Imagine for a second you needed to make a part, as well all do in the robotics industry. This part does not need to be exact, but it has some critical dimensions, such as holes in a drive rail, or slots in a moving piece. here is the kicker, you don’t have a CNC machine or other computer based manufacturing machine. What do you do to make such the piece would come out with about the same amount of precision that a standard CNC machine would have? You do have access to a printer to print out scale plans, and are equipped with power and hand tools. I ask this question because although my team has access to a CNC machine, sometimes its easier just to make the part by hand, especially if its small, not more than 6 inches in any direction. We do seem to have some trouble, even after measuring twice, to get that critical dimension correct. holes are off, spacing is either tight or loose, etc.
My team has lasercut cardboard and wood as a template, and clamped them to the piece. We have also drawn measurements onto the piece.
If you can, setting up a jig is a good way to get precise holes, in the past we have used our drill press and the included table pieces to drill hole patterns.
Also, we have a drill guide that makes sure the holes are straight.
Here are some tips:
-Understand the tolerances that you require. Is hole-to-hole spacing particularly import for the fit of a certain component? If so, put it on the drawing either with a tolerance callout or a note. This is especially important if different people are designing and fabricating the parts.
-Use a combination square to lay out holes. You can find these in standard, metric or combinations of both.
-Mark lines on each axis with a fine tipped writing tool like a pencil
-At the intersection point, mark a precise dot
-Use a center punch, I prefer a spring punch, to create an indent for the pilot hole
-Use a pilot hole placed precisely on the indented punched mark from the previous step. Using a smaller drill bit will reduce the tendency for the bit to “walk” away from the desired location.
-Starting with a large hole is recipe for disaster, the bit will walk away from the desired location.
-Use nice drill bits - consider “jobber length” drill bits for cutting aluminum and steel. Coated bits will help to keep the bits sharp throughout their lifetime.
-Choke up the drill bit as much as you can in the chuck on your hand drill or milling machine to reduce the tendency for the bit to flex while drilling.
Sheet metal layout was something I was taught during a mandatory metal shop class in 7th grade, back in the middle ages. There are no longer any mandatory shop classes in our school system or in the one I attended so we teach it to the build team with varying degrees of success.
Sometimes when we need a few identical (-ish) parts made, like gussets, I make a bit of a contest out of it. We run off some copies of the CAD drawing at 1:1 scale and each student gets a copy and enough aluminum sheet for the part. We use glue stick (not hot glue) to adhere the drawings to the aluminum sheet. Holes get center punched directly through the drawing. The paper pattern stays adhered to the aluminum until all the cutting and drilling is done. The part gets soaked in some water and the glue stick dissolves from the part.
The pretty parts go on the competition robot. The rest go on the practice 'bot.
One thing to consider is to design the part so it does not need machining operations that are difficult to do without the machines you don’t have.
For example, avoid having to make slots. Keep the number of holes that need precision placement relative to other holes, to a minimum. In general, try to select materials that are as close to the needed size/shape already, so machining is minimized. Find COTS parts that will fill the need, as is. Think of a different mechanism design, for example one that uses a hinge instead of a slide.
We’ve been building robots this way for a long time, it’s not that difficult. It does take some creative planning.
Some parts can be made by hand with relatively low precision, such as gussets or long 2x1 rails. Using products like the Ozzboards drilling jig can help create standard hole patterns to interface with.
I was taught to print out the part drawing on a 1:1 scale and spay glue it onto a sheet of aluminum. Then, center punch all the holes (manual works better than the automatic ones, I’ve found) and center drill and drill all holes using a drill press. Finally, bandsaw the gussets out and sand to finish. You can get very good results like this.
Another common tactic that I used in high school was to mill all the holes in a plate (such as for a gearbox plate) using a manual Bridgeport mill, then bandsawing out the plate using a spray glued template to get all the fancy, low-tolerance curves in.
Paper can be removed via acetone or hot water based on the glue used.
Metal bluing, straightedges, squares, and punches can achieve a few thou tolerance if you have a bit of patience.
Yes, they can. I made this as a freshman in high school machine shop class, using a hacksaw, files, and a drill press. I measured it recently, aside from the hole being off center a little bit, it’s within a couple thousandths of the drawing it was made to.
I wish I had the patience to teach our freshmen to do things like this.
Dial calipers, in addition to things already mentioned. Hand tools can be used precisely
For plate parts spray adhesiving on 1:1 print outs (super 77 is what we’ve used) and a vertical bandsaw works great, then use a center punch and drill press for the holes.
Or by the brand name Dykem.
Do you guys have a surface plate? A surface plate with height gauge makes for a really good marking tool if you are starting with stock that has been squared.
Use a manual machine.(With or without DRO)
Pretty much this. CNC machines generally don’t improve accuracy compared to manual machines; they reduce cycle time.
For small parts, a manual mill. It’s simple, easy, and we can get parts within a couple thou (It’s not the best machine out there, has a little slop in it that we haven’t figured out how to completely eliminate) easily enough. It’s easy to do the hole pattern to mount a gearbox, for example.
After that, match drilling. While a CNC would open up other options, we design to our capabilities… and match drilling lets you do most of what you want. For example, our arm uprights this year. spend a little time clamping them together properly, and drill the bearing holes through both uprights at the same time. Flip the piece over and drill the mounting holes at the same time. Very easy to get everything lined up properly that way!
That’s a great reference doc, thanks!
Where do we get this 5 bit adapter for our hand drill?
Manual machines are the answer though. We did our first few years exclusively on manual mills and lathes, although if you’re outfitting a shop for FRC purposes and have the money to buy either a knee mill or a CNC router, go for the router. If that’s not an option, then design simpler and make do with bandsaws and drill presses (or hacksaws and hand drills, pick your poison).
Dykem blue, lay out with scribe marks with a t-rule, Optical center punch
We don’t use these specific ones, we’ve had ours for more than five years so I don’t know where we got them. They’re wonderful.
A wide-tipped permanent marker will also work for doing the layout.
I just might succumb to *T.A.S. and buy the t-rule and optical center punch for my home projects.
*Tool Acquisition Syndrome
This post touches on a topic that rarely gets discussed, thinking about the manufacturing process.
The drawing @BordomBeThyName attached shows how the gusset is fastened to the two tubes using three bolts. This eliminates the necessity to mark out 5 or 6 of the holes indicated with the drill bits on the tubes. They can be match-drilled through the gusset.
Furthermore, it it possible to only have to mark the position of one of the holes along the length of the tube shown with two bolts. By lining the edge of the gusset with the side of the tube and lining the hole in the gusset with the line showing the position of the hole, then clamping the gusset to the tube, the two holes for the bolts can be match-drilled in the tube. After the two bolts are installed in the first tube, the second tube can be aligned with the gusset and clamped, allowing the third bolt hole to be match-drilled. The other 5 holes can then be match-drilled.