[umniy2000]

Dear colleagues,

I need a second opinion on fixturing and machining one of my parts. It is a component of our product line, the annual quantity is very low, about 10-20/year. We make them every few years, enough to stock for the next 2-3 years worth of demand.

It is a steel cylindrical part, 1” DIAX2.75”L. In the past we used to make it complete, including turning, milling and cross-drilling on the CNC lathe with live tooling turret. This time our manufacturing decided to turn it and then do milling/drilling work (flats, slots, cross-holes) on a vertical CNC mill as a separate operation. They also took up on making the milling fixture without consulting with the engineering department, and that’s what I need help with.

The fixture looks has a large steel base bolted to the table. Then there is a steel bar about 1X2.5x12 bolted to the top of the base on its narrow 1” edge. There are 6 1”DIA holes bored in that bar to insert 6 parts located horizontally. The part is 2.75” long, so they are sticking out from the bar by about 1.75”; that’s where the milling/drilling is done.

Now, clamping. That rectangular bar has a slot about ¾” wide milled along its top edge and deep enough to expose about half of the mounted parts diameter. Then there is a clamping bar spanning along the entire length of the first bar and bolted to it with a dozen bolts. That clamping bar has 6 corresponding half-bores, matching 6 parts location. So the part is inserted the main bar and clamped from the top by a matching half-moon in the top bar.

Actually, there is a second 6-position operation on that same base, similar to the first, where they do more milling work and again clamping 6 parts with a single bar.

I believe that this concept is fundamentally wrong. There will be variations in the parts diameter after turning; variations in the fixture machined holes and half-moons in the clamping bar. This may result in un-equal clamping of 6 parts and some of the parts not being clamped at all. I am not sure if the clamping bolts can elastically bend the clamp bar every time to conform to slight variations and looseness.

My second concern is use of multi-position fixture for this job. Typically we try to do that to optimize machine usage and cycle time. But for such low production quantity it may not be effective, since setting up and tweaking in each of 6 positions every time we make the part and passing the first piece inspection on all positions may be counter-productive. A simple single-position fixture, like a Chick Vise with a V-shaped clamp milled into the jaw, seems a natural solution.

I attached a sketch and I am hoping the general idea is clear.

Thank you for the input.

[Geof]

...I believe that this concept is fundamentally wrong. There will be variations in the parts diameter after turning; variations in the fixture machined holes and half-moons in the clamping bar. This may result in un-equal clamping of 6 parts and some of the parts not being clamped at all. I am not sure if the clamping bolts can elastically bend the clamp bar every time to conform to slight variations and looseness......
Thank you for the input.

The concept is fundamentally okay; for 60 parts per run it is bordering on a waste of time. We use fixtures with a similar split clamping design for dozens of different parts but these are for repeat runs of up to 1000 three or four times a year. You are correct about the split clamp using a long bar being not very accomodating of OD variations; we find the OED has to be maintained with less than +/-0.001" otherwise parts can spin in the holder. Alternatively the clamping bar can be split into sections each clamping two parts in a balanced manner.

It also seems a bit illogical to have two fixtures and relocate the parts for the milling operations. Ours all clamp onto a base mounted on a rotary table so that all the milling can be done using a single fixturing which results in one alignment procedure and all the milled features have the correct relationship to each other. If a rotary is not available an alternative is to have the fixture that clamps the parts, attach to two or more bases in different alignments so it is not necessary to re-align six parts, just one full assembly.

But what are you going to do? It seems you are faced with making it work.

[One of Many]

Yes, I'd agree, the production numbers hardly warrant the investment in labor intense custom fixturing. Far too much fussing around with bolts just to do secondary machining on six parts or 1/3 of the total run.

I'd think a basic 5C collet block or manual indexer would hold the parts on center better than a vee block nest. For that matter a gang of inexpensive collet blocks might be worth it. There are tooling suppliers that offer these ganged collet holders ready made, but nothing inexpensive enough to be absorbed by that annual quantity.

DC

[umniy2000]

Hi, everyone!
I am going to revive this post. This little job has been postponed for awhile, and now we are back at it. The operator is having problems setting up this fixture. One of the issues is that the parts are not sitting square; every time he clamps them, they are to a different degree out-of-squareness. So, the shop people decided to use shim stock under the parts, trying to get them clamp straight. They will have to put a small piece under each part in each load. I am against shims in production fixtures, since they just mask the problem, are un-reliable and may cause problems every time we need to make this same part in the future. My shop people believe that it's O.K., as long as they can get by this time. I am not saying they cannot make any good parts, but I believe it will be hit and miss and against good machining practice. Am I being "a stickler" in trying to solve the problem, rather than masking it?
Thank you for the input.

[Geof]

Shimming makes no sense. The holes in the fixture and the semicircular clamp should be sized so the parts sit square and level and are clamped into that position. Your sketch indicates that the part passes through two holes in the bottom of the fixture so how are they getting out of square?

[cncjunky]

Hi, everyone!
I am going to revive this post. This little job has been postponed for awhile, and now we are back at it. The operator is having problems setting up this fixture. One of the issues is that the parts are not sitting square; every time he clamps them, they are to a different degree out-of-squareness. So, the shop people decided to use shim stock under the parts, trying to get them clamp straight. They will have to put a small piece under each part in each load. I am against shims in production fixtures, since they just mask the problem, are un-reliable and may cause problems every time we need to make this same part in the future. My shop people believe that it's O.K., as long as they can get by this time. I am not saying they cannot make any good parts, but I believe it will be hit and miss and against good machining practice. Am I being "a stickler" in trying to solve the problem, rather than masking it?
Thank you for the input.

No your not being a stickler, your operation sound mighty rinky dink rigged together junk. Sounds like you need to build a new fixture and quit wasting time messing with garbage.