Learn me CAD...: Off-topic discussion forum: Grassroots Motorsports Magazine

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Feb. 7, 2012 7:47 p.m. Hungary Bill Reader

I've stumbled across an Air Force machine shop where the reservists are in need of a bit of training (or so their supervisor says). Today a friend of mine asked them to make weights for his bow's stabilizer and I brought them a couple drawings for an engine/trans-axle adapter kit found here: http://guerillamotive.com/wordpress/?page_id=183

This is going to be my first experience with CAD since I drew 3-dimensional boxes in high school so here are a few questions:

If I understand correctly this needs to be drawn on a program that will save it as a ".gif" file for the CNC machine, correct?

Is this something I could do (theoretically) in a couple of months on my home computer without risking a divorce?

Is there a place I could download a CAD program that meets those needs (preferably free?)

A machine shop once made this exact thing for $1500 for another person... I was hoping that most of that was "machine shop fee's"... would I be saving a ton of money here if I supplied the required metal to these people and brought them $100 of booze or is this just a pipe dream that wont pan out as expected? (the booze is already the agreed upon price)

Is there an aluminum option that would be a suitable sub for the 1045 steel that was called out in the specifications?

This is all for a GT40 build I'm slowly making progress with and, although I'm not ENTIRELY sure I want to mount the Toyota V8 to my 012 trans-axle, it IS an option I was considering and I didn't want to miss an opportunity to save a metric-ton of cash on an adapter if I DID decide to go that route. (my budget for this build is incredibly small)

Lately I've been considering the Audi 4.2l V8 as my preferred engine option since it already shares bellhousing patterns, clutches, starter, etc... which would be readily available in a time of need from "Joe Local" auto parts supplier. (K.I.S.S. I guess)

Thoughts?
Feb. 7, 2012 8:36 p.m. BradLTL HalfDork

Google SketchUp is an option. It's fairly simple to learn but has some limitations.
Feb. 7, 2012 8:45 p.m. slopecarver New Reader

Sketchup is an option, as is autodesk 123d which is I believe more powerful. Both are free.

As for what you will give the machine shop it depends on what the part looks like and what machine it will be made on. If a CNC machine they will want a .stp (STEP) file which is a universal solid model file format along with a .dxf drawing specifying certain things such as thread callouts, chamfers, surface finishes, materials, tolerances etc. They would load the .stp file into a CAM package which will generate the code to run a CNC machine.

If by manual machining they will want a the dimensioned set of drawings ideally in .dxf format if you are just going to email it to them, or if it's just a buddy doing it for some beer them a printout of the drawing would suffice.

If it is a smaller part the machine shop might just rummage through the scrap bin for something that will work for free, or you could do that rummaging somewhere else for free. If it is a larger part then a machine shop will already have the connections and weakly material supplier deliveries so they could get you larger raw materials than you could at retail prices, unless you rummage through some large scrap bins.
Feb. 7, 2012 8:47 p.m. ProDarwin SuperDork

I'm not sure I understand the question: You want to model the trans-adapter shown in the link above? Just so the machine shop doesn't have to work off a drawing?

My experience with free CAD software is the following: It sucks. Now part of that may be because I am spoiled from using Solid Edge all day. I've messed with some, but just didn't have the patience to learn them.

The CNC machine will read G-code. G-code can be generated by any CAM software (there are some free-ones out there), however it is unlikely that any machinist would allow you to generate the code. You will likely want to supply your 3d file to the machine shop in a .STP, .IGS, or .X_T format. From there, the shop will generate the G-code and send it over to their machine.
Feb. 7, 2012 9:25 p.m. Wonkothesane Reader

As is hinted at above, but not directly said, you're mixing up a few steps.

Step 1: Talk to the nice people who will be doing this for beer and see what information they want to see. They may just want to see a print and they'll do the programming (entering) of the data themselves. They may want you to hand them a model (Cad drawing file) that has the holes in the correct location and size already, etc.

