Bearings selection / suggestions for a long spindle suport

[harryn]

Hi, I am attempting to build a DIY cnc wood router. Like many people, I would like to have a longer than typical Z motion.

A little more detail
- Router - Bosch 1613 evs - approx 3 hp, variable speed, 1/2 in collet
- Bit - typical 3/4 in straight sided, 2 flute carbide insert bit, approx 1 in cutting length
- RPM - approx 20,000
- Material - soft woods
- Estimated cutting force (cnc applied force) 25 lbs at the bit (somewhat of a guess, could be as low as 10 lbs)

The general idea
- The router is mounted with the shaft horizontal - not vertical
- Mount a 1/2 in x 18 in precision hard shaft in the router collet
- run the shaft through a fixed support bearing
- Mount the router bit on the end of the shaft (perhaps with another collet)
- Allow for up to 12 inches of linear motion, leaving a nominal 6 inches on the router side of the bearing
- The Z axis motion – not more than 2 inches per second, 1 inch more likely

The obvious challenges
- Mounting a collet on the end of the shaft
- Finding a bearing that allows rotational and axial movement
- Keep from creating an incredibly dangerous setup.

This is a hobby and I have kids and myself to protect. If you think I am really doing something crazy dangerous - please tell me. At the same time, I would really like to do this if it is feasible. (under say $ 200 – 300 )

Misc useful info
- Tolerances - acceptable goal - approx +/- 0.1 inch
- Bearing life - at least 5 hours - ideally 25 hrs or more

Search results so far:
- I searched through a lot of posts on cnczone for spindle info and bearings. Certainly I did not read everything, but got enough of an overview to get a sense of the challenge and info needed
- SKF web site - nearly overload of info, but after looking at the bearing general info, I got the feeling that a precision bearing is not right for this application. I could easily be wrong here. I also saw their sleeve bearings, but did not do a detailed analysis.
- McMastercarr - SAE 841 bronze sleeve bearings.

Sleeve Bearings info
- P max = 2000
- V max = 1200
- PV max - 50,000
- 1 in long sleeve bearing
- 1/2 in shaft (only because that is my collet size)
- PN 9440T25


So, I did a quick estimate for the bearings

At max extension, the bearing load is

Load = 1 ft / .5 ft x 25 lbs = 50 lbs
P = load / (shaft dia x bearing length) = approx 50 / (.5 x .1) = 1,000 - within spec

V = shaft surface velocity in fpm = (shaft dia in x pi x rpm)/12
V = (.5 x 3.14 x 20,000 ) / 12 = approx 2,600 - so about 2 x spec


PV = approx 2,600,000

So, this would indicate I would be running this bearing at something like 50x its rating, which seems really bad. I can imagine methods to get this down to perhaps ½ of that, but not down to 50,000.

So,
- math check please.
- Suggestions

BTW, I am not asking anyone to put their liability on the line on this. I will go back to the actual bearing supplier for more detailed selection, but I would like to get pointed in a general direction with the family / type of bearings one would use for this kind of application.

Changing the application slightly, it seems like this is not so far in concept away from a gun drill fixture, except for the much lower speeds. If the router part will not work, could I at least find a way to use it with a drill – perhaps to 1,000 rpm ?

Thanks

Harry

[ger21]

If you think I am really doing something crazy dangerous - please tell me.

Imo, yes. As soon as that shaft flexes even a little at that rpm, everything goes bad very quickly i would think.

Not that it would be any safer, but you should be able to cut with a 3/4" bit at around 10-12,000 rpm. I use a 3/4" two flute indexable knife bit at work and I never spin it faster than 14K, even at 300ipm.

Not sure what you plan on cutting, but you might want to look into doing it in slices. And you can buy bit extenders to give you an extra inch or two. Or get a 6" long bit like these. http://www.vortextool.com/standard/p...fm?groupID=330

You might want to try this. Get a very long (5"+) straight bit and a very short one (less than 3") . Make a very fast, heavy cut with each. You should see a noticeable difference. Then imaging it 12" longer.

[brunog]

Harry,
IMHO,
increase the shaft diameter to approx 3/4 inch to reduce flexing, I would get away from bronze bushings and go for rolling bearing, something like SKF 6204, I would suggest you PM NCCAM (i hope i have his user name ok) as he has a lot of bearing experience.

Best regards

Bruno

[harryn]

Hi, I contacted NC Cams.

Ger21 - I appreciate the input on the dangers, that is why I am asking - it makes me nervous as well. In one particular project, slicing will not work, so if this does not pan out, I guess I will just tell my buddy I cannot help him with the project.

[harryn]

Well, to make a long story from nc-cams short - The general idea is feasible but of course more complex.

The only minor challenge is that if it is done even the slightest bit imperfectly, it will kill me. The tolerances required are so tight, that they can only be achieved by an extremely experienced machinist in a temperature controlled room. I looked at it a bit more mathematically and dang if he is not right.

I guess I will need to avoid projects which need this feature set. Maybe that is a good thing.