View Full Version : Flat bottom bores.

02-26-2002, 06:36 PM
I have been reading another post about boring heads and was wondering if anyone had any suggestions about how to bore a perfectly flat bottomed hole with a standard boring head in a milling machine. I know it can be done with an automatic boring and facing head but what is the best way to do it with a plain boring head without getting a lot of chatter at the bottom?


02-26-2002, 11:03 PM
I've been wondering this myself. I believe achieving a flat bottom with accurate depth using a boring bar is a difficult operation. What I do is drill a hole in the center which is slightly deeper than the depth of the bore that I need. I progressivley bore out the opening, but going about 10 thou less than the required depth. Once I've got an accurate bore opening, I then work on accurately "turning" the bottom of the bore from center out, and the extra depth that I drill intially make this easy. Unwittingly, you may cut a wider bore at the bottom, but for most applications, this is not important. Another way woul be to bore, and then machine the bottom accurately using an endmill and a rotary table, but alignment and the sides of the endmill scraping the bore opening may be a problem.

I'd be interested to hear how the pros do it without the use of a specialized tool.


02-27-2002, 01:35 AM

As far as using a boring head goes - it is just a matter of selecting the proper boring bar designed to make a flat bottomed hole. Travers, KBC, and most likely all the other majoe suppliers have them - just check the catalog. As in all things, some minor comprimises may have to be accepted - a blind hole like this is most likely for a bearing or a sub assembly and only needs to be flat near the edge (usually as a stop or reference register). Going to the extra effort to make an perfectly smooth bottom is a waste of time in most cases (better to have a little clearance space instead for the shaft, etc.).

The easiest way is to use a CNC mill. Threads, for instance are milled with CNC machines on odd pieces at a very low cost (except for the CNC mill and the single or multi-point thread mill). So holes are a breeze!

To give you an idea how cool this really is, a California company uses Haas mills to manufacture pure teflon acid pumps for the semiconductor industry. Zero metal is used in these as metal contamination as low as 1 part in a billion can ruin the wafers. To make a long story short, a lid screws onto the body - a thread is present just under the edge - a riser about one inch farther in is threaded on BOTH sides. The body is threaded to mate it. If you think about this, getting the threads all started in the right position would be impossible without computers! I am impressed! This story was in a past issue of their CNC magazine - you should be able to read about it or find it at www.HaasCNC.com (http://www.HaasCNC.com)

Sorry about going off topic - again.

[This message has been edited by Thrud (edited 02-27-2002).]

02-27-2002, 11:33 AM
Most perfectly flat bottomed holes are a wild pipe dream of some dimwhitted engineer or draftsman. It's easy for them to draw them, should be no problem for the machinist, right.

Those guys need to get out into shop sometime, and see if they can make what they draw up, a reality check of sorts. Lots of them don't have a clue.

Kind hard to do a flat bottomed hole even with a automatic head, since the head won't let tool go to center.

Mill out bottom square with cnc, then you will need cutter less than 1/2 diameter due to end angle of endmill. Been trying to educate boss on this point. Been doing some counter bores with cnc, endmill needs to be small enough to clear center hole as it mills circle, and go around more than once.

kap pullen
02-27-2002, 02:03 PM
Use an end mill and rotary table.
Or bore it, and leave a little relief in the bottom corner.
Then you can carefully mill the bottom out to the relief without touching the side.
A fllat bottom boring bar will likely chatter.

02-27-2002, 03:08 PM
I'm not a pro, but...

I ran into the problem just this morning. I bored the hole using my lathe.

Otherwise...the rotary table idea is good. Getting a flat bottom with just a boring head is pretty difficult. I think Kap's right -- a flat-bottom boring bar is going to chatter, as soon as it's cutting any signficant width.

02-27-2002, 05:05 PM
To help eliminate chatter when doing a flat bottom hole with a boring head, your rpm's should be lower when you hit the bottom of the hole....If you are doing aluminum, try going really slow, like maybe 80 rpm's or so ...what happens is the tool "peels" the aluminum, and gives a mirror like finish...but it works almost as good in steel...try experimenting with negative and positive angles on your tool...negative angle will tend to "scrape" which has a strong tendency to chatter, while a positive shear cut will shear the metal beautifully, but becareful not to "dwell" at the bottom of the hole...Dwelling seems to make things chatter a lot for me...You should cut the bottom of the hole and then get the heck out!!!


02-27-2002, 07:56 PM
Speaking of boring flat bottom holes, I have a part to make for a small Stirling engine that I am making, that has me scratching my head. It is the displacer tube made of aluminum and weight is an issue. This thing in a closed end tube .965 OD and .928 ID (.0185 wall) with a flat closed end that is .040 thick. The length is 3.61 inches. Instructions say make from solid bar. What do you do first and how do you hold a thing like this? Also, I don't have CNC tools.


