Measuring the diameter of shallow holes

DMS said:
I am making some parts in delrin that involve a shallow hole 0.118" deep (3mm). The diameter is 0.866" (22mm). It's also a bearing fit (a pair of these parts fit over a 608 bearing), so I need to be pretty consistent. I am going for a fairly heavy press, as friction is all that will be holding these in place. My problem right now is measurement. I have been using telescoping guages, but the hole is so shallow, and the plastic is so forgiving, it is hard to get a consistent measurement. I'm thinking somebody out there has to have a better way.

One other thing; this is a test batch (6 pieces). If it works out, I need to make another 12. It took me about 2 hours to make 3, mainly because I was measuring 3 times after every pass before I was sure I was really "there".
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Make up a SPADE DRILL BIT of the correct diameter, or modify a store bought one. Your only cutting plastic.

fixit
 
fixit said:
Make up a SPADE DRILL BIT of the correct diameter, or modify a store bought one. Your only cutting plastic.

fixit
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I'm sure I can cut it, but would that be accurate enough? I tend to think of drills as being ballpark sort of things.

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elf said:
Wouldn't a Go-No Go gauge work well for this?
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A go/no go gauge would let me check pass/fail. I suppose if I had a full set of pins in that size range I could make it work. Unfortunately pins in that range seem to be pricey. I think the other methods posted would get more use in my shop. I do want to pick up a set of pins at some point, but just not sure if I need anything greater than 0.5"
 
I had another thought after reading about spade drills. How about a 22mm, center cutting, end mill, Enco has them for $22. With a good set up it should give you pretty much a reamed hole, and will be sharp enough to cut Delrin just fine.
 
DMS said:
I'm sure I can cut it, but would that be accurate enough? I tend to think of drills as being ballpark sort of things.

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A go/no go gauge would let me check pass/fail. I suppose if I had a full set of pins in that size range I could make it work. Unfortunately pins in that range seem to be pricey. I think the other methods posted would get more use in my shop. I do want to pick up a set of pins at some point, but just not sure if I need anything greater than 0.5"
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I was expecting you to just make the gauge :)
 
elf said:
I was expecting you to just make the gauge :)
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I can make the gauges, but unless I am missing something, they just tell me if I am pass fail. I would need a series of gauges to see what size I was at before my finish pass to see how big that final pass needs to be right? Am I missing something here? Or is the plan just to sneak up on it till the "go" side just fits? That doesn't seem like it would be much faster than my current method...though maybe more consistent and accurate...

After that little bit of musing, seems like it may be a good thing to do to dial things in, and then I think I could use the method Jim suggested. I made a boring bar holder last night to help things along; I'll try things out today.
 
DMS said:
I can make the gauges, but unless I am missing something, they just tell me if I am pass fail. I would need a series of gauges to see what size I was at before my finish pass to see how big that final pass needs to be right? Am I missing something here? Or is the plan just to sneak up on it till the "go" side just fits? That doesn't seem like it would be much faster than my current method...though maybe more consistent and accurate...
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You might want to make a stepped plug gage with 2 steps. When I use these I machine the smallest step 0.0100' larger than the target bore; when it slips in I know I am close. The next step depends on which insert I am using. If my nose radius calls for a finish depth of cut of 0.003" then I make the next step 0.006" larger than the target bore. When that slips in I dial in my finish cut and will usually come in on size, or very close to it.
 
What about buying a 7/8" (.875")forstner drill bit for wood and grinding the OD down to the exact size that you need. It's only .009" to grind off the OD. Chuck it up in a hand drill and spin it as you touch it off on a grinding wheel. A little at a time, checking and touching up as necessary. It shouldn't take very long at all to do_Once the drill bit is made and set , you wouldn't even have to check your holes anymore.

image_24219.jpg
http://www.harborfreight.com/http-w...tride-coated-forstner-bit-set-39812-html.html

That's a whole set in the link, but I'm sure you could buy just the size you need at Home Depot or some other place that sells wood working tools.

Marcel

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I tried just trusting my dials, and was able to turn 5 out in about 30 minutes, fit was much better too.

My main issue it turns out, was stubbornness. I was using as small HSS boring bar. Too small for my quick change holder to repeat accurately. I made a boring bar holder to bring it up so that it repeats. I'll post some pics of the parts I am making later, pretty pleased with how they turned out.

I'm still glad I asked the question, I at least, have learned a lot here ;)
 
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