How to make precision hole to receive 6mm ball bearing?

[j ferguson]

I've included photos of what I'm trying to do. The device shown is an adaptor for my Chinese 6040 Router to enable it to cut foamboard parts to make r/c planes quickly. It clamps on to the router spindle and uses the reciprocating .025 wire to cut the board - very much like a sewing machine. I call the part which has the stