Multi-Part Fixturing

[Geof]

Mr Thinks-Outside-The-Box a few weeks ago asked what the application was for a chunk of hot rolled steel that I had made a lot of holes in, here is his answer; it is a fixture for holding multiple parts for multi-operation machining.

We make many parts that are cut from round bar in a lathe then have milling operations on up to four faces. One is shown in the first picture, 2.250" dia. aluminum puck on the left finished object on the right, end facing the camera faced, blind semicircular tee-slot machined in this face, circular through hole on center, flat on what becomes the top with one tapped hole and one blind hole, part code engraved and serial number engraved. If we made these in the umpteen kazillions we would have multi-million dollar machines and robotic loaders and all sorts of fancy equipment like that; for a yearly volume of a very small fraction of a kazillion it is necessary to be more modest.

Second picture shows a fixture base mounted on a rotary table in a Haas Super MiniMill. Third picture is the fixture loaded with parts on the bench, a torque wrench is used for tightening the bolts to ensure they are all the same tension. Fourth picture is the loaded fixture on the rotary base. Fifth picture is 15 minutes and 2 seconds later with all the parts finished and the final picture is the fixture ready for reloading on the bench.

There are several advantages to this type of fixturing: Dimensions that have a critical relationship are easy to keep correct because everything is done at a single fixturing. Multiple parts can be processed which divides lost time due to tool changes over all the parts. In this example there are two holes with one of them tapped; even though five parts are done the tool change time is still a good portion of the total drill/tap process. With two fixtures one is loaded while the other is running which reduces machine dead time.

I don't have a tight grasp on the time it took to design and make the fixture, combine and rework two previously separate programs and prove everything out so it was ready to hand over to the machine operators. This is the kind of thing I do for the company and nobody keeps track of my times, not even me; it was certainly less than 80 hours and probably more than 30. However, even at the 80 hours it would be recovered within less than a year on this part alone and the fixture will be used for at least five different parts.

[HuFlungDung]

I like it

[thkoutsidthebox]

Thanks for the reply Geof. I was actually thinking about this the other day, but I didn't want to harass you for an answer. I was starting to get a suspicion that I had missed the relevant thread.

Thats facinating, the fixture is actually more complicated than the parts its used to make!!! ?? At least to my untraind eye .

[Geof]

Originally Posted by thkoutsidthebox ....the fixture is actually more complicated than the parts its used to make!!! ?? At least to my untraind eye .

Yes, once you get past using a vise or just clamping things to a table your fixtures are often more involved than the part. Although if you look at things correctly a vise is often more complicated than the parts it holds when you get down to it; Kurt duoLock vises are very complicated internally.

I served my apprenticeship a bit earlier than CNC came into relatively common use in a place that did small scale production volumes of things like firefighting equipment, domestic water pumps, valves and fittings for dairy farms.

Pre CNC one emphasis was on maximizing efficiency by making multi-point tooling to complete a number of dimensions in one operation; also to have the tooling define sizes rather than rely on machine graduations. Another emphasis was on fixture design that was easy to load and which located the part on previously machine surfaces or which held the part through two or more operations with the fixture locating by reference surfaces in bases in one or more machines.

With CNC the need for multipoint tooling disappeared because the machines are so precise and can change tools. I also think to some extent the emphasis on effective fixturing also was downgraded to some extent. It still does exist in truly high volume work with pallet changers and things like that but it seems to me a lot of small volume shops still fumble around with vises and clamps when a little ingenuity could improve things. Even on non-repeat jobs sometimes it might be possible to have a net saving in time by using some time upfront to make fixtures that increase throughput. Even if the fixture building time plus shorter production time only equals what it would have been without building a fixture it is worth it. And....if it happens that the job does come back then the second time through and everything following has a lot of gravy in it.

[pdoherty]

Geof,

Could you elaborate a little about the rotating base vs. the fixture itself?

It looks like you have designed the base to be universal so that you can use it with a variety of fixtures.

Very cool design and well executed!

[Geof]

Originally Posted by pdoherty Geof, Could you elaborate a little about the rotating base vs. the fixture itself? It looks like you have designed the base to be universal so that you can use it with a variety of fixtures. Very cool design and well executed!

I suppose you could call it a One Sided Horizontal Tombstone. It is made so different fixtures can be used and it is also the reference point for Work and Tool Offsets. There is a reference hole at the center of the base that is used for primary Work Coordinates, G54, 55, 56 and all the Part Work Coordinates are taken from here using G52.

The Tool Offsets are taken to a Tool Setter standing on the base; these are the values on the Offset Page in the machine. Then a second value is entered into the Z for the G54: This second value is the difference between the top of the Tool Setter and the Z zero plane for the part. We normally make this the finished surface of the top of the part.

The reason that more than one primary Work Coordinate is used, G54, 55, 56, is because the parts get processed on two or three sides and need different Z values for the different distances between the Tool Setter and part Z zero plane.

The base is dropped below center so that tool clearance is maximised, the one shown is on a Haas Super MiniMill with the extended Z clearance so there is not a whole lot of head space. Also dropping the base puts the parts closer to the rotary centerline and reduces the torque load on the rotary brake.

The fixtures are made for groups of parts. Most of our stuff is either 2" or 2-1/4" OD so it is possible to handle several different parts with a common fixture. The tricky part is the making sure the fixture doesn't fall apart because the different clearance holes and cutouts for access to the different parts interesect. The fixtures are all in pairs so one is being loaded while the other is being processed.

[pdoherty]

So it looks like the you are using the outboard support available from Haas and that you have mounted it and the rotary unit on individual sub plates that I'm assuming are keyed to the table.

Does the outboard support have a bearing or bushing and does it just slide on the journal that is attached to the left hand 'crank arm' of the base?

I really like this. Thanks for taking the time to share it with us.

[Geof]

Yes and yes.

The Haas outboard support just has a hole in the casting no bearing or bushing.

The plate for the Rotary Table is permanent on the machine table and the Rotary is switched on and off this plate. I did a thread showing this a while back; Super MiniMill Vise/Rotary Changeover

There are alignment screws and keyway so the Rotary goes back to with 0.001" of the same place each time. Also the Rotary is far enough away from the spindle centerline that it is not possible to crash it.

[Mitsui Seiki]

That's a really nice looking fixture!!
I've done a few myself .

[timmydabull]

very nice fixture thanks for sharing that with us.

[panaceabea]

Thanks for the pics Geof, Here is my rotary jig . Holds 10qty forgings , jigs can be changed for other parts

[radioactive]

Geof,

Sorry if this is a noob question, but when designing a fixture like that to hold multiple round parts. What size ID of hole in the fixture did you machine, knowing that it will clamp down on a part that is "X" inches OD.

For example if your parts are 2.000" OD, what size ID of circle did you machine out of the fixture?

Would you machine the hole the same dimension or ever so slightly smaller to ensure a tight fit once clamped?