The question about drawbar for milling machine.

To all the forum members who imply that you shouldn't over-tighten the drawbar - I agree (and I don't over-tighten) - but that was not my point. I just don't like giving anything a mild tap with a (in my case copper) hammer when a less neanderthal method can be applied. I am dealing with small lightweight lathe and milling machines with appropriate small and lightweight bearings but with the same frictional forces applying to the tapers as for hulking great brute machines. It seems to make sense, if it can be achieved, to apply some model engineering finesse to the procedure.

Colin

I did mention it in a thread a few months back, you still need to apply the same force to the end of the tool to break the taper, just depends on whether you want to apply it slowly or in one quick burst. machine size does not really come into it, as small taper spindle will have less friction and less bearing surface than a large machine with a bigger MT and bigger bearings.

This is why the designs that bear against the moving quill are better than one on the outer casing of the machihe as there is no force against the bearings, your swing out bar will still be pushing down on the spindle bearings.

J

I would really like to know how the percussive force of tapping the top of the drawbar with a hammer compares with the forces exerted by applying a heavyish cut.

I'm no automotive engineer, but I know that items on my car fail quicker from percussive potholes than from prolonged steady oscillation on motorways. But the point about the moving quill is well taken - just that, in my case, it looks more trouble than it is worth to create such a solution.

A comparison of 'percussive force' from tapping a drawbar and 'forces' from a heavyish cut is not easy. In fact the two are different things, measured in different ways. The force bit is easier - we are all familiar with forces of gravity, Pounds force, Kilograms force, and Newtons (as well as various easily forgotten stuff from the sixties and seventies). Force is a vector quantity, and so it involves the direction of the load as well as the load in Newtons etc, so in full, the force exerted on a tool requires a knowledge of the load on it, and the direction of the load (and both will vary as a cut is taken, of course).

Percussive force is not the same sort of thing, it involves time as well as force. Hit with a copper hammer and it distorts as it hits the surface, spreading the impact out over time. Hit with a hard steel hammer, of the same mass and at the same speed, and there is much less distortion, and the load is applied more quickly, so it has more effect in separating the parts. Unfortunately it also has more effect in damaging the bearings etc. So, my point is that a comparison of the amount of steady force, with the amount of force in an instant, is not possible. Rather like not being able to compare speed with acceleration, I suppose.

Of course, some machining operations, especially interrupted cuts, do involve the sudden application of force, and this too could be described as 'percussive force', (and they are what causes tooling to chip), but this only occurs at the commencement of cutting for a very short time, not continuously.

Now I have read this through, it could be explained better, - I just hope it helps someone. Perhaps it might help them to explain it better, too?

Cheers, Tim

Many years ago, my physics master taught me that a suddenly applied force produced twice the force of a gradually applied one. he illustrated this by drawing graphs of force vs time.

The gradually applied force resulted in a triangular diagram, whereas the suddenly applied force showed a rectangle, of double the area.

Reverting to my earlier post about my method of breaking the taper, as with any puller of this type, if tightening gradually does not break the taper, a shock to the already tightened puller, almost always produces the desired result.

The exception seemed to be removing the rear brake drums on 1940s/50s Vauxhall cars (probably true for any semi floating axle). A blowlamp applied to the drum then did the trick!

I am not advocating doing the same to your milling machine, even with a heat gun!

Howard

Edited By Howard Lewis on 29/12/2017 12:05:40

Please don't confuse the Impulse of a force F x t ( the are under the Foce against time graph ) with the applied force.. If the same mass masses of copper and steel hit an object with the same velocity the force exerted on the object will be less because of the distortion of the copper and hence the increased time in contact ( the rate of change of momentum will be less in the case of the copper )

The problem with the ' it it wiv in ammer ' approach is the difficulty of applying just enough force to release the taper and no more .

Brian

Cackhandedness and machine tool ownership don’t go well together. When I was a young lad they never let any students on their summer break anywhere near machinery. Only trained youngsters were allowed.