Conventional fixturing

[Nathan the Machinist]

Since we were just on the subject of fixtures and some unconventional approaches, I thought I'd do a thread about some regular old conventional fixturing.

Most of my fixtures are used in a mill, so for me the most common kind of fixture is some form of soft jaws machined to hold a part in a vice on a mill.

It is pretty common to machine a part on more than one side, so two presentations per vice is not unusual, but three can be problematic because the vice will bottom out against two of the three, leaving one loose.

This is a steel soft jaw fixture that has been reused for a couple different parts. Two things to take note of. One, I often like to bore a hole in the fixture to pick it up with a coax, for me it is easier, faster and more accurate if I want to put it back on in the future. And notice that I'm gripping round parts but I didn't make my pockets round. I like three contact lines. A round pocket will only make good contact if both the part and the pocket are exactly the same unless you really crank down on it and get significant elastic yielding. Otherwise a larger pocket will only contact in two lines, and a smaller pocket will contact in four places in an uncontrolled manner. On a part like this, either can lead to rocking. So for something like this, I like three planes, 120 deg apart, one of which is just a regular hard jaw.

These jaws are steel. I always use aluminum if I can get away with it, but aluminum wasn't going to work for this part.




This is a variation of the same thing, steel jaws to hold Tai's hammers. There are some heavy cuts in 4150, so this fixture is steel and beefy, while allowing access to the surfaces to be cut.

A very popular and easy to make a fixture. Drill and tap holes around the perimeter of your part. I thread the screws in from the back side, and hold the part down with nuts. Most folks just run the screws in from the top, but doing it from the back, the threaded posts are rigid so they don't rock away from the part as you tighten up. You get a better grip and the part stays flatter.

And last but not least, the humble bondo fixture.

This is a great easy fast way to hold onto odd shapes. This one was used to countersink some holes an off-the-shelf spring hinge. I probably ran 100 parts through this fixture and it was still locating accurately.


I hope something here is helpful to some of you folks.

 

[blindhogg]

Excellent post, I enjoy reading how the pro's fixture. Being a hobby machinist myself I get alot out of posts like this.
Loving the Bondo fixture, that is one I will have to try one day.
CW

 

[AVigil]

Awesome.

 

[Tai Goo]

Nathan, I use epoxy putty like you do with the bondo, for holding odd shaped parts in a vice. Not for machining, but just for handwork. The putty sets up real fast, not sure how long the bondo takes? I usually wax or grease the part so the epoxy putty doesn’t stick. Great trick.

 

[Rick Marchand]

I used and made fixtures quite a bit when I worked in a T&D shop. Ours where MUCH larger.... some were 120" x 280". I once worked on an 32,000lb 8 station dial base.

The bondo fixture is cool, Nathan. I've seen Wally Hayes using that rezin stuff to hold things in place on engraving bases.

 

[Mark Behnke]

Thanks for posting. I bought an old BP mill in the summer and I'm collecting basic tooling. Don't know a thing about machining so these tips from a pro are so valuable.

When you refer to soft jaws, would that be nonferrous metal, AL., copper, brass or would mild steel be considered a soft jaw material also?

Thanks Mark

 

[SteelSlaver]

Thanks for the great tips Nathan. Bondo sets up in less than 30 minutes depending on how much hardener you put in it. Never thought of it for fixtures though.

 

[little mac]

Soft jaws can be pretty much anything but hardened steel. Mild steel, alum, brass, copper, even plastic. Generally they are a sacrificial part. Maybe that's not the best term for it. As nathan said they sometimes get reused for other things. Eventually they have been so used they are best disposed of and remade with fresh stock. You want the material to be fairly cheap and easily machined in such a case. In some cases, you may want to dedicate the machined soft jaw for the same task at a later date. In this case you may end up with alot of dedicated fixtures, which means going thru stock like candy everytime a new job comes up. Again this puts you back to wanting fairly cheap and easily machined. I haven't priced brass in a while but seems it's pricey and copper does anything but machine easy. It is some gummy stuff and clean cuts are not the easiest thing to get. It also tends to be very maliable, meaning if you get the vice really tight, it will deform and your referance points may not be where you planned. For most hobbiest, the variation won't amount to a hill of beans, but for guy's like Nathan, it's everything.
I'd suggest useing mild steel or alum.

 

[Nathan the Machinist]

re: soft jaws

I always use aluminum if I can. I buy aluminum soft jaws in bulk, they're not terribly expensive. It's kinda like sanding belts, use them like they're free. If I'm running some parts off parallels and it's more than just a few, I'll just cut a ledge into some soft jaws, etc. When you're busy, time is money.

Sometimes a job will require steel jaws, which I make from 1018 cold rolled.

If I'm running a production job I'll often use a torque wrench for running up the vice to eliminate that variable from the process. This also helps extend the life of soft jaws, because over doing it it what kills those. I have some aluminum fixtures that have lasted years and run thousands of parts.

Hard jaws are hardened steel. However, even these are usually mild steel with hardened inserts and wear pads. I'll use hard jaw fixtureing for running raw castings, and large volumes of steel tubing and hot rolled, which are things that eat up everything thing they touch. However my shop here doesn't ever do much of that kind of stuff.



re: bondo:

Bondo is sweet manna from heaven. Perfect reproduction, dirt cheep, ready to use right now. However, it isn't always strong enough for real work, it stinks up the shop, and even though it can position a part with good repeatability, you still have to orient the part accurately. It is not always suitable for contact with certain parts of a knife like the handle because of the solvents. For this reason, the epoxy puddy Tai is talking about is probably better for knife work.

You all might want to take a look at polycaprolactone. This is a polymer that feels a lot like one of the more rubbery nylons. It melts around 160 deg F, and because of relatively low thermal conductivity can be safely handled and molded while at this temp. You can heat it up in hot water, or in your oven to melt it, then you can mold it around your part just like the epoxy puddy. Once it cools off you're ready to go. You can reheat it and use it again. It is non toxic (I think it is the same stuff used to make bioabsorbable stitches). I picked up a couple pounds of it sold as "Polymorph" on ebay for 20 or 40 bucks.

 

[Mark Behnke]

Thanks Nathan, little mac
After filing a 304SS guard slot for a Loveless style hunter for hours I was convinced I needed a mill ( doesn't take much to justify a new tool). That will be first project, and I also want relieve liners at the pivot on folders, a jig for setting stops on folders, Oh and maybe try the IKBS etc etc. The excitement is building.

Great info thanks for sharing you knowledge.