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Hold Downs
 The right
picture shows how an
odd
sized piece of birch plywood was held to a
table that did not have a hold down system.Drywall screws with fender washers were used. A piece of cardboard was placed under the work so the letter could be cut without the table being carved as well. Double sided tape is used in conjunction with hold downs when the parts have internal cut outs that would otherwise fly free. This simple system works well, but a set of hold down clamps makes set ups quicker. ..........................
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 The hold down on the right consists of a wooden knob with a tee nut insert that is threaded onto a section of 1/4-20 threaded rod. The threaded rod is either screwed into tee nuts that are pressed into the bottom side of the table bed, or it is attached to a foot that slides in the tee slots. The foot of the threaded rod is a piece of 1 x 1/8 inch steel flat bar that is drilled and tapped for the rod. Loctite, epoxy, or solder secure the threads in the foot so the rod will not turn as the knob is tightened. A locking nut can be used with deep tee slots, but the nut will not fit in the commercially made tee slotted MDF from home centers. The hold down clamp is a piece of 3/8 or 1/2 inch stock with a slot cut in it. This system is used with the tee slots on the belt drive and rack/leadscrew tables. It is also used on this shop's metal table. |
 Commercially made knobs are available for similar systems, but a batch of them can quickly run up the cost. The knob shown here can be made on the CNC machines with the g-code that is linked at the bottom of this page. The knobs were designed to be cut quickly with horizontal roughing cuts, no parallel passes. Because of this they are not highly finished, but they only take a few minutes to carve. |
| The code was post-processed for
Mach 2, but can be used with TurboCNC
by deleting lines 1 to 6. The spindle speed (line 6) is arbitrary
when the speed is not controlled by the software. The feed rate is 60 ipm and should be set to a rate that the table can handle, either by changing the g-code or by using the Feed Rate Override in the controller's software. Codes M03 and M07 are spindle and auxiliary (vacuum for the tables here) and 0,0 is just inside the lower left hand corner. The knob is approximately 2.5" in diameter. The code is for 3/4" stock and the clearance plane is at 0.81 inch. Z at 0.6300 should lightly cut into the top of the stock which will leave some material at the bottom of the cut; Z at 0 is the bottom of the cut. |
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For set up, the Z axis can be lowered until the bit rests on the top of the stock. Then the Z axis' DRO is set to 0.63. In Mach this is done by clicking the DRO box and entering the desired value and then hitting Enter. The X and Y can be Zeroed. ....................
The code is a text file and can easily be edited by opening it in
Notepad. From the Menu click Edit and then use Find and/or Find and Replace. These simple editing tools make changing a feed rate or clearance plane etc. easy. Save the file to your computer and rename its extension to one that your software understands. MDF works very well with this cut; there is no grain to split other than the laminations, and they make removal of the knob from the stock easy. Simply "peel" the finished knob from the stock. The code was made for a 1/4" round tipped bit; a flat end will work also, but the knob's edges will be stepped rather than smooth. This code works well on my machines and computers, but you assume all responsibility when you use it on yours. Knob G-Code |