What are Reasonable Goals for Home CNC?

[SteveHGraham]

I have a fundamental question about computerized machining. What are reasonable expectations for home CNC?

When I got started in this, I read superficial claims saying CNC was not only more versatile than manual machining, but more accurate. Now I'm reading that this is not true, at least for hobbyists. While you can do a lot more things using CNC, your accuracy will actually decrease unless you spend a lot of money and/or time making accurate machines and teaching them to cut correctly.

I am not a good machinist, but I can mill parts accurately enough to get within tenths on a micrometer. I can do almost that well on the lathe. Today I read that if you use CNC programs that require tools to change direction while touching the part, you have to work really hard to get within a thousandth of spec, and you are likely to end up five or more thousandths off.

The backlash we don't worry about on manual machines is a gigantic problem for computers; they can't compensate when the direction changes during a cut. The little inaccuracies in screws don't seem to mean much on manual machines, but they cause problems in CNC. You may actually have to measure your screws' lead inaccuracies manually and map them into your PC in order to protect yourself, and to get rid of backlash, you will need not just ballscrews, but EXPENSIVE ballscrews. The cheap ones can have over .005" of wobble in them, which is about like a new manual machine.

One of the major allures of CNC is the ability to make curves. If curved parts are off by .005" here and there, it can cause a lot of problems. That's a very sloppy tolerance. On the other hand, I can fire up the manual machines and make one round part that slides into another so accurately, regardless of angle, that it will barely let air pass as it enters.

What, then, is home-grade CNC good for, assuming I don't want to spend ten grand on a mill and then spend six hours a day getting it to function? Should I think of CNC as useful only for parts that don't require precision? Should I see it as suitable mainly for roughing? Should I expect to have to mill or grind the parts manually to correct them? Should I give up looking for accuracy except in parts that don't involve direction changes?

It's tough to know what to do without being able to identify goals.

[DavidF]

It's tough to know what to do without being able to identify goals.

Theres your starting point....

[oldvan]

Perhaps a trip to the land of YouTube and watching a handful of vids of people making amazing creations using CNC would inspire you. Many are composed of scrounged stepper motors and hardware store threaded rod.

THIS:

http://www.youtube.com/watch?v=1J87F3lVQYI

or

http://www.youtube.com/watch?v=09vAqSJP6zs

or

http://www.youtube.com/watch?v=gvemr8wcbXk

or

http://www.youtube.com/watch?v=Q4PKkJu6Doo

or any of the hundreds of others.

[Dave_C]

Steve,

Don't give up on accuracy but, you never get more out of a CNC machine than you put into it. In other words, cheap is cheap and quality is quality. Same goes for manual machines!

While I did not spend big bucks on my G4004G conversion ball screws, I did go to great lengths to eliminate backlash. I have zero backlash but that required me to make a double ball nut set up riding on belliville washers and preloaded. Right now I'm tracking down to tenths for the first 12" of Z and spot on for the whole X axis.

So it can be done but at a cost and that is where I think most home machinist get miss lead.

Dave C.

[Marty_Escarcega]

The backlash we don't worry about on manual machines is a gigantic problem for computers; they can't compensate when the direction changes during a cut. The little inaccuracies in screws don't seem to mean much on manual machines, but they cause problems in CNC. You may actually have to measure your screws' lead inaccuracies manually and map them into your PC in order to protect yourself, and to get rid of backlash, you will need not just ballscrews, but EXPENSIVE ballscrews. The cheap ones can have over .005" of wobble in them, which is about like a new manual machine.

One of the major allures of CNC is the ability to make curves. If curved parts are off by .005" here and there, it can cause a lot of problems. That's a very sloppy tolerance. On the other hand, I can fire up the manual machines and make one round part that slides into another so accurately, regardless of angle, that it will barely let air pass as it enters.

What, then, is home-grade CNC good for, assuming I don't want to spend ten grand on a mill and then spend six hours a day getting it to function? Should I think of CNC as useful only for parts that don't require precision? Should I see it as suitable mainly for roughing? Should I expect to have to mill or grind the parts manually to correct them? Should I give up looking for accuracy except in parts that don't involve direction changes?

It's tough to know what to do without being able to identify goals.

This is why its best to start with a CNC machine designed this way versus converting a manual machine. A CNC machine has been designed and engineered to work together with all the components. The only concern then is the wear on the machine. MOST CNC machines had a lubrication system to keep key components oiled up. (Design feature to help make them last)

Are you wanting to make super precision parts or learn? Do you NEED tenths accuracy? What do you want to make?

[warmstrong1955]

Am I off here....but wouldn't you want to buy or build the machine, after you have a goal in mind?


Bill

[SteveHGraham]

Well, see, I assumed it would just work. I didn't realize I was getting the base model, and that the stuff to make it function as well as a manual lathe was all optional.

When I got my big lathe, I didn't sit down and say, "How many thousandths of backlash do I want?" I never had to think about things like that.

[ctwo]

I thought you bought ballscrews? Even if they are not of the highest tolerance, what they should provide (as well as the machine as a whole) is repeatability. Even an ACME screw should provide that. You may not have absolute accuracy, but you should have very high precision. There is no reason you cannot work within the limitations of what you have just as you would when operating manually. The understanding of how to compensate is similar, even if the approach is all different.

I will also say that if you are machining to within a couple tenths, then you are an excellent machinist! When I bought my factory Bridgeport CNC, I had no illusions and understood that getting that machine, even with very high quality ZERO backlash screws, chromed ways, etc., would be a challenge to get accuracy to +/- a thou. That does not mean that I cannot produce parts that are to my measurement capability, which is getting seriously challenged at the tenths level. For example, it was difficult for me to pick the right sized balls to reball my Z-axis ball screw. I went through all sorts of contortions to ensure the number I was coming up with would match up to a very high tolerance commercial product. I got one right on. The other was at least a tenth off, probably three...so I had to try another guess, and that is all it was, really.

I did not buy nor do I expect my CNC to improve the quality of my machining*, but I do expect the machine will provide some capabilities that are not possible, or just difficult, otherwise. CNC is great for volume production and certain kinds of prototyping work** because you can machine so many more features or more complex features on a part with fewer setups.

* although, the quality of my machining is starting to inch out the capability of my machine, so the class of tool here is the real factor, not that it is CNC. In that sense, I bought a knee mill because it is a better machine than a column mill/drill. The CNC part was a bonus.
** high tolerance prototyping is going to require high absolute accuracy in your machine.

That is just my amateur armchair take on it.

I watched several of the Tormach knife making videos, the two younger guys that have been running the channel for years now. That is a good application for home CNC.

[SteveHGraham]

I am not a good machinist, but I have found that if I use a micrometer, the dials on my Chaiwan mill will get me to within less than a thousandth of where I want to be, when dimensioning simple parts. Maybe it's blind luck.

[SteveHGraham]

Now I'm wondering if it's possible to knurl using CNC and a single-point tool.

[magic9r]

Now I'm wondering if it's possible to knurl using CNC and a single-point tool.

A cut knurling tool might be a better bet.

- Nick

[SteveHGraham]

I think it would be neat to knurl stuff that isn't flat.