Precision Leads to Accuracy

Lack of precision, not so much.

This is actually not a ShapeOko post for a change. Well, it’s related…

One of the things I sell in my Etsy store is loading blocks for reloading ammunition. These are finished planks of maple with various sized holes drilled in them.

I have a pretty good system for drilling the holes using spacers that I made out of MDF (medium density fiberboard.) Below is an example of the normal blocks I make.

Loading Block
Loading Block

It has 50 holes in it, on 5/8″ centers. I start at one end and do each row, stacking spacers as I go. It works out pretty well and takes about 15 minutes to drill.

One of the things I want to use the ShapeOko for is to make the holes by boring them with a router bit, but I’m not quite there yet.

Then I got a custom order for a square block with 100 holes in it. No problem, I’ll just do the same kind of thing with the spacers.

Square Block and Spacers
Square Block and Spacers

I had figured out where the start point for the first hole should be and was drilling the first row. When I got to the end of the row I noticed that there was more space between the edge of the hole and the edge of the block then there should be.

Kind of a lot more actually. Enough to make my inner perfectionist sit up and take notice. Enough that I can’t sell this block. Hrm. What happened?

The blocks are 6-7/8″ square. The holes start 5/8″ in from the edge and are spaced 5/8″ apart. They should end up centered. But they were not.

Why not?

It turns out that my 5/8″ spacer blocks, instead of being .625″ wide, are actually .608″ wide. That’s only .017″ off. About 1/64″. Doesn’t seem like that much, does it?

It’s not if you aren’t trying to center a row of 10 holes in something. On my normal loading blocks it’s no big deal, because they are not centered the long way.

But over 9 spacer blocks that .017″ adds up to .153″ – more than 1/8″. And that’s a fairly obvious difference from one side of the block to the other.

Luckily I had extra blanks made up, so I fudged the difference and centered the next three that I made, but it was annoying to have to scrap one. Well I didn’t really scrap it, I’ll use it myself but I can’t sell it.

And now I’ll have to make new spacer blocks. And I’ll have to measure all the rest of the various sized spacer blocks that I have.

It was only .017″.

And boring the holes with the ShapeOko will avoid this problem altogether. Maybe I’ll get that sorted out before I have to make more loading blocks.

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