Part edge Taper

[LancoUSA]

So...... we have a cnc cell here that runs kurt vises. we hold our parts on shop made vise jaws that have steps for the part to sit on like paralells. I am getting taper in the zdirection on the edges that are aligned with the vise jaws. We have tried different tooling, climb vs. conventional cutting, finish passes. I also check that the guys were not over torqueing with a torque wrench and then again with a load cell to measure force on the jaws. I also had them run the parts with standard Kurt hard jaws. Nothing seems to change the taper except for clamping on a huge chunk of material way down inside the vise, and machining up above the jaws. This is how I ruled out tooling and the head being out or something weird. So My problem lies in the vise...... Any one have any Ideas? Thanks.

[htm01]

you can't rule out tooling without indicating holders and tools, usually have better luck with solid holders over collet. also level and tram of table.

[Del.]

Try putting a block in vise that you know is square all the way around and check with indicator to eliminate vise problem.

[LancoUSA]

We used the same tools holding by 4mm and then sinking the part down inside the vise 38mm gave us no taper. I don;t think my problem is my tools. if it is my tools I am not as worried about that. I am getting .0007" wall taper in a 14mm tall part. The taper is only present on the edges that are aligned with the vise jaws. The other two edges square with the vise jaws have no taper.

[htm01]

if your vise and part are square to begin with then the vise may be flexing, how are you holding the vise down? you may need to clamp the vise in more than one spot.

[kh012]

it probably has to do with the clamping pressure of the vise. the vise compresses the part at the base, then you cut it.

when you release the part from the vise it springs back.

the only way we have found to get rid of this with kurt vises is to use very lite torque for the finish cuts.

on some of our parts we are down to around 25 inch lbs. or less.

 

(edit) you might try adjusting the screw on the movable jaw on the vise. you can tighten it up

to get less movment or slop in the jaw.

 

HTH Ken,

[LancoUSA]

ken,

I actually went out on the assembly floor and got a load cell to check the tightening pressure when certain foot pounds were applied. i had one guy tightening to around 400-600 psi repeatedly and one guy tightening to 1200psi inside the jaws. the weird thing is that you get almost the same taper on those edges between the two guys. Just for fun i got the handle and puts a lot of torque on the handle, not sure how much. but I got it up to 5000psi. I did that just to see what it would go up to. I think it is definately bowing the part. I think tightening and then loosening for finish is not a good answer for us, but I will look into it.

[Newbeeee™]

I'm sure Ken has it above ^^^

Put the part in the vice and nip very lightly. Put an indicator on the part. Then tighten the vice to your normal tightness and you'll probably see the indicator move.

It's really easy to get a few thou as you can put tons of clamping force on a part using a vice, with very little effort.

 

Slight tangent could be strength of part. As you cut the centre of the component out the part gets thinner and weaker as the material is removed. The vice pressure stays the same and the moving jaw gradually moves as the strength of the part dissapears in swarf. So effectively, your clamping pressure isn't constant while the part is being made.

I hate vices for production work.

 

OT/ I remember during my apprenticeship, I put an indicator on the bottom of a cutter in the spindle of a pre-interact (Bridgeport BOSS).

With one hand on the head I could flex the casting a couple of thou plus and minus on the indicator just by pushing or pulling with not much force.

[LancoUSA]

all our production work is sent out. We do one offs almost all the time. so we use the vise for 90% of our work, and we use 16 common tools so we hardly ever change the standard tools. We just add the extra drills and reamers. it works out really well. except for the taper on those two edges. i did the indicator thing first before the load cell. i ended up with .0005 to .0008" movemoent. I used two indicator on the bottom close to the jaw and on the top edge of the part. It is just driving me crazy..... By the way. I think our biggest problem is most of our parts are aluminum. So they flex or bow easily.

[kh012]

if the part shape allows it I generally try and do all the roughing and drilling with a high vise torque on the part.

then add an "M0" to retorque the part for the finish tools. this is usually on a part by part basis but we do it

on all types of mat'l. thin walled alum. can be really fun like prg. an arc to cut a straight wall to compensate

for the part flex in the vise.

