Precision clamping devices can be custom designed for applications and can vary over a wide range of industries. David Jones, precision workholding manager for Emuge Corp. in West Boylston, Massachusetts, talked with Gear Solutions about the importance of workholding and designing precision clamping devices.
What role does precision workholding have at Emuge Corp.?
We are a precision cutting tool manufacturer, first and foremost. We began offering workholding solutions because we needed a better way to hold tools and parts in the machining process. Originally starting with our German parent company, EMUGE-FRANKEN, we are one of the leading workholding solution manufacturers worldwide, especially for demanding, challenging applications. We have a lot of collective knowledge — in fact, EMUGE-FRANKEN is celebrating its 100-year anniversary this year.
I make sure that our field salespeople have the workholding solutions they need in their toolbox and also ensure communication with our engineers in Germany is concise and accurate. Sometimes, there’s a little bit of a different way of thinking engineering-wise in Europe compared to over here, so we have to be able to accurately relay that information. I’m basically a facilitator, if you will, of technical details. There are a lot of T’s to be crossed and a lot of I’s to be dotted. We sometimes call it “attentive engineering.”
A finished part’s dimensional accuracy is directly proportional to the clamping device and its application — what are the challenges?
What that means is you need to hold the workpiece so it doesn’t move in the operation being performed. And I think that’s something that most people agree with — you don’t want the part moving once you put a tool to it.
It can be a little tricky from there, in terms of what the workpiece has available for areas to hold on to — geometrically speaking. In other words: Is there enough surface to hold the part, or is there a very short surface area available?
If you don’t start with accuracy, you’re going to lose even more accuracy moving forward.
You want to use as much of an available surface as possible to average out the tolerance. When holding to a turned surface, I can get a good, nice grip. When holding on to say, a cast surface, it is challenging because it may have some taper to it, and it is a rough surface to begin with. I want to hold on to as much of that available surface as possible in order to average out those highs and lows of the casting.
How does this procedure relate to gears?
Emuge workholding solutions for gears include hobbing, shaping, shaving, and inspection operations— grinding as well. And for many of these processes and with most round gears, say in a transmission for a car, you’re holding that gear in the bore. Generally, you’re going to mount that bore somewhere on a shaft or drive. And the tolerance goes from the bore to the pitch line and its teeth on the gear. What you want to do is hold that bore and make it your primary datum for the operation or operations being performed. Some of these operations, such as hobbing the teeth, may call for very high transferable torque values. So, in some cases, a little extra tailstock or axial face pressure is applied to the gear in order to overcome these higher torque values and keep the gear stable during the cut. In this instance, the geometrical relationship between the gear bore and the faces is very important. Remember, you want the bore to be the primary datum, so if one of the gear faces has excessive runout to the bore of the gear, it may strongly influence the final part quality.
If there is excessive runout to the bore of the gear, you would do something different with that face, including not having it touch a solid surface but having it touch a wobble plate of sorts. In a gear, if that bore doesn’t run back to the teeth, it’s bad news. That’s why that bore-to-face geometric tolerance needs to be good to begin with — maybe a few more pennies in the customer’s bank, if you will, to get a good part out the other side.
Basically, if you hold that part and the face has an influence on your geometric tolerance, the gear teeth are not going to run out to the bore when you’re done.
What’s the difference between the final assembly datum and the preferred workholding datum?
Final assembly datum is when you have all components assembled, and now you’ve got to mount that assembly to a flange or whatever device it’s going into. That assembly has accuracy to the surfaces that it’s going to mate to. If that assembly is made up of several machined components and the final assembly has that 10-micron run out to a taper mounting face, how accurate do the components need to be that make that assembly carry a 10-micron runout?
If I have 10 building blocks and, at the end, I need to have five to 10 microns runout, then each of the 10 individual building blocks has to be a portion of that tolerance. In essence, it has to be that much more accurate than the assembly is, because as you start to add those blocks together, that tolerance starts to build. That’s why you really want a preferred workholding datum so that the outgoing component and subsequent assembly are as accurate as possible.
The machining surface should have a datum that goes back to where the workholding is, if possible. And that’s something engineers should think about when they’re designing a part.
Why is having the clamping locations with tolerance important? And how does it help produce a better workpiece?
If I’m going to hold something from say, a cast surface, it is inherently going to be less accurate because that tolerance on the cast is not as accurate. For example, when turning a round bar and half of it is cast, and the other half is machined, when holding that round bar on the cast end, some inaccuracies result simply because of the less-than-desirable cast surface. Now, if you hold that same workpiece on the machined side, then you automatically have more surface contact to the workpiece, which affords options such as higher transferable torque values for your machining process or even perhaps less workholding force as well.
