Machining Your Own Vs. COTS

We’re getting a plethora of big new machines next year and have been having little mini debates about whether we should machine things like wheels, and other parts or just buy COTS versions. We’re also debating about if should use our new CNC for the precision or use the good old measure twice cut once method on bigger parts, holes, and cuts.

Please post your all your opinions on how we should handle these machines, my current thoughts are that we are going to be using them way too much and use them for things much more suited for a normal saw or drill press.

Well, I was going to vote for “no, never use them… ship them up to us and let us play with them”, but didn’t see that option. :slight_smile:

What I would suggest is that you use them for everything you possibly can in the off-season to learn their strengths and weaknesses and to get good with them so that you can produce parts reliably and quickly.

Then you can make intelligent decisions about how to use them during the build period. It isn’t as simple as “small parts” or “large parts”… but rather which parts are most suitable for CNC machining, and what parts can you make with CNC that were not available to you in the past.

Get creative… you have now broken free of 90 degree corners and straight lines!


The answer is it depends.

If you have something that needs to be extremely accurate, or you need to make a bunch of them, or it has angles and contours, then you’d want to use the CNC.

If you have a simple part you can make with a bandsaw and a power drill, you wouldn’t want to use the CNC.

My formula for buying COTS vs making your own would be to decide:

Can we get the same functionality out of a COTS part as a custom?
How much more will it cost to go COTS?
How much longer (or shorter) will it take to get the COTS part?
Am I going to spend more than price premium of the COTS part to actually make it myself?
What else could I be making with my CNC, that isn’t available as a COTS part?

If the answers to those questions favor COTS parts, then it’s probably a worthwhile investment.

to add to Cory’s comments -

use the off-season to learn the machines.

then, during build time -

if the part an easily be purchased, purchase it.

focus your design and fabrication time and attention and your machine tools on the parts that you cannot buy and need to be custom made for your robot.

One thing that becomes obvious after a few years of this competition is that you can’t make everything yourselves. If you can buy what you need, that’s better than making it, unless you’re under severe time constraints (which almost never happens to us, since we’re literally within driving distance of McMaster-Carr).

Personally, I would reserve the new tools for the complex parts, and try to stay away from using them unless they’re actually necessary.

Doesn’t it also depend on the precision required for the product and cost/quality ratio?

Or, as in the case of sprockets (one example), you can get COTS and modify them using the machines, e.g, lightweight them.

I would say to do a combination of the 2. If you can purchase something that works just a well as what you can make and is cost effective then by all means purchase it. That will save machining time for other more complex things that need to be machined.

As far as using mills to do large things, I somewhat favor it, especially for fabricating drivetrain plates, or things like that just because the accuracy is so much greater than could be achieved with just measuring and a drill press and band saw.

I don’t know, our robots have never had exact right angles- the only time I even tried was with a friend of mine, unfortunate freshmen that we were. The only piece on the '06 robot that was perfectly parallel to the floor, and the robot worked better without the piece. :rolleyes: We always say our robot is built on 89 to 91 degrees, if that accurate. And competitions always gives our robot curves… (in the polycarb)

Anyway, if you can’t justify using the CNC for machining an easily purchased part other than the “coolness” factor, buy the COTS. However, consider whether a machine-competent team member or a machine-competent member and a machine newbie has something to do and how soon you need the part. Machining for the team is an excellent way of sharpening (no pun intended) skills while involving as many members as possible. Also, CNC machines and mills are excellent ways of attracting attention for your team, so think about summer workshops for new team members. For some reason, people are always interested in the big and dangerous, i.e. robots. And remember your safety glasses!

Don’t get so wrapped up in the “I can make anything on the CNC machine!” mentality that you spend ten hours making a single wheel when you can buy a Colson wheel for $4.50 (and with minimal custom machining) and have it do the same job.

It’s all about the cost to benefits ratio, or the return on investment. It it makes sense to machine it via CNC, do it. Otherwise, don’t.

Engineering is not about designing everything as custom parts, or making it all “in-house”. It’s about finding the most effective and efficient solutions with regards to cost, time and resources.