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Not the master machinist I thought I was

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  • Not the master machinist I thought I was

    I recently got sick of the compressor on my prehistoric Cat 922B loader putting copious amounts of engine oil into the air tank, so I decided to do something about it.. I pulled the compressor off and as expected the bores were heavily scored and the pistons were also scored..One a lot worse then the other... I initially thought oversize pistons would be the go, but while the service manual mentions them the parts manual makes no mention of them..

    A phone call to the aftermarket/wreckers parts supplier and yes they had a "good" compressor in stock and for only $850..OK, maybe not...

    So why not bore it out and make up two new pistons and rings to suit, after all I am a master machinist in my own mind....

    I have done a fair bit of machining but usually that is usually non accurate work or plain but accurate simple work..A shaft here and there, a stub axle or four, keyways, threads, just basic stuff..

    I bored and honed the bores first..A simple job and quite easy to do...Next was the pistons..First up was measure the old piston and get some drawings..I use the free solidedge 2D program for my drawings..I find it quick and easy to use and quite powerful...

    I already had the CI rod in stock so chucked it up in the four jaw (because my 3 jaw does not have a 50mm bore), turned it to near size then sneaked down to the required size, the first one I got spot on....Next was the grooves for the rings..A shaped HSS tool did the trick but since I have no DRO on the lathe I relied on the top slide to get the grooves in the correct spot, tricky to keep remembering how many turns you have done to get it to the correct position... The oil ring groove has a taper below it and as I have no way of measuring the taper I guessed it and it seemed OK...

    I parted it off and started the next....It went pretty much the same but I made it a tad smaller then I would have liked...Still within the tolerance the service manual gives but only just..

    Next was boring out the inside of the pistons..Disaster struck me there, upon reading the diagram I drew I went about boring it out to what I thought was the correct diameter, only to realise I had read the wrong figure on the drawing...The old saying..."I drilled it twice and it is still too big" came to mind..

    A few choice words later and a cup of coffee and a check of the emails and the HSM site I was cooled down and went back and made a new piston.. It went straight forward but again I went took too much off the OD then I had liked...In fact I made it the same OD as the one I stuffed up....This working to fine tolerances is hard when you are relying on just the dials of the lathe..

    Next was milling out the bit for the con rod... Something I believe I only wasily accomplished because of my new DRO on the mill....It made it super easy to find where I wanted to cut..I am still very much an amateur on a mill, a lot more knowledge is needed for one of them then with a lathe..

    Next was boring the holes for the gudgeon pin (wrist pin for the US) Here I struck the trouble of lack of tooling..I had the required adjustable reamer 7/16 was the size of the needed hole..But I had no collet to fit the initial drill and had to go with a ISO30-->MT3 adapter and then a MT3 shank drill chuck..So it ended up about 6 inches long without a drill, which did nothing positive for the runout...

    Anyway I stuck the piston in the vice, got it lined up with the help of the DRO and drilled the first hole....I then chucked a small boring bar to enlarge the hole the tiniest amount so the reamer would enter and do it's stuff ( I really have to invest in a proper drill set) I had done several test holes previously to this and it had all worked out well but murphy intervened and for some reason my "drill" took out more then expected and when I used the reamer it took out bugger all metal..Upon testing fit with the gudgeon pin it was snugish but there is a tiny bit of will be loctite to the rescue when it comes to fitting. The second one went much better and turned out as expected..

    The little 1/16th holes was the next challenge..There is six drilled in below the bottom ring for what I presume is oil holes...They are on an angle...measuring this angle to what I thought was 33 degrees I tilted the mill head over..Mounted the rotary table on the table and chucked up the piston... When I got them all drilled I realised the angle was far too shallow and that it looks to be closer to 45 degrees..oh well, it should still work...

    Anyway I only have the rings to do now and I will do them later this week or on the weekend..What this has taught me is the skill needed to make accurate items is not something easily attained..I thought I was fairly good with a lathe but working to thousands and sub thousands does take a lot more skill then you think..These pistons are a bit rougher then I hoped and it is yet to be seen if they will last 1minute, 1 day or 10 years..

    I am sure you can work out which one is the home made one..

    Precision takes time.

  • #2
    I'll be interested to see how this continues.
    Making pistons is not trivial, they are normally both a little oval and a little barrelshaped.
    But let's see.



    • #3
      Keep it up.

      Good job and good write-up Ringer.

      You sure do dive in at the deep end don't you?

      Please keep us posted as to progress and eventual outcome.


      • #4
        Originally posted by Benta
        I'll be interested to see how this continues.
        Making pistons is not trivial, they are normally both a little oval and a little barrelshaped.
        But let's see.

        I think that is more relevant on internal combustion engines, where the latent heat will be very much higher, and is used to compensate for the different rates of expansion between the thin walled piston, and the thicker wall sections around the gudgeon pin, and the greater expansion due to more heat on the piston crown.

