Need Ideas for Precise Hole Spacing

MrWhoopee

MrWhoopee

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I want to modify the fixed jaw on my Perverti-vise for an adjustable stock support. This will serve the same purpose as a stop on a horizontal vise. What I have envisioned is a piece of 1/4 x 3/4 flat stock about 1.50 in. long with two 1/4 dowel pins spaced 1.00 apart through the 3/4 face. These pins will slip into holes in the fixed jaw, providing a consistent support for the part. The fixed jaw will have about 6 pairs of holes spaced 1.00 apart vertically. I would, of course, like the holes in the jaw to be a very close slip fit on the dowels, which requires that the spacing between the holes in a pair be VERY closely controlled. I only have a vertical mill (no jig borer), and the holes are on the small side for boring, so I'm looking for suggestions as to how to produce them. I have a 1/4 in. reamer which produces a hole that is perfectly sized to slip on a dowel, I just have to figure out how to precisely and repeatably produce the pilot hole.

I have couple of ideas, but I'm not going to taint the pool with them. So let's hear it.
 
Here's an old school idea.
Make up a drill jig using drill bushings.
Only problem I see is the cost. Roughly ~$15/ea. if you do it up with liners it could get a bit expensive.
Get a Removable/Renewable 1/4" and use that for all your drilling.
(Here's one on eBay for $9, but it's kind of long @ 1-3/8")
If it's not going to see a lot of use you could just use a plate with the Slip-fit thru holes for the bushing and use copious amounts of anti-seize.
That way you'd have repeatable spacing.
 
First make sure your head is properly trammed. This will minimize parallax errors.

If you have a DRO on your machine, then that is close enough, or use a dial indicator to position. Then just center drill, pilot drill, and then ream. You could use your jaw as a reaming guide, it might be able to correct a small position error. I would experiment a bit with a scrap piece before I did the work on the real part.

I normally use a 0.251 or 0.252 reamer for a slip fit on dowel pins, 0.250 can be a bit tight unless the one that you have is cutting a bit large.
 
I'm familiar with tool makers buttons, that gets me into position but doesn't solve the actual production of the hole in that location. I do have a DRO, which should get me close enough on location. My primary concern is the tendency of drills to wander (if ever so slightly) and reamers to blindly follow the drill.

Per middle.road , if I'm understanding correctly, I'm considering making a "reamer guide" with a pin that locates in the first hole after it's reamed and a second hole (previously reamed) to guide the reamer into the second hole of the pair.

My other thought is to spot drill, drill 3/16, plunge with an endmill (to correct drilled hole location), drill to pilot size, then ream. A 6mm endmill would be perfect, no need to pilot drill.

I fully expected the reamer to produce an on-size hole, which would be a press-fit (.0002) for a dowel. I was surprised to find that the dowel was a very close slip fit, almost too perfect.

There would probably be no harm in the holes being a thou or even two oversize if necessary, it's just contrary to my nature. That's why I became a machinist in the first place.
 
MrWhoopee said:
I'm familiar with tool makers buttons, that gets me into position but doesn't solve the actual production of the hole in that location. I do have a DRO, which should get me close enough on location. My primary concern is the tendency of drills to wander (if ever so slightly) and reamers to blindly follow the drill.

Per middle.road , if I'm understanding correctly, I'm considering making a "reamer guide" with a pin that locates in the first hole after it's reamed and a second hole (previously reamed) to guide the reamer into the second hole of the pair.

My other thought is to spot drill, drill 3/16, plunge with an endmill (to correct drilled hole location), drill to pilot size, then ream. A 6mm endmill would be perfect, no need to pilot drill.

I fully expected the reamer to produce an on-size hole, which would be a press-fit (.0002) for a dowel. I was surprised to find that the dowel was a very close slip fit, almost too perfect.

There would probably be no harm in the holes being a thou or even two oversize if necessary, it's just contrary to my nature. That's why I became a machinist in the first place.
Click to expand...
Use the bushing to drill the hole and then the ream should follow the hole for what you're trying to do.
come to think of it, you'll be doing a 'C' letter size drill for reaming...
Is your reamer (.2505") by chance? Cause normally dowels are (+.0002").
 
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