What’s wrong with delaying processes to ensure accuracy until the final operations?
What you put into something is what you get out, and if you don’t have good tolerances going in, it is hard to compensate with machining processes later on. In some cases, it can be done with slowing down the tooling pass and making a light finish cut. But time is money, and we want to produce parts as accurately and efficiently as possible. A good tolerance is important from the beginning, because you cannot expect to put a bad workpiece into a precise workholding device and still achieve optimal results.
Generally speaking in a turning operation, the workpiece turns, and the tooling stays still. In a milling operation, the tooling moves, and the workpiece stays still. Both need to be accurately held, so the workholding processes are critical from beginning to end of the machining process. And the manufacturer may not be using a precision workholding device, designed and built like Emuge provides. It may be something as simple as a three-jaw chuck in the lathe, but it still needs to be accurate. That first cut is going to potentially set the tone for the rest of the manufacturing process, so you definitely don’t want to wait until the end to address accuracy issues.
What other issues need to be addressed when working with those higher tolerances?
Repeatability is key. That’s not only for your workholding that has been designed and built to the tolerances in a drawing, but also the blanks being used. When blanks are received from their supplier, and the tolerance is different than the device it is designed for, there may be repeatability and consistency issues. That repeatability is important because that keeps scrap rates down.
It can be a perpetual cycle. For example, when a workpiece is not accurate, and you are holding on a bad surface, then you can have chatter and excessive tooling wear. Also, for example, there is wear to the workholding device because of variations resulting from lot-to-lot differences in tolerance. To compensate, the workholding device has to stroke farther and farther.
It’s like a paperclip; you can only bend it so many times. A paper clip is designed to do one thing. If you bend it the other direction so many times, it’s going to break. And that goes for your tooling, too. If the clamping element in your workholding has to move farther because of a bad tolerance lot of blanks, then it is clear you will have additional wear and, over time, a potential failure of the element.
How does insufficient rigidity compromise a workpiece quality?
I like to use the analogy of a pencil. Why do we hold a pencil down at the business end where the lead is and not at the eraser? Because when we hold it at the eraser, there’s no rigidity. The rigidity is down at the business end where the lead is. You don’t even have to hold it tight, but you get much more rigidity by holding it close to where the business end is being operated. If you think about it as a cutting tool or milling cutter, same thing — you want to hold that cutter down there where the business end is, not way up on the shaft. It’s the same for workholding. You want to get as close to the action as possible. It simply adds rigidity.
How does the geometry of a workpiece dictate the end-stop location?
The end stop is basically where the workpiece gets loaded into a workholding device, and physical metal-to-metal stops, so it indexes the workpiece at a specific location. At that point, the actual clamping pressure is applied, and the workpiece is held. Many of our designs have a slight axial pull in the clamping cycle, which physically pulls the workpiece toward the end stop, helping to achieve a very rigid design. If the rigidity is not present, issues such as tooling wear and surface-finish degradation begin to show their faces.
Conversely, if the face that’s touching the end stop is not good and it doesn’t have a good run-out to the primary workholding location, then we may not use what we call a solid-end stop, which is basically a solid piece of metal that generally has a ground surface. What we would do in this case with a bad face runout is put a wobble plate in there as a stop. We still have surface touching on the wobble end stop, but it’s not a primary location. That surface is allowed to float a little bit and find its home. So, we still add some rigidity to the application, but we limit the influence this face has on the runout of the workpiece.
What’s the purpose of the wobble plate?
We’ll use a shaft, for example. With a shaft, there is a diameter to hold on, as well as a left and right face. When loading the shaft into the chuck, one of the faces is going to go to an end stop to locate the shaft within a device.
If that face that’s inside hits the end stop and it isn’t flat or isn’t square or it doesn’t run out to the bore, then you may have a fixed runout issue in that shaft before it’s even clamped. We don’t want that to happen. If that face is bad, we will put in a wobble plate for an end stop.
The wobble plate is still an end stop, and it still has some rigidity, but it lets the workpiece find its home as it’s coming through the clamping cycle, so that the clamping location is still the primary location, and this bad face had little influence. If we had a shaft, and the backside of its face was cut at 4 or 5 degrees (as an example only), it doesn’t look like a significant angle, but for workholding it is. In the end, you haven’t even put chips on the floor yet, but you already have runout issues.
What’s the importance of having a rigid workholding solution?
Rigidity is key, and that goes for almost all machining operations. Rigidity will result in better tool life, better surface finish and better machine life. It boils down to rigidity, rigidity, rigidity.
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