View Full Version : 1988 Jet JMD-18 with cracked head casting.

04-30-2009, 11:51 PM

I recently acquired a used 1988 Jet JMD-18 Mill/Drill. I did not notice until disassembling the machine to haul it into my basement that the head casting was cracked near the lower head locking bolt. I don't think I got taken by the previous owner as he no longer needed a mill and all his tooling came with it.

I now have to either attempt a repair or try and find a replacement part. Welding or brazing the casting is going to be difficult and time consuming at best, or expensive/impossible to have done professionally as I live in a remote area of MN. A new part would be expensive if I could even find one that would fit. From the text on the manual and a decal on the motor that says "RF-20.25.30" it seems as if the mill may be a rebadged genuine RF-30.



http://i33.photobucket.com/albums/d65/natewoz/100_0863.jpg (Link due to 4 picture rule)

These two show the crack. It extends all the way from the vertical slot at the back of the head to where the pictures show it. The crack opens wider as you tighten the locking bolt.



Here are my questions:

1. From the pictures/ your experience, is this a genuine RF-30?

2. Any chance that a head casting off an RF-31, newer JMD-18, G1006, or similar clone would fit and work with my mechanical parts?

3. Any sources for said parts that are reasonable? (Best I've seen is $300 from Grizzly)

4. Has anyone ever had a professional casting repair shop fix a similar crack? What was the repair bill?

Since I am new here I'll provide a little background on myself. I have been around machining for the better part of my life. I came out of a very strong machining/manufacturing high school and continued taking machining classes in college as part of my major/minor (Technology Education/Industrial Technology)and served as a TA as well. I have worked as a research project machinist and have experience with most all manual machines as well as CNC. My present occupation is as a technology/industrial technology teacher in a 7-12 grade setting.

Thanks in advance for your input/advice.

ETA: Change picture links to actual images.

tony ennis
04-30-2009, 11:55 PM
Manual in the 3rd photo, "Complex Machine." :D

04-30-2009, 11:59 PM
Possible quick and dirty fix- relocate the bolt?

05-01-2009, 12:01 AM
I don't think relocating would work too well. In between the bolts there are no flat surfaces to bear the head of the bolt and there is webbing.reinforcement around the factory holes for rigidity.

05-01-2009, 12:01 AM
Manual in the 3rd photo, "Complex Machine." :D
My favorite line from a Rong Fu manual ..."Do not put machine in the sunshine place."

05-01-2009, 12:05 AM
My favorite line from a Rong Fu manual ..."Do not put machine in the sunshine place."

My manual says exactly that! Odds look good so far that it is a genuine Rong-Fu. Gotta love that Chinenglish! :D

05-01-2009, 12:07 AM
I don't think relocating would work too well. In between the bolts there are no flat surfaces to bear the head of the bolt and there is webbing.reinforcement around the factory holes for rigidity.
Looks as if there is plenty of meat there, maybe you could cut a spot face for the bolt to bear against?

05-01-2009, 12:44 AM
I moved the motor and pulled off the lift/lower gear assembly. Unfortunately it doesn't look like it wants to stop even though it cracked through to an opening. you can barely see it in the picture, but there is a very fine crack leading straight down from the crack inside the lift/lower opening.



doctor demo
05-01-2009, 01:12 AM
I moved the motor and pulled off the lift/lower gear assembly. Unfortunately it dosn;t look like it wants to stop even though it cracked through to an opening. you can barely see it in the picture, but there is a very fine crack leading straight down from the crack inside the lift/lower opening.


I can't tell exactly from the pics...but have You thought about pinning it like is done for a head or block repair?
I did it years ago on a back gear casting on an old lathe and it worked out for for the guy I was helping.


05-01-2009, 01:22 AM

I have only seen a few pictures here and there. Can you enlighten me?


doctor demo
05-01-2009, 01:53 AM

I have only seen a few pictures here and there. Can you enlighten me?

Well they say a pic is worth a thousand words...I don't have a pic, so here goes.
The pins I used were tapered, cast iron , threaded with a square head.What You do is drill and tap on the crack. Then screw in the pin and cut flush. Overlapping the first pin and the crack, drill and tap again and screw in another pin.
Tedious ,but it does work providing the crack is where it can be accessed to be drilled and tapped.


Jim Hubbell
05-01-2009, 02:30 AM
The pinning worked well on my FH Ford V8 FR engine but it seems as if it would tend to force the crack only wider in this application. I wonder if a close fitting splint with many small bolts might not hold things in alignment.

05-01-2009, 04:11 AM
I'd look around for a PROFESSIONAL weldor to do the repair. You will probably get a lead at a engine machining shop. They will know people who can successfully weld castings. There are always cracked heads.

Some people can do them and some people think they can do them. The engine machinists know the difference.

I have hear that these mills are OK so I'd invest the money.

If you want to see some casting weld Magic look at the threads started by castweld on WeldingWeb.

