| CNC JIGS AND FIXTURES Ok so now that you know how CAD CAM works from the last article you will need to know how to hold those parts that you're going to make. Over the years it has been drilled into our heads that material is expensive but in reality when it comes to CNC machining material is economical. Setup time is a large percentage of the cost of one off or small runs. Often times it can cost 3 to 4 times as much to set a job up as it does to run it. CNC run time is actually fairly reasonable because the machines are so efficient. So with this in mind if you where to cut your material close to size and then try to hold it some how with specially built fixtures it may take as much as 3 to 4 hours to get set up and running the first part. The cost for this $180 to $240 whereas if you where to cut the stock larger and just clamp the stock in a vise and walk around it the extra material cost maybe $10 to $30 depending on the size of the part. 
   Another way to save is to do the hand finishing. With a die grinder and a vibrator sander you can put the finishing touches on the part yourself.    Some times if you are running a bunch of small parts you are better off to nest the parts in a large block of stock. Also by nesting the parts you reduce the number of tool changes and the cost. Another way to save your self some money is if there are some secondary operations that you can do yourself. Lets say that you are making a large engine mounting plate and you have 5 different size holes in it. If you where to have the CNC drill all the holes to the smallest sized hole and then when you got the part home you could just drill them to size on an ordinary drill press. This would save the CNC operator the time of setting up 4 additional tools. On the other hand tapped holes are best done on the CNC. They tap incredibly fast and accurately. There is a trick to CNC tapping. The tick is not to drill the holes to small. A lot of machinist think that if you drill a smaller hole that you will get a better thread. This is not the case and I have spent many hours convincing journeymen machinist of this fact. To attain almost maximum holding strength on a tap hole you only need 55% thread depth. Any more and you gain very little extra strength. Most thread charts give you drill sizes based on 70% to 80%. For example if I were CNC tapping a 3/8-16 thread where the chart recommends a 5/16(.3125) drill I would use a P drill (.323). The extra .010 will reduce the chance of breaking the tap buy about 95% and actually produce a cleaner better fitting thread because there is more room for the chips to clear the hole. I use spiral flute taps for CNC tapping because they curl the chip up and out of the hole and they like to run at high speed. For example on 6061 T6 aluminum I tap at 600 to 700 RPM or on mild steel at 500 RPM.       There will be times when you want you need to machine both sides of a part. This can be accomplished with some relatively inexpensive fixtures. For example if I was machining a motorcycle engine case. The first step would be to clamp the block of material directly on to the mill table. To do this I use 2 ground flat bars that fit snugly into the back T slot of the table. Then I use toe clamps to pull the stock down on the table and push it hard against the flat bars.  For the flip side I make up a fixture plate to blot the case onto and then clamp that plate to the table. It is very important to make sure that the fixture plate is perfectly flat and square. I generally setup the plate on the NC mill and mill all 4 sides in 1 shot. To hold the plate down for this operation I drill some extra holes in an area of the plate that will not interfere with holes needed to hold the part. I grind both sides of the plate to insure that it is perfectly flat. Then I drill holes in the plate that match the holes in the case. Some of these holes are reamed for dowel pins and it is these pins that ensure that the part is perfectly aligned on the plate. Some of the holes in the case are taped for hold down purposes and then drill out to size after. This plate can be used of more that one part. For the cases I have one plate that works for both sides. Now it is a simple mater to setup this part for machining I just clamp the fixture plate up against the flat bars with the toe clamps and then touch off of one corner for X0, Y0. The nice thing about CAD CAM is that all you need to do is lay out the program from some point of reference. It does not have to be on the part and in this case is off the corner of the fixture. This is about the only way to index the second side of the case because there are no machined surfaces or holes to index from.    An easy way to hold round stock or parts is with a lathe chuck clamped to the table.  There is currently a post running on the stipbike.com bulletin board under V-Twin Racing called CAD CAM if you are interested come over and have a look. There are posts from CNC rookies and masters all over the world.  JERRY and AL |
