Matching Spacing of Existing Blind Holes

[Tmate]

How can you match the hole spacing of blind holes when you can't use a transfer punch? You could use dial calipers and do a bunch of math to determine the spacing or you could do this:

In this case, I needed to fabricate a bracket to mount a magnetic scale reader head for a DRO to the back of the saddle on my mill. The saddle had existing 1/4" x 20 threaded holes, and I wanted to use those rather than drill and tap new holes.

In the first photo, I knew that the holes were aligned vertically, but didn't know the distance between them. I needed to space the holes in the bracket to bolt up to the mill.

I first determined the location of the first hole and drilled it. I then machined a short stub of drill rod that would slide snugly into one of the existing threaded holes, but not so tightly that it couldn't be easily pulled out. I turned a point onto the stub that sticks out of the hole 3/16" or so.

I applied Dykem Blue to one side of the bracket being fabricated, and then bolted it loosely to the mill using the hole already drilled. I then scribed an arc onto the back of the bracket using the pointed stub. I scribed a line across the arc the same distance from the top as the first hole. That gave me the exact location of the second hole, which I center drilled and then drilled.

 

[lis2323]

A set of these may work

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[extropic]

Yes, that's one way to get it done.

If you do much of that sort of thing . . .

McMaster-Carr

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[markba633csi]

You can also stick the butt ends of two tightly fitting drills into the holes and measure between the same facing sides with a caliper and straight edge. I do that sometimes.
-M

 

[benmychree]

More than one way to skin a cat ---

 

[C-Bag]

My favorite way since finding out about here on H-M is the Sorensen Center-mike. No muss no fuss. Easily within .001 or less.

 

[benmychree]

My favorite way since finding out about here on H-M is the Sorensen Center-mike. No muss no fuss. Easily within .001 or less.
Yes, a valuable tool, I also have one, they were manufactured by Brown & Sharpe.
View attachment 365949

 

[C-Bag]

Sad thing is they seem to have gone up in price. When that thread showed up several years back on the Sorensen most picked up the 8” version for $3-40 on eBay. I didn’t see any for less than $80+shipping. If someone buys one make SURE that little placard of how to use it along with test plate are both in the box. It’s useless otherwise.

 

[benmychree]

Sad thing is they seem to have gone up in price. When that thread showed up several years back on the Sorensen moist of picked up the 8” version for $3-40 on eBay. I didn’t see any for less than $80+shipping. If someone buys one make SURE that little placard of how to use it along with test plate are both in the box. It’s useless otherwise.

I would say that they were never cheap, the two that I bought (two different sizes) were a good deal higher than that, and it was over ten years ago, having said that there will always be a time on EB when nobody is looking for one, and a seller that set a low starting price, I have bought a good many things at low prices. I never bid on any item before the end of an auction, using E Snipe, and have ceased buying much of anything; got too much stuff already, just ask my wife ---

 

[MrWhoopee]

They make a tool for that, transfer screws