Boring a tapered hole.

[Captain Jerry]

I bored a tapered hole on my mill using my new 2.5" boring head (cheap import) and I don't want to do it again. Where did I go wrong? I was just boring some test holes, trying to get aquainted with this new tool. The material was 3/4" thick 6061 aluminum, pre-drilled to 3/4" diameter. I was attempting to open the bore .001" at a time to get a feel for the adjustment. All was going well until I checked and found that one of three test bores was tapered about .002" over the 3/4" depth.

The table was locked and not moved after the original hole was drilled. The mill is an x2 and I suspect maybe some slop in the column gibs is at fault but it only happened on one out of three.

On anther point, the grub screw marred the surface of the boring bar. The depth of the threaded hole that holds the grub screw does not seem deep enough for a BB and there is no flat on the bar. What's the best way to handle this?

Thanks for any help you can offer.

Jerry

 

[b.lindsey]

Jerry,

Was the dovetail or moveable part of the boring head locked? Might help to know wich of the three holes was tapered....first, second, or third?

Bill

 

[Peter.]

Sounds like the tool rotated in the holder. Do the marks on the shank of the tool indicate that as a possibility?

 

[kvom]

How does the tool get fed in the Z-direction on your mill?

 

[Metal Butcher]

Some times a bore will end up 'looking' tapered based on a variable feed rate. If a pointed tip on the boring bar is used, and the quill is lowered by hand it can miss and leave uncut material behind giving the impression of a taper caused by other reasons. I check the bore from top to bottom using a telescopic gauge, and repeat a cut without advancing the cutter to clean up anything that can be missed using a variable hand produced feed rate.

I never break the set-up and remove the work piece to check the bore. That's just asking for trouble.

Edit; I Hate to admit it, I recently bored two holes through 3/4' material and one checked out showing an hour glass shape with a small diameter in the middle. I re-cut the bore with out changing anything, and the middle area opened up to the proper dimension. The only conclusion I could arrive at is that my feed rate was a little faster half way through the bore, and the results would have been better if I had bored the holes using my lathe.

-MB

 

[Captain Jerry]

Thanks for all of the useful replies. I have a lot to learn about this tool.

Bill, I think that all three screws on the dovetail were tight. I was being carefull and haven't had enough use of this tool to get sloppy with familiarity. But maybe....... That would cause a taper wouldn't it?

Peter, The tool could have rotated. I didn't like cranking down on the grub screw and messing up the shank. That could have caused the taper.

Kvom, The Z feed on the X2 mill uses a rack and pinion on the column. You can engage a dog clutch on the feed lever to use fine feed but its one of the main faults of this machine. I used both manual feed and fine feed in my trials.

MB, I don't have a complete set of measuring tools and checking the bore at the bottom is not easy. I made some more holes today and the goal was to bore holes that would be a thumb press fit to a piece of bar that I had turned and measured to .825". I didn't get any taper today but I found it difficult to get the fit I wanted. There is a LOT of backlash in the adjustment screw. I finally resorted to setting up a dial indicator against the tool tip and was able to get adjustments of about .001" at the tip, (.002" in the bore) but trying to sneak up on the .825" was beyond me. I overshot every time.

I was using a fairly slow RPM, just a bit above chatter speed. Using the fine feed it is almost impossible to get a continuous feed from top to bottom. To improve the finish, I increased the RPM an used a slow steady manual feed on the up stroke. Just taking the spring out of the bar. Near mirror finish but straight and oversize.

Is it possible to get .0005" adjustment on these tools and if so how?

Jerry

 

[Metal Butcher]

MB, I don't have a complete set of measuring tools and checking the bore at the bottom is not easy. I made some more holes today and the goal was to bore holes that would be a thumb press fit to a piece of bar that I had turned and measured to .825". I didn't get any taper today but I found it difficult to get the fit I wanted. There is a LOT of backlash in the adjustment screw. I finally resorted to setting up a dial indicator against the tool tip and was able to get adjustments of about .001" at the tip, (.002" in the bore) but trying to sneak up on the .825" was beyond me. I overshot every time.

