CNC routing of small parts

      How to keep small parts from wandering during machining. January 16, 2001

I have small drawer parts that are moving around when machined on my Komo VR510 router, spoilboard, 25 hp vacuum, 800 ipm, 18000 rpm. What is the secret to keeping my wandering parts in line? We are already routing pieces twice. The first pass cuts through 90% of the depth. We are using up down .5 bits. Machine, collets, etc are in great shape.

Forum Responses
Nested-based is a major challenge. We rout small parts first, thus ensuring that the highest degree of holding power is available when we need it. You can also use a ramp tab option, along with the standard dual pass approach.

We use the "machinistís friend" duct tape--one strip in the center or two strips spaced out. I put the tape so it is routed through last. In conjunction with the tape, use a scrap piece of wood that holds suction well and cut an "L" out of it and lock it down with separate pods or independent gasket hole on story board next to the piece. Try a new sharp bit and slow the speed down. Double check your gasket for wear and make sure it is bumped up next to itself.

If youíre using the software I think you're using, use the heli-in lead in/ramp, and wrap about half of the geometry. That way, as you come around the piece and are approaching the parting point, you are taking a thinner and thinner cut.

Alternatively, do not make that last pass. Take the board off, and run it through the TimeSaver (or whatever brand) and sand off the back 20 or 30 thou. This doesn't always work, but I have used this approach on some pretty small parts.

Also, try using a smaller tool--it will push less. Skip the up/down and go for a conventional down spiral tool. It will help hold the parts to the spoilboard. The spoilboard will act as a chip breaker.

A 1/4" tool will cut surprisingly well even in 3/4" thick ply or MDF. You probably will not be able to push a 1/4" tool as hard as a 1/2", but yield may offset cycle time penalty. My favorite size tool, for most sheet stock applications, has to be the 3/8 spiral. Second would be 1/4". 1/2" tooling certainly has its place, but the economy of the smaller sizes lends to their appeal.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
The one note that seems missing here is to cut *all* the 90% cuts first (at least for the small pieces), and then do the last 10% cuts. If you do each small piece separately, and they are nested, the first will likely be okay, but the cutting of the next piece, at 90% of the thickness, will still generate a lot of force, with a slightly (and locally) reduced vacuum. By doing all the 10% cuts last, the low vacuum is matched up with the low-force cuts, and should be better. I just had a similar problem, and this solution worked excellent.

Comment from contributor B:
To machine small parts on a route I have used a few tricks.

One: Machine the part bigger than needed, then come back and trim down to the finish size, sometimes machining partway through as you are doing. I use this especially if shaping the finish part (with delays to remove sprap parts from table).

Two: Feed control. Slower puts less pressure on the part. New bits are better than used bits in this scenario as well.

Three: Use some form of clamps if possable.

Four: Use another board to place next to the actual piece to "pinch" it against the stops. Note the other board must be sucked down as well.

Five: Using a ramp-in and ramp out sometimes helps even when ARCing in and out. It sometimes covers up some movement of the part. I normally start with a 25mm to 1mm ramp, then adjust to within tolerance of the company specified tolerances. (Note: this doesn't stop moving parts. It only compensates.)

Six: Use down cut bit to cut partway through the board to push down on the board for better suction, then cut all the way though with up/down or up cut.

Seven: Narrower bits, but they have to cut slower.

Eight: Make sure that the operator of the machine keeps the table clean as possible. Task your best people with these "special" jobs. Maximizes suction.

Nine: In a few instances I have seen where the part just left a knot on the end of the part as it was cutting out. I have added line to program to come back and ramp-in to machine off the knot.

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