# Thread: Two Questions

1. Junior Member
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## Two Questions

On many threading dies both sides are tapered and I find it difficult to ascertain the leading edge. It seems on most the labeling, i.e. 1/4-20, is facing outwards, away from the direction of feed, but occasionally i would swear this ain't so. What you say? It's driving me nuts, pun intended. The second I would like to think I originated over fifty years ago while working for Cummins engine Co. goes like this, piston comes to dead stop at TDC and BDC (or as the Brits say inner and outer DC from horizontal engine days). There are three logical geometric places where one might assume max piston velocity occurs. All are wrong except on an engine which has never been built and on that engine all are correct. Name the three geometric points, the engine which never has been built and the location of maximum piston velocity on an actual engine. thanks for your time, Ian.

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One side is more tapered, teeth are probably crisper one way for better cutting.

3. Senior Member
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On all my thread cutting dies, one side has a real taper and the other side has a one or two thread chamfer. Start cutting with the tapered side. Sometimes, like when I need a full thread as close to a shoulder as I can get it, I turn the die around and use the chamfered side VERY CAREFULLY.

In an engine, WHICH USES A STANDARD CRANK SHAFT AND CONNECTING RODS, the piston will reach maximum velocity at TWO points near the half way points between top and bottom dead centers (90 degree points). The exact location of these two points depends heavily on the exact geometry and dimensions of the crank shaft and connecting rod and it will be slightly above (towards top dead center) those half way points. I leave it as an exercise to the reader to do the math. Calculus would help here. You will also need some simplifying assumptions, like the flywheel speed of the engine is totally constant over the course of a single revolution. If that is not true, then you would need to know, IN DETAIL, exactly what the rotational velocity is for every angular position of that flywheel or whatever serves as a flywheel. This would depend on many factors, friction and the resistance of the load being two that instantly come to mind. Oh, and the exact lubricating oil would have an effect. Too many variables!

As for that engine that has never been built, there could be many possibilities, not just one. So the question will have many answers. One possibility would be an opposed piston engine.

I doubt that very many engine designers ever worry about this. One would have to be deep into the design of an actual engine before it would become significant. And even then, it probably would not make much difference in most designs.
Last edited by Paul Alciatore; 08-10-2019 at 04:11 PM.

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If memory serves me good the max piston speed is when the rod is tangent to the the circle made by the rod throw on the crank. The position is dependent on the rod length.

lg
no neat sig line

5. Junior Member
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Originally Posted by ian radin
On many threading dies both sides are tapered and I find it difficult to ascertain the leading edge. It seems on most the labeling, i.e. 1/4-20, is facing outwards, away from the direction of feed, but occasionally i would swear this ain't so. What you say? It's driving me nuts, pun intended. The second I would like to think I originated over fifty years ago while working for Cummins engine Co. goes like this, piston comes to dead stop at TDC and BDC (or as the Brits say inner and outer DC from horizontal engine days). There are three logical geometric places where one might assume max piston velocity occurs. All are wrong except on an engine which has never been built and on that engine all are correct. Name the three geometric points, the engine which never has been built and the location of maximum piston velocity on an actual engine. thanks for your time, Ian.
May be I was not clear on die question. Of course, I am aware of taper, but some times that doesn't help. My question, has any body ever seen a die where the lable end is not away from the direction of feed? As for engine riddle, three distinct logical geometric points- crank @ 90 deg (horiz),piston at half stroke and con rod and crank throw at 90 deg. to each other. All are wrong except when the con rod is infinitely long compared to crank radius, then three points coincident at crank @ 90 deg. and all are correct. IN real engines with a con rod length/crank radius of say 4 ( they range from the low 3's to high 4's),max piston velocity occurs at about 76.51 deg either side of top dead center and the angle between con rod and crank throw is about 89.4 degrees and the piston is about 44.7% through it's stroke. No fancy math required to solve, just a knowledge of sines and cosines.

6. Senior Member
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Hi,

Yes I've seen them put the lead in side on the marked side. Ain't no standard to follow. And just because it was one way the last time you bought a die, doesn't mean it has to be the same this time. So look both ways before you cross the street.

7. Senior Member
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The starting side should be obvious by the size of the tape as mentioned, regardless of which side of the die is marked.

JL.....

8. Senior Member
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American made dies always have the marking on the lead IN side (taper)
The reason is-When you put a die in a die holder, the holder usually has or may have a flanged back and
the die size marking must be visible.
This is particularly true of Hex dies!!

However with Chinese or import dies...ALL things are possible. would not surprise me
Rich

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Perhaps my answer wasn't clear. When I use a threading die, I pay absolutely no attention as to what side the label is on UNLESS it says, "Start on this side." or something similar. If it does not specifically specify which side to start with, I LOOK AT THE TAPERS, not the label.

There are many, many people who have manufactured dies, in many countries. Do you really think they got together and agreed as to which side with the printing should be?

So I went into the shop and inspected my drawer of 17 threading dies. Some had the labels on the side with the start taper and others had it on the other side. About a third of them had labels stating which side was the starting side. ONE OF THOSE WAS WRONG. I will say it again, just look at the tapers and go with the longest one. DO NOT WORRY ABOUT THE EXACT ANGLE OF THE CUTTING EDGE. Many lathe tools have negative rake. I doubt that the makers of the dies are overly particular about the rake angle. And there is no clearance angle except on the tapered area, so they probably work equally well from either side.

Originally Posted by ian radin
May be I was not clear on die question. Of course, I am aware of taper, but some times that doesn't help. My question, has any body ever seen a die where the lable end is not away from the direction of feed? As for engine riddle, three distinct logical geometric points- crank @ 90 deg (horiz),piston at half stroke and con rod and crank throw at 90 deg. to each other. All are wrong except when the con rod is infinitely long compared to crank radius, then three points coincident at crank @ 90 deg. and all are correct. IN real engines with a con rod length/crank radius of say 4 ( they range from the low 3's to high 4's),max piston velocity occurs at about 76.51 deg either side of top dead center and the angle between con rod and crank throw is about 89.4 degrees and the piston is about 44.7% through it's stroke. No fancy math required to solve, just a knowledge of sines and cosines.

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