Imperial Cap Drill Bit Size

[randyrls]

Hope everyone had a great Christmas or חנוכה שמח (Happy Hanukkah!)


Any suggestions are appreciated. How to get a close slip fit?

I am making an Imperial and the cap drill bit (17/32") is one I do not have. I ordered one but it is a bit oversize by about 1/16" (too large). I try to make my brass tube a close slip fit (no slop) . A 13.3mm (really 13.3mm) size is a bit undersize. I used a micrometer and pin gauges to measure all sizes. So what to do?

I've thought of:

1. Using an adjustable reamer set (I have) but not sure it goes to that size.
2. My boring head and boring bar to dial-in the size
3. Getting a 13.5mm drill bit. My metric set stops at 13mm.

 

[egnald]

Your Granger store should carry 13.5mm bits - for about $30 if you are so inclined to go with #3. I guess that's the direction I would take anyway. - Dave

 

[duncsuss]

I get most of my twist drills from Victor Machinery - there's a minimum order, used to be $25, but I never had trouble thinking of stuff to add when I was in the candy store.

 

[randyrls]

Thanks Guys; I ordered the 13.5mm drill bit and will see how it fits. If Not the boring bar is still an option. The adjustable reamer stops at just under 1/2"

 

[randyrls]

Well! That didn't end well. The drill bits I ordered are ALL too small. (13.4mm and 13.5mm) I don't think it is possible to drill a hole smaller than the bit?? I may have to get a good one from MSC.

I FOUND a larger set of adjustable reamers that I will try.....

 

[duncsuss]

I don't think it is possible to drill a hole smaller than the bit??

It absolutely is possible - depends on the material.

Take ebonite (hard rubber) for example. After drilling with any particular bit, if I stop the rotation before withdrawing the bit it'll stick in the hole. I often find I can't turn the drill bit around and push the shank into the hole which that exact drill bit just made.

My guess is that the material deforms - maybe it stretches a bit as the cutting tip and leading edges of the flute whoosh by. Then when the drill stops spinning, it returns to its un-deformed shape/size and clogs up the hole.

 

[egnald]

I don't think it is possible to drill a hole smaller than the bit??

Everything has a tolerance. Most inexpensive drill bits are manufactured to the low side of their tolerance. The factors that drive this include an assumption of the tolerance, concentricity, and runout (TIR) of the drilling process which not only includes the bit itself but also the drill, chuck, the characteristics of the material being drilled, and the holding mechanism for the material. In addition, for precision machining, it is desirable to have the drill provide a slightly undersized hole so the more precision reamers can do their job correctly.

For example, according to a precision cutting tools chart that I use, a standard 1/2 inch twist drill has a typical manufacturing tolerance of -0.0077" to +0.002.

Even though most drill bits themselves are manufactured to the low end of the tolerance, the holes that they produce are almost always oversized because of these factors. Drilling a softer, more resilient material on a well aligned lathe using precise tools such as a four jaw chuck to hold the work and collet chuck to hold the drill bit instead of a Jacobs chuck, can provide a hole that is indeed smaller than the specified dimension of the drill bit.

For the fit you are hoping to achieve, I think sneaking up on the size with reamers may be the only solution.

That being said however, when I find myself with the hole in a blank feeling a little bit too tight for my liking, I make a crude, jerry built hole sander by cutting a slot in the end of a wooden dowel into which I place a strip of 800 grit sandpaper. I usually chuck the dowel up in a hand drill and use it to sand the inside of the hole until I am satisfied with the fit of the tube.

Please let us know how you wind up solving the problem.

Regards,
Dave

 

[Curly]

You could try and saw a slot in a dowel, wrap some sandpaper around it and then chuck it in a drill and sand the hole to the size you want. Less costly than reamers you won't use often.

 

[randyrls]

Dave; I believe you are correct. I was so focused on the "under sized" that I didn't realize the problem was spring back in a soft material (wood). I even have instruction sheets that I wrote two different drill sizes for synthetic vs wood. The wood needs a slightly larger bit size than synthetic. I am using a milling machine for drilling so, very accurate, and a Paul Hoffman vise. I even mark the orientation of the blank so I put it back in the same position. I already had the reamer set but the spring back seems to be worse as the reamer doesn't have any shearing action. It is a scrape. I have a full 115 piece set of Chicago-Latrobe bits that I scored many years ago.

Pete; I already had the larger adjustable reamer set, so no expense, but it isn't meant to do deep holes AND the hole size isn't uniform with depth.

Duncan; I think you are correct.

I think I will just bite the bullet and use the 17/32" bit and use Gorilla foaming glue for the blanks. This is branch wood (English Yew) and I will turn the entire branch blank down to round, cut the blanks out and drill the pith out to prevent cracking, dry in desiccant beads, and drill to 17/32", glue. I'll post when finished.

I believe I made the under size drilling problem worse by sneaking up on the size I needed.