Announcement

Collapse
No announcement yet.

1988 Jet JMD-18 with cracked head casting.

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • J Tiers
    replied
    Originally posted by dkhntr04
    I suspected a bad casting but your first idea seems logical as well since the previous owner had it in his garage for a decade in Duluth, MN where avg. winter lows are in the -20 to -30 range or colder for the better part of the winter.
    Oh, OK, that's pretty warm ......... I was thinking -40+, but it might work the same either way.....

    Either way, that casting seems to have gotten the bad spots from a whole run rolled into one unit.

    Leave a comment:


  • MTNGUN
    replied
    Atta boy, dkhntr04. Well done !!!!!

    Leave a comment:


  • dkhntr04
    replied
    I suspected a bad casting but your first idea seems logical as well since the previous owner had it in his garage for a decade in Duluth, MN where avg. winter lows are in the -20 to -30 range or colder for the better part of the winter.

    Leave a comment:


  • J Tiers
    replied
    You probably don't have any answer to this, but it needs to be asked.........

    How on earth did the head get THAT MANY cracks in it all over? It seems to be "partly shattered"..... if that can be said without twisting logic too far.

    There is no particular reason for them, as far as the unit being dropped, etc. Dropping may break off a piece, but rarely makes the whole thing "start to shatter".

    The only thing I can imagine is that the maker did NOT take any precautions as far as releasing stresses in the casting, and that the machine was then left in a very cold environment. You said you were in a remote area of MN, which might be rather cold in winter (I am from up there). Possibly locked-in stresses and cold-related shrinkage were able to start all those cracks when the prior owner left it in his shed. Or possibly it was simply a defective casting to begin with, and they were started back at the factory.

    It just seems odd that your unit would have so many cracks all over it. The fact that many seem to be at changes of section, or "corners" of the casting, tends to suggest a foundry problem, one that might have been visible when the casting was assembled. Jet may have said "paint over it and ship it".
    Last edited by J Tiers; 05-03-2009, 10:21 AM.

    Leave a comment:


  • Astronowanabe
    replied
    that a great fix!

    now just don't leave in the sunshine again.

    Leave a comment:


  • dkhntr04
    replied
    i. And here are a few pics after primer and paint. After wire wheeling off the old paint and body filler I primed with aerosol auto primer. After 45min I topcoated with Rustoleum grey gloss paint. Application was with my Husky conventional gravity feed sprayer with the paint thinned 2:1 with Martin Senour fast drying enamel reducer.






    Many thanks for all that contributed to this thread. Your assistance literally saved me the cost of a brand new machine as I had determined it was not economically sound to purchase a new head(It also needs about $200 worth of electrical controls and surface grinding to flatten the table). Thanks again, and happy machining.
    Last edited by dkhntr04; 05-02-2009, 11:37 PM.

    Leave a comment:


  • x39
    replied
    Originally posted by dkhntr04
    Some of the beads are not the prettiest but in my judgment are sound.
    Nothing wrong with those beads, they're just a bit "muscular". Just kidding, nice repair.

    Leave a comment:


  • dkhntr04
    replied
    f. Here is another shot of the major crack from a different perspective.



    g. Here is a view from inside the column bore. As you can see the bronze penetrated well here. I cut the excess back using a HSS cutting tool on the die grinder. I then used the largest diameter sanding drums I have for the die grinder to gently finish it out. I cut the area of the repair about .030" or so deeper than factory to ensure that it will fit and function properly when placed back on the column.



    h. A quick shot here of the completed raising gear area. I finished the mounting surface with a series of flat files. Plenty of clearance for the bead.

    Leave a comment:


  • dkhntr04
    replied
    Well, I think I have it fixed. Many thanks to Ries, TIG brazing worked quite well. Following a typical groove welding procedure I prepped the cracks by grinding a U shape groove over the top of the crack to about 50% of it's depth. I'm not sure if that is the best way to do it, but it seemed to work out.

    I do not by any means consider myself an authority in this area, but I would like to share a few things I learned/observed for the benefit of the forum.