        Nice work Ringer, keep us posted.
        I've been doing exactly the same for the past few months, but on miniature compressors for instrumentation, making pistons and cylinders with a 17mm dia and 10mm piston length. However, running speed on these little things has been around 4000rpm and they run unlubricated, apart from a drop of oil on assembly. It's been an interesting R&D exercise.



        • #5
          You will never fail if you never do anything, and only learn if you try. There was a sign in one shop I ran that read; "mistakes made while you wait."

          That looks like a perfecrly good piston to me, and it can't be any worse than a worn out one. You can't always count on the dials on a lathe as the tool might remove more or less material than it is moved for a variety of reasons, a DRO presents only a minor improvement.

          Pistons on IC engines are oval to compensate for expansion from heat. An air compressor does not present this problem.
          Jim H.


          • #6
            Fantastic! It looks as though you have a good start on it.

            I'd say don't worry. Keep going and when its together, running and better than before, you'll wonder why you didn't do this sooner.

            Civil engineers build targets, Mechanical engineers build weapons.


            • #7
              I might possibly suggest stopping with the turning tool when a few thou large, and then moving to working it down using a lathe file or abrasive on a stick, etc.

              The rough surface from turning always measures larger than it really is. Particularly in "sub thousandths" , where the finish roughness may occupy a significant space relative to the tolerance.

              Looking at the two pistons, the made one, while it looks very good, is noticeably duller, and therefore presumably rougher, than the other.
              CNC machines only go through the motions.

              Ideas expressed may be mine, or from anyone else in the universe.
              Not responsible for clerical errors. Or those made by lay people either.
              Number formats and units may be chosen at random depending on what day it is.
              I reserve the right to use a number system with any integer base without prior notice.
              Generalizations are understood to be "often" true, but not true in every case.


              • #8
                Don't forget to compare the weight of the new and old piston, they should be comparable (although on a two-piston compressor it's probably not a major problem).



                • #9
                  Nice work. Thanks for taking the time to write this up and post pictures. Just one comment from the peanut gallery - do the new pistons weigh about the same as the old ones? I don't think the compressor spins all that fast but if there's a significant difference in piston weight with the new ones you might have some unwanted vibration.

                  Edit: Looks like Benta and I are on the same wavelength. He posted the same comment from Germany while I was typing away in the USA. Kind of like Newton and Liebnitz....great minds think alike!
                  Last edited by Langanobob; 03-11-2009, 11:40 AM.


                  • #10
                    Nice job Ringer. Don't worry about the undersize if it is inside the tolerance range. The pistons on compressors and steam engines and old hit and miss engines are mostly straight sided.

                    The only thing you have to be concerned about is having the wrist pin exactly 90 deg from the side of the piston. The location of the rings is not extremely important and if your off up or down a few thousandths it won't matter. The depth of the ring groove is not as important as the side clearance of the ring in the groove.

                    It's always fun doing stuff like that and I'm glad you could cool off and continue the job.
                    It's only ink and paper


                    • #11
                      Yikes - and don't forget to deburr those guys. Nice work, great recovery, and don't you feel cleansed for baring your sole


                      • #12

                        You are better than you might think. Pretty nice work.

                        A suggestion for your lathe work Ringer. Instead of relying on the dials for repeatability. Use a dial indicator on a magnetic base. Put the tip up against the backside of what ever you are moving, i.e. the cross slide or the compound, or tool post, and use that to measure your in-feed as you machine. Think of it as a "poormans" DRO.

                        If you think you understand what is going on, you haven't been paying attention.


                        • #13
                          VERY nice!!!

                          THANX RICH

                          People say I'm getting crankier as I get older. That's not it. I just find I enjoy annoying people a lot more now. Especially younger people!!!
                          People say I'm getting crankier as I get older. That's not it. I just find I enjoy annoying people a lot more now. Especially younger people!!!


                          • #14
                            Good job Ringer, only change in procedure that I would have done is make the pistons first, and got the finish that I wanted on them, and then bored and honed the cylinders to the pistons.


                            • #15
                              If you got a worn out, sloppy ole piece of typical yankee iron and can no longer rely on the dials being accurate, because EVERYTHING is just plain flogged out, then why not use the dial indicator to get your final measurements.
                              Just use the magnet and stick it down near the top slide handle, so it attached to the saddle, then swing the dial tip over till it touches the top slide. OK, you gotta remember that one thous on the dial, is two thou on the cut, but hey, it works.
                              Saves beating your brains out with that good ole "first class", worn out yankee rubbish, but then again, why not go get a brand NEW Tiawanese Hardinge HLV clone,they must be OK, as the yankees even use them in their Hardinge factory now-a-days.