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Milling a slot

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  • Milling a slot

    Hello. I've got a real newbie question for you all. How do you go about milling a slot? I've got a HF44991 mill. I'm trying to mill a 3/16" slot in a piece of brass. The slot needs to be about 3/8" deep and anywhere from 3-5" long. I've tried with varying degrees of success, but none quite right. I've tried to plunge to the depth I need with a center cutting mill and then slow feeding the cutter at the full depth of the cut (if that makes sense) but I got a lot of chatter. So I tried to slow the speed down to about 500 rpm and speed up the feed, and that didn't work so well either. Lots of chatter and a lot of ridges in the cut. Then I tried to mill the slot in cuts of about 1/16" at a time but the bottom of the slot was offset a little, like 1/32" or so. In the machine shop trade secrets book the author said to mill a slot with an undersize mill, then go back with an on size mill, so I tried that and ended up with a broken 1/8" mill.

    Any suggestions? I know that the HF desktop mill probably isn't the best unit, but it fit in my budget, both price wise (My mom got it for me as a gift) and size wise (I live in an apartment) I am using a 2-7/8" screwless precision vise I got from enco and my mills are 4 flute hss. I also ordered a couple of 2 flute mill to see if that helps, but I haven't had the chance to use them yet.

    Also, can anyone recomend a basic book on using a mill? I've bought several books lately hoping to find some good info, but I don't really have anything on just basic use of a mill. Theres lots on lathes, but not a whole lot on mills. The machine shop trade secrets book has been the most usefull, but its more geared to pro's, so there isn't anything in there for a rank beginner like me.

  • #2
    You cannot use amill by drilling to appropriate depth and using it like a wood router even in wood you need to cut a little at a time same with metal try making sure everything is gripped tigh especially in the vice and take your time then make very light cuts and return to make more till required depth is met.Alistair
    Please excuse my typing as I have a form of parkinsons disease


    • #3

      Milling cutters are not- repeat not designed for slotting. They are designed to surface an article.

      A slot drill- is a slot drill!

      A 3 page dissertation on the subject is available. Geo H Thomas writing in the Model Engineers Workshop Manual by Tee Publishing as follows

      "Furthermore, the sides of the slot will be very rough, looking as though they have been nibbled away by rats"

      When you get your slot drills, your speed should be 4000 rpm!

      From one who has kept rats!



      • #4
        Yes, you need a slot drill. It is two flute and the cutting edges of the flutes on the face do not meet at the center. This allows for plunge cutting.

        This piece was milled with a slot drill using a milling setup in my lathe. Although it may look like the slot isn't the same depth all the way across it is, it's just the light.

        [This message has been edited by Evan (edited 06-18-2005).]
        Free software for calculating bolt circles and similar: Click Here


        • #5
          If at all possible I always try and use a horizontal saw type cutter for slotting.
          Much better results can be obtained.

          It all depends on what equipment you have available though.



          • #6
            Thanks for the quick replies. I had never heard of a slot drill, and I just did a quick google search to see what I could find. The first 5 pages of hits I checked didn't come up with a single US supplier I could order from. At least not that I could find. Any recomendations on where to find some? I also tried the usual places I order stuff from, like Enco and MSC and no go for them either.

            Thanks for the help, I'm a total newb and I'm learning by doing and reading as much as I can.


            • #7
              It is a UK term. I'm in Canada in a very small town in the west but I can get Dormer slot drills locally. Try someone that has Dormer products.
              Free software for calculating bolt circles and similar: Click Here


              • #8
                Are the gibs on your mill adjusted fairly tight? Does the table shake? if it does then you may need to tighten the gibs. it is an adjustable strip that runs in the dovetail that the table slides on ther is one for each axis. They are normally a screw that you adjust which pushes the gib strip in or out. the gib is tapered (normally) so that is how the slack is adjusted. Might not be the problem you have, but worth learning about if you want nice looking work.


                • #9
                  A two flute end mill is what the British refer to as a slot drill. A four flute end mill can also be used for slotting. Either a two flute or four flute can be used for plunge milling if they are center cutting. Most endmills sold these days are center cutting.

                  A 3/16" slot 3/8" deep is quite deep for the diameter, and the mill will flex if forced too much.

                  Get the speed up to 2000 RPM or better, if that is possible, and only cut about 0.050" or less depth at a time. Feed the table slowly. Make sure the gibs are tight, the table should feed smoothly with no side to side movement. Lock the table movement not being used and the height movement.

                  If it is a blind slot, it will be difficult to clear the chips, vacuum them out, or use low pressure air to remove them after each cut.

                  The same advice applies to the 1/8" mill, and it would be best to mill an undersize slot first and finish to final dimension with the 3/16".

                  [This message has been edited by JCHannum (edited 06-18-2005).]
                  Jim H.


                  • #10
                    JC is correct.Look at any endmill catalog and look for center cutting endmills.As stated,most are that way .


                    • #11
                      The best way to cut a long kerf is with a slitting saw. I only use end mills when I have a relatively short piece of work that must end abruptly and I cut no more then 1/10D of the cutter on each pass. So a .25 kerf would mean .025 depth of cut.
                      If you use an end mill use one with LESS diameter then your slot. Make your initial cut then clean up the remaining .005 or so on each side in a separate pass. This will give you the nicest cut.
                      On small machines don't do a 'climb' cut, most are not rigid enough.


                      • #12
                        Just a couple of questions here; Are you using a collet to hold your end mill or are you chucking the cutter in the drill chuck? Do you have every thing shortened up as much as possible? If your milling 3-5" and only have a 3" vise are you experiencing the part moving,vibrating, while outside of the vise jaws?
                        As for books doesn't Sherline or Taig have a micromachining book that is highly regarded. I've not read it but others have spoke of it.
                        no neat sig line
                        near Salem OR


                        • #13
                          Thanks for all the input. I've got a lot of learning to do. I haven't checked the gibs, but I've not really noticed any table shake at all. Its a good thing to know.

                          I am using collets with my mill, the first thing I did when I got it was take the chuck out and put it in a box.

                          I think that part of my problem is that my vise might not have been clamped to the table tight enough. I have more strap clamps now, so I'm going to add a couple and see how that works.

                          My machine can do 2500 rpm I think, so I'll turn it up and see how that does.

                          The most successful try so far did look like rats had nibbled the sides away. I want the sides smooth though.

                          I bought a slitting saw arbor and some blades and I thought to try that also, but one end of the slot is closed.

                          I picked up the desktop machining books from sherline and they seem to mostly be advertisements for their products. They do have some usefull info, but nothing on making slots.


                          • #14
                            Everybody has pretty much abandoned the real answers to this question. Shapers and horizontal mills.
                            THAT OLD GANG 'O MINE


                            • #15
                              A two flute slot drill is not a two flute end mill. The flutes on a slot drill are asymmetrical. This can be plainly seen in this photo of a Dormer 1/4 inch slot drill that I used to mill the above example. The slot drill and the end mill are not the same.

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