The Necessity of Driving a Part down on Parallels

wackyvorlon · Feb 1, 2009

I've been wondering about the necessity of using a hammer to drive a part down on the parallels. I've had the part bounce up at times and other times it's been marred. Tonight I decided to test it.

I put a 1-2-3 block in the vise, on parallels. I then put my indicator on the block, and watched as I tightened it. No movement.

To my mind, this indicates that it's not necessary to drive the part down. Am I right in my thinking?

Seekins · Feb 1, 2009

Do that 50 times and see what kind of consistency you get. Doing it once is not a accurate test. Every part is different. I have some that seat perfectly and some that need some persuasion from a mallet.

wackyvorlon · Feb 1, 2009

I did test it a couple times. Mind, the vice itself may be a consideration. It's new, made by GS Tooling(part of Sowa). It has the mechanism for downforce.

The real Leigh · Feb 1, 2009

When a part is properly seated you should not be able to move the parallels with finger pressure.

How you achieve that condition is a matter of personal preference. I use a plastic-faced hammer and tap the work into position.

You're not supposed to mash it with a sledge hammer. :eek:

If you damage the work you're hitting it too hard or using the wrong tool.

- Leigh

stannp368 · Feb 1, 2009

Workpiece on parallels

There are too many variables involved but every time I setup, I tap lightly w/ a lead hammer & then feel for snugness on the parallel bars. Remember, nothing is perfectly square & parallel & can easily be off a thou or two

EPAIII · Feb 1, 2009

The need for this is largely a function of the type and quality of vise you have. Many inexpensive vises (drill press vises) have a movable jaw that is somewhat short and the screw applies force to the center of it. This, in turn, can cause that movable jaw to rotate in a manner that moves it's face and the part that is resting on it, in an upward direction. So the part rises and also rotates since the other side is against the fixed jaw. Here is a drawing that illustrates this:

This generally only happens when a part is supported on parallels so the bottom of the movable jaw can move closer to the fixed jaw as shown in the drawing.

The vises that are made for milling have longer movable jaws and better overall quality/tolerances so they do not show this kind of behavior or at least do so to a much smaller extent. But they cost a lot more than the less expensive, drill press style vise.

Another solution for SMALL parts is the type of vise called a machinist's vise or a screwless vise (not really screwless). These vises have screws that are oriented at about a 45 degree angle downward so they are simultaneously pulling the movable jaw down and closed. This also eliminates the upware motion of the part being held. But they are relatively small and not suitable for larger parts or heavy cuts.

HuFlungDung · Feb 1, 2009

If the stock is perfectly parallel sided, it may stay seated on parallels using an angle lock vise. The older crap vises would/could not even do that.

But if you are grasping the mill edge of a piece of stock, then the parallelism of the opposing sides is not guaranteed. Indeed, quite often the mill edges will be convex, and if you clamp them below the center of convexity, then you are virtually guaranteeing lift-off because you've got two opposing leverages working against one another. The angle lock jaw cannot compensate for out of parallel with its own action. However, the fact that the moving jaw does pull down to the vise bed means that you can at least seat the part on parallels with a hammer, if you apply the right amount of force to create movement, but not so much as to create rebound.

wackyvorlon · Feb 1, 2009

I should see about checking it with a feeler gauge. I will have to experiment more. I might see about putting an indicator on the part when I tighten the vise on a regular basis, to see if it moves.

Metalcutter · Feb 2, 2009

All these replies have good reasons and address many variables.

You almost have to look at it as though you were holding a piece of rubber. If the two edges of the part you are holding on have a very slight "V" taper the part will tend to bow under vise pressure.

One "gotcha" happens when using soft jaws if the step in the jaw was cut with a worn end mill. The corner radius will raise the part when clamped.

It's very difficult to get variables "beat down to their ankels."

A one two three block is pretty square all over, so that helps a lot to identify the actual vise errors.

Regards,

Stan Dornfeld Quad-I.com

Mike Nash · Feb 2, 2009

HuFlungDung said:
If the stock is perfectly parallel sided, it may stay seated on parallels using an angle lock vise. The older crap vises would/could not even do that.

But if you are grasping the mill edge of a piece of stock, then the parallelism of the opposing sides is not guaranteed. Indeed, quite often the mill edges will be convex, and if you clamp them below the center of convexity, then you are virtually guaranteeing lift-off because you've got two opposing leverages working against one another. The angle lock jaw cannot compensate for out of parallel with its own action. However, the fact that the moving jaw does pull down to the vise bed means that you can at least seat the part on parallels with a hammer, if you apply the right amount of force to create movement, but not so much as to create rebound.

I have found my 5" "Kurt Type" Enco vise was really bad to flex the fixed jaw which caused the workpiece to lift up and rotate. The 6" same source replacement (that looks even more poorly made) has less of this fixed jaw lift. But I definitely noticed that I could keep my workpiece down on the parallels better with light taps from a dead blow hammer. Heavier blows do indeed seem to cause the workpiece to bounce up off of the parallels. And even Kurt advises not to strongarm the handle on finish passes, but to back off the screw pressure somewhat after any hogging cuts are made since there is flex even in their vises due to the amount of pressure that can be achieved with the screw

Justsomeguy · Feb 2, 2009

wackyvorlon said:
I've been wondering about the necessity of using a hammer to drive a part down on the parallels. I've had the part bounce up at times and other times it's been marred. Tonight I decided to test it.

