Machining precision required to make custom gearboxes
 

My team is currently making a (simple) custom gearbox in place of the COTS gearboxes we were planning to use that ran out of stock.

My question is: Do you really need thou precision to make decent gearboxes? Because after looking at some other threads, it seems like even measurements as small as .003" can make a significant difference in the performance of a gearbox.

Re: Machining precision required to make custom gearboxes
 

I would say you should be able to relatively easily hold .002" on position of hole centers on a decent quality manual mill with a somewhat experienced operator. That would be good enough for a custom gearbox.

Re: Machining precision required to make custom gearboxes
 

What about oblong holes? Our DIY gantry-style CNC consistently cuts oblong bearing holes that are exactly correct in the 'X' direction, but approximately 0.001-0.002 off in the 'Y' direction. That's about as close as they could get before needing to move on to other things.

As the guy CAD'ing our first-ever custom (non-drivetrain) gearboxes this year, is there anything I can do to make this a non-issue? The concern for us is gearbox longevity in ~120 matches between March & November.

Re: Machining precision required to make custom gearboxes
 

Your probably going to be fine, the 20DP gears commonly used in FIRST are purposely larger than necessary so they can handle being abused in low precision gearboxes and high sudden torque situations. Usually the failures of gears in FIRST are either due to gears being too small and cracking at propagated stress points of the hex shaft or flexing and allowing the gear mesh to become far too loose (the VEX Ball shifter 3rd stage seems to have this issue). In the scheme of things the amount of time FIRST robots run is very short and wearing your gears to the point of failure is very unusual unless something is really wrong like the mesh being too tight. This why its common to add .002" to the C-C mesh of gears in FIRST.

Re: Machining precision required to make custom gearboxes
 

Quick correction: gears in FRC are 20DP (which is actually a very odd pitch for some reason, most sites only sell 16p or 24p) and are actually way undersized. If you compared the tooth of a Vex gear to that of a Boston Gear gear, you'll notice that the Vex teeth are slightly thinner and have large sections of the root of the tooth taken out.

I second that you need .002" precision at the minimum. Personally I prefer .001". If you do not have a mill, please don't attempt this mid-season- there is almost a saturation of gearboxes available from WCP, Vex, and Andymark.

Re: Machining precision required to make custom gearboxes
 

Do you have backlash or is it just lead error? Can you program backlash compensation into the machine configuration?

Re: Machining precision required to make custom gearboxes
 

I do not know, but I will ask those questions. They spent about 6 hours re-calibrating a new bit Saturday, but since I was knuckles-deep in drive train gearboxes I don't know the specifics.

Re: Machining precision required to make custom gearboxes
 

If you don't have a CNC or a manual mill, you can do this manually. With the typical FRC gears and a simple gear box. you can lay out and drill holes using a machinist scale, scribe, prick punch, a drill press, and a little OCCD. Clamp & drill the side plates together. This takes a lot of care, but is very doable.

Re: Machining precision required to make custom gearboxes
 

While what you describe certainly could be done by someone with very steady hands, good eyesight, and great attention to detail, I would argue that any team which only has a drill press shouldn't be considering building custom gearboxes given the plethora of great COTS options available.

Much more valuable to expend that effort on other areas of the robot.

Re: Machining precision required to make custom gearboxes
 

Are there any other teams in your area that have access to a CNC mill or water-jet? Most teams are happy to support other nearby teams with machining or needed parts. FIRST is really a community, we only compete on the field.

Re: Machining precision required to make custom gearboxes
 

A lot of OCD and a quality drill press. Even with the plates clamped together, our drill press isn't straight/rigid enough for me to be comfortable with 841 trying this [with aluminum gears].

Yes. Highly recommend finding the extra ~$100-200 to buy-not-build.

Consider milling undersize and finishing with a 1.1250" reamer - biting the bullet and getting a reamer has allowed 841 to use our not-super-rigid benchtop tools to rough out bearing holes at the right location (on say a WCD rail or mechanism arm) quickly and a little undersized, and then finish the holes to a really nice bearing fit.

Re: Machining precision required to make custom gearboxes
 

That is how I have done this in the past to deal with our Taig cnc. I grabbed a 1.124 reamer off eBay that I'd like to try.

I would venture to think that there is backlash in the router that can be corrected with software (LinuxCNC/Mach/etc.), but I haven't seen their setup.

Re: Machining precision required to make custom gearboxes
 

We do have a reamer, but have had mixed results when using it on oblong holes. Using a micrometer on a sample after under-sizing still shows a position margin of error of around +/- 0.001 in a dimension.

In any event, since the gears arrive tomorrow we'll likely just make the gearbox and report back in November. If we see signs of premature wear during our planned 50-climb test, we'll probably get steel gears and just brute force it. It's in an easy-to-replace spot and we're building a spare as it is.

Re: Machining precision required to make custom gearboxes
 

On a DIY setup, that's pretty respectable. You could try running a spring/multiple finishing passes to see if that helps remove hole eccentricity.

Re: Machining precision required to make custom gearboxes
 

I agree that you are better of buying the solution when one is available. The COTs choices have grown tremendously in the last few years. But one stage is fairly easy with 20 DPI gears. More than 2 stages becomes really difficult. I always take stated accuracies with a grain of salt. :]

In terms of drill presses. We are fortunate to have a heavy 1/2 press with good bearings. But I get as accurate results by using a 1/4 in Ryobi and either a short 1/8" bit or center bit.

Re: Machining precision required to make custom gearboxes
 

This makes sense. A reamer will not change the center position of a hole, it will follow the existing hole. If a roughed hole is not on center position wise you'd need to use a boring head or plunge with a correct size end mill to fix that. Neither of those solutions will help you if your machine is positioning incorrectly to the center location though.

Re: Machining precision required to make custom gearboxes
 

I agree it is doable. On my team we made a stainless steel template for the toughbox mini. Which allows us to clamp it to the material drill pilot holes out and slap a gearbox on anything in minutes. So i guess our process is this.

1. If you are going to be repeating patterns get your hands on some stainless and make a decent template either by drafting the dimensions onto the metal or by tracing the pattern off of something else and finding the centers. only drill it out to 1/8" for pilot holes

2. Clamp the template to the piece you want a gearbox on and drill out your pilot holes.

3. Bring your holes into size for the smaller holes (<=1/4") we just use a drill bit but step your way up to keep it straight. for the 7/8" and 1-1/8" holes just buy a regular hole saw. the normal hole saw will be over size so you have to put it in a drill and run it on a grinder or belt sander for just a second or two. this will bring the size of the saw under size. pull the 1/4" drill bit out of it and just shove a 1/4" rod in there this will help to keep the saw straight. Bring your pilot hole to 1/4" then used the smooth shaft on the hole saw to bring it to size. for the holes less than 7/8" and greater than 1/4" I highly recommend every team owns a rotabroach kit specifically this one https://www.mcmaster.com/#4093a81/=15ybt9g

4. The bearing holes are undersize so open them up with a deburring tool. another highly recommended if not necessary to pass inspection tool https://www.mcmaster.com/#4253a16/=15ybuy8
Just take your time with it, try to keep the cut perpendicular to the surface of the material and keep checking the bearing until you can press it in snuggly

It isn't fancy but it works. I think it is hugely advantageous to be able to integrate a gearbox into just about any surface. Then you can start doing weird stuff like perimeter mount gearboxes to save space in your bot. There are a lot of COTS gearboxes but it is awfully nice to feel like you can crank out exactly what you need in an hour or two.

this is a test set up on a piece of scrap;