How do we mount stock to mill gearbox plates

We have a CNC mill (but are mostly teaching ourselves how to use it) and are trying to make 1/4" aluminum gearbox plates and have everything figured out except how to mount the stock to the mill. We have searched for tutorials but cannot find anything. Should we clamp it in place on the bed? Drill holes and screw it into the sacrificial plate? Clamp it in the vise? Do we need tabs so it doesn’t fly off at the end?

It would be best if you sent a pic of your set up, but for us we screw most of our parts to the sacrificial board so that we don’t need to use tabs. Work-holding is definitely dependent on the situation though.

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What we typically do is rough the plate out to size with about an inch on all sides. We use toe clamps to hold the plate in place while the machine handles all the bores and pockets. The last two operations are a contour around the out side to leave tabs, and then a contour around the outside to remove the tabs from the previous op. Typically we get something long enough to hold the part as it removes the last few tabs. The deadblow hammer typically serves this purpose. For parts which are inconvenient to hold down at the end, we just leave the tabs on and then band-saw the part out and sand off the tabs.

An alternate technique which we’ve used occasionally is the superglue trick, which NYC CNC details well here: https://www.nyccnc.com/super-glue-fixturing/

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We use fixtures to mill almost everything that is not a tube. The setup takes a little longer than using a sacrificial plate but allows us to not have to use full engagement on our end mills. An example process for gearbox plates would be

  1. CAM(1) fixture plate using the CAD of the gearbox plates (bolt holes only)
  2. CAM(2) gearbox stock plate (bolt holes only)
  3. CAM(3) gearbox features (bearings, pockets, contour)
  4. Probe corner of vise (We typically just line up the stock for 5. by hand)
  5. Use CAM(2) to drill holes in all the gearbox stock
  6. Probe corner of sac plate while in the vise
  7. Use CAM(1) to drill holes in sac plate
  8. Tap holes in sac plate while still in the vise (you could tap using the mill - we are scared to)
  9. Mount gearbox plate
  10. Use CAM(3) to finish it out
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Are we talking about a CNC Gantry Mill (aka CNC Router) or a CNC Vertical Mill (like a Haas or Tormach)?

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Vertical Mill.

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Step clamps and T slot nuts? Could add a waste board under it if you’re concerned about milling through the stock.

If you can afford it, I recommend a milling vise that can accomodate the stock. You’ll want a set of parallels as well.

Some of this depends on what you have and also the cutting path you have planned

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We’re always concerned about milling through things.
If placing it in the vise, do yo use tabs? I am wondering what happens when the last tab is removed (or weakened) and the piece moves with the cutter spinning.

To a certain extent, the design of the gearbox plates influence how you make it. If you have a simple vise and parallels then it is much easier to make gearboxes that have two relatively large parallel sides that are from the original piece of stock . It’s even easier if you are willing to use the stock edges on all sides. Several AndyMark gearboxes are designed this way and work great.

If you want something Fancy you could still use a vise and machine the internal features first, including mounting holes. You can then machine a sacrificial piece to hold your part down that matches up with the mounting holes. After that simply fasten your part to the sacrificial plate and machine away the excess. If you fully remove the extra material as chips then you don’t need to put in extra mounting holes to hold it down. If you only want to contour around the perimeter of your part with a slotting style operation then it would be wise to put a few fasteners through the scrap areas so they don’t go flying. An unsecured piece of material that contacts a cutter can go flying and can also damage the endmill/cutter! With a sufficiently slow tooth speed this can be less of a concern but it should not be a game plan.

Something that I have learned over the years form a few veteran machinists. If you respect that parts must me secured well to be machined properly and safely you can have a lot of creativity when it comes to how you do something. One machinist that I knew seemed to have a unique trick to make the process much easier for each and every situation they encountered. With him, there was no “standard” way of doing anything.

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Mill vices are useful, but not for holding onto plates directly. You will absolutely warp the material and risk throwing larger pieces.

Use a larger sheet of alu. plate (.625 is good), and mount it to the table or flip the movable jaw around on your vice to hold it. Deck it flat after clamping it.

Some people get fancy with drilling/tapping holes for fasteners but that’s a lot of effort. I use parallel clamps to hold the stock onto the sacrificial plate. Others have mentioned it, but I also tend to remove all the material for internal pockets so no small bits are thrown. Tabs around the outside contour are more then secure (outside is machined last anyways).

There are plenty of ways to skin a cat, but just to prove how bad of an idea it is to clamp your plate directly in a vice, put an indicator in the center of the part as you tighten down. You’ll see it move. An then as material is taken away, you will loose your clamping force. Bad bad bad.

We have MDF mounted to our table, and we screw our plate stock down to the MDF.
Then run a pass to drill holes in each of the parts and any scrap pieces which we screw into place. Final step is to cut the profile for each of the parts.

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Until we got a router this year, virtually every single gearbox plate we’ve made in the last 10 years that fits inside a 4" x .25" bar has been made by holding the part in vise jaws. We use tabs and leave 0.25" of meat top and bottom (in Y) and 0.25-0.5" per side in X.

It works just fine.

We’ve done this on some that are 5" or 6" tall, but that gets into sketchy territory.

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