Work-Holding Techniques

[Alex Nelson]

Hey all,

Anyone who's even dabbled in machining knows that good work-holding and fixturing is critical to making good parts. It's also very hard to find good solutions on the scale of the Pocket NC or similar micro-mills. I figured I would share what I've come up so far with in the absence of ready-made solutions.

The most versatile work-holding tool is a strap clamp. These are usually beefy pieces of black-oxide steel that can be trusted to stay put under the most intense vibrations and cutting forces. Nobody sells strap clamp sets, however, that are small enough for the Pocket NC machine. Inspired by this design, I designed my own for use with M4 socket-head cap screws, and 3D-printed them on our Form2 SLA printer. I tapped them for 1/4"-20 threads so I can use set screws or socket head screws to set the height. I tested them by securing a Sherline 3-jaw chuck to the machine bed. The chuck allows me to quickly and easily position stock directly over the centers of rotation. Below are some photos:

   

[isaac...@endomastermedical.com]

Hi Alex,

I found this guy from the comments of the link that you posted. He is selling these small clamps.

http://www.rwlsystems.com/clamps.html

Maybe he could make a batch specifically for the PocketNC thread size and everyone could benefit from his batch volume pricing.

[jaker...@gmail.com]

I bet those could be milled using the vice on the pocket NC!

I think this makes sense to be an early project when mine arrives in a couple more months!

[rwl...@greatlakes.net]

Yep, that's my site. I've been making these little clamps for myself for many years. I worked in the prototype aerospace industry for many years and there were no off the shelf clamps that would work. Once i started my own company, I again started making these for the shop and thought I'd offer them to others. I haven't done much in the way of marketing them, we have several production jobs that keep us pretty busy. But I do have the ability and capacity to manufacture these clamps in quantities. The fixtures are done, programs are written, and all the heat treat and oxide processes are in place. We've made several thousand of each size over the last 6 or 7 years. Mostly in 4~6 pc qtys. I'd love to offer a set specifically for the mills or lathes, but need some input on what is most needed as far as sizes and quantities. So check my site or send me an e-mail of what you need or suggestions for a kit and I'll start making them. www.rwlsystems.com

[Jake Rember]

If you make some of these which fit the Pocket NC. I would probably buy some, or additional holding fixtures for the Pocket NC.

[Martin de Blois]

I like this chuck idea. Most big 5 axis set-up have this chuck mounting option.

I own a watchmaker lathe, and being able to combine components is nice.

What is nice about those 3 jaw self centering chucks for watchmaker lathe, is that they are fairly low profile. 

That gives you more work envelope in this context

This looks like a Sherline chuck. It is operated by turning the knurled ring using small rods inserted on the side. 

They come in different mounting options, as per back thread.

See this link. http://www.sherline.com/3jawpg.htm

Now, what would be nice is to have a mounting plate that is mounted on the PocketNC plate using pre-drilled threaded holes.

I guess the best way to do this would be to CNC the fitting in place, and for ultimate precision, mark position (index)

This would offer self centering set-up for any part analog to lathe jobs, with +3 axis bonus.

I believe brass would be hard enough for this job.

Question is, could the thread be cut on the PCKNC?

Some older watchmaker chucks, like Levin, made such chucks with 3 screw back plate, but they are more expensive and have to be picked from Ebay...

The best thread option would be 3/4-16, as other threads are finer. Attached drawing.

Last word: I have been watching PocketNC since their Kickstarter launch.

I checked every week for their blog update.

I have been in contact with them. 

I have looked for reviews and videos about the performance of the machine.

Have not ordered yet. 

But let me tell you that after checking this forum, I am ready to order.

This small company is just a nice story.

And then, on top of that, husband and wife are driving it.

And like its not perfect enough, they have a new baby in the the midst of it.

My hat to this couple ! Nice job.

Martin de Blois

[Bryce Bell]

Here's my attempt at a chuck adapter for one of those sherline chucks. It's made out of aluminum and cut on my little manual lathe. Haven't been able to try it yet, but it seems solid. Could cut another 1/4" or so off the height by counterboring the screws and cutting a thread relief. I use the hole in the center of the table to center the adapter.

It does end up taking quite a bit of room on the machine. I may end up making something similar to the little vise that comes with the machine, but with a self centering arrangement.

[Martin de Blois]

Bryce, 

Nice job !

Did you machine the base plate on your lathe ? (looks like).

I guess you could countersink your screws and get it lower. 

The thread is a challenge here. Getting it all the way to the base would be the best.

I ordered a flange bolt of the same thread from McMaster. I am planning on shaving the head, and keep a wide flange for mounting.

Will get it ready for my machine, coming in June.

Would you mind sharing the dimension of the spacing for the 4 mounting screws on your base plate ?

Your idea of using a centering pin makes this easy to set-up. Nice. What is the diameter?

(what kind of lathe do you own?)

[Bryce Bell]

The adapter is all one piece and the only thing machined. I thought about machining a flange to mount a modified bolt, but didn't want to order a bolt. That would probably be the most solid way of doing it. 

That was as close as I could get the threads to the flange on my lathe. (Grizzly 7x12 Mini Lathe)

The mounting holes are 4 evenly spaced holes on a 1.26" (32mm) bolt circle (there are 8 holes at this location on the machine). The centering pin is .256" (6.5 mm)

Originally I was hoping that there would be some way to mount the chuck directly against the machine base without using the 3/4-16 threads. Such as pressing the lower ring of the chuck into a larger flange or drilling some holes into the lower section of the chuck, but then I realized if you are holding it by that lower section of the chuck, trying to work on something mounted in the chuck would be able to loosen the chuck jaws relatively easily, so that wouldn't work unless you also had some kind of locking mechanism for the chuck. 

With the threads we are also running the risk of unscrewing the chuck since we aren't always loading it in the same direction like you are on a lathe. May have to add some Loctite or play around with some other ways of giving those threads more resistance to unscrewing. 

[Martin de Blois]

Ok, thanks for the information on dimensions.

You are right about the thread being "unsecured". I didnt see that...

What comes to my mind is using the chuck operating pin hole as a register. 

The top portion of the chuck that supports the jaws is the same solid as the threaded part.

So using a rod that can be secured to the work table, and into the chuck  top portion pin hole would actually prevent rotation of the chuck in relation to the table.

Then the chuck jaws are operated by rotating the lower ring alone, with a single pin.

I would like to be able to port the chuck back and forth between the PocketNC and the lathe.

What do you think?

[Bryce Bell]

I think that would work, but whatever is holding it would either need to be made specifically for your setup or with some adjustability to it so you can be holding it exactly in the position where the chuck bottoms out on the adapter.

Another way I was thinking was cutting the threads on the adapter into segments and putting a taper then a tapped hole down the center so you could have a screw in the center that could be tightened down to sort of wedge the thread segments outward and lock the chuck in place.

[Martin de Blois]

I finally got this self-centering chuck mounted on the B table. Made a centering pin to keep it centered on the B table.

I am reading a run-out of .0035" total, I can work with that for now. (I hurried a little in some steps) 

I will see about the need to fix the main body position to the mounting plate, so that the height is always the same.

Otherwise, as you mentioned, rotation of the chuck main body on the mounting screw will change the chuck height.

But right now, it looks pretty good on the repeatability.

Will start using it soon and report further findings.

See attached images.

[diego Garcia]

Thats awesome Alex!!!
When you start using the chuck to machine pieces post some pictures!

[soere...@gmail.com]

I have also ordered my Pocket NC, and im am planing to build something simular to a wax chuck. Just with super glue, as Clickspring did in his video. Hopefully it will work.