 

Good Luck Ken,

[BenK]

Put the part in the vice and nip very lightly. Put an indicator on the part. Then tighten the vice to your normal tightness and you'll probably see the indicator move.

 

 

^^^^^^

+1

 

Also check your soft jaws to make sure they are not tapered. Also check to make sure there is no corner radius in the bottom of your jaws pushing on the part.

[kh012]

(Also check your soft jaws to make sure they are not tapered. Also check to make sure there is no corner radius in the bottom of your jaws pushing on the part.)

 

 

we sometimes use a dovetail cutter to under cut the fillet in the jaws to prevent the part lifting off the jaw.

 

Ken,

[LancoUSA]

I was thinking the same on the soft jaws with the corner radius from the end mill, how erver small it would affect the holding of the part. THat is why i had them try running the parts with stock hard jaws from kurt with paralells. No change in taper. I am leaning more towards tightness of the vise. I just have to figure out what we can do to make acceptable parts and still hold it in the machine without launching it.

[jeff]

grab a scrap piece of stock, mill around all 4 edges and check for taper on all the sides.

 

Are you taking a finish cut?

 

Try a reduced shank endmill where the flute length is minimal to reduce deflection?

[LancoUSA]

Yes, we have tried finish passes and all sorts of other tricks too. the average plate is only 10-14mm tall. So i am not reaching down the length of a tall part to need reduced shank tools

[LancoUSA]

Yes, we have tried finish passes and all sorts of other tricks too. the average plate is only 10-14mm tall. So i am not reaching down the length of a tall part to need reduced shank tools

[McLaren]

Have you put an indicator on the highest point of the fixed jaw to see if it is moving under torque? If your fixed jaw flexes your part will go with it. Also make sure the floating jaw set screw is snug enough to prevent lift.

[heavychevy2155]

Also make sure when you change hard jaws they are not lifting up when you bolt them to the vise body. I always snug the jaws, then tap them down and finish torquing them tight. It doesn't matter if you cut fresh steps in the jaws. Because as soon as an operator puts a part in them and hammers the living hell out of it to making sure it's seated, the jaws will move then the parts come out tapered.

[LancoUSA]

As much as I am not a chevy fan..... Just kidding. I will check into that one and check the hard jaw flexing. Aren't Kurt vises supposed to pull the movable jaw down towards the bed? with tha angled knuckle thingy? I have not checked snuggness of that either... Thanks guys I will look into those two.

[PLANET]

How are your vises clamped down? Most shops I have been in use the D688. If you only bolt down the 2 spots in the casting, over time the vise will bow up off the table in the rear. I've seen that make a difference in parts. It's hard to see, try slipping feeler stock under it, also may see coolant squirt out when tightening. Also have seen chips get under the rear of the vise & hold it up. I always try to put at least 4 clamps on the vises.

[LancoUSA]

Planet I have not looked into that either..... I will check that out. OUr stuff is all new with the machines....We do only clamp down using the two screws....and have vises set side my side. We have a maintanence schedule to take them off once a month to be cleaned under them.

[McLaren]

That would show taper even when you clamp the part all the way down into the vise, which you said it doesn't. I think you can rule out chips under the vise, or the bottom of the vise bowing up.

[LancoUSA]

As long as I faced the part and did not face the back side just saw cut it or something all edges would be square to what was machined in that operation.

[Kevangel]

If I understand this right you are saying the top and bottom are not parallel after cutting (Zaxis)...is this like a single pass facing cut with a 3 inch type face mill? If so maybe your head is out..if your facing with a x-axis feed over the part..try using the Y axis to face with and see if any diff..a little hard to understand without a diagram....

 

If not out put an idicator on back vise jaw when free and zero, then tighten and see if it lifts...angle locks are suppossed to pull downward...this should not be that hard to solve..

[LancoUSA]

Kevangel,

It is have taper on the two edges aligned with the vise jaws in the x axis. I have ruled the head or anything on the machine being out. I sunk a part all the way into the vise and milled above the jaws. It milled square with in .0002".....