[How do I?] Drilling tapered holes

[keeena]

What a good option for "drilling" small tapered holes? I need to make 2 steel indexing plates for my dividing head. The indexing holes have an included angle of about 4.84° (the plate is 0.272" thick and the index hole is 0.140" on one side, 0.117" on the other).

I will have 607 holes to drill....could use a 3rd axis right about now.

Searching around, it seems a tapered endmill may be best bet? The closest McMaster has is either 2° or 3° per side (4° or 6° included angle) and I'd have to get carbide (HSS not avail). McMaster link As an aside: would you use this to drill (they are center drilling) or would you drill a pilot first?

Another option I came across is to make a single-flute cutter; not sure I want to go down that road and I'd think it would need heat treating. And outside the box: I could probably make my own pin, thus a different (or no) angle other than a chamfer in the plates. I'd have to make 3 plates in that case though.

Suggestions welcome!

 

[RJSakowski]

McMaster sells reamers for tapered dowel pins but the taper is 1/4" per 12". From your dimensions, you would have a taper of 1/2" per 12".

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[keeena]

Those combo drill-reamers would be perfect!

Unless I'm missing something, wouldn't I need a rate of taper to be about 1" per 12"?

 

[Superburban]

Searching around, it seems a tapered endmill may be best bet? The closest McMaster has is either 2° or 3° per side (4° or 6° included angle) and I'd have to get carbide (HSS not avail).

I think it was on here where I read not to use them for drilling, they will dig in and jam until they break, or the drill stops. I do not know if it was hand drilling, or a drill press if that matters.

 

[Illinoyance]

If you want a .500 per foot taper use a Brown & Sharpe taper reamer.

 

[RJSakowski]

Those combo drill-reamers would be perfect!

Unless I'm missing something, wouldn't I need a rate of taper to be about 1" per 12"?

.140-.117=.023 diameter difference; .023/2=.0115 radius difference; .0115/.272=.0422"/1"; .0422x12=.507"/12",

 

[keeena]

.140-.117=.023 diameter difference; .023/2=.0115 radius difference; .0115/.272=.0422"/1"; .0422x12=.507"/12",

I totally agree with that math which determines the angle for one side of the bit. TAN-1(0.507/12) = 2.42 degrees, aka 4.84 degree included angle. Based on how they specs this type of bit on McMaster: the rate of taper spec would be called 1.014"/12".

We're saying the same thing. I just was clarifying that if shopping based on the spec they use (rate of taper): it would be ~1/12. Either that or I'm up too late for math.

 

[NortonDommi]

How critical is that taper? How close is a standard off the shelf taper reamer? Drilling and reaming is the best way. use a depth stop to get your final dimension hole on one side. Good luck getting it 100% on the other.

 

[RJSakowski]

I totally agree with that math which determines the angle for one side of the bit. TAN-1(0.507/12) = 2.42 degrees, aka 4.84 degree included angle. Based on how they specs this type of bit on McMaster: the rate of taper spec would be called 1.014"/12".

We're saying the same thing. I just was clarifying that if shopping based on the spec they use (rate of taper): it would be ~1/12. Either that or I'm up too late for math.

You are correct. Looking at the drawings for tapered dowel pins, they are specified as the difference in diameters divided by the separation.

It would seem implausible to me that a manufacturer would use a design requiring a custom tool rather than off the shelf. You may want to remeasure your plate. Tapered holes are difficult to accurately measure anyway and one with dimensions that small make it all the worse.

If you are correct in your measurements, you may want to consider using a 3º per side tool. The locating pin will still accurately center on the hole.

 

[Nutfarmer]

It would be easier to machine a new locating pin to fit a straight drilled hole. Most of the indexing plates I have seen have straight holes with a small chamfer. Just drilling that many holes will be a chore let alone taper teaming them. What make of dividing head are you making plates for ?