02-27-2002, 08:26 PM
I made one of those for a different engine. I cheated, and got a piece of aluminum tubing out of the scrap bin, trimmed it up for the body, then made end caps. Threaded the rod end , and put a small step in it to hold the back head of the displacer. Screws together tight.
Lots less awarf running around that way, and it seems to work just as well as if I made it solid.

02-28-2002, 01:14 AM
That's just crazy, man! Use a tube with an endcap - loctite it on. Or if it is used in high temperture epoxy it, pin it, dinky wee screws, press fit. What ever works will be just fine...

It takes an engineer to dream up something stupid like that (probably from Harvard). As I said usually you just need a register near the edge - the middle of the hole really should be slightly recessed for shaft or bearing clearance - real need for a flat bottomed hole is rare and usually just the result of poor judgement on the part of the designer or just cosmetic in nature (and therefore can be ignored usually).

02-28-2002, 11:44 AM

You make a very good point. I'm an electrical engineer by education and profession, and I've taken mechanical drafting courses in school. It wasn't until I started to machine as a hobby that I realized how poorly we were trained as engineers for the real world. It's so easy to draw splined cruves, sharp edges, deep holes with all sort varying internal dimensions. It looks pretty on the paper but it no relationship to the real world where things need to machined with efficiency and economics in mind. Needless to say, learning to machine myself have helped me open my eyes to engineering in whole new way http://bbs.homeshopmachinist.net//smile.gif Happy machining.


metal mite
02-28-2002, 08:58 PM
I'd bore your piece first and flat bottom it(if you really have to have it that way).
Use like 1.125 material.
Turn the first 5/8 or so to finish o.d. size.
Make a neat fitting plug to fit inside.
Insert the plug to the bottom, and chuck on the plug and tube in a collet. Gently turn the rest of the od to blend the finished portion.

Or turn bore the tube leaving lug in the bottom.
Counterbore the bottom end to fit your plug (a little deeper than the plug is thick).
You can now ROLL the tube over, with a ball bearing, to retain the plug kinda like a tin can.
Probably need to make a mandrel to support the thing while rolling it.
Might need a little sealant in there too(teflon tape maybe).


03-01-2002, 04:24 AM

That is quite refreshing to hear an engineer admit that! And a very good observation. I am glad your machining experiences have had a positive impact on your way of thinking. I find there is always at least three ways to look at anything: your way, their way, and the "right" way.

Our local University works on hybrid/alternate fuel vehicles. The Professor in charge makes the students crawl under the cars to experience mechanical enigineering at its finest. It is quite funny to hear all the cussing and swearing from the students as they try to remove components. The Professor then reminds them "remember this when you are working for Ford/GM/DB Chrysler - now you know what mechanics have to put up with every day!"


03-01-2002, 04:42 PM
Come on you guys, you are giving mechanical engineers a bum rap. All of the mechanical engineers that I know have been building things since they were kids. Our criteria is, could I make this part in my garage on a lathe and a Brigeport. Actually, the machining people are way ahead of us.They can do things that I never would have dreamed of a few years ago. Very handy when you can't do it on your lathe and Brigeport, but it doesn't pay to stray too far.The only thing I will admit to is that the people I know were building things in the 30s when you built it yourself or did without. Old farts uber alles.(Idon't know how to put those little marks above the u)

03-01-2002, 09:16 PM
you don't need to put any umlauts on the "U" .....we jamaicans know what your talking about...thanks Mon

brent mon

03-01-2002, 09:22 PM
It is good they have been building things for so long - did any of them work? I hear Lego's can be quite the challenge when you have your head firmly lodged up yer butt (must be from Harvard).

I met a good engineer once - when he was wrong he was quite CIVIL about it! (har-har)

This has been a paid announcement from the "pick on engineers" foundation.

03-02-2002, 12:11 AM
That's ok thrud. In their capacity as the official receivers of all blame,engineers perform a very useful service to humanity. And they can take comfort in the fact that they have room to grow and improve. Perfection,after all, is a dead end.

kap pullen
03-02-2002, 01:34 AM
There's an exception to every rule.
You must be him.

03-02-2002, 09:25 AM
Engineers come in two flavors.
One is the kind you find here. We have actually made things with our own paws. Sometimes you can actually make <the product/item> the way we say to do it.

The other flavor is a book-larned type, who is very good at theory, but *a bit less able* at real, actual, physical items. If one of these offers a suggestion on how to actually do something, you probably can't do it that way!
(Apparently many application notes are written by this second type)

03-02-2002, 11:42 AM
I think it is the duty and responsibility of machinists everywhere to help educate engineers. Remind them of important things such as coming in out of the rain.

I have met a few that impressed me, some others I've just about got trained. One engineer at work gives me these vague prints, he tells me to just make it work. I appreciate that.