05-01-2009, 10:14 AM
I don't think relocating would work too well. In between the bolts there are no flat surfaces to bear the head of the bolt and there is webbing.reinforcement around the factory holes for rigidity.
Looking at the images you posted of the inside of the casting, I'd say you're right. Looks like welding is the best option.

05-01-2009, 10:15 AM
would this work?


05-01-2009, 10:46 AM
It could be welded by someone who knows what they are doing. But ...... by the time you pay the welder, you won't save much compared to the $300 Grizzly replacement part.

If it were mine, I'd grind out the cracks (which might be a chore in itself, since some of it appears difficult to access), preheat the dickens out of it, and weld her up with nickel rod (about $35 per pound).

If you are not comfortable doing the repair yourself, you might as well spring for the $300 Grizzly part.

05-01-2009, 11:38 AM
It is difficult to get an idea of the area needing repair due to the closeup photos. In many cases, it is possible to drill & tap the broken part and bolt a strap bandaid to hold the pieces together. Ugly, but cheap and effective.

If it were mine, and I could not bandaid it, I would V it out and braze. Preheat the area well, and slow cool by burying in vermiculite. If it works, you are out only the cost of the gas & brazing rod. Brazing cast iron is not too difficult to master, welding is a different story.

In either case, drill a small hole at the end of the crack to prevent further propogation.

05-01-2009, 12:31 PM
I see several recommendations to have this thing welded but it has been my experience that those Chinese iron castings are nearly impossible to weld, some are in fact impossible. I speak from experience, well over 35 years repairing and rebuilding mining equipment, and during that time I saw everything from machine shop equipment to small items like vises, etc. Almost all of it was a very poor candidate for welding but most parts could be successfully brazed with Bronze filler metal although some of it could not even be brazed! If you decide to go the welding route brazing would almost certainly be the best choice and don't underestimate the strength of a properly done brazing repair since when dealing with these castings it will often be stronger than the parent metal.

05-01-2009, 12:38 PM
I agree with those who suggest that welding or brazing is the way to fix the cracked head. With sufficient V-eeing and good preheat, it will weld or braze very nicely.

I one brazed an exhaust manifold on a 4 cylinder Mercury car of some sort for a friend. (Capri?) It was broken off just above the exhaust pipe flange. We beveled the broken parts, fitted them up and then heated the manifold for about 20 minutes with a torch. I don't know the exact temperature but we got it very hot

I used standard brazing rod with the white flux and my O-A torch with a large tip.. Once the preheat was done, the brass flowed beautifully and made a neat repair. We let it cool for the remainder of the day. I understand that it was still holding when he sold the car several years later.

I think a professional welder with the proper nickel rod could do a nice job for you if you don't want to do it yourself. Proper preheat and proper post-cooling could minimize any warpage.

Good luck.

05-01-2009, 12:38 PM
weld her up with nickel rod (about $35 per pound).

I have serious doubts as to successfully welding even with Nickel rod but at $35 a pound you must be talking about Ni99 rod which would not be necessary here. Most people seem to think that because it is so expensive the Ni99 must be a stronger rod that would be less likely to crack than the MUCH cheaper Ni55 but that is a misunderstanding. The Ni55 is a better choice for more than just price because the extra money for the Ni99 buys you the ability to machine the the weld unlike the cheaper (and slightly stronger) Ni55. So unless the weld will need to be machined after welding the much Cheaper Nickel rod is the one to use, that is if this thing turns out to even be weldable.

05-01-2009, 02:24 PM
Since the crack does cross a gasketed sealing surface (If I read the pics correctly), I would tend toward brazing too. Even if you cannot machine that surface back to flat, you will need to work some magic with a hand file to get that to seal again...and that is much better done with brazing.

The other issue is that the iron may well have some embedded oil. Lots of soaking in lacquer thinner may help, but oily iron does not braze well, so get it clean first.

As advised, v the crack and some would advise to drill through the end of the current crack to keep it from spreading. That hole can be brazed back shut. It's been years since I even brazed cast iron, but I have seen some good repairs....and some not so good.


05-01-2009, 04:28 PM
IMHO brazing with a good bronze alloy rod (not the auto parts store variety) from a welding supply would be the way to fix this thing. This would make for a very sound and strong repair and the resulting weld would machine easily, beautifully in fact. The problem with brazing is that most people seem to think that it is an inferior process that results in a weld that is little more than a glue joint but it is in fact a very good way to repair an iron casting that rivals, and often exceeds, the parent metal in strength.

05-01-2009, 04:35 PM
I have had good luck repairing things like this with Tig brazing, using a silicon bronze filler rod.
No preheat, no post heat, no burning off lots of paint, just surgical repair.
Grind a V, nip in there with a 1/16" diameter silly bronze filler rod, DC, argon gas, takes ten or fifteen minutes, and you are done.

