Newbie to Benchtop CNC... trying to get started

[andrewreed1]

Hi all,

I am sorta new to Bench top CNC milling. I have done CNC CAD work in the past, so I'm good there and I understand basic milling concepts. But what I am lacking in is the actual hands on of setting up raw materials and cutting. There seems to be so many ways to setup for actual cutting.

Most of my experience was from PCB drill/routing, with some other experience with flat sheet routing as well. Just barely got into full 3D milling and changed careers. So, I am familiar with cutter deflection, cut direction, those kinda concepts. Even went through some training for Uni-graphics, but I always worked on the computer side. Plus, the PCB cnc programs had standard pinning programs or the PCBs to be cut were pinned from the inside and the sheet materials were screwed down into a wood backup piece of material.

Now I am involved with Scale RC aircraft and have recently come into possession of a Roland MDX-15 Bench top 3D mill. It was mainly used to mill jewelers wax, but I intend on using it for aluminum to cut parts and also to cut master molds for plastic injection.

What are good sources for information, and more importantly, illustrations on how to setup for cutting & milling. How do I setup for holding the workpiece? What are the best endmills to use? So far, I have not been able to locate a decent "how to" for a beginner. I have been reading articles from cnccookbook.com, which are great, but those articles seem to be geared more for intermediates and not so much for a noob like myself.

Any help or info you can give is greatly appreciated!

BTW, I already have a 3d model of a simple part I want to cut, but have no idea exactly what to do next. I do have some pieces of aluminum stock to try, and will get some wood as well to start out with...

[Memran]

That is a very open-ended question, and the open-ended answer is 'it depends...'.

For work holding the are a few commonly used methods, and your selection is always going to depend on what you are making and the type of cut you are doing.

Examples:
2D profiling (or 2.5D) - You are probably going to use mill clamps

Holding your stock on top of some sacrificial stock, on your X-Y table. Ensuring that the toolpath is not going to crash into the clamps.

For Parts where you are going to have to mill top and bottom, you will probably use a milling vice (and possibly a custom work holding jig). Usually, you use precision parallels in the vice which the work rests on, so that the part to be milled protrudes above the vice.

For 4 axis work, your stock will most likely be held in a chuck or on a rotary faceplate attached to your 4th axis. The other end will be held with a tailstock (same as on a lathe) is necessary.

There are also vacuum chucks, on which you place your stock, and it is held by a vacuum created by a vacuum pump. Obviously you cannot cut right though your stock using this method, unless you have an intermediate sacrificial piece to between.

If you are milling something magnetic (unlike aluminium), there are elecromagnetic work holding devices too.

So it all depends on what you are doing.
If you show us what you want to make, we can probably offer more useful and less generic advice.

[andrewreed1]

Here is a screenshot, with rough dims of the model (see attachment). I figured with the .16 slots, I could use a 1/8" flat endmill. The machine currently has a 1/8" shaft collet/holder. The machine also already has a horizontal clamp installed. I might opt for a ball nose endmill, but for such a simple part, does it really matter? number of flutes, etc?

So, I am guessing I should be able to use stock that is wider than the piece required and clamp it. Maybe a piece that is 3" long, 3/4" high and .3"-.5" deep.

If I also understand correctly, since the material is rough I will want to mill the top surface flat first, then flip the piece over since I now have a flat reference surface that should level the rest of the piece.

From there, I should be able to mill the rest of the piece? Or should I just mill the piece as it is viewed now, and build a jig that would hold the piece inverted so the bottom could be milled to the proper height? Plus, I will be adding a hole at some point in the shorter upright in the middle of the part, from the side. Again, would it be easiest to cut a recess in another piece of material (a jig) that the part drops into, so the machine just has to run over to the location and drill? Or, would you simply clamp the part and drill it that way? So many options...

And yes, this is one of those types of parts that might get produced more than once. So, a jig down the road is better than manually mounting and locationg the hole each time, I assume?

Thanks!

[Memran]

http://www.youtube.com/watch?v=4mhT1a28qO0&feature=g-vrec&context=G27e51c0RVAAAAAAAAAQ]2.3 Machine Tool Basics -- Mill Workholding -- SMITHY GRANITE 3-in-1 - YouTube

Someone smarter than me needs to talk to you about cutter choices.

[awerby]

on a knee mill, but the MDX-15 has about 2.5" of clearance under the Z axis bridge, and that's without counting the tool hanging down.

I'd suggest putting a couple of square holes in the base plate, so you can use carriage bolts that come up from underneath to hold down some strap clamps (like those shown in Memran's picture). Space them so as to leave room for your workpiece between them, with a little room to spare in front and behind.

Use very light cuts if you're trying to mill aluminum, since the spindle is easy to bog down. Spray the workpiece periodically with WD-40, and try to keep the cut free of chips. The standard spindle on the MDX-15 holds 1/8" shank cutters only, so that should narrow things down. You'll want flat 2-flute 1/8" endmills for roughing, and an assortment of smaller tools down to 1/32" or so, flat and ball ended. Don't bother getting anything much smaller, since the 6500 rpm spindle isn't fast enough to run them at a reasonable feedrate.

Andrew Werby
ComputerSculpture.com — Home Page for Discount Hardware & Software

[andrewreed1]

Thanks guys, that's exactly what I was looking for!

Andrew, so what you mean by "light" cuts is for the tool to take shallow bites in each pass? I take it this means I might not need to buy endmills with long cutting surfaces and, considering the already short Z height, short shanks as well, unless I need a long reach bit?

