4140 internal stresses

It is generally accepted that all parts have stress, the result of the manufacturing procedure, as you alluded. Hot rolled materials tend to have much less.

Good shop practice dictates that one roughs parts, leaving sufficient material for finish machining, with total cleanup. The less material remaining, the better, as the piece will more closely represent the finished object, and the small amount of material removed in the process should make very little difference in movement of the part. I expect that unless you have a very critical dimension, less than .0002" tolerance, you should be fine.

Nice job. I spend a lot of my time trying to have folks understand that roughing is a very important part of machining. Some don't pay attention (until they have the pleasure of reviewing the resulting scrap, anyway).

Time can be a factor, but the roughing should release much of the stress. If you have doubts, allow the part to age a day.

Harold

I backed off the chuck jaws a tad before I came in , rechucked it after a couple of hours . After recentering it I found about .002 runout on the last area I'd machined . Looks like it might have had a bit of stress . I'll finish it up today .

We call it "Roomalizing"!

Speaking of which, would you have a piece of 4140 that is 6" x 2" x 3/4" or thereabouts that you could sell?

Nope , all I have is rounds in 1.250 and 1.5"

"Ed Huntress" wrote>

Shirley I do . More knowledge is always a good thing .

Thanks anyway. I need to make an arm that holds a carbide square like an arm on a pair of scissors. The current one is cold-roll and is flexing too muck under pressure. Ya' think 4140 would be a better choice?

Probably not. The modulus of elasticity will be the some. The yield will go up, so it will bend farther before it takes a set, but it will flex just as far with the same load. Redesign the part to have more meat in it.

Paul K. Dickman

The flexing -- that is, the spring rate without taking a set -- is the same for all grades on steel, within a couple of percent. More correctly, they all have about the same Young's Modulus, and it doesn't matter how they're heat treated or work-hardened. The modulus will be the same.

That is, except for stainless, which is a little springier (lower Young's Modulus).

I was afraid of that but I can add 20%-25% where it counts. What about hard D-2?

My cousin told me that, I was hoping he was wrong. I could make it out of Ti and/or go from 5/8" to 3/4" or a scooch more.

Not titanium! It's MUCH springier than steel. The Young's modulus for steel runs around 30 (don't worry about the units). For titanium, it's around 16 -- about the same as gray cast iron (14) . Aluminum is around 10.

Tungsten is a little over 60. Tungsten carbide is close to 70.

On the other hand, stiffness is a function of the *cube* of thickness (called "plate stiffness"). So making your part a little thicker will improve its stiffness by a lot more than you might think.

In your example of going from 5/8 to 3/4, the part would be 1.2 times thicker but 1.73 times stiffer.

Tom Gardner wrote in news:meidnU-ZdcGZqffMnZ2dnUVZ snipped-for-privacy@giganews.com:

I'm a little rusty on this stuff, but for a simple rectangular bar, I believe the stiffness goes up as the cube of the dimension. You shouldn't have to add much to make it significantly stiffer. 5/8" to

3/4" would almost double the stiffness (halve the deflection). Adding a rib could do a lot, if there is room.

Doug White

A rib, or better yet use U-Channel so the face and stiffening ribs are all one integral piece, and a chunk of thick bar at the top to bolt your inserts to - go as physically big as you can. Modify the machine to make room if need be.

If this is one of those gidges you have mentioned in the past where you're using the carbide inserts as a hard-faced guillotine and anvil-blade to chop wire to length, don't count on the machine and mount rigidity to keep it aligned...

I'd have a finger from the upper jaw extend below the lower jaw where they overlap, and a few roller bearings with a fine-threaded gap adjusting method. And a jam nut to keep it there.

Physically keep the upper and lower blades from crashing, even if the machine gets out of alignment.

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That'll do. The part's 10 years old but the new wire it cuts is higher tensile.

Room for another 1/8", 5/8" to 3/4". Thanks!

Yep, you must have seen it! The cutter uses a 1" shaft riding in Timkin tapered roller bearings that this arm is attached. I use 3/4" square x

1/4" thick, coated HSS inserts. I've tried every grade of carbide, even custom made they get tiny chips and and they all self destruct in very short order. The HSS works great and they are only $5 and I get 8 cutting edges on each.

I think the arm sometimes gets too much pre-load pressure from the mechanism that tightens the stationary blade. It gets readjusted every time we do a blade change.