Milling Wrench Flats

[ranchak]

This is not a BobCad problem, but I know there are some really smart guys on here and I really don't want to hear all of the BS on the General Forum. I have been having a problem machine wrench flats on some round parts. I have the same problem on my cnc mill and the manual mill. First off I am using two square collet blocks, not imported ones. I am holding the rod on each side of the area that I want to mill the flat on. The problem I have is that the flats are not the same depths. One side will be deeper than the other. I am setting the blocks on parallels, machining in the Y axis. I am careful to place the blocks on the parallels and I even tap them down before tightening the vise. Both blocks are clamped in one vise, although I had the same exact problem on a longer part where the blocks were held in separate vises. The only thing I can think of is the blocks may not be parallel?? I'm pretty sure that it's not operator error, but you never know! I'll check the runout of the part, but I don't think there is enough to be causing this problem. I did check my Z height when I previously experienced this problem, if I remember correctly it was about 0.003" - 0.005". I haven't measured what my flats are but you can easily see that the flats aren't the same depth. Any thoughts??

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[jrmach]

I will tell you the ideal way to check things.Surface plate with height gauge/with indicator in it.Set gauge for height on part for one side,then rotate ,compare measurement,repeat.You can apply what I just said in the mill by using indicator in drill chuck and collet holder on mill bed.
I manual machine about the majority of my parts.That being said,if you do much of any kind of collet indexing,you need to get a 5c collet indexer.It is a must if manual machining much.You will never use those collet blocks again.

[BurrMan]

Those wrench flats are practically invisible!!!

[RCaffin]

A photo of your config would be really helpful.

I machine a lot of flats on small rounds, no problem at all.

Cheers

[fizzissist]

Sounds like the collet block's outside is not concentric with the inside, or you're using a lousy collet that isn't clean, or something...
It's really that simple.

[jrmach]

A pic would be helpful.I am trying to figure out why it is that you are using 2 collet blocks? ?Not too many parts would require that.And if you needed support on the other end I can think of at least 3 ways off the top of my head that would be faster,easier,repeatable,and just better.Pic of what the set-up is with part,I would be glad to make suggestions for best way.Done so many hexes and flats,I would hate to guess how many.

[ranchak]

I'll post a pic tomorrow. The collets are Crawford brand, they are new and haven't been used much. I don't notice any runout when using them in the lathe. I have had the same problem machining the wrench flats with two different sizes of collets, makes me suspect that fizzissist is correct about the collet block. The reason for two collet blocks is the parts are sometimes are long and there are flats at each end of the part.

[JEST]

Is most likely the collet blocks. Is the time a height gauge comes in real handy!
It is staggering how quickly a machinist' eye will be drawn to a size difference of only a few thousands of an inch.

Remember also... The length of the chord is affected at both ends for a change in base radius. Meaning any discrepancy in Z, is "double" error in the chord length (as your eyes show)

[ranchak]

Here's a pic of my setup. I checked the block with an indicator, within .002" on all four sides. I'm about ready to get another set of blocks.

[JEST]

lets use that .002 off...
the arc highly effects the width of the flat. At that diameter (.5?), a .002" Z shift, means a difference of .0046" in surface width for the matching 180 flat. Your eyes will see the 2% change rather easily.

[jrmach]

Are you sure something like my pic isn't going on?Also,a Little support on the very end would help.Maybe Planar Shaper Gauge,or tail stock center .

[JEST]

I didn't think he had any taper issues....hmmm

[holbieone]

sounds like your center of rotation is off

[ranchak]

holbieone, what do you mean? concentricity?

[ranchak]

I don't have a taper issue, I have used an extra block on the end of the longer rods to add stability. I know there can be some error in the material itself, but I can't believe it's what I'm seeing.

[Dylwad]

Is the collet bore perfectly centered in the block? Are you just flipping the block upside down in the vise or rotating the part in the collet between sides?

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[ranchak]

Flipping the block.

[holbieone]

Flipping the block.

i was think of the error you would get by rotating or flipping the block

remember any error you get on one side is double by flipping

[Dylwad]

Touch off the z on each side

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[jrmach]

Are you sure something like my pic isn't going on?Also,a Little support on the very end would help.Maybe Planar Shaper Gauge,or tail stock center .

To elaborate a little further,,,,,the pic I am showing is exaggerated,but if the taper that is ground in the collet holder is at an angle,this is the result you would see.Indicate top of PART,go to other side of collet(rear side) check height of PART.Rotate collet BLOCK,repeat.Total of 8 measurements.Your material is straight,right?