Step 2a (Assuming you have a physical print): Transfer that drawing into a CAD program. I'm not too familiar with the free ones, though Autocad 123d & sketchup are the most popular of those. It looks like the part is fairly simple so they should be able to handle it I would think. Like some of the others, I play with the higher-grade stuff myself..

Step 2b (Assuming you have a digital model): Find out what format they want it in from the machine shop, if it's a 2d print, then you'll need to output it as a DXF drawing. If it's a solid model, it can be anything as mentioned above. ASK THEM :)

Step 3: Buy Beer.

As mentioned above, the software that takes a drawing or print and actually makes the code that runs a CNC machine is called CAM software, not CAD. Cad is the Computer Aided Design, CAM is Computer Aided Machining. Most machine operators won't let a program onto their machines if they didn't write it/oversee it. There's a LOT that can go wrong, sometimes more than can go right :)

If you need any futher help, sounds like there's a few of us CNC guys around for further clarification.
Feb. 7, 2012 9:52 p.m. Schmidlap HalfDork

Honestly, the part is simple enough that a good CNC programmer could make it from a 2D drawing as quickly as from a 3D model (assuming the 2D drawing is provided in a dxf or dwg format that he could import into his CAM software). Even working off a paper drawing, some of the machinists I've worked with could draw it in their CAM software as quickly (or quicker) than I could in Solidworks. It looks like it is about 3/4" thick with pockets milled about 3/8" deep so really all he'd be doing is grabbing a few paths and setting some depths. So yes, it's something you could do in a few months without getting a divorce. With modern CAD software though, modeling that in 3D is just as easy as making a 2D drawing so I'd model it 3D and create a 2D drawing and give them the option of using your model or drawing.

As for using aluminum instead of steel, I assume the spacer bolts to the engine and then the trans bolts to the spacer? If so you'd probably be fine using 6061-T6 aluminum but use helicoils in the aluminum to prevent issues with the threads. Once you model it though, you can easily plug in different materials and compare the mass of the part in steel vs aluminum. My guess is that the weight difference will be minimal, but because the piece requires a fairly large piece of stock, the cost benefit of steel will override the slight weight savings from aluminum.

Bob
Feb. 8, 2012 5:39 a.m. Hungary Bill Reader

Looks like I knew less than I thought I didn't know :)

The real reason I was looking at trying to do as much as possible with the CAD programs (I have 2D printouts of the adapters) was that during our conversation they originally quoted me a week's turnaround. I felt kind of bad making them do a weeks worth of work for peanuts so I asked "well, what if I brought you an electronic, 3D version?"

To which he said it would cut the time down to three days... I felt better about that.

I downloaded google sketchup once a while back, It looked like it was more for a house (had wall options, doors, yard, etc) so I deleted it. I must have downloaded a funky version, but I think I'll look into autocad 123d.

Thanks a bunch guys, I'm going to meet them again today for lunch and I'll run this new info by them and see what turns up.
(now to drink my coffee)

-Bill
Feb. 8, 2012 9:11 p.m. BAMF Reader

Wonkothesane wrote: Most machine operators won't let a program onto their machines if they didn't write it/oversee it. There's a LOT that can go wrong, sometimes more than can go right :)

QFT for sure. I do a lot of CNC routing of odd 3D items. We have been asked by clients if we'll take their G-code. I have made my share of mistakes, and I've been using the machine and our software for years. If don't get it right each time, no way someone from outside our shop will on the first time.
Feb. 9, 2012 6:10 a.m. Hungary Bill Reader

I talked to them a bit yesterday at lunch and shared a bit of my new found knowledge (thanks everyone!) and they were pretty happy with the idea of getting a 3D drawing via Autodesk 123d and doing the finishing work themselves. They even offered their program to me if we had some down time, but I don't think my supervisor would like that one bit.

I'm actually getting pretty excited about this, spending near $1000 on an engine adapter wasn't really something I wanted to do with the wife's approved budget plan.

We'll see how this turns out, worst case scenario, I end up with some pretty cool paper weights.

-Bill