Tuesday, November 29, 2016

One Step Forward, Two Steps Back

After last Sunday's post, I spent some time considering how to drill the 4" long hole and finally bought a special drill bit.  Not content to go with a simple, cheapo drill bit with a long flute length, I went and researched the proper angle for the cutting point on the bit, found that "bigger numbers are better", and then found an exotic, 6" long, cobalt steel bit with a 135 degree angle.  I had to order that because it's not a big box store kind of bit.  Standard drill bits tend to have 118 degree angles, which is a good compromise, but I figured I wanted every possible advantage I could get.

Still, how do you drill a 4" long, 1/8" diameter hole without snapping the drill bit, or having it wander into the next county?  I think a proper fixture and then doing my best to not exert any sideways forces onto the bit are the ticket.  No handheld drill; use the Grizzly mill, taken apart as it is, as a drill press.  Like this:
The cross slide is clamped to a fixture that holds it at 90 degrees to vertical.  Use a center drill to spot a hole where the 1/8" hole goes.  Perhaps (I'm not decided) drill a preliminary hole with a smaller bit, like 1/16".  Then drill the long hole without moving anything except the mill's head stock. 

The fixture is a piece of scrap aluminum Mrs. Graybeard gifted me back in '04 or so, when I first started metalworking as a hobby.  It's actually a waveguide switch and probably a military or space program surplus switch at that since it's (get this, cognoscenti) an S-band waveguide switch.  It's built the way you'd expect Milspec hardware to be built: seems to have redundant switches in it to let "the system" know if it switched or not; all of the hardware is stainless on aluminum, and it's just built like a tank.  I'm sure the angles on this are going to be pretty close to the accuracy I'd get from an angle plate because the dimensional accuracy required out of a waveguide system is pretty high.  

With that step forward finished, I thought it was time to cut the oil grooves.  I drew a set of curves I'd like the groove to look like and wrote a CNC file to cut the grooves on my Sherline-based system, then moved the cross slide over to the table on it.  While setting up the cross slide to be worked on, I noticed that the X-axis had changed from the place I initially set it.  Having that happen while working on the cross slide would be disastrous, so I started double checking to make sure I could see what was going on.  Sure enough, I found that my X-axis was losing motion.  The motor kept turning and seemed normal, but the leadscrew would stop turning.  At one point, I wrote a little "torture test" file that just moved the table back and forth 8 inches.  After about half an hour, the left end of the motion had drifted right half an inch.  That would have made a mess out of the oiling grooves. 

Then the troubleshooting began.  I don't want to get into too much detail here, but Sherline uses a funky system in their motor mounts.  Still, it may be odd, but the Y and Z axes use the same system and are both fine.  The leadscrew for the X-axis ends in a small (half inch long?) portion that's threaded 1/4-20 and ends in a small, conical taper that engages the matching taper on a coupler that couples the motor to the leadscrew.   The coupler is attached to the lead screw with a 5-40 screw down the axis of the leadscrew, and a 1/4-20 nut pulls the coupler tight against ball bearings on one side and pulls itself tight against the ball bearings on the other side. 

When I first started troubleshooting this problem, I could stick a hex key into that hole on the bottom near the right end (in this picture) to immobilize the coupler, but keep turning the lead screw (and moving the table) by turning another hex key in the 5-40 screw.   The two systems that are supposed to snug everything together were working separately. 

Both the 5-40 and 1/4-20 screw threads looked a little damaged.   I ran a die over the 1/4-20 threads on the leadscrew and returned them to normal-looking threads.  Both the preload nut and one out of my hardware box ran much better on the leadscrew 1/4-20 threads.  The small screw, #5-40, is an odd size and while I had some spares, putting one into the end of the leadscrew made the new one look as damaged as the old one.  I bought a tap to clean up those threads, too.  Along with several pieces of spare hardware. 

Meanwhile, I'm waiting for the parts I ordered and working on other things that need to get done. 

13 comments:

  1. 4" deep with a 1/8" bit? In aluminum? Hm. I am no machinist, but a short drill to start, plenty of good lube, and peck drilling to clear the chips,and attention to speeds. Depending on the alloy. aluminum can gum terribly. What is the hole for?

