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Flat bottom bores.

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  • Flat bottom bores.

    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?

    "The truth is incontrovertible, malice may attack it, ignorance may deride it, but in the end; there it is." Winston Churchill

  • #2
    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.



    • #3

      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

      Sorry about going off topic - again.

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


      • #4
        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.


        • #5
          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.


          • #6
            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.

            Try to make a living, not a killing. -- Utah Phillips
            Don't believe everything you know. -- Bumper sticker
            Everybody is ignorant, only on different subjects. -- Will Rogers
            There are lots of people who mistake their imagination for their memory. - Josh Billings
            Law of Logical Argument - Anything is possible if you don't know what you are talking about.
            Don't own anything you have to feed or paint. - Hood River Blackie


            • #7
              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!!!



              • #8
                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.



                • #9
                  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.


                  • #10
                    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).


                    • #11

                      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 Happy machining.



                      • #12
                        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).



                        • #13

                          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!"



                          • #14
                            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)


                            • #15
                              you don't need to put any umlauts on the "U" .....we jamaicans know what your talking about...thanks Mon

                              brent mon