chuck question

Well, actually, friction applies whether it is constrained or unconstrained. If the surface between the chuck and the backplate had no friction, it would be pretty easy to start moving the chuck sideways-as the first bit of motion would stretch the bolts very little-then as you moved the chuck further, the bolts would begin to stretch more and more for each unit of motion, and it would get more and more difficult to move the chuck.

And I'm not trying to be a jerk, but just because it is common to use the incorrect term, doesn't make it right! (at least, until it becomes so common that the new usage gets put in the dictionary) :) Also, we are talking about a technical subject here, so I think it's important to use correct terms, especially for the benefit of those who don't know this stuff, and may be confused. It's kind of like quoting torque in ft/lb, which is very common, but totally incorrect. Try plugging it into an equation, and if you don't see the error, you will get completely incorrect units when you are done.

Ray C said:
It's even more complicated than that. The phenomenon you mentioned is called static and and dynamic friction but that only applies to unconstrained motion. In this case, the bolts are constraining the chuck so, it would take a minimum of 9000lbs (to begin overcoming the static friction and then overcome the bolt tension) to move the chuck. My point was to outline the general principals involved and get folks to realize that you'd need to put the weight of two automobiles on the chuck before anything could possibly happen. I also made a light attempt to show folks how to make simple clamping force calculations. I should have mentioned that the values are taken from a standard chart.

It's pretty common when describing a problem to use the word "force" as an improper noun or as an adverb. The units of the final value take precedent and clear-up any confusion. Many folks say things like "The force of the crash caused damage to the vehicles". -It makes perfect sense but it's wrong. Collision encounters always generate impulses -not forces. -But we still understand what is meant.

Ray
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I see a great deal of information.... Thank you very much...


Ray

awander said:
Well, actually, friction applies whether it is constrained or unconstrained. If the surface between the chuck and the backplate had no friction, it would be pretty easy to start moving the chuck sideways-as the first bit of motion would stretch the bolts very little-then as you moved the chuck further, the bolts would begin to stretch more and more for each unit of motion, and it would get more and more difficult to move the chuck.

And I'm not trying to be a jerk, but just because it is common to use the incorrect term, doesn't make it right! (at least, until it becomes so common that the new usage gets put in the dictionary) :) Also, we are talking about a technical subject here, so I think it's important to use correct terms, especially for the benefit of those who don't know this stuff, and may be confused. It's kind of like quoting torque in ft/lb, which is very common, but totally incorrect. Try plugging it into an equation, and if you don't see the error, you will get completely incorrect units when you are done.
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I have a 6" 3-jaw chuck mounted the same way as Ray C has done. Been running that way for over 12 years. I've had several "wrecks" on it with part off blades in the past. It so far has had no affect on the runout of the Chinese chuck. BTY, It has less than 1/2 thousandth runout!

The chuck is held on to the adapter with three M8 Soc HD cap screws. Made up with torque from two good "smacks" from a 10 oz ball peen hammer.:))
 
Andy, good points. I always get frustrated when force and pressure are used interchangeably in science documentaries. If anyone should know the difference it would be them :)). Nevertheless, I understand Ray's point - that there is a LOT of force holding the chuck to the backplate if the bolts are properly torqued. The backplate is cast iron as I discovered when I took the first cut on it (lots of flying sandy swarf) and the chuck body is presumably steel. According to a friction table, the coefficient of friction (CF) for that combination is approx. 0.4 unlubed. My bolts are 3/8". Assuming grade 5 torqued to 35 ft-lb (per torque table), the resultant force is 5,600 lb per joint for a total of 16,800 lb clamping force. Multiplying by the 0.4 CF you get a theoretical friction force of 6,700 lb - still a lot of force. I doubt any cuts I'll be making will be enough to break that. Ray says he hasn't experienced any slippage using this technique, so I think I'll be in good shape.

Also, just to add my own observations to the discussion, it should be noted that the clamping load doesn't distribute itself evenly between the backplate and the chuck body. The pressure loads would be highest near the bolt holes and get progressively lower as you move away from them due to the elasticity of the metals. Since friction forces are independent of contact area (at least in theory) the total friction force is constant no matter how the loads are distributed. Just my 2 cents.

Allen
 
I feel like I'm re-taking Statics 101 all over again. Not that I need a refresher, but .......

Anyways most of us hobby machinists don't let stuff like this stop us from doing it anyways! We find out "quickly" if it doesn't work or does work.

Have fun!
 
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