    I am not clear on the 5-40 screw- is that to tension the coupler on the taper? It is the taper that is acting like a collet to hold the coupler to the leadscrew? Is there a keyway or something to keep it aligned?

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    1. It's actually a bit worse than 4" deep in aluminum. It's 4" deep in cast iron. It's going to take time to do this, but your approach is what I'm thinking. Plenty of Tap Magic, and never drill more than 1/8" without clearing chips. Right off hand, I don't think I have a 1/16" bit that would drill more than 1" in.

      For what it's worth, Hoss said he did it freehand with a battery powered drill, and told me I was over complicating things.

      I think you're right on the 5-40 screw: it pulls the taper closed. The 1/4 20 nut has a little cutout milled in it so that the coupler can actually go below the surface of the nut. I think Sherline's idea was that the two fasteners end up holding everything fixed and the taper drives the leadscrew (!), but I can't see that a taper that small would do any good. I'd much rather have a Woodruff key or something like that.

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  2. Canned air with a straw will clear chips (all over everything)

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    1. I just finished cleaning up around the big lathe from just that.

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  3. A lifetime ago I worked in a machine shop on a Brown and sharpe handscrew machine. We got a job in that had a really long hole in bar stock and the machinist who set it up used a D-drill. Since it was new to me I asked him about it and he said that the D-drill will self center as it drills. Another time, a different shop the setup was a regular drill, slow feed followed by a set of reamers for a close tolerance result.

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  4. I would not try to predrill with a smaller bit. A 1/8" bit with a 135 degree split point has a very small non cutting edge anyway, and the sacrifice of rigidity combined with low chip clearance may be just enough to break off a a 1/16" bit. If you have a short 1/8" bit in good condition, start with that and save the long one till it is necessary-that will retain the edge and help keep the flex down.
    Also, you may be best with no oil at all on cast iron- see what they have to say on machining forums. I do a fair amount of drilling cast iron and it drills pretty easy. It sort of crumbles out of the hole. Sometimes the oil will make it pasty and harder to clear. See what others say about it.
    Is there a sacrificial area you can drill a test hole first without destroying the part?

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    1. Thanks. I think they say to use oil, but I should check again. I'm all but certain I have a virgin 1/8" bit, of the typical jobber style. If not, I can get one. FWIW, Hoss said he just drilled the hole. No pilot hole, no succession of hole sizes.


      I did a little work in cast iron last week, and noticed that the chips were magnetic and stuck to the cutting tool. I assume that eventually can cause a problem, so that's what led me to thinking of peck drilling and lots of chip removal efforts.

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  5. Just looking at my 1950 copy of "How to run a lathe", SB recommends dry drilling on cast iron. It is the only metal listed for "dry".
    Machinery handbook 1970 lists "dry" under cutting fluids for drilling cast iron, with a note that light oil may be used to keep the metal dust down.

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    1. Thanks for looking that up. I'm questioning whether or not it really is cast iron. I mean, it's clearly cast, but I can't say if it's some steel alloy. I need to research a bit more.

      When I milled a pocket on it, the chips broke up into little grains, like large sandpaper grains. The sliding surfaces are scraped flat and are quite smooth. I've read that cast iron can tend to keep sand in it, but perhaps the sliding surfaces are machined past that layer and then scraped flat.

      Really, the only reason I'm saying cast iron is that everyone describes these tools as largely cast iron.

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  6. Didja order from McMaster:

    https://www.mcmaster.com/#standard-drill-bits/=15a6ynm

    They generally deliver within two days...

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    1. I ordered this one from MSC Direct, because "I had a coupon" (as my wife likes to say). It was here in 3 days, but free shipping. I have accounts both places.

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  7. "When I milled a pocket on it, the chips broke up into little grains, like large sandpaper grains. "

    Sounds like cast iron. No idea how much the quality of cast iron can vary regarding machining ease.

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  8. Cast Iron in general is a cool material. I used a sharpened file as a scraper to scrape the ways on an old 1940's mill. Used a surface plate to blue them. Worked great, but should have taken the time to braze a carbide tip on the scraper, as it dulled pretty fast. Amazing how accurate you can get a surface with some bluing and a flat for reference.

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