Need some help from the guild on drilling some holes.

Joined: February 25th, 2015, 1:30 am

November 22nd, 2017, 9:38 pm #1

Okay, so, I just landed a new job at a small machine shop where I am Blissfully overqualified. I'm going to have free time on my hands filling out my 40 hours, and I plan on staying Late.

So, I need to drill some holes in a personal project sooner or later and I need some guidance.
I've got a whiz bang brand new CNC knee mill at my disposal, I can buy in tooling, and I'm going to be making fixturing for this, but it's tricky for a number of reasons. first of all, the Subject:
http://mjmotors.tumblr.com/post/1582917 ... rakit-from full writeup.

Those stud holes are in the wrong place for my application and there's Cast Iron about halfway down. I've gotta bring them In towards the bore a couple mm, I forget how many. Studs are 6MM diameter.
Whole thing is around 50mm deep. But modding this and making an adapter plate for the intake is all I need to finally have a 'correct' cylinder for This engine:


I was thinking of using a straight shank 7MM drill bit, slow, with a steel jig plate half an inch thick guiding the bit and lots of cutting fluid.

Oh, and a BIG shout out to Doc, and everyone here. I got this job because I've sat here and read the ancient scrolls penned by cranky elders for sixteen years.
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Joined: November 30th, 2014, 1:36 am

November 22nd, 2017, 9:45 pm #2

I'd drill those holes and tap them to fill them, then drill new clearance holes using an drill if there's mostly old material, an endmill if you're starting in the fill. If you use permanent Loctite to glue the screws you fill with they'll stay for the machining, without you'll just break an endmill or drill. The reason for filling the old hole is to make sure that you have enough area for the gasket to seal.

If you really want to risk it w/o filling you have to use an endmill to take the material out, a drill will really want to wander off and break.
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Joined: January 1st, 1970, 12:00 am

November 22nd, 2017, 10:43 pm #3

Where's the iron start? I'm assuming a top flange?

If that's the case, what I'd do is drill and tap the bottom (crankcase side) of the holes, about 1" deep, for a fine thread only a little bit bigger than the new bore size. IE, if you have a 6mm hole now, and need to move it inboard 2mm, bump it up to a 10mm thread.

Turn and thread a chunk of aluminum rod to fit- doesn't have to be a perfect fit- and screw it in with a generous helping of red Loctite.

Once it's cured, saw it off, machine flush, relocate the hole, and drill most of the way through.

If the iron liner has a flange at the top, turn the cylinder over, relocate/reindicate the hole to be drilled at the top, and as noted above, use an endmill to gently drill through, and hopefully down far enough to meet up with the new hole from below.

I think it'll work, without more data, but it's also worth keeping in mind this is not the only way to do it either. For example, you could, assuming there's sufficient meat to the cylinder, drill it oversize, ream, and press in a solid bar of aluminum, with no threads. Then just redrill the new hole location.

It's also possible to plug the holes in the crankcase with threaded slugs as above, and redrill to relocate the studs instead of the cylinder holes.

Always more than one way to skin the proverbial cat.

Doc.
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Joined: February 25th, 2015, 1:30 am

November 22nd, 2017, 11:46 pm #4

Bottom of the cylinder looks like This:

So somewhere in there, it bulges out around the stud holes. I'm probably going to have to go the drill/tap method and put some alu rod in there.

Changing the stud location can be done, but then I'll have to machine the head too. And it kills compatibility with off the shelf kits.

Carbide drill or regular HSS?
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Joined: November 23rd, 2014, 9:43 pm

November 23rd, 2017, 9:22 pm #5

You don't want to be trying to pick out bits of broken carbide 2" down a Ø6mm hole. If you're worried about wander drill it undersized and follow it up with the correct size drill slower to in effect "ream" it to finished.
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Joined: November 30th, 2014, 1:36 am

November 26th, 2017, 10:39 pm #6

Bottom of the cylinder looks like This:

So somewhere in there, it bulges out around the stud holes. I'm probably going to have to go the drill/tap method and put some alu rod in there.

Changing the stud location can be done, but then I'll have to machine the head too. And it kills compatibility with off the shelf kits.

Carbide drill or regular HSS?
Carbide is brittle as hell and in my experience will break most any time you try and drill by hand (meaning that either the work or the drill is free). I always fixture something in my mill or DP if I'm going to use carbide.

Nothing here looks like it would need carbide, anyway. Carbide is used in a couple of situations - where the material being drilled is high hardness (like 50+ Rockwell C) or when it's abrasive (high silicon cast aluminum). In the latter case you can make do with HSS drills if you're willing to accept lower life. (I'm ignoring the few cases where you simply need the additional rigidity of a carbide drill and hoping you don't point out carbide point drills).

If you have a high hardness material and you want a drill by hand you could go with a cobalt drill, they're pretty tough but not brittle as a carbide drill is.

Draw up what you're doing and post it here in this thread, maybe some fresh ideas can come out of it.
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