    1. Silicon Bronze rod works great. I used 1/16" from Praxair, Item #PRS 61801 Which totaled $18.14/lb. 3/32" is a little cheaper. I used about 1/3 pound for all the repairs.

    2. I was using a Lincoln 175A Square-Wave machine. I saw the best results using DCEN and 110A maximum setting (material was about 3/8" thick). I was using 100% argon shielding gas.

    3. I heard some of those awful high pitch cracking noises a couple times. Inspection showed that it was the brazing that cracked. I simply reheated the bronze and cooled it slower (see next item).

    4. Using a standard propane torch on the bead and/or peening with a hammer immediately afterward eliminated any cracking in the braze.

    5. When starting a bead, add a drop of rod right away and keep your arc on the bead. No matter how low of current I ran the spot at which the arc touched the casting would start to melt. It worked best to get a liquid glob of filler going and then heat the casting through that glob.

    6. Once the base metal got up closer to brazing temperature I would then move the arc right to the edge of the filler glob to heat the casting more directly. As a result, the filler would then run freely allong the base metal. When you do this it does appear as if you are puddling the cast iron at first. You are not, the braze flows to the point where the arc contacts the casting very fast and in a stream as thin as paper which is hard to see.

    7. I contacted Enco because I was curious to their pricing for a new casting. The guy I spoke with from their parts department said the castings for the newer RF-31 would not fit the RF-30 and sourcing a casting for this mill would take 16-18 weeks. I have no other info to substantiate this. No price was quoted.

    Here are some pictures I took toward the end of the process. Some of the beads are not the prettiest but in my judgment are sound. I decided that homely beads were better than risking cracks by re-heating to smooth them out.

    a. This picture show the major crack at the parting line that contacts the column and a couple of the other small cracks I found.



    b. Here is the main crack after brazing.



    c. This is where the crack ended, right in the corner of the pocket where the head raising crank attaches. There is plenty of clearance in there for me to leave a tall bead for reinforcement.



    d. I found a 2.5" crack leading up along the area where the head widens to go around the column. I placed a 3-bead multipass in the fillet to repair/reinforce.

    Last edited by dkhntr04; 05-02-2009, 11:32 PM.

    Leave a comment:


  • x39
    replied
    Originally posted by A.K. Boomer
    Do you have any idea what that really means?
    LOL!, Yeah, you're probably right. Probably a source of recurring mirth at the engineering dept. water cooler at the Red Blossoms of May No. 9 machine tool factory.

    Leave a comment:


  • A.K. Boomer
    replied
    Originally posted by x39
    My favorite line from a Rong Fu manual ..."Do not put machine in the sunshine place."

    Do you have any idea what that really means?

    Its an ancient Chinese insult --- there telling you that you can "stick it where the sun don't shine" its a very serious slander second only to "bah fung goo"

    Leave a comment:


  • dkhntr04
    replied
    Disassembling brought more fun presents. The bottom column bore of the casting is cracked at the parting line and I also found 3 other hairline cracks as well. I plan on going making the trip to get some silicon bronze rod tomorrow. Thanks for all the input.

    Leave a comment:


  • Oldbrock
    replied
    I'm with banjoallen, I have seen a large crack in a marine engine cylinder stitched and chain plugged. A super system with no heat distortion and strong. Peter

    Leave a comment:


  • dkhntr04
    replied
    I definitely picked the right forum to jump in with, the wealth of knowledge is amazing.

    I am going to try and braze the crack this weekend. The TIG approach is very tempting as I have access to a TIG welder and have done a great deal of it ( I teach basic steel and aluminum TIG welding in my classes).

    Ries,

    I take it you strike up an arc but not with enough current to cause a puddle to form? Which polarity provides the best results?

    If I go with the standard torch heating, must I pre-heat the entire casting or can I preheat the local area and heat less as I gradually make it further from the crack? Thanks

    Leave a comment:


  • x39
    replied
    Originally posted by Ries
    ...Tig brazing...
    Neat, I gotta try that! Once again I'm amazed at the the breadth of knowledge on this board.

    Leave a comment:

Working...
X