Is it possible to get .0005" adjustment on these tools and if so how?
Jerry
[/quote]

Jerry, Ignore the back-lash. To advance the cutting edge of the boring break loose only one set screw and snug it back up. now repeat on the next screw, and finally the third one. Turn the calibrated dial clockwise to the calculated mark against the pressure that the snugged set screws provide. Now you can tighten up the three set screws. If you pass the mark on the dial when your advancing the bar, then you need to back up past the back-lash and start all over again by advancing till you can get a light cut, re-measure, and advance the bar the appropriate amount. Trying to back up and re set the bar to your original dial setting rarely works due to the back lash. You can get the line on the boring head in between the calibrated lines on the dial, and cut an additional 5 tenths if needed along with your final cut, ie .0035".

I'm curios, what are you using to measure the bore?

 

[dreeves]

When I bore I always do a spring cut. As the others said cut a second time without moving the adjustment on the holder. Its amazing that it will still cut a little more on the second cut. I also will sneak up on the finished size in .001 to .002 cuts. Just my 2 cents good luck and keep at it.

Dave

 

[Captain Jerry]

MB

I don't have a precision bore gauge. My method today was to turn a short piece of brass bar to the desired diameter and measure with a micrometer. With the compound angled at 30 degrees and using cosine offsets, I was able to hit .8250" on the bar and tried to sneak up on it with the boring head. As it turns out, this is only a no-go gauge so as I snuck up on it, it was no-go, no-go, fall through. Tomorrow, I will make a new gauge with steps of .8250", .0005" under and .0015" under. With this gauge, I should be able to get better control as I get close.

Jerry

 

[Metal Butcher]

MB

I don't have a precision bore gauge. My method today was to turn a short piece of brass bar to the desired diameter and measure with a micrometer. With the compound angled at 30 degrees and using cosine offsets, I was able to hit .8250" on the bar and tried to sneak up on it with the boring head. As it turns out, this is only a no-go gauge so as I snuck up on it, it was no-go, no-go, fall through. Tomorrow, I will make a new gauge with steps of .8250", .0005" under and .0015" under. With this gauge, I should be able to get better control as I get close.

Jerry

Sounds like you have a good approach by making and using gauge rods. I haven't tried that method. I use a 'good' set of telescopic gauges that work for me. I measure the gauge with the quick and dirty dial calipers, and switch over to using a micrometer when I get to about .002" of the target size.

Precision bore gauges would be great, but their far too expensive for me.

-MB

 

[Maryak]

A good spring type inside caliper will get you within 0.0005".............When I say good I mean one that is free and doesn't go boooyyiing every time you move the adjusting screw :.............. and the ball ends are balls, not a series of bumps

I have to agree with MB telescopic gauges are the "ants pants" for measuring bores, (I'm a cheapskate who likes a challenge ;D).

Best Regards
Bob

 

[Mainer]

How is the boring head held in the spindle, a taper shank to fit the spindle, or a straight shank in a collet? In my experience, a taper shank is a much more rigid setup that is less prone to flex and variation. When I changed from a straight to a taper shank on my boring head, I got better results.

I also tighten the gib adjustment screws before every cut. The routine is, loosen the gib screws so there is just a slight drag, make the cut feed adjustment, retighten the gib screws, cut, repeat.

 

[kvom]

I have seen a technique for setting the bore size using a v-block and height gauge on a surface plate. This assumes that the body of the boring head is round to the accuracy needed.

Place the head on the vblock and measure its height above the table. Subtract the radius of the body, and that's the height to the center of the body and thus the 0 point. Then measure the height of the tool tip.

My boring head has an R8 shank, but I've never been able to get accuracy better than .001, esp. with the brazed carbide tool I have I need a stiffer tool to get good results in steel. In aluminum it works OK.

 

[Captain Jerry]

Thanks for the replies.

My boring head is mounted on an R8 shank. I need to get a feel for turning the adj screw against the drag of the dovetail gib screws. I have seen the method of setting the diameter on the surface plate but with the limited Z height of the X2 mill I can't always get the shank into the spindle with the tool installed. I have to install the head and shank and then insert the tool.

I was at Cabin Fever last week and picked up a couple of telescopic gauges but I put them down and bought something else. DOH! I'll keep my eyes open for a good inside spring caliper. Ah, the challenge of cheap!

Jerry

 

[Metal Butcher]

Thanks for the replies.

I was at Cabin Fever last week and picked up a couple of telescopic gauges but I put them down and bought something else. DOH! I'll keep my eyes open for a good inside spring caliper. Ah, the challenge of cheap!

