Extremely basic machining question.

[touser]

Hello everyone, i know this question is going to sound ridiculous but please bear with me as i am as new as it gets to the world of machining. I just gained access to a 2hp enco drill/mill and would love to learn how to use it, unfortunately i have no local help. My question is about material placement and cutting. If i want to cut a square mounting plate out of a larger piece of aluminum do i just put the aluminum in the vice elevated above the surface of the rest of the mill so the cutter doesn’t go into the mill and proceed to cut out the square part from the large piece of aluminum? Because when i attempted this by the end the part i was attempting to cut out was holding itself up by a very small sliver of metal and by the time i got to the end of the cut it snapped off and the part came out square 3/4 of the way around but the last edge was jagged. I must be missing something extremely simple? Should i put a piece of wood between the metal and the table so the cutter can still go though the metal without harming the table but the wood supports the part being cut out? Once again i'm very sorry for what must be an idiotic question and if anyone can recommend any good resources on basic milling: books, websites etc i would really appreciate it. Thanks in advance!

[HuFlungDung]

Touser,

That's not dumb, it's just the first step in you becoming an expert

You have a fixturing problem, we call it. Yes, you could use a sacrificial spacer under your part. But, it won't hold the cutout. You've got to control everything. That means, any part that gets cut loose, needs a place to go, or else, it must be clamped down so it can't move either. In many cases, it is better to reduce the cutout to chips, but in this case, you want to save it because it is your part. So, stop cutting before you get to the last line of the cut, and apply a hold down clamp of some sort. Then cut it.

Or, you could leave a tiny sliver of thickness along the bottom of the cut, joining the part to the stock. After machining, then you can cut this out with a knife or some shears and remove the remaining "tab".

[touser]

Thank you for the reply HuFlungDung i appreciate it! You say on the last leg of my cut to clamp down the piece being cut out. In order to do this i would need to have a sacrificial spacer (wood?) under the part throughout the whole cut so there is something for it to clamp down to correct? Also, how would I go about clamping down the piece assuming it is in the middle of the table, is there a special type of clamp for this or will I just have to come up with something? Thank you again!

[HuFlungDung]

Yes, you should begin with the spacer underneath the entire part.

You'll have to use your imagination. A strap clamp can be used to reach from somewhere else (close) on the table. If you are really clever, you can position the cut to occur over top of a T-slot in the table. Then, pass a stud down through the cut, through your sacrificial spacer (which you will have a pre-planned hole drilled through) and into a nut waiting in the Tslot. Then apply a small strap clamp across the cut line, clamping the cutout whilst using the outer stock as a clamp support.

[NeoMoses]

another option is to use a low melting point alloy (LMA), often called fixturing metal. There are many alloys available that melt under 200F, so it is fairly easy to melt them, and not difficult to deal with.

With LMAs, you can leave the thin sliver like Hu recommended, then fill the gap with LMA. when it freezes, flip the part over, and face it off. This way you'll get a nice surface finish on both faces of the part.

Clamping sets, such as the one found here (http://www.use-enco.com/CGI/INPDFF?P...&PARTPG=INLMPI) can be a good way to hold unique parts.

[Ken_Shea]

Another means of accomplishing this is to use a slightly thicker piece of stock, how much thicker would depend on the material height and type but say for aluminum .750 desired thickness a 1" stock should be fine, rest the stock on a set of parallels (or something flat)with the say .200 being below flush with the top of the vice. Cut your .750 thick part and there will then be a .250 flange at the base. Turn the part over on parallels and face off the flange excess. If your stock is already the proper thickness the thin web at the bottom that Hu mentioned is really easy as well.

[touser]

Thank you all for the help i really appreciate it! You have cleared up alot of questions i had. I suppose my last question would be do the same techniques apply to cnc mill's? Because as i understand it all of the techniques listed so far require you to stop the machine and reposition the part or at least apply another clamp.

[DareBee]

The most common machining procedure is to make your rectangular block by facing it then flipping it in the vise and facing again. Do this either 4 or 6 times, if only 4 times (longer skinny parts) often clean up the ends by side milling to keep it square. There are a lot of different procedures to use to keep the part true. Once you have a sized and true rectangle than start machining in the details.

In my experience it is extremely rare to just cut a workpiece out of the middle of a block of raw material, but sometimes it is the way to go.

[gibbsman]

If the the part that you require does not have any kind of flatness requirement then all you need to do is as specified earliar. Clamp on a piece of material thicker than required, machine the profile then flip the part around and then face to size.

[Nono]

Ideal would be to rough cut it out with a band saw (in my case 5 or 6 gallons of elbow grease) mill it square then flat.