Material Hold down 101

[dneisler]

Ok, I am new to all of this metalworking. I have machined several small parts and it was a pain to figure out how to hold them down and mill. I am sure I was doing it wrong. So here I have a new part I am about to mill in the next week or so. Small 3" x 3.5" x 3/8" which I have some 3/8" x 3" stock on hand.

Here is a picture of the part I want to create.


Machine: Sherline 5400 CNC

I was thinking of setting it on top of some 1/4" Scrap and just clamping it down. But how do I machine to the profile of it? Should I use some 1/2" material and clamp in vise so that 3/8" is exposed. What is the correct way of doing this?

[Geof]

Drill the holes first then use these to bolt it down to a piece of material that is clamped to the table. Then machine your perimeter.

[dneisler]

So I clamp the peice down and I drill the holes, the two biggest, 10-32 and 1/8". So after I drill, I need to drill again in my scrap for the bolt I want to use and tap it, then bolt it all down and clamp the whole thing down.

Then how do I locate the exact 0,0 of the part so the rest is machined at the right place.

[mrk]

I am new to this but would the following work?

The following are some newbie thoughts but it might work and someone else can fill in the details.

Step1: Drill and tap a hole in a plate of scrap that you bolt to the table. (i.e. this is now your simple fixture plate)
Step2: Place the stock on top of the fixture plate and hold it in place with strap clamps (i.e. clamp it down).

Step2: Drill the holes that you can access.

Step3: Bolt it down using the holes you just created and since the fixture plate is under the part you are ready to go. Don't remove the strap clamps until you have the plate bolted down (this way it will not move when you bolt it down).

Step4: Now that it is bolted down you can remove the strap clamps.

Step5: Machine the profile

It also looks like you have one transverse hole (i.e. the hole that comes in from the side). I figure this would be a second setup but you can index to a fixture that you build for that purpose. The fixture would need locating pins for a couple of the holes that you drill from the first setup. It also might be possible to do the second setup in a vice using side of the part to locate on.

[HuFlungDung]

There are many ways to hold a part, and no one correct way for all parts

On my old open knee mill, I used to machine parts like that right on the end of the bar stock. I had enough stock sticking out clear of the vise to do all the profiling except for the 3" flat end. I would usually make some kind of a support post that bolted to the table to support the stock for drilling. So when all operations were complete, then I would pause the program, and add a strap clamp (or use a bolt hole through the part if available) onto the finished end, bearing down onto the support post. Then, do a cutoff operation to finish the long flat cut. Machine the cutoff path a little bit in the rough, and not quite through, perhaps leaving .02 attaching the part to the bar stock. Then, move to the exact finish line and profile finish the end face of the part, leaving the .020 for a final cut. Even then, the tool can want to grab the part and gouge it, so you may need to dream up a fingerstop to prevent such an accident at the end of the final step.

Now in a machining center, its not to handy to have 10 feet of stock hanging out so I would probably go with the 1/2 thick stock method and perhaps make a set of soft jaws to hold the completed part for a final facing operation when it is flipped over.

[Geof]

mrk has it pretty well complete.

Regarding locating the part you can use the holes. You know where they are relative to your 0,0 so you know where 0,0 is relative to them.

[cdlenterprises]

Either way works. I'm more likely to go with holding on excess stock (if ti's available) and machining the profile the whole part (profile, holes, etc) This way all of the features have been cut at the same time and are as good as the machine is location wise.

[Verfur]

If you use the bolts to hold the part down, then use a small end mill to do the profile as the bolt size and # of, to part size is small (ie. clamping force is very limited) for finish of profile. You could glue it or stick it down to a base plate then machine it, then peal it loose via heat. Myself I would use the soft vise jaws to do the trick fast acurate and I can put the horse power and torque to it but then again I would be using a 30 horse machining center.

The side hole is no problem if its on plane with the side of the part it is drilled through. Simply put the part in a vise locate the side and the end with a edge finder drill hole !!!! please space up part in the vise and dont drill new holes in my good vise LOL

[dneisler]

Thanks for lots of good information. I am going to give it a try this weekend and see if I can get one of them work. I have some things I want to make several of as well, so I am trying to learn more the proper way of doing things, and more about fixturing as well.

[bigvinney]

I am new to this as well, but I found an easy way of holding down the parts. My setup works great for milling aluminum plate, but may not be rigid enough to take realy deep cuts.

First I buy a piece of machinable plastic (I forget the name, but they use it for cutting boards) slightly larger that the part I need and about 1" thick, costs about $5. I clamp the plastic to the table, then clamp the aluminum ontop of the plastic. In the program code I drill the holes first and I use an 1/8" endmill to peck drill the holes but I drill them through the part and about an 1/8" into the plastic. I then bring the machine to home position (G28) and use my electric drill with a 1/16" drill bit to drill the holes almost to the bottom of the plastic. I then use 1/8" sheet metal screws with a hex head so I can use a ratchet to screw them in.

I then remove the clamps holding the aluminum and I machine the profile out (make sure when moving around you give clearance for the screws ontop of the part now). Once the perimeter is cut I clamp the part back down, remove the screws and drill the holes to the correct size.

One note, for this to work you will need to mill down that lowered profile on the top first before you can bolt it down.

Vince