Making and measuring small holes (<8mm)

R

rronald

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I was making a shaft to hold a rotary encoder today. Perfect job for a lathe. So I thought.

To mate the encoder to a pulley, I needed to create a hole for the encoder shaft. When I measured the shaft on the encoder, it appeared to be about 5.97 to 5.98 mm. I tried to sneak up on creating the hole with this dimension, but failed. Two problems:

1) The only way I know that I can measure the hole diameter is with calipers or by test fitting my encoder shaft. Neither work too well. Calipers aren't that accurate and the test fit provides no guidance until the hole is "bigger" than needed. Snap gauges don't seem to go that small.

2) I have boring bars and such. But I don't have anything that could bore anything as small as a 6mm hole. I used an imperial letter "A" drill, but the hole ended up a bit too big. Are there any other tricks that I'm missing?

RR
 
Drill bits typically create slightly oversized holes; pin gages are a good way to approach small holes and a micrometer is more accurate than calipers.

Small boring bars are available (Micro 100), but are carbide and will snap if your lathe isn’t rigid. @Jim F is right: a hand ground HSS boring tool would be best.
 
As the name implies, Small Hole Gages are the tool for measuring small holes.
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(Holy crap, those things have gotten expensive!)

Did you simply drill with the A drill? If I was trying to hold close to size, I would drill about .010 -.015 undersize, then "ream" with the A drill. This prevents the grind of the drill point from affecting size.
 
I second use of small hole gauges. Most small shafts I've encountered are relatively standard sizes and usually can be matched well with specific diameter reamers. 5.97mm is within 0.001" of 6mm. I doubt you will get a closer fit with a boring bar unless you are quite skilled at the boring process.
 
I have gauge pins. They could be used, just fit them in, and see what size fits.
As far as small hole boring, I save all my old high speed steel end mills and I also have blank 1/8 , 1/4 3/32 (I think) hss round blanks. Easy enough to grind a boring bar out of one .
 
There are metric reamers but I for one don't own any- I wouldn't use them often enough
I do like Mr. Whoopee and drill undersize then finish size- usually comes out very close especially using cutting fluid for the final drill

There are other little tricks also: an A drill and a 15/64" are supposed to be the same size but if you had both you could measure them
and see if they are slightly different- I might try the smaller one first in that case
 
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If close tolerance matters, buy the appropriate reamer. Remember: Drilling is for making a rough hole. Boring is for getting a hole on center. Reaming is for getting the hole to the correct diameter. Sometimes you have to do all three sequentially.
 
I have both the small hole gauges and a gauge pin set that tops out and .25" along with the fractional inch pin set to .5"
I use the gauge pins more than the small hole gauges but both work pretty well. The small hole gauges don't work well or at all in holes with a bottom less than 1/2 diameter deep.

You could also grind a D-bit and make test holes with it to sneak up on the fit that you want.
 
rwm said:
If close tolerance matters, buy the appropriate reamer. Remember: Drilling is for making a rough hole. Boring is for getting a hole on center. Reaming is for getting the hole to the correct diameter. Sometimes you have to do all three sequentially.
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I agree completely and I often do just that and as often as not, just because I like doing it that way.
 
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