not like it is not cutting at all.

Am I just kidding to think that I can bore a hole with my mini-lathe? I could post a video if that would help. It looks like the site supports videos......

Richard
Are you producing much swarf when you are boring? Does the swarf look like little curls of metal, or is it more like fine dust? Does it make scraping sounds when you are trying to bore the hole? Very little swarf, dust-like swarf or scraping sounds indicate a problem that needs to be corrected.

If there is a problem like I mentioned, your boring bar probably isn't aligned properly; or it's not of very high quality. I have a set of (cheap) brazed-carbide boring bars, of which perhaps just one or two are capable of actually boring a hole. The rest rub against the side of the hole, someplace other than the cutting tip. The carbide on the rest needs to be reshaped and I don't have the tooling to do that. Not yet, anyway.

I gave up on the cheap stuff and bought a decent insert boring bar from LMS that uses CCMT inserts. That made a HUGE difference. If you are going to be mostly turning softer materials like aluminum, brass and plastic they also sell HSS boring bars, which can be resharpened. The cobalt alloy is more expensive but will hold up a long time. I've used it to bore bearing pockets and achieved a friction fit -- in aluminum. With a 15 YO 7x12. So you can, too.

Even when you have a good boring bar, it has to be aligned correctly so the cutting tip is very close to the spindle center -- or it may rub anyway. And your starting hole has to have a large-enough ID so the boring bar can go in without rubbing. I think you probably are good there but it doesn't hurt to check.

It took awhile for me to figure out that it wasn't just my lathe (or me), but was due to those crappy boring bars.
 
I have a 7 x 12 mini-lathe. I am trying to bore a hole to match a shaft, which measures slightly over 0.750." I have an 11/16" drill, so after a pilot hole, and an 11/16" drill, I started to bore. It seems like I am not making much progress. 11/16" is .6875. 0.750 - 0.6875 = 0.0625. Looking at the dial on the cross slide, one revolution should be 0.040". I believe that the cross slide motion should produce a change in dimeter of twice the motion of the cross slide. (Am I wrong?).

You might be. Some lathes calibrate the cross slide dial to read tool movement, others calibrate it to read diameter change. It is up to you to understand your lathe. Make a sharpie mark on the cross-slide, turn it 10 or 20 or some other convenient number of revolutions, and then use a 6" scale to see whether it moved 0.040 or 0.080 or 0.020 per rev. (I'm guessing from the 0.040 in your post that it has a metric lead screw...) Once you understand how your lathe works, write it down for future reference.

So a single revolution of the dial should change the diameter of the bore 0.080". However, I have turned the dial several revolutions, and the diameter is still less than 0.750".

You should be measuring the bore after every cut. Later, once you have confidence in your machine, tooling, and process you can skip some measurements but you should always be measuring as you get close to final size.

If you don't know how much you are taking off (or not) on each pass you are working totally blind.

What tools do you have for measuring inside diameters? For 0.750, the options are:
A) Inside jaws on your dial calipers. Only accurate to a couple thou on a good day, several thou on a bad day. And can't measure deep holes.
B) Telescoping gage and outside jaws on the calipers. Good to a couple thou, some skill/practice is needed to operate.
C) Telescoping gage and micrometer. Can be good to a fraction of a thou but skill and a good touch is needed to get that accurate.
D) Gage pins. Accurate to a thou or so, no skill required, but most people don't have a set.
E) Dial bore gage. Can be accurate to better than a thou, but not something most beginners have. And still requires a bit of skill as well as a standard to set it to.

I strongly recommend option C. If you don't have a set of telescoping gages, get some and learn how to use them. The cheapest import sets are under $20 (Harbor freight has a set for $17.99). Mid-range imports are $20-50. Mitutoyo is $150-200 for a new set of 6, about $100 for a set of 3 that covers up to 2" holes. Less for used sets on eBay. Let your budget be your guide, but keep in mind that you will use them every time you do an ID measurement.

I am using a boring bar that sticks 2" past the end of the holder. I can see the lack of rigidity in my lathe, of course. I can see some chips, so it is not like it is not cutting at all.

Deflection is almost certainly part of your problem, but 0.040" or more seems like a lot. Maybe the boring bar isn't tight enough in the holder, or the holder is slipping on the compound, or the cutter/insert is slipping in the boring bar.

Are you trying to bore 2" deep? The boring bar should stick out as little as possible to achieve the hole depth that you need.

What kind of cutting tool are you using? How sharp is it? Is the geometry appropriate for the cut and the material? Are you sure the height is set right? All of those things could cause it to rub instead of cutting, which in turn might make something deflect or slip.

Am I just kidding to think that I can bore a hole with my mini-lathe? I could post a video if that would help. It looks like the site supports videos......

Richard

You should be able to bore a hole with that machine. The deeper you go the harder it will be to hold a tight tolerance, but 0.040 isn't a tolerance at all. Something is grossly wrong.

Try posting pictures of your tooling and setup. Video might be useful, but photos are probably easier and often better.
 
Thanks for your comments. I will post some photos. The shavings do look like dust, even under magnification. I did the simple experiment to see if 0.040 on the dial advances the cutting tool .040”; it does. I only have a digital caliper. The bar is 3/8" diameter, and I am trying to bore almost 2 inches.


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You don't mention how big of a boring bar you are using, or what the setup is beyond "a boring bar". As is alluded to in the previous posts, you need to have the boring bar at the right position AND the right rotation. A perfectly square cutting tool (i.e., with a straight vertical face) will never cut as it will rub on the bottom edge of the tool.

You may just need to review your tool position and setup. You need enough clearance angle so that the bottom edge clears the curve of the bore. In a .6875" hole, a 1/16" thick insert (or brazed on piece) with no built in clearance angle would need to be turned down at least sin-1(.0625/.34375)=10.475 degrees. (Assuming I did my trig correctly!) Drawing yourself a picture is helpful. Tool deflection just makes this worse ...

editted to add: I think it actually works out to 2*sin-1(.03125/.24275), i.e. 1/2 the tool height / radius, which in this case is very close to the same number.
 
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I reground the tool to put in side relief, and I can see the difference in the chips. I still have dust, but mixed in are real curved chips, so that is progress. I do not have a diamond wheel. I typically use HSS tools.
 
If you just chucked up a 11/16 bit and got through your material your lathe should have plenty of power and running rigidity.
Your dials could be direct reading or not. You’d have to verify that by putting a DI on the cross slide and check. Yes your correct whatever you move the machine takes x2.
Make sure everything is tight that doesn’t have to move. Are you see chips when boring?That’s a clear indication of what’s going on.
 
I suspect that @homebrewed hit it on the head. Clearance is a big issue when boring , especially when boring smaller holes. I regrind my carbide boring bars to increase clearance. I grind around a70º angle between the top face and the side face of the carbide. Check in the bore. I also angle the boring bar slightly towards the front so the heal of the carbide isn't dragging. Finally, rotating the boring bar or raising slightly above center can buy some clearance. This is sometimes necessary when using insert tooling as it isn't practical to regrind the inserts.
 
+1
The brazed carbide tooling needs plenty of fine tuning/grinding to produce a good result.
I think many of us have purchased those sets over the years. They can be made to cut quite well.
You’ll get there, sounds like you are making good progress.
 
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