So I’ve made a swerve design that is hopefully gonna be manufactured, but whether that comes to fruition or not, I’m still wondering on how other teams fixture COTS gears to be cut on CNCs. I’m not too familiar with CNCs and definitely not knowledgable on fixturing, but I’m mainly gonna be redirecting info here to someone who does
For the swerve, there are 20DP gears going from 16T to 82T, both steel and aluminum, and without the ability to use the center hex bore for fixturing, as these will be bored out for bearings in most cases.
A few ideas I had(take these with a grain of salt) were 3DP parts with bolt holes for mounting and a hole in the shape of the gear to hold it, or something similar out of laser cut wood. My concerns are heat/vibrations but again I don’t know if those are significant factors.
The machines we’ll have access to are the Tormach PCNC 1100 3-Axis Mill and the CAMaster Stinger II 3-Axis Router, both of which have 4th-Axis attachments.
I’ve done this for aluminum gears on a Tormach PCNC using the following procedure:
-3D print a square fixture with the negative of the gear profile in it. You want this to go partially through printed part but not all the way. The whole height of the gear should be in it, though, so the whole part should be 0.75”-1” tall
-You want the gear to be a very tight fit. Hopefully a press fit. For me, printing it on a Markforged produces a nice press fit.
-You still need an additional way to affix the gear to the print, though, as the press fit alone won’t be enough to hold it through the cutting process. If your gear has a known hole pattern (like Versa gears), and you are not intending on removing those holes, you can make matching holes in the 3D part and bolt it together. If it doesn’t have a hole pattern, you can use the hex in the middle: have your 3D printed part have a hex “shaft” sticking up, with a vertical hole in it. Then use a bolt and a washer to hold it all together.
-Securely mount the whole thing in a vice which is known to be square with the machine.
-Use an edge finder to find the edges of the printed part. This needs to be very precise.
-use very shallow cuts, because even with all of this, your gear won’t be as secure as it would be if you were directly clamping it.
-heat shouldn’t be a problem if you are otherwise taking shallow cuts and use good bits with a coating.
Rereading your message, you may not have the opportunity to use either the central hole or versa holes for fixturing in what you are doing. In that case, you can design a second piece that acts a a “lid” that just covers the parts of the gear that you are not machining (like teeth and any material near them). This part then needs to bolt to the lower part described below.
Here’s an example of this that we used this year on our arm. This was for a sprocket but the idea is the same. The piece on the right is the “holder” and the piece on the left is the “lid”
Another option would be to use superglue or epoxy on a flat face of your gear. NYC CNC has published an explanation of how they do it here:
Like the other workholding methods, you can’t go super fast to hog out as much material as you might on a vise, but it’s pretty reliable, especially if you’re using a big gear.
Another option would be to drill the mounting holes on your first setup, and then mill out the features on a second setup using the mounting holes to locate and clamp down your gear. If you publish a drawing of your gear, we can give detailed advice on what might be best.
The ideas the others have posted are all good options. If you have a true CNC mill you can also swap the vise jaws for aluminum ones and physically mill a contour into the jaws and seat your gear in that. The contour can just be an idealized circle and you want to cover greater than 50% of the circle on your gear to reduce the point loads on the gear teeth so they won’t deform. You can modify gears much faster and mill it out more aggressively if you do it this way.
I completely agree with RoboChair. Soft jaws for the vise in your Tormach would be the method I’d use.
Of course, you need a quality vise (Kurt or similar) and aluminum soft jaws that you can make or buy.
If you have a Tormach, you probaly already use a CAM or even program manually. The program is a simple circle that is the diameter of the gear you want to cut.
Be sure to clamp the jaws tight when milling your circle!!!
You can clamp the jaws closed on to any scrap item so that the gap + aluminum vise jaws are sufficiently large enough to hold the gear, yet there is enough jaw material to machine away and provide clamping pressure once you remove the scrap.
With soft jaws, you could also bore out the hex for your bearings in the same setup. No need to set up an additional operation just for the center hole.
You will need a boring head (preferred method for accuracy and roundness), but you can also interpolate the bearing hole too.
…Many teams interpolate their bearing holes without concern, but know that the hole may or may not be “perfectly” round. It might be slightly egg shaped or other. You should still be able to press your bearing in.
Just be careful with this and mose methods, of you are machining sprockets or gears to match (i.e. common shafts) they need to be loaded exactly the same into the fixture. You don’t want one side to preload the system by be a half tooth ahead of the other.
Similarly, if tensioning chain, the tensioner blocks/turnbuckles need to be in the same side so they don’t try to twist and preload the common shafts.
The only reason why the steel jaws are not mounted is because they were not already on the vise.
There is no part stop. For every gear that was machined, after clamping we indicated X and Y. We only made 3 of these gears total. One for each robot and a spare, so it wasn’t bad.
I’ve used the soft jaws method before, but something about it always gives me some pause. In order to hold the gear in place, you need to tighten the vice quite hard. When you do that, it deforms the printed part a little bit - (hence the “soft” in “soft jaws”, presumably!). Since you don’t really know where that deformation is going to occur, you kind of lose the ability to precisely locate where the gear is relative to the vise. For example, if your 3D printed piece is designed to be 3.00” when the gear is resting in it, but it shrinks to 2.99” when tighten the vice, where is the center of the gear relative to the back edge of the vice? It seems like it could reasonably be anywhere between 1.49” and 1.50”.
Maybe this is too much precision to actually care about, but it always makes me think twice about using soft jaws.
Aluminum soft jaws are great if feel like making/buying them, but printed jaws should work too. I definitely recommend indicating or probing the gear no matter how you fixture it. I have put a gage pin that was a light press fit into the hex and probed that, and also indicated the boss which is the more accurate option.
You can indicate off of both the fixed and moving jaw after clamping and use the center as Y zero. And also indicate off the left and right of the soft jaws and use that center as X zero.
To minimize the deformation tighten the vise very gently and use a small diameter cutter with conservative cuts to minimize cutting forces. When it’s just a few parts, cycle times don’t matter. 5 minutes is fine for a part that in production would be 30 seconds.
“Soft jaws”, as I referred to, are not 3D printed. They are typically made from aluminum. They replace the steel jaws in your vise and are then machined to fit the widget you want to hold. The AL jaws will be significantly more rigid and repeatable when compared to any printed, plastic soft jaw.
Soft jaws can also be made from unhardened steel (vs the hardened jaws that came on the quality vise originally) for durability in a production environment. The aluminum ones will last hundreds, if not thousands, of parts.
The vise only needs to tightened hard enough so your clamped part doesn’t move enough to be out of tolerance, or break itself, or the cutter.
My strategy would be to cut a hole in a spoilboard for the raised boss at the bore then just use finger clamps to hold down the edge of the gear. That way you can use the same fixture for multiple gear sizes, you can clamp real hard without distortion and clamping force won’t change based on skeletonizing the part.
