Custom Gearbox Without Automated Machining?

My team does not have any access to a CNC, water jet, laser cutter, etc. The best tool we have to machine parts is a Bridgeport mill without DRO. I am wondering if anyone has experience making custom gearbox plates with limited automated machining capabilities like us. If we use a calipers to measure, and a center punch to mark the holes, is it possible to obtain center distances that will be accurate enough for the gears to mesh properly?

You should be able to get the precision needed using the dials on the handwheels.

Since we normally use R8-size bearings that have a 1.125" OD, it may be a bit difficult drilling the bearing hole. I’m not super smart with machining, so I could be wrong, but it seems like a 1.125 annular cutter might be the easiest way to cut those holes. (a machinist should chime in here).

It really depends on the tools you have.
What will you use for the bearing holes? In a CNC you would use something like a 1/4’’ endmill. Using that i doubt you can get good results with a manual mill.
If you have a good enough way to make nice 1.125’’ holes, i’d think that you should be ok.
With that said, remember to add the 0.003’’ to the distances of the pitch diameters for smoother runs in your gear. We recently learned that lesson the hard way.

Team 1991 recently got a 1.125" end mill and have been using a 1.125" step bit that worked flawlessly. I’d suggest getting the step bit because it can also be used in hand drills. I’m not by any means an expert machinist but have used these both with no issues with the hole or spacing.

Hi there,

Yes, last year my team manufactured custom gearbox plates on a traditional mill, even though we did CAD them first to make sure that they fit everything properly. It’s a pain but if you have a capable machinist you should be fine.

It’s possible if you know what you’re doing with the Bridgeport and pay attention to backlash.

To make the bearing bore, I’d probably use a boring head. If you don’t have one and don’t want to buy one, you can also buy a 1" drill bit ($10 for the cheapest reasonable one) and a 1.125" ream ($50).

I wouldn’t use calipers and a punch. The center to center spacing of the gears needs to be within a few thousandths for the gears to not turn to dust in a short period of time.

Is it possible? Yes, especially if you have a machinist that understands backlash in the handwheels and a mill that’s been taken care of.

Is it the best use of your resources? Color me skeptical. Unless you have a very specific need, finding a COTS option would likely be better use of your finite time and machine access during build season. Making a gearbox plate manually is time consuming, and there’s a noteworthy chance that you’ll have to correct mistakes in the initial attempt (or even start over from scratch).

Bridgeport? Want to also do other bearing sizes? Get a cheap boring head. You’ll be able to make A+ holes of any size.

All of the gearboxes used on 716’s robots are made on a manual Bridgeport. I am still making them as, at least for us, it is lower cost than buying. I have upgraded to a DRO to make it easier (Shooting Star BT-2, excellent quality, easy to install). I recommend a boring head for the bearing holes.

Check out my gearboxes in the white papers. All have been made on manual machines.

+1 for Andy Brockway’s white papers. This is the guy and these are the papers that got me started in custom gearboxes way back in 2004.

You can do much better than that if you use the dials on the cranks. If you have someone around the shop that has been using that machine, ask them how to put two holes into a piece of metal exactly x.xxx" apart and they should be able to show you.

That said, you can make gearboxes with low precision tools in some cases. We made this gearbox without a mill. It was made using a center punch to mark the holes and a Rotex punch machine to make the holes. It’s not an accurate process, so we made several plates until we had some that fit the dimensions we needed. If you have enough darts, eventually you can hit a bullseye. I wouldn’t try that for a gearbox with more than one critical dimension, but it worked in that case.

This is way too aggressive. You’re not going to get good results with 1/16 stock removal per side.

Your pre drill should be 2-3% smaller than your finished hole. 1-1/16" in this case.

Just use the dials on the handwheels. Ordering a DRO is in your best interests, as they save a massive amount of time that is otherwise spent worying about backlash and tracking position. A “competent” machinist will not need a DRO, but it helps a ton for students who are learning. Add 0.002" or 0.003" between gear bearing holes to make sure that small tolerances don’t kill the gearbox.
I usually just use a 1/2" drill, followed by a 3/4" endmill and then a 1" endmill to bring the hole larger, then finish with a 1.125" endmill. This makes a tight slip fit for 1.125" bearing OD, although not a press fit. You should always seek to retain bearings on a shaft with “positive” retention, which is things like shaft collars, screws, etc.

You will not usually need to make a custom gearbox. For drivetrain, I urge you to go COTS all the way to save time. For anything else, try using clamping gearboxes and bearing blocks from Vex, unless it’s super heavy-duty. After exhausting those options, then, I would look into custom. Follow KISS principles in design when manually machining a gearbox.

Machinist were building gearboxes way before CNC and DRO although those are great tools if you have them. You can lay them out with a machinist scale and a scribe. Youtube is an incredible resource for this kind of stuff. Here a couple of videos comparing the different methods. I would recommend machining the side plates in pairs the whole match.

Hole layout part one

Hole layout part two

On youtube: mrpete222 has many great videos on old school machining. THATLAZYMACHINIST is another good source.

I love mrpete, but still strongly recommend against making FRC gearboxes with only prick-punch layouts. In the second video he compares the accuracy of three different methods (one of them being DRO) and he is still only getting around +/-0.005" on his holes- which is still better than the great majority of students will do. That kind of tolerance is unacceptable for even Vex gears. At the very least, use the mill’s built-in dials to drill your critical holes like those for bearings.
This Old Tony is another good resource for machining.

we went with calipers and punches, takes a wile, a few bumps along the way, but eventually we got it. Good Luck!

Did you do this on a manual mill? or a drill press?

I’ll add Keith Rucker as a good machinist on youtube:

He’s even milled gears for a blacksmith blower using a horizontal mill and dividing head. No DRO on that mill either.