I put a 1-2-3 block in the vise, on parallels. I then put my indicator on the block, and watched as I tightened it. No movement.

To my mind, this indicates that it's not necessary to drive the part down. Am I right in my thinking?

It doesnt move. But I would think that its possibly not seated right. Use a surface gauge and an indicator set to 'zero', hammer it and re-check. Is it still zero?
Alot of possibilities and variables have been brought up!
Tony

Mike C. · Feb 2, 2009

I'm with Leigh. You are bashing the part. Another common problem is overtightening. Unless you are really hogging material, standing on a 3ft cheater bar is not required to keep the part from moving.

dp · Feb 2, 2009

When the sides of the work against the jaws are not parallel a bolster bar is useful to allow the part to gracefully align itself to the fixed jaw of the vise. Here's a quicky tutorial:

http://www.jjjtrain.com/vms/mill_squaring/mill_squaring_06.html

AccessDbGuy · Feb 2, 2009

What I was Taught

Back in my training days (old history), we were taught two techniques:

a) Use pieces of clean (!) paper. Place them (more than one!) between the work piece and the parallels or the bottom of the vice. Set the vice jaw. Try to drag the pieces out. If one or more slip away easy, and one or more remain in place, the bottom of the work piece was not evenly flush with the parallels/vice. Start all over again, using a plastic/leather/copper-headed hammer to set the work piece, re-try the paper.

b) If the sides of the work piece that face the fixed and movable jaws are (sort of) parallel, place a piece of round stock, placed horizontal, between the work piece and the vice's movable jaw. This permits minor out-of-parallelism to be ignored to some extent.

Your mileage may vary.

Stan Db

HuFlungDung · Feb 2, 2009

In this age of high tech, has anyone made parallels with an integral strain gauge, battery operated, with red and green LED's on each end to register contact?

Perhaps this feature could somehow be engineered into the vise bed itself?

michaelab · Feb 2, 2009

There have been several threads on this subject. I use a kurt vise and never use a hammer. I hold parallel and squareness all day long.
I have also worked in shops that do not allow this practice of hammering the vise in any way.
My question to anyone who hammers the vise is, have you tried not using a hammer and measuring the results?
There is an adjustment screw on Kurt type vises that decreases the amount of clearance on the movable jaw.

David Carlisi · Feb 2, 2009

I find that repeatability of any vise is relative to it's quality and amount of wear. A good vise will repeat very well, as long as you apply a more or less equal amount of pressure to tighten. It sometimes helps to apply a slight amount of downward pressure with one hand while tightening with the other. From time to time, I use a dead blow hammer to tap (not pound) a part so that the parallels will not slip. I do this when I'm working with lesser quality vises.

wackyvorlon · Feb 2, 2009

michaelab said:
There have been several threads on this subject. I use a kurt vise and never use a hammer. I hold parallel and squareness all day long.

What I wonder about is mention of movement in the parallels. I can slide them from side to side, but I don't detect any vertical movement. Mind, this belief isn't backed up by indicator readings. It occurs to me that if the bottom of the part is generally flat to within a thou, you wouldn't need much room at all between the bottom of the part and the parallels to allow them to slide.

michaelab · Feb 2, 2009

wackyvorlon said:
What I wonder about is mention of movement in the parallels. I can slide them from side to side, but I don't detect any vertical movement. Mind, this belief isn't backed up by indicator readings. It occurs to me that if the bottom of the part is generally flat to within a thou, you wouldn't need much room at all between the bottom of the part and the parallels to allow them to slide.

.0001 is all you would need to have a parallel slide freely. The vise and the parallels are precision ground. It does not take a lot to allow the parallels to move freely.

HuFlungDung · Feb 2, 2009

michaelab said:
There have been several threads on this subject. I use a kurt vise and never use a hammer. I hold parallel and squareness all day long.
I have also worked in shops that do not allow this practice of hammering the vise in any way.
My question to anyone who hammers the vise is, have you tried not using a hammer and measuring the results?
There is an adjustment screw on Kurt type vises that decreases the amount of clearance on the movable jaw.

I don't hammer the vise, I hammer the part, using a dead blow hammer. Using the 'slide the parallels' method of checking the quality of downness, I can safely say that the parts I install in the vise are rarely if ever, perfectly down on both parallels.

I also use my fingertips to feel the part as I hammer it. You can feel it move an almost imperceptible amount if its not down by the way the reverberation changes.

The Necessity of Driving a Part down on Parallels

Plastic

Stainless

Plastic

Diamond

Hot Rolled

Diamond

Diamond

Plastic

Titanium

Aluminum

Aluminum

Diamond

Banned

Aluminum

Diamond

Plastic

Stainless

Plastic

Plastic

Diamond

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