Jerry

Be careful what you buy. Buying 'cheap' can come back and haunt you. Filling in the gap with a missing piece can be expensive and difficult. And with used measuring tools your on your own with out the warranty that buying 'new' provides.

I bought a inexpensive 4 pc-set of 'work shop grade' squares that turned out to be a waste of my time and money.

I bought a better second set (USA in a 'shop hardened case') that was more money, still not good enough.

I bought 3 individual higher precision squares, they hit the spot with accuracy. For what I spent on the first two I could have bought the top quality ones in the first place and still had money left over!

When your ready to buy a set of telescopic guages I suggest buying the best quality. You will get a quality tool that's not only more accurate, but also a pleasure to use. And when you close-up shop they will have a very high re-sale value. And unlike a cheap cutter or drill bit, they won't be thrown away.

hint, hint.
http://www.google.com/products/cata...+telescopic+gauges&cid=15814160384991541721#p

-MB

 

[mzetati]

On anther point, the grub screw marred the surface of the boring bar. The depth of the threaded hole that holds the grub screw does not seem deep enough for a BB and there is no flat on the bar. What's the best way to handle this?
Jerry

sounds like You have hardened grub screws and unhardened shanks on the BBs. Been there, seen that.
I either filed a flat, or turned a groove in the 'already marred' ones. Had ideas about making myself a brass tipped grub screw for the boring head, but never got a round tuit.
Marcello

 

[LongRat]

CJ - one more point that hasn't been mentioned: Check your gib screws on the z axis of your X2. If you have any slack here, the head will rock right to left and you'll get an ovalised bore only made worse when you retract back up. When you are measuring be sure to check a few places around the bore at a similar depth to ensure roundness.

 

[Blogwitch]

Jerry,

Even though I have a fairly large mill, I still use an Imperial el cheapo 2" boring head that I bought many years ago, exactly the same as they sell nowadays.

I used to have the same sort of trouble as yourself when I used to use it on my mill/drill, that was until I started to use the fine feed. Using a slow counting method while turning the wheel, even when retracting, to ensure I had a constant feed, all my problems disappeared. A real PITA, as my hand used to ache really badly when doing a final finish skim, by slowing down the feed to about as slow as I could reliably go.

I am lucky in that I now have a power feed on my Z1 axis (table) and that allows me to bore up to about 3" deep perfectly, purely because it keeps to an exact constant feed. I have just bought a smaller (40mm diam) metric one, and I am hoping that should allow me to bore up to 5" deep if the hole is larger than 40mm.

The scale on the side of the boring head should be, one mark removes one thou from (actually adds to) the diameter of the hole, so by going to half a mark, that should give you the 0.0005" that you are after. Allowing you to get super accurate cuts. I used to use my old one even for metric cutting, and I could easily get within 0.01mm (0.0004").

Another problem might be the angle you are feeding the boring tip to the surface being cut. If the cutting tool is kept razor sharp (usually by grinding off the bottom of the tool), and keeping zero rake on the top (front) face it should be able to cut almost any metal. I have always set my boring tools with the top (front) perfectly parallel to the dovetails on the boring head. See C-o-C.

I hope this helps with your problem


Bogs

 

[Captain Jerry]

Thanks Longrat and Bogs

I think I got things under control now. When I get a new tool like this I like to get to know it and since these things don't come with any instructions, I just use the old "Scientific Method"... do something and see what happens. I had originally thought that orienting the cutting face to provide a little positive rake would make a better finish in aluminum but came to the conclusion that a zero rake works best. I have only used it on aluminum so I'm glad to hear that zero rake is right for other metals as well. Now that I know that, I will go ahead and file a flat on the shank which will solve the problem of scarring from the grub screw. I am a zealot about sharp tools. I believe that a keen, polished edge is the most important factor in any cutting situation.

I think that the taper came from my experimenting with the face angle. If the grub screw was not tight it could have allowed the bar to rotate as the load increased, going from a zero rake to a negative rake and less diameter.
It only happened once and hasn't happened since.

The Z-axis fine feed is a PITA on the X2 mini-mill. The knob is just over an inch in diameter and if the gibs are kept snug takes a full hand grip which then makes it almost impossible to see the dial. When I get around to it, I will replace it with a bigger knob with a handle.

Thanks for all the help.

Jerry

 

[Blogwitch]

Glad you now have things under control and have sorted your problems yourself.

Bogs