Reply with QuoteI think that it would be far more useful to know what the finished part looks like instead of
what the vice looks like.
How to measure clamping force on a vise ?
Finish my last run and noticed a few of my thinner walled parts have a slight .022" dip on them. I took everything apart and couldn't find the problem until today. When face milling the back side of my parts off I'm clamping too hard on them and deforming the parts by .022" . So instead of playing around with it and ruining more parts I thought I'd ask you guys first. I'm not really sure how much force I need to clamp the material at the speed and feed I'm facing. Last thing I want is the stock flying at my head or a broken tool...
6061-T6
Tormach 1.5" Face Mill
.02" doc
4340 rpm
40 IPM
Climb
As for the vise I'm currently using
shars.com - 6quot 670V CNC Milling Machine Vise 00004quot
How are you guys measuring how hard to clamp your vise down to avoid this kind of a problem ?
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I think that it would be far more useful to know what the finished part looks like instead of
what the vice looks like.
I agree.
Finished part would help. Stock thickness, etc.
I ran into this issue myself. A vise wasn't the best tool for the job when running thin parts. I use a magnetic chuck for sheet steel. I use a clamping plate with toggle clamps for sheet aluminum. I use a few different vises and jig plates for other setups on different material.
It may be that you need to build a dedicated tooling or jig plate to hold these parts if you intend to do a lot of them.
Lee
I am looking to move over to a subplate for a lot of my smaller parts if I can for machining off the back. Interested in this thinking as well. While I agree a picture of a part could help, I think what twocik is asking is what should you look for. Would hate to have to post a picture of every part I want to make.
What I do is after I mount one of my parts I run an indicator across the top of the part while I am setting the Z depth. I can tell if the clamping is actually warping the part of not. At least this is how I found it on a few parts and have started holding them very lightly with very light cuts or mounting on a subplate.
Experience. While wearing a face shield or working with an enclosed machine.Originally Posted by twocik
If your workpiece bows you are clamping too hard.
If it gets pulled out of the vise you are clamping too light for the depth and width of cut and the feed.
If you cannot get an acceptably small enough amount of bowing by reducing the depth, width or feed and grip pressure, without throwing parts all over the countryside, then you go to some other way of holding the part.
An open mind is a virtue...so long as all the common sense has not leaked out.
"I think that it would be far more useful to know what the finished part looks like instead of what the vice looks like."
I was just asking in general how to measure clamping force on a vise. I think if any of you tool makers are listening, wouldn't be a bad feature to start adding in a vise if possible. I'll upload a pic of the part if this would make it easier...
Stock thickness is .5", part .38" leaving just about .140" to face off. One problem is my part at the top has a very thin wall on the top pocket of the part at .125" depth.
"Would hate to have to post a picture of every part I want to make."
You know normally I do post a pic only because a pic is worth a million words...... Doing it now
"Experience. While wearing a face shield or working with an enclosed machine."
Yea still working on that..Always something new to learn, but is grey area that would be a very nice feature to have on a vise.
"If you cannot get an acceptably small enough amount of bowing by reducing the depth, width or feed and grip pressure, without throwing parts all over the countryside, then you go to some other way of holding the part."
Could be another great thread....
This is exactly how I taught my step son to drive. Told him when he tried to stop at a red light and he went into the intersection he wasn't applying enough brakes. If the car behind him hit him in the rear, he was applying them to hard.Ahhh.. Nothing beats experience! (sorry, couldn't resist that).
If you wanted to actually measure clamping force you could do it with a spring or a hydraulic cylinder and pressure gage to give you numbers you could calculate pressure with. But this isn't what you want to do is it? I have never had to do this but I would suggest using a torque wrench for repeatable clamping. (Bock makes one but it probably on fits their vises.) Of course, you'll have to determine the correct torque or clamping force by experimentation. Maybe you could put a dial test indicator on the part and stop clamping when you see deflection. Obviously, that's not going to work too well if your operation is thinning the part where the deflecting forces are being applied. I do not understand what the part looks like or how big it is. If it is small maybe you could change it to a 4-axis operation by using an indexer. Stick a rod of your material through the indexer, machine one side, rotate 180 deg. mill 2nd side and part w/ an end mill next to the indexer. I have had to do this with some parts because of deflection and registration problems.
I'm sure this is really easy to figure out for this part, but what I was asking is what would be the best way to measure something like this. A torque wrench is a good idea, but would need to find a socket. I guess the good old feel of expierance is best...
Probably not the best pics, but only had a few minutes take them. My 2.5 year old starts school tomorrow and guess what... will not go to bed !!! I'll try and take a few better pics if these don't get the point across. That little gap between the vise and pocket on the part has a wall thickness of .08" at .125" depth.
