Taper pins - another not so dumb question

dbb-the-bruce

Dave
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I guess today is the day for not so dumb questions.

I'm going to try to use taper pins for a semi-permeant assembly. So off to McMaster-Carr to spend money first!
Browsing taper pins, taper pin reamers and taper pin drill/reamers, I find that there is not one standard taper but 3 different tapers published:
  • 1mm per 50 mm
  • 1/4" in 12"
  • 1" in 50"
Which at first I found a little baffling, particularly because the metric sized reamers were listed at 1/4" in 12"!

Well a slope (taper) is a slope is a slope wether you measure it in furlongs or poppyseeds. That means that there are 2 different (common) slopes for taper pins:

1:50 - 1mm per 50mm, 1" in 50"
1:48 - 1/4" in 12"

1:50 == 0.020
1.48 == 0.0208

Applied to a 1" long pin, the 1:48 pin is going to be .0008 bigger at the small end. That's pretty close. Are these ment to be interchangeable? I could see that in REALLY important alignment and precision cases you would want them to be as close as possible to exact. A 1:50 pin in a 1:48 hole is going to jam on fat end first and a 1:48 in a 1:50 hole will jam on the small end first.

For a short (1" or less) hole it's pretty damn close, you are going to drive the pin in anyway. For a really long pin, only one end of the pin is going to do any of the "holding" and that could really matter depending on the application.

Fortunately, in my rush to impulse buy, I ended up with all 1:48 except for a drill/reamer combo bit (it didn't come in 1:48). Worst case I can use the drill and then ream with the 1:48 reamer I bought to open up the bottom end a tiny bit.

So that's the first question - 1:48 vs 1:50?

Second question, making the taper pin hole. It seems that the common method is to use multiple dills of successively smaller diameters and then use a reamer to get the final taper. Then there is also this drill/reamer combo I found (and bought) - seems like smarter to do the job all with one tool but now I'm not so sure.

So: drill, drill, drill and ream or drill/ream with one bit?
Pros and cons? I imagine that "reaming" action of a drill/reamer is somewhat less precise than using a dedicated reamer to finish.

Thanks,
Dave
 

Just for fun

Tim Young
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Hi Dave,

I would think either would work for a 1" long pin. And also for such a short pin, I believe you only need one size drill bit. No reason to drill multiple sizes.

Tim
 

rabler

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I use the 1:48 (1/4 per foot) as, at least per my understanding, that is the classic used in older machines, such as the Monarch lathes that I have rebuilt. Unless you are going with a really long taper, drilling just isn't that accurate in terms of diameter, so I just drill one size hole hopefully about 5 thou smaller than the small end, and then ream the hole with the matching taper pin reamer. That takes a bit of reaming but works fine.

If you look at the taper pin sizes, the longer lengths overlap in diameter. I buy overlength pins, loose seated them, marked the ends (protrusion) and saw it to length before final install. Easier than trying to get a perfect match between pin/hole length and reaming diameter. That way I can also clean up older taper pin holes with a reamer without having to ream all the way to the next size. The taper pins are fairly soft, so a brass hammer will seat them pretty well.

Getting them out on old machines can sometimes be a challenge, the usual issue is when the small end has mushroomed (been intenionally mushroomed?) so it looks to be the larger end, and you are driving it out the wrong way.
 

MrWhoopee

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IIRC, the 1/4"in 12" is the traditional standard in the U.S. All I've ever done is drill one size just below the small diameter of the pin and ream for taper. I see while I've been typing that @rabler has covered it pretty well. I wouldn't try mixing the two different tapers, even very small differences will result in a pin that won't stay in. Speaking of that, while working with my automatic threading stop, I noticed that it wasn't catching the half-nut lever as easily as it had been. Examination revealed that the taper pin securing the lever had worked its way almost completely out. This on a 60 y/o machine.
 

dbb-the-bruce

Dave
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I wouldn't try mixing the two different tapers, even very small differences will result in a pin that won't stay in.
Yes, thinking on this further and relating to experiences with slightly messed up morse tapers - pins that won't stay in was a concern. I think a lot of us have had the experience of a center or something that just wouldn't stay in the spindle or tail stock.

Also, I did search this site for posts on taper pins and one of them linked to this really good chart/info on pre-drilling multiple sizes and reaming:

taper pin drill chart

All of the 1" lengths require a single drill.
 

woodchucker

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I drill multiple sizes.
Then I take on of my taper reamers and run it in.
Then I put my taper pin in and using a dremel cut off the long protrusion, or if they are short, I might leave them showing.

I start with the smallest drill size measured on the pin, then mid way through drill to that point with an appropriate sized drill, and sometimes will or won't drill the last step the largest, which just gets a little entry. The reamer joins them all up.
 

SLK001

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Get a reamer to fit the 1:48 pins you bought. If you use a 1:50 drill/reamer, then "set" the 1:48 pins (RE: pound them in), you'll probably have to drill the pins out in the future, since it will be deformed. A taper pin is meant to fit tight, then be able to be easily removed by a tap on the small end. A deformed pin is pretty much permanent.
 

dbb-the-bruce

Dave
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Get a reamer to fit the 1:48 pins you bought. If you use a 1:50 drill/reamer, then "set" the 1:48 pins (RE: pound them in), you'll probably have to drill the pins out in the future, since it will be deformed. A taper pin is meant to fit tight, then be able to be easily removed by a tap on the small end. A deformed pin is pretty much permanent.
Thanks, that's pretty much the conclusion I came to.
Fortunately, I got the correct 1:48 reamer. the drill/reamer (does both) is 1:50. So I will use the drill/reamer to make the hole and finish with the correct 1:48 reamer.
 

projectnut

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If you don't want to go through the hassle of multiple drills or excessive reaming, you can purchase drills for taper pins.

They're expensive, but worth the money if you're doing them on a daily basis.

The company my wife's grandfather worked for used taper pins on many of the machines they built. I inherited drills for #2/0 thru #10 taper pins. I don't use them often, but they do come in handy when needed.
 

dbb-the-bruce

Dave
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If you don't want to go through the hassle of multiple drills or excessive reaming, you can purchase drills for taper pins.

They're expensive, but worth the money if you're doing them on a daily basis.

The company my wife's grandfather worked for used taper pins on many of the machines they built. I inherited drills for #2/0 thru #10 taper pins. I don't use them often, but they do come in handy when needed.
That's exactly what I did (what prompted this thread):).

If you notice, the taper listed for all of those drills is 1:50 not 1:48. I kind of assumed that they were interchangeable (the tapers are "close enough"). Based on the response here and logical thinking, the answer is "sure you can probably get away with it (a 1:50 hole with 1:48 pin) but your are going to deform either the pin or the hole".

I guess that if you are using the pin as a permanent attachment that will only be taken apart for a major repair then it's probably OK because you are going to use a new pin and clean up the hole anyways and are probably replacing the part that's being joined anyway.

If you are going to repeatedly assemble and disassemble the joint (my use case) then you REALLY want the pin and hole to use the same taper.
 
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