Then there are the Junior Engineers, often College students, I've had some fun with these kids. When I was going to Junior College I could have made it into the drafting and engineering department of a local plant. Mininum wage, not for me, I was running piece work at a local job shop making quite a bit more. They probably wouldn't have let me wear bib overalls either.

One always wonders what if, about career moves. Wish someone had made me take Calc back then.

03-02-2002, 12:09 PM
I agree with all of the talk about engineers...

Personally, I have a Mechanical Engineering Degree. But I choose to Machine instead....I started machining when I started going to school...I really wanted to be an engineer because of my strong math background, so I started machining and I just loved it...so when I got my degree, I just put it on the shelf and continued to machine...I know a lot of engineers because I went to school with a lot of them, and I wish they knew something about machining, because it would be a lot easier on machinists......


03-02-2002, 12:32 PM

I have a similar background. I have a B.S. degree in Engineering Physics. I didn't go to college right out of high school but worked as an automotive machinist for about 3 years before starting to school. After I graduated, I worked for Ingersoll-Rand as an engineering co-op. I quickly realized that I didn't like the "Corporate" atmosphere of a large company. I just wanted to be in my own little corner of the world crunching numbers but noooo...I had to fit the mold and play the game. That is why my degree is sitting on a shelf at home and I went back to the machining trade. I now teach machine shop at a local vocational school and don't see myself straying too far from the trade in the future. It has been too good to me and I thoroughly enjoy it! The only thing I ever saw an engineer do with his hands was swing a golf club!!!

I will step down from my soap box now.


03-02-2002, 01:27 PM
At the risk of becoming a flat bottom bore myself, I have to put my final 2 cents in here. I have a little list of those who wont be missed, at least by me.
At the top of the list, of course, are the big wheels who have never designed anything and wonder why it takes we knucklhheads so long to do the job.And then there are those who may have been pretty good design engineers in their day but have been promoted to their level of incompetance and the guys who have always worked with computers and accept any number the conputer lays on the as gospal then they wonder why the damd roof collapsed at the firs snow.And the draftsman who can't do simple arithmatic, specializes in dutch projections and never saw a drafting convention he couldn't flout.After all those, the drawings go to the shop and old Joe will spend the first couple of days trying to find a mistake.if he finds one you have to go down to the shop an let him tell you what a bunch of dumbheads engineers are, and you say yeah Joe ,thats a mistake and I'll tak care of it. And if you move real fast, you wont have to spend the next 20 minutes listening to old Joe tell you how the part should have designed. It isn't that old Joe's ideas wont work, but it's a little late. Worse than that, if you try to get oldJoe involved early, he will probably say that that isn't in his job description. Have fun you guys.

03-02-2002, 02:56 PM
I have to add one more note to the above diatribe. What I have said applies to big companies with captive shops. For the last 20 or so years, I have had the priviledge of working with a couple really top flight outside shops. They don't have time to play gotcha. If you explain the requirements and ask for their input, you get it, and if for good and sufficient reasons you can't do it their way, they say ok and get on with it. Gotcha is a tremendous waste of time and talent.

03-03-2002, 12:12 AM
Having "wasted" much time at university, trade school, and some jobs I have come to the jaded conclusion more thought is put into "politics" and far too little into actual solutions.

I have a book titled "To Engineer is Human". It documents failures like the Tacoma Narrows Bridge. Anything serious happens and an engineer is on the hot seat. But they are not the only ones to blame - a professor once told me "That's the way I learned it, and that's all you need to know!" So much for the scientific method and discovery. I do not have time for closed minded nincompoops. Oh sure, they may have been "brainwashed" into doing it the traditional way, but that is no excuse for being buttheads to those "below" them...

That is what is wrong with (most of) the "professionals" - poo for brains.

And to bore a flat bottomed hole in wood use a forstner bit (just trying to stay on topic)!

03-03-2002, 01:40 AM
Thrud , my daddy who had a pile of assigned heat transfer patents , once told me that the study of engineering was diametrically opposed to creative solutions,it only taught the status quo ,...incidentally , he was highly ticked when M.I.T. dropped shop for me's . i saw the letter he fired off to the dean ...i have mentioned it before ,but in 1945 , i saw engineers come into the shop swearing, putting on aprons & running the part that someone said could not be done ,...& i have seen senior machinists come into my dads office mad as a wet hen asking ,who the he-- drew dis tam ting?......( & it WAS redrawn)......different times, different folks..2000 employees ...went thru depression w/out a layoff.....cut back to 3or 4 days so nobody lost his house....& that was when u needed a %40 to %50 down payment!!!!
best wishes

03-04-2002, 12:36 AM
I think I like your dad - it takes moxie to ream the Dean out!

I wish there was more hands on experience for the professionals and more theory for the trades people - a meeting of minds half-way. Would not hurt either party and give better perspective of the others day to day problems. Cross discipline experience would benefit everyone. It will never happen - too bad.

I have a dream...

08-04-2006, 08:12 AM
What about a D bit? That should work. Vic