Cans and Cants of Machining

[DMAT]

So im looking to design up some parts to be machined. What I'm wondering is the things that can and cant be done by typical machining processes, I would think mostly through a CNC or similar machine.
This way I can design a part that is easier to make and hopefully keeps my costs down.

[toyranosaur]

Hey Dmat,

I don't usually give machining advise since I'm not that all-'round knowledgeable, but the difference between drawing (Where anything is possible) and practical manufacture depends on the equipment, mostly. I have a 3-axis- left/right, forward/backward, & up/down known as the X,Y, and Z axis. Soon I'll have a 4th axis, which is like a lathe attachment sorta, but is for, say, engraving around a horizontal cylider and similar.

The subject is difficult to fully explain to anyone who has not seen machining done, but a couple limitations. One is, no 'organic' forms or curves from the top of the part, generally. This requires 3D machining, and needs programming for that which is not a huge deal, but the machine time is: It takes a LONG time to cut, and this is reflected in the price (As is anything that eats time.)

The big one is Ops, or operations required. Say you want to cut all sides- This may mean making several setups, most on a vise w/Al. jaws, which each jaw is another program. So, say you're doing a part that needs holes drilled/milled, etc. on several sides- This may mean 3-5 different setups, so 6-10 different programs: Things get pricey fast doing this.

Try to stay so the part can be done mostly from the top and the bottom if possible to avoid setups and ops. I must 'Stage' my ops- Take the trannys, for instance. Machined the outer features, all at once, then tore down that setup and did another for all the inner ops. Them programmed & cut a Delrin holder to be able to drill & tap the top & bottom holes. Now, if I already had the 1st op done (The outer features) and someone wanted to get a few after that, it's too late- I've already torn that down and am working on the next step, so can't practically go back & do a few more outers. I always do a few extra as some inevitably get chewed up locating & de-bugging later ops, so I may have some extras at the end or not, depending.

It would be much easier to see a drawing, and focus on what's not ideal specifically. In industry, they're trying to bring CAD guys, programmers, and machinists together more to avoid the drawings that can't be machined practically, if at all, to save time and problems. I wish I had a nickel for every kid that learned some CAD & said on a forum "I've done the hard part, now I just need someone to cut it." You are wise to check into this first!

All I know is, I've seen a million threads with guys that post their pretty 3D renderings of a part, and talk about how this will will be available soon etc.: 99+% of the time, this never happens as they find out just how much one needs to know to make an actual part.....

If you like, you can Email me an example, and I can 'splain why something may be undoable, or simply done differently, or?? Again, this will be within my perspective, and not generally true of every machinist: But a lot of things, while technically possible, are abandoned as soon as price enters the picture. I get asked all the time if I can do such-and-such: The answer is usually Yes, but it is not practical price-wise. This is why I no longer do 1-off custom work, all the time invested in understanding someone's part only to have them choke on the price (Not that I blame them.) Chris REC

[DMAT]

Redesigning parts to lower the operations needed is in my plans. Id like to make it as simple as possible to the point I could try to make it myself. I am designing this to be mostly done from top and bottom. The only things I cant get around is the holes in it. Some of the questions id wonder is like.

What should be the minimum radius you'd want to put on an inside corner. I know you couldnt make it perfectly square but what is reasonably small radius ?

How much extra material should I give for a hole to be drilled. say im doing a 3/16 hole. would I be ok with using a 1/4" for the total thickness or would I want to bring it up to 5/16.

The parts im trying to do were only done in plastic from what ive seen. I know some things have to change to allow it to be made in aluminum.

And how you talked about cost. I understand what im getting into. It is one of the reasons I'm trying to simplify the design so I dont start looking at several grand to make a few parts. Another thing im trying to do is lessen the tolerances needed. +/- 0.01 inches has to be a lot easier than doing the thousandths / ten thousandths you did.