To measure the clamping force one could install a strain gage on your part or even on the vise jaws. Strain gages is easy enough to make or buy. It was a rite of passage in EE instrumentation class (decades ago ) to make one’s own strain gage and metering electronics. Omega engineering sells already built strain gages. The Strain Gage The advantage of the strain gage on the part itself is that one could apply the minimum force needed in order not to deform the part. That is one of the reasons I opted for a pneumatic 5C closer for my lathe so I could easily set the clamping force using a simple differential air valve when holding thin walled Turcite parts for turning. But as been mentioned it would be easier to just determine by experience just how much pressure on your vise and how aggressive machining you can get away with. However a strain gage vise jaw pressure measurement system might just make an interesting DYI project. A strain gage meter could also be useful in setting bandsaw tension.
Don
Couldn't you take the part and send to some sort of FEA software. You know how much torque one turn of the vise can generate. You can use the software to tell how much force on the sides start to deform the part and perhaps determine the amount of torque or turns needed to apply with the vise?
It is not really feasible to try and measure the clamping force that is needed because there are so many variables such as the shape of the material, the coefficient of friction between the vise jaws and the part, whether the vise jaws are perfectly parallel or slightly splayed, etc, etc.
But once you have learnt from experience what works with one part it is possible to extrapolate to other parts and treat them in an equivalent manner. Then the device(s) shown in post #2 in this thread is very handy for getting repeatable clamping much better than a torque wrench.
http://www.cnczone.com/forums/genera...implified.html
An open mind is a virtue...so long as all the common sense has not leaked out.
It feasible to measure the forces directly on the part using a strain gage. By measuring the forces directly on the part it doesn't matter what variables the vise has.
Don
Certainly you can measure the force acting on the part. But you have no idea whether the force you measure is adequate to prevent the part moving under the cutting conditions you are using.
An open mind is a virtue...so long as all the common sense has not leaked out.
With a strain gage one could determine the maximum force so that the part is not bowed. The amount of force needed to machine would be determined experimentally.
Don
BTW Personally I would just experiment, adjust the feeds/speeds/vise pressure and perhaps trash a few parts till I got the right results. Remember however the OP’s original question and the thread subject was how does one measure the forces on the vise. A strain gage is one way that could be done.
The problem with a strain gauge is you need some strain to measure. Strain in this case being distortion of the workpiece. No distortion equals no measurement of strain, therefore no force value.
Phil
unless your running a million of the same part then over analyzing will only prove to be a waste of time because your next parts will be different
the easiest and quickest trade practice is to put a dial to the center of the part , then tighten till the dial shows some movement then back off some , if your doing multiple parts then set a torque wrench ,
if removing a lot of material and or if precision is needed then another thing that is fairly typical practice is to leave extra material after hogging , have an m0 , back off the vise pressure , tap down the part( if needed) , then finish cut
A poet knows no boundary yet he is bound to the boundaries of ones own mind !! ........
Bingo.
Phil
Bingo.
Phil
Originally Posted by dertsap
...... the easiest and quickest trade practice is to put a dial to the center of the part ......
The point I was trying to get across is that it is possible to measure strain by deflection or clamping force directly. But there are so many other factors you do not know what amount of strain/force is needed to hold a part adequately.
As an example:
http://www.cnczone.com/forums/genera...t_machine.html
An open mind is a virtue...so long as all the common sense has not leaked out.
Workholding is an art in itself. It is important to understand the dynamics of what you are doing and design the work holding in consequence to that.
For example when you face mill, you have a large force in the direction the cutter is moving, a lesser force perpendicular to that and comparatively little lifting force. So if you are clamping a part in a vice setup in the usual orientation where your clamping force is applied in Y and you are milling in X then you are trying to resist the greatest milling force by friction alone. As you have seen, with thinner more flexible stock you will have bowing problems.
So how do you get around this? Some options (with no regard to practicality in your specific instance) are:
Mill in the Y direction, this way the greatest force is resisted by the vice jaws. This allows you to significantly reduce the clamping force.
Use a pinned soft jaw or a good vice stop and machine towards the stop. This gives you a mechanical limit to movement and allows you to reduce clamping force.
Build a fixture using a low-profile clamp like a Mitee-Bite bulldog to hold the part against stops.
The linked book is a good read on the subject. It is a bit hard to come by and a bit pricey if you can find it. However Chapter 2, which sets out the basic principles, is in the Google preview. The rest of the book is in cookbook form where it lays out a problem and a potential solution.
Setup reduction through effective Workholding
Another book by the same author "Jig and Fixture Design" is readily available at Amazon etc...
bob
