Hold Down / Clamps for Desktop Milling Machine / Shapeoko / xCarve

thingiverse

Another mix of the standard Milling Machine hold downs. I wasn't happy with the performance or design of the others, and the lack of source files to customize always annoys me... so, I drafted up a set in Inventor 2014. These are setup to work with 5mm bolts but you can customize the source files as needed (or scale your STL along the width axis to elongate the 5.5mm opening for the clamp bolt. If you scale the length, you'll need to scale all of the parts correspondingly or the mating surface between the step blocks and the hold down bars won't align anymore tho! Update: I changed the hold down bar to remove the bevel from the top side. This allows for easier printing (print the bars "upside down" with the teeth facing up) without a need of support or fear of warping during the print if printed without. Instructions Print Settings: When printing the parts initially, I printed them at a high percentage of infill for strength. I believe my initial prints were done with a 60% infill w/rectilinear fill pattern. This lends itself to being very strong for clamping and does a good job at resisting cracking or delamination of the print while in use, but at the cost of using a LOT of filament for an otherwise small part. So, after printing a bunch more and tested each, I'm changing my recommendation on print settings. I now find, for myself at least, that the means for these to be strong, yet light and print quickly, is using a higher perimeter count... Going with something closer to 10 or 15 gives you some solid shell for the connection points, which will remain rugged after lots of use, yet only require something like a 20% infill and a 200 micron layer heights. For print settings for the Arms I most frequently print them with 8-10 perimeters, and 6-8 solid layers for top/bottom. With all that, then going with 20% infill is all that's needed really, as the infill at that point is just providing mass vs strength of the part.You do need enough infill for when you are clamping down, especially if you don't have a large washer to distribute the load, to prevent the clamp arm from being potentially crushed if you have to really crank down on a part. Print Orientation: For the actual printing orientation on the print platform, I have been most successful by orienting them as such: Steps - Print on the side, with the teeth facing sideways. I find it's best to print these in pairs with the teeth facing each other, with the part oriented 180 degrees to one another, such that the pair then forms a large rectangle with the space between the two. Give yourself about 4-6mm between them, which should allow your slicer to lay down some brim that bridges between the two parts for your first layer. Bars - Print on back, with teeth facing UP. Use: There is a reason for this design for clamps having been used for 200+ years... but you DO have to use them correctly... and that method MAY not be completely obvious. If your bars are bending under clamping force, you are clamping too far away from the work. Remember, when clamping parts, especially for milling, clamp with the bolt as close to the work as you can get it. Don't clamp in the middle or further away, or you will 1) put a lot of stress on the clamping bar and 2) you get MUCH less clamping force. That's just plain ol' leverage/physics! I see photos online ALL the time of people clamping things just terribly wrong on their desktop milling machines (and even full sized Bridgeport Mills) where you can see their clamps bending in the middle from the bolt in the wrong location, and people using the step blocks at the wrong heights (level or too low typically, tho sometimes too high also) so very frequently... and it's just because they were not taught how to use the clamps correctly. Hopefully this helps if you didn't previously know. :) The point is to clamp JUST slightly above down onto the part. The flat of the clamping bar should NOT be flat against the part. What you are doing is trying to clamp with just the edge where the flat transitions to the bevel. This translates the maximum force to a single point, holding it down. With plastic printed parts, we are limited by the strength of our material... but with metal clamps, you can generate TONS (not exaggerating) of force with very little amounts of clamping force. This is why the end of the bars are beveled... to make the bottom of that "V" of the clamp has a small surface area. As I said, this may not be obvious at first, especially to someone who hasn't been trained in machining and are just figuring it out as they go along... but if you consider the physics (google Clamping with Inclined Plane and/or Coefficient of Friction if you are REALLY interested in all this) you'll quickly see where and how to use this style of step block clamp and why it's pretty much THE method utilized by machinist first and foremost whenever they can. So... Set the bar such that there is a slight decline from the high end of the steps down to the part being clamped. You want the clamping bar to be set 1 or possibly 2 steps (no more) above the surface of your part / stock... so the bar should NOT be level, and it should only have the corner of the clamping bar contacting the material, not more (aka a flat from the bars main body, or the flat of the bevel of the end of the bar... and the bar should never be level or lower than your material! This is why there are SO many teeth on the step block, so you can get JUST the right angle for clamping. Also, always use the shortest clamping bar you can. Remember, when machining, it's ALL about rigidity Rigidity RIGIDITY! Sometimes you have to use a longer bar, especially with the SO2 where your clamping points are somewhat limited in their spacing (lacking T-Nuts with the ability to position almost anywhere) but just use the shortest clamping bar you can, and put your clamping bolt as close to the work as you can manage.