You can also get Nickel Tig rod- which is a lot less wasteful than trying to stick weld nickel, at the prices they get for nickel rod these days.
Some welding supply places still will sell you either of these rods by the pound, which means you just buy a few.
Nickel will stick pretty good to most cast iron, and is a strong weld. Again, with Tig, no preheat or postheat needed.
I did a lot of this at Repair Days, at the Metals Museum, in Memphis, a couple of years ago, when I volunteered- and the range of quality of cast iron that came in was pretty broad- some high quality american antiques, and some bottom of the barrel chinese imports. The nickel tig rod stuck to all of em, just fine.

(by the way, Repair Days is a hoot- I heartily recommend anyone volunteering- you meet the most amazing metalworkers, have a great weekend, and help a great cause- )

Find someone with a tig welder, and it should be a relatively quick fix.

05-01-2009, 05:19 PM
Would you expound on tig brazing with the silicon bronze rod (either here or in the welding forum)? I have seen some work done with it (after the work was complete) and it's a very neat process...no brazing flux mess or black suet marks etc. It seemed to lend itself to a very good looking bead in the applications I saw...and...importantly for us who machine stuff...it's machinable after the fact with just about anything. If anything, you have likely annealed the base metal.

What baffles me just a bit is that TIG welding (or any welding) involves melting the base metal. Brazing differs in that the base metal is not melted, just heated to a temp that will melt the filler rod. Its pretty easy to moderate the metal temp with an oxy-acetylene torch. Theoretically, it ought to be easy with a TIG torch since you also have a pedal to vary the arc intensity. On the other hand, the arc is...well...arc temperature (very hot):D I would think that at its minimum, you would too readily be melting the base metal as you warmed it enough to melt the rod??? With an O2 torch, you can always pull the flame back if you start to see the sparks that betray the fact that the base metal is melting. Back off too far on the TIG pedal, however, and the arc goes out and your rod is stuck in the puddle.

However, like so many things, I over-complicate it and then try it and figure out it was not as complicated as I made it to be in my head, so maybe this is not so tough after all:eek:


05-01-2009, 05:33 PM
By using the foot pedal, its pretty easy to braze with a tig machine.
I have been doing it for 20 years or so now, and its not rocket science.

You just dont use as many amps.

I used to produce a line of candlesticks which required attaching a 22 gage stamped steel leaf to a 3/8" round bar "branch", and tig brazing was the perfect application for that- you heat up the 3/8" round, add a dollop of brazing rod, and gently work it onto the leaf. The proverbial "razor blade to a propeller shaft" problem.

With cast iron, its actually easier than those radical material thickness variations- You heat the base metal, in this case the cast iron, up enough so that the silly bronze rod melts when it is touched to it, then, you actually work ahead of the braze with the torch- you dont melt the filler rod with the arc, you allow the hot cast iron to melt the filler rod, while you are heating the next area of cast- its easier to do, than to describe.

Tig brazing also works great on thin wall tubing. I once took a class from Ron Fournier, famous Detroit auto fabricator. While he is mostly known for his skills at metalshaping, and gas welding aluminum, he also has fabricated more tube frames for cars than most of us have seen in our lifetimes- and he usually uses a silicon bronze filler rod to braze together square and round tubing for this. He says it causes less heat distortion in the thinwall tubing, and is just as strong as welding.

05-01-2009, 05:51 PM
Ries has it correct; think of TIG as no more than an inert, tightly focused flame. The base metal doesn't melt, you just need to get it up to the temp to melt the brazing filler.

However, TIG is extremely picky for cleanliness. I disagree with him about the "grind and go" comment. On any casting that has had oil in it(or anywhere near it) , it can be problematic to get it to weld/braze properly because of oil permeated in the casting in the crack. The best method I have found is to use the TIG torch as a "wash clean"; go over the cracked area with the arc at a lower current setting to burn out the oil, stop, cool, clean with solvent, wash with arc again, stop, cool clean. You can see when you are ready when the arc smoking and carbon deposits go away.

05-01-2009, 06:26 PM
Jeff is right about oil.
Especially engine or auto parts.
Oil can really soak into cast iron, and screw up welds or brazing.

But the pics of this mill drill dont show a particularly oil soaked crack.

05-01-2009, 06:50 PM
...Tig brazing...
Neat, I gotta try that! Once again I'm amazed at the the breadth of knowledge on this board.

05-01-2009, 07:05 PM
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).


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

05-01-2009, 08:33 PM
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

05-02-2009, 12:07 AM
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.

A.K. Boomer
05-02-2009, 09:01 AM
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"

05-02-2009, 10:40 AM
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.

05-03-2009, 12:13 AM
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.


05-03-2009, 12:27 AM
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.


05-03-2009, 12:28 AM
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.

05-03-2009, 12:31 AM
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.

05-03-2009, 02:27 AM
that a great fix!

now just don't leave in the sunshine again.:)

J Tiers
05-03-2009, 11:17 AM
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".

05-03-2009, 11:23 PM
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.

05-04-2009, 12:36 AM
Atta boy, dkhntr04. Well done !!!!!

J Tiers
05-04-2009, 09:47 AM
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.