[jwscab]

ordinarily, you use the largest radius that will comfortably fit where it will go. the smaller the radius, the smaller the tool bit, which becomes exponentially weaker as you go down in diameter. So use the largest radius that will 'fit'. It also depends on the features, you might be able to use a 1/4" endmill for 90% of the operations, so you use that mill into a corner.

as far as how much 'meat' to leave around a hole, ideally, you have at least the diameter of the hole thickness all the way around, so a 3/16" hole center drilled would want 9/16" thick material. obviously that's huge compared to the parts we use. You can get away comfortably with half the hole diameter per side, so something 3/8" is good. For instance, in most a-arms we have, they are 1/8 inch diameter hinge pins, and 1/4" thick arms, so the 'half' rule of thumb applies. the pins are also inboard of the leading edge by at least half a diameter.

beyond that, you need to know strength of materials and engineering principles to safely engineer a hinge pin with less meat around the pin.

[DMAT]

So one of the odd things im gonna try and get made is a battery cup side.

Id start off with a 3/8 thk bar. add the holes for the 4-40 screws. afterwards the areas around the hole can be machined down thinner and cut to the final shape.

This wouldnt be a full cup like associated does but instead just the sides. The battery in the car would straighten it up but it would put all the force of the battery shifting position on the screws. Honestly i Probably wouldn't even need to thin it down after the hole is drilled but there needs to be a little design work in it.

I know odd part, but I'm going for an all metal build and its one of the parts I cant find.

The main parts im going to try and do are

rear bulkhead
battery cup sides
stealth tranny brace

[GoMachV]

I'm sure you have probably seen this but just in case....
Taken from here: http://www.rc10talk.com/viewtopic.php?f=35&t=21280&hilit=Askbob+metal

Looky what came out of the mailbox today....

[jwscab]

unless you absolutely want it to look like the factory pieces, you can easily do a different design to get what you need.

see this battery brace I made for this car at the top of the thread. Could be made entirely of metal if that's what you want.
http://www.rc10talk.com/viewtopic.php?f=35&t=14211&hilit=junkyard+dog&start=15

I've made aluminum bulkheads from 1/4" plate and then some sheetmetal for the battery 'cup'. it's a 2 piece assembly but it works. unfortunately I have no pictures of this.

tranny brace is very easy to make from a piece of plate.
Made one here for rc10johnny:
http://www.rc10talk.com/viewtopic.php?f=37&t=24652&p=272569&hilit=trans+brace#p272569

[DMAT]

Sigh....I hadnt actually seen that thread since i started trying to do my metal build. I guess mine wont be the complete same but it will have a few differences in the parts.

I have a battery brace already coming in, just sorta liked the idea of doing the battery cup sides to keep it looking like they were sorta there. I could just do a rod in place if i wanted.

That bulkhead was a one off so the odds of getting another one like it is ill to none. So thats why im doing a redesign.

When it comes to these parts. I could make some of the simpler ones but it really ends up being lack of machinery and tools. I dont really know anybody who works at a machine shop that I could work with. So im stuck finding a shop to do the work.

about me. I did take a machinery class in college that let us get hands on with a mill, lathe, saw, and drill press. We did a few simple projects to show we understood how to use them but its been years since then and most of that knowledge has since disappeared.

[328isjohn]

I get asked all the time if I can do such-and-such: The answer is usually Yes, but it is not practical price-wise. This is why I no longer do 1-off custom work, all the time invested in understanding someone's part only to have them choke on the price (Not that I blame them.)

This is funny because I get asked all the time if I can make stuff for people .... And my answer is always the same....

No I'm a welder.

[DMAT]

I'm one of those cad guys. bow before my impossibru design lol

[328isjohn]

Show us what you have drawn up...

[EvolutionRevolution]

I'm one of those cad guys. bow before my impossibru design lol

One-piece hollow sphere?

[askbob]

One more item I'd like for that build is an alloy battery cup like the original... hint, hint Chris.

[DMAT]

I'm one of those cad guys. bow before my impossibru design lol

One-piece hollow sphere?

I have a theory on how to make that work.

It envolves black magic and explosives.

[marlo]

That rear bulk head is a work of art, its things like that , that make me want to go back to custom builds, how much was that anyway?