I'm thinking about buliding my own 3D printer from scratch.
Is it better to buy a starter DIY kit and try to build your printer around it, or to order separate parts for printer, and then to combine a printer?
From a general point of view, there are a few things to consider.
If you buy a kit:
The way I look at it, the better option for you depends on how you want to spend your time. That is:
If you don't already own a 3D printer, I would recommend getting a kit, simply because struggling with trivial things like parts not fitting together can take away the fun for many people.
I built my first printer from scratch, though it's fairly similar to an oversize MendelMax 2. It was a good learning experience, but very frustrating at times. Overall, I think I'm glad I did it that way, but a kit would have gotten me printing much faster and a bit cheaper.
Here are my main impressions from the experience...
You should already be familiar with most of the basic skills involved: soldering, wood- and/or metal-working, electronics, etc. And don't forget software -- I wrote a lot of Arduino code for various things along the way.
You'll need to be diligent about searching for answers online, because there's no manual to go to. This group should make it a lot easier now than it was last year.
Some problems will have great answers online; some won't, or the answers will be hard to piece together. I found that stepper motors have a "lore" all their own, with several subtleties I didn't expect.
You'll have to decide how "from scratch" you want to go -- I bought completed Arduino Mega and RAMPs boards, but you could buy a kit or components for those instead, or even design your own electronics altogether.
You'll make mistakes and spend money to recover. For example, my Y-axis motor couldn't move the build platform nearly fast enough (too much inertia), so I had to re-work that axis entirely, using a much more expensive lead screw, plus bearings, blocks, couplers,....
If you think you might want to sell the printer later, I imagine it would be easier if it's a known design.
That said, it's a blast except when you get stuck; and the satisfaction when your first successful print comes out is pretty great.
Oh, one other hint: especially for hardware, buy in bulk. You can probably get 100 of a certain bolt online for the price of 10 at your local hardware store. Give or sell the leftovers to help others get started.
A few months ago I bought a cheap (220 USD) Prusa i3 kit from China and put it together. Putting it together was fun and still a bit of a challenge. It was also nice to have pre-configured firmware already loaded onto the Melzi board that came with it.
So for the downside: The board (Melzi) that came in the kit only supports one extruder so if I want to add more extruders later I have to replace it. This means that in that case I would have paid for two boards.
Maybe you should ask yourself what your #1 goal is. If you are mostly interested in having a printer for use in other projects, then a kit might be what you want. If your main interest is having something to tinker with, then maybe you should go all in and make everything from scratch.
Three great answers have already been posted, and it has been extremely interesting to read them. I shall try not to repeated what has already been said.
I have sourced the parts separately for three different printers:
I have been coding Arduinos and Pis and building robots and quadcopters for a few years now. Then, in November 2016, because I needed a prop guard for a ZMR250 quadcopter that I found hard to obtain, but easy to print, I started reading about 3D printers (mostly RepRap wiki, and then individual blogs of straight forward builds, as well as design modifications, of Prusa, P3Steel, Wilson and Delta/Kossel printers), watching countless construction videos and asking questions here on SE 3D Printers, and reading other's questions and answers, as well as going through eBay for hours at a time, looking up parts and making numerous Bill Of Materials (BOMs) and blogging the information that I gleaned. So this gave me a good grounding and starting point for when I did get around to ordering. In fact, the process is still on going...
After ordering the parts, in December, piecemeal, I then had to wait for a month for the parts to arrive from China, during which time I read some more, and revised what I had already learned.
I then, finally, got to work on the P3Steel, in January, but two and a half weeks later, before I had finished it, I had to move to BKK for an extended period.
I suffered delays with the P3Steel build due to postal latency, obviously, but also, some partial kits where missing critical parts (see Is the 8mm x 20mm bearing axle for the X-axis idler (of a P3Steel) a custom part?), so I had to get them machined in Thailand (because it only costs around $3 to get something machined here). Hopefully, when I get back to the UK, I should have everything to hand and be able to finish the build in a few weeks maximum
Once in Bangkok, I started sourcing parts for a Wilson II, and then, subsequently, a Kossel, mostly because the aluminium and steel rods are a quarter, to a half, the price that they are in Europe. Also, I had to go through the ordering process again, getting parts from China for these two printers - however, the parts from China only take two weeks to arrive to Thailand, not a month or so, for the UK. The Wilson II parts I plan to take back to the UK, in order to complete the build there, hopefully printing the plastic parts on the P3Steel, when/if the P3Steel is completed.
Note, that seven, or eight months, down the line from when I first took an interest in 3D printing, I still haven't completed a single printer, yet. However, I sure as hell have learnt a lot. Note: most of the delay is due to the six month relocation away from my printer build in the UK.
Also, due to my reading of the modification blogs for the Wilson and Kossel, I have recently been re-purchasing upgrades, before I have even fitted a bolt together, for the Kossel and Wilson, let alone completed either of the base builds. For example, I have just purchased Chinese aluminium vertices, rather than the plastic PLA prints that I got from Sintron. So I have ended up with a fair collection of spare (redundant?) parts, but again, it has helped me gain a great insight as to what works well, and what does not.
To reiterate that which TestGeek has said, one major tip I would have is (and I read this on a forum when I was first getting into 3D printer building), if you are sourcing the parts separately, is to buy bulk (get packs of 10 pcs, 50 pcs, 100 pcs), and buy more than you require: nuts, bolts, bearings, GT2 belt, GT2 pulleys. The price comes down phenomenally, and you can resell the spares, locally, for about as much as you paid for the whole lot, online, thereby covering, or almost covering, the cost of the printer. Plus, spares come in handy for further builds down the road. Don't buy anything from the US (unless you are already in the US, I guess) - the import/postage fees are outrageous.
In the same way as you learn more from building a kit as opposed to buying a pre-built printer, you will learn more sourcing the parts separately, but it might cost more, in redundant parts. Also: