Narex Vhu boring head feed repair

mksj

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Purchased a Narex boring head from an eBay vendor a while back, it appeared to be in excellent condition and the seller did not state that there were any issues or problems with it. I primarily do boring and wanted a bigger boring head, and this model has 1 3/8" (36mm) facing travel and can bore outwards to 9" diameter with 0.0005" resolution. I had not been familiar with the facing operation, so I finally go around to checking it out. You can set 3 rates of feed when facing, 0.001"/r, 0.002"/r and 0.003"/r which is accomplished by turning a rotating ring which sequentially engages 3 sets of pins that engage a star gear ratchet which feeds the head. Trying the first position, I had no feed, the second position only 1 pin engage (0.0005") and the third position 3 pins engage (0.0015") when I should have had 6.

So the next step was to figure out how to dissemble it and figure out what was the problem. Most of the disassembly is straight forward, remove the arbor, and the flange ring it attached two. Then remove a heavy C clip and washer. The tricky part is then to remove the clutch ring and set ring which is indexed by a very small spring and ball bearing which is around 1 mm in diameter (so easily lost). There is then an engagement ring for the drive pins, they are spring loaded and turning the ring pushes on the heads and the tips then engage the star drive.

Upon examination of the drive pins the first set the heads were sheared off and one on the second set. There also seemed to be some damage to the remaining pins. Most likely what happened was the clutch that kicks out the feed was set too high and when it encountered the stop they were damaged. Setting the kick out clutch tension per the manual needs to be just enough for it to feed w/o kicking out until it hits the stop. So one needs to have a very light touch on the clutch adjustment, and when I checked it, it had far to much tension.

Parts for these boring heads are hard to get, must be purchased in sets, and are stupidly expensive. Some of the parts also had a 12 week lead time, so either they are on the slow boat from the EU or they do intermittent runs on the parts quarterly. Whatever, I was a bit concerned about the replacement pin dimensions and if they needed to be heat treated. Given that the remaining pins had deformed, I surmised that they were not heat treated, and actually appeared somewhat poorly finished. They can only be purchased as replacement sets of 6 and with their springs and will run around $200.

I decided to make my own replacement pins out of O1 steel rod, dimensionally the tolerances were under 0.0005" and required multiple operations in my 5C chuck. The 5C collets is excellent for this type of work were you are holding 0.2" in the collet and machining the other end of the pin. I cut the first pin steps and OD using a DCGT insert, parted it off and then flipped it to get the exact length and cut the reverse step, and the used a radius cutter to cut the top round profile. I made 8 pins (two spares) and reassembled everything. Checked it on the mill and everything works as it should. Some pictures below.

Narex VHU36 star drive.jpg
Narex Pin Profiling.jpg
Narex Pin rounding cutter.jpgNarex Pin cutting the head round profile.jpg
Narex Old and replacement drive pins.jpg

QC check
Narex pin dimesions drawing.jpg

Head assembly
Narex base plate with pins insertered and control ring, set ring with ball detents.jpg
Narex control ring with spring and ball bearing detent.jpg
Narex hold ring with adjustable clutch, washer and snap ring.jpg
Narex washer with pin and snap ring installed.jpg
Narex flange hold ring.jpg
Narex R8 Arbor installed.jpg
 
Same as the set I have down the basement . :encourage:
 
Very impressive :encourage:
 
Impressive work Mark!


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Nice work! Looks like a quality piece.
But I have to say I'm really jealous. Been wanting a boring/facing head for awhile, but can't justify the cost right now.
 
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