So just to recap, the clamps would clamp the workpiece in areas where the cutter isn't programmed to go. With the workpiece under the clamps, I would then have a sacraficial piece of material under that, so I don't cut into the worktop.

[kregan]

You know if you go flybarless you wont need that part !!!

Just joking!

I would consider milling that part in out of flat plate, then milling the slot in the top in a second operation while it is held in a vise. That would avoid having to face off the bottom and then flipping the part to cut.

[arizonavideo]

"so what you mean by "light cut"

This means to buy extra cutters because you will need them.

I would get a 5 pack of 1/8" square bottom carbide 2 flute China cutters to start out with.

Your depth of cut for each pass will be like .01" to .100" so you will be doing lots of passes.

A small blower like a hair dryer or the like will keep the chips away if the spindle does not blow them away.

[andrewreed1]

I would consider milling that part in out of flat plate, then milling the slot in the top in a second operation while it is held in a vise. That would avoid having to face off the bottom and then flipping the part to cut.

After pondering everone's input and looking at other options and methods on the net, I think you are right. I think I will approach this as a two or three stage process...

Step 1> Cut a plastic/metal base plate to size and drill/tap a grid of holes for clamping purposes.

Step 2> Make/Buy bolts and clamps to fit plate.

Step 3> Clamp appropriately sized flat stock to the base plate (thicker than required, or just over the minimum thickness), with appropriately sized sacrificial material placed between the base plate and the part material. Make sure material is larger than part size, and that clamps are placed outside of the contour area.

Step 4> Load the appropriate drill bit and locate Z0 on the MDX-15.

Step 5> Drill the required hole for the part (prior to cutting the contour).

Step 6> Load the appropriate endmill and locate Z0 on the MDX-15.

Step 7> Mill the contour with the longest side running in the positive X-axis, and the next highest side running in the positive Y-axis. Z-axis is the material thickness, so as long as the flat stock is flat, there will be no issue there.

Step 8> For a repeatable process, develop a jig where the part can be held vertically. This may be accomplished by cutting a recess in a larger piece of plate with two locating holes, which can be used in concert with a nc-program that drills the same holes in the MDF material prior to running the second cutting program. The jig is then pinned to these holes, the part loaded vertically in the jig and the jig clamps locked onto the part accordingly.

Step 9> Load the appropriate endmill and locate Z0 on the MDX-15.

Step 10> Mill the remaining contour of valleys.


I know this sounds rather lengthy in steps, but I'm not quite comfortable locating programs to parts on the machine yet (I've never done it). But I can understand absolute coordinate systems, and I understand that if your programs drill locating pins, and then the rest of the program is based around those pins, and the part is located based on those pins, then your program should be spot on.

Otherwise, if I have left out a step (or details in a step), please let me know.

Once I get all the materials together, I may shoot some videos of the entire process so you guys can see what I did and give feedback. This way, others can benefit from all the input.

Thanks!

[andrewreed1]

"so what you mean by "light cut"

This means to buy extra cutters because you will need them.

I would get a 5 pack of 1/8" square bottom carbide 2 flute China cutters to start out with.

Your depth of cut for each pass will be like .01" to .100" so you will be doing lots of passes.

A small blower like a hair dryer or the like will keep the chips away if the spindle does not blow them away.

Arizona,

So a 10 pack for around $15 on eBay ought to get me started? Square bottom, since the part and cuts are rather square, and I am not yet going for nice smooth edges (I can debur via sand paper).

Maybe I should do the same for the pins as well, just order some 1/8" carbide drill bits. I can then use the shaft of any broken bits/endmills as pins in the MDF.

So, you suggest a starting DoC of .100", and if that breaks a cutter, then back off to say .050", and keep backing off by half until I stop breaking bits?

As for chip clearing, will a vacuum suffice? I may have a way of attaching some vacuum hoses close to the milling head.

Thanks!

[awerby]

The general rule of thumb, for any machine, is not to cut deeper than half the diameter of your tool at a time. Since your largest tool is .125, that's .0625. But this is a very light-duty machine, so you should cut that in half (or more). Try a depth of .030, and if the spindle bogs down or the tool breaks, adjust it to be an even lighter cut.

For drilling, you don't need carbide tooling, but investing in a set of 1/8" shank drill bits in various sizes would be useful. Make sure that any endmills you get are center-cutting, or you won't be able to start in the middle of your part - you'll have to enter from the edge. If you do get center-cutting tooling and plunge in, use software that will do a linear or helical ramp-in, or you might lose steps in Z.

A vacuum cleaner alone will not be sufficient to pick up oily aluminum chips. A blower might help, or getting in there with a chip brush. If you want pins, get or make steel pins - broken carbide cutters aren't the best candidates.

Andrew Werby
ComputerSculpture.com — Home Page for Discount Hardware & Software

[andrewreed1]

Make sure that any endmills you get are center-cutting, or you won't be able to start in the middle of your part - you'll have to enter from the edge. If you do get center-cutting tooling and plunge in, use software that will do a linear or helical ramp-in, or you might lose steps in Z.

Thanks, Andrew! I ordered some China endmills to start off with. They are 3-flute though, not 2. They are center cutting, however.

If you want pins, get or make steel pins - broken carbide cutters aren't the best candidates.

Sorry, I meant Cobalt shanks.