Boring flat bottomed holes?

[TroyO]

I recently got my first shot at using a boring head in my mini mill. It worked, after a fashion but I have some questions.

I was trying to bore a flat bottomed recess for a large nut (Mini mill column nut to be exact.) and I was able to complete it but I chipped one carbide tipped boring bar and had a lot of chatter in the process.

I started by drilling a 1" through hole then set a depth stop and started by taking small passes further and further out by adjusting the dial on the boring head. After a few passes and getting progressively further out it would bore OK until I hit the bottom where it would chatter like a banshee.

So, specifically I am wondering what angle do you set the head of the boring bar in to the boring head at? I tried to keep it as parallel to the boring head travel as I could eyeball... is that right? Is there a better way than "That looks about straight"?

Finally... any tips for reducing the bottoming out chatter, or ideas what I was doing wrong in the first place? Maybe you aren't even supposed to flat bottom by boring in the first place?

Any other tips for boring operations?

 

[doc1955]

You should flatten bottom out I usually grind up a flat bottom drill and go in and flatten out the bottom.
Funny you should post this I'm about to start the hot cylinders for the engines I'm building and that is what I'll be doing to me it is the simplest thing to do and I just hate to chip up my favorite boring bars.

 

[techonehundred]

I just got through doing this for a stirling engine. I was doing the displacement cylinder which was .950" id and 2.875 deep with a flat bottom. what I did was to drill my hole to depth with the largest drill I had. then, I was given a 7/8" center cutting endmill. It had a 3/4" shank which worked in my quick change tool post. and I used that to give me a big enough opening so that I could finish it with my boring bar. This worked great.

 

[gbritnell]

Hi TroyO
It's just a characteristic of a flat surfaced cutter. When I worked on very large and rigid machines the same thing would happen. The only way you can prevent this from happening is to slow the speed way down when you're near the bottom. Once the chatter is put into the surface it's darn near impossible to take out.
gbritnell

 

[Mainer]

A truly flat-bottom hole is difficult to achieve. I generally make-do with the result I get from first drilling a hole so the tip of the drill is at finished depth or slightly beyond if permissible, then plunging to finished depth with an end mill of appropriate diameter.

In your case, where the hole is apparently "large" or at least you don't have an endmill of that diameter, I'd use a somewhat pointed boring bar with only a small cutting area on the front, and enlarge the drilled hole in a series of steps using the boring head, paying careful attention to depth so all the cuts were the same depth. You'll end up with a slightly rippled bottom to the hole, but if your steps are "small" it will be pretty flat.

It might be easier to bore on a lathe, where you can feed a toolbit across the bottom of the hole to make it flat.

And there is always the trick of drilling a through hole then silver soldering on a plate to close the bottom of the hole.

 

[Ken I]

You get a boring and facing head

http://www.lighttoolsupply.com/catalog/Manufacturers/Narex/Narex-Universal-Boring---Facing-Head-Sets?productID=3649

This can traverse in order to face the bottom of bores or to perform undercutting and circlip grooves on a mill.

It has a ring which when held stationary (generally a rod bearing against some fixed surface or stop) causes the internal mechanism to traverse the boring bar out towards a pre-set stop.
You then reverse to unwind or stop and move off centre to extract the tool.

Not something found in most machine shops let alone home workshops.

I've used one - a nice piece of kit - particularly the very fine radial adjustment it provides.

Ken

 

[Loose nut]

A bit expensive to buy for a home shop but if you have access to old Model Engineer magazines they did have a facing head, a star feeder type, that did the same thing that you could make. From the 1970's I believe.

 

[Stan]

My guess is that you have no relief on the end of the boring bar. If there is no relief, it will rub on the bottom of the hole, causing chatter. The cutting should be done by the edge of the bar, not the end. A properly ground and set boring bar will cut each successive cut a few thou deeper than the previous cut if you don't have a stop set to limit the depth.

 

[doc1955]

You get a boring and facing head

http://www.lighttoolsupply.com/catalog/Manufacturers/Narex/Narex-Universal-Boring---Facing-Head-Sets?productID=3649

This can traverse in order to face the bottom of bores or to perform undercutting and circlip grooves on a mill.

It has a ring which when held stationary (generally a rod bearing against some fixed surface or stop) causes the internal mechanism to traverse the boring bar out towards a pre-set stop.
You then reverse to unwind or stop and move off centre to extract the tool.

Not something found in most machine shops let alone home workshops.

I've used one - a nice piece of kit - particularly the very fine radial adjustment it provides.

Ken

We have boring heads similar I believe they are a walhopper not sure of spelling but they can face a bottom and also cut grooves and also cut tapers. They are extremely nice boring heads but I know they are totally out of my budget. I have either ground a flat bottom drill or used an EM but one thing with an EM they are not flat on the bottom. As stated once you get chatter in the bottom of a hole it is tough to get rid of.

 

[o.h.cam]

Yes the Wahlhaupter is a great tool.Used them occaisionally on the boring mills I used to run.Price is a little too prohibitive for a home shop but I have seen a few used ones on some surplus tool sale sites

 

[TroyO]

Hmmm, maybe I did better than I thought to get it done at all then.. LOL.

I ended with a decently flat bottom at the cost of 2 dirt cheap carbide tipped boring bars, so I wasn't particularly unhappy with the state of affairs, I just figured I had done something wrong.

The hole recess ended up being 2" or so, about 1/2 inch deep. There were some chatter marks in the recess but it was flat enough to do the job.

I do have a lathe but it couldn't swing that big of a plate around. I like the idea of that facing/boring bar but at the same time I don't see it coming up often enough to be worth the investment.

Overall... the carbide tip does go as close to parallel as possible? Or do you hook it "in" at a slight positive rake? (I do that sometimes on the lathe for larger holes).

Ohhh and one last "dumb newbie question" (LOL... don't fall for it, it's a trap.. I WILL ask more. ;-).... How tight should the boring head gibs be set? I snugged them up so a decent amount of force was needed to turn the adjusting screw but I didn't totally lock it down after each adjustment. Should I have fully locked it down after every adjustment?

Overall.. here is a pic of the recess in question... it worked so I will call it good.

 

[hammers-n-nails]

i usually set the gibs just tight enough that i can definatly feel the backlash in the adjustment screw then tighten them down pretty good for cutting. i think getting about the same tension on the adjustment screw each time is pretty important when trying to get an exact diameter.

 

[pete]

TroyO,
Your boring head must be locked down after adjusting. Since I don't know what boring head you have, I'll assume it's a well designed one that you bought. The set screws on the gib strip should be adjusted to give you a smooth movement when using the dovetail adjustment screw but there should also be no excess play. After you've made the adjustment to set the boring head to your next cut you use the center set screw on the gib strip to lock the head from further movement. This is the only screw you use to lock and unlock the boring head dovetail. The other two set screws are only for doing infrequent adjustments to the gib strip to compensate for wear. Try Googling "Criterion boring heads and shanks" There's a lot of very good and free information on that site about precision boring from a very well respected manufacturer of well made boring heads.

Pete

 

[TroyO]

Awesome, thanks for the info. So, just leaving the dovetails tight-snug isn't enopugh. Next time I'll be sure to pop that center screw tight each time for a full lock.

The boring head is from Grizzly, I think... (I got it a while ago but haven't used it.)

http://www.grizzly.com/products/2-Boring-Head-Combo-Set-R-8-Only/G9322

It seems decently made, although maybe some hand fitting/polishing of the dovetails wouldn't hurt. (It hasn't hurt any other tool I own, LOL) As to well designed... well, I don't really have a point of reference to go on but it seems suitable to the tasks it will find on a mini mill.

 

[pete]

TroyO,
Nope, The head has to be locked to prevent movement. You have backlash for any type of screw that's used to provide movement. And locking the head is a definate requirement to get your bores to the correct size and a good surface finish. You don't have to reef down on that center bolt like you were torqueing head bolts on a car engine. The forces for single point boring are usually pretty light, But tighten it up a lot more than just snug. I'd guess your head was built in China, India, or Taiwan but there's no excuse for the manufacturers not to include a proper set of user instructions. Good to see you spent the money and bought a boring head. IMO a person only has half of a mill without one. There a very powerfull tool with a few more uses than just boring holes. Once you get used to using it you'll wonder how you managed to go thru life without one.

Pete