Questions about drilling / reaming in cast

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scattermaster

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Hey guys,
I have contracted to do some mods on some hydraulic pumps and I need some guidance.
I need to enlarge some dowel pin holes from a nominal, 12mm to 16mm diameter. About 15mm deep.
I have to maintain the hole locations to within a couple thou total between the 2 holes.
I can indicate the holes, so theoretically I'm OK with that step. As long as the tooling doesn't walk.
It's the enlarging process that I'm mostly concerned about.
The finished hole diameter TOLERANCE is 16mm (0.6299") at the smallest and 16.018mm (0.6306") at the largest. So, I have a 0.0007" tolerance.
After measuring a couple of the new pins I think I need to be at or close to the larger end of my tolerance.
SO,
I know I'll need to use a reamer to finish the hole, but how much should I drill first? In other words, I don't know how much material I should expect the reamer to remove.
Also, I'm concerned about the drill bit not staying concentric since the original hole is quite large. (Not like a normal pilot hole) Maybe I shouldn't use a drill bit at all?
Is it good practice to use different size reamers and enlarge it in steps?
The piece is cast but that all I know about the material. It's just a pump housing.
Any advice would be appreciated.
What tooling would you suggest?
thx,
 
Drills will wonder.

Boring bar will not.

A guided drill that rides in hole will work, like a step drill.

How deep are the holes?

Harbor Freight has a step drill with very long steps.

Not metric, but there may be a metric version.

A step drill is self guiding if the steps match before and after size.

You can drill from both sides then if required use regular drill to finish as the pilot hole is in place from step drill.

Final action would be to ream to size to insure single pass inside without any steps.
Sent from my SM-G781V using Tapatalk
 
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Maybe consider using a .625 (5/8”) end mill IF your confident that you can get over center of the existing hole. The operative word here is IF. If you can, then you should have no trouble reaming the remainder to your target diameter.
 
Agree with @Eyerelief, plunge with 5/8 endmill, then ream. At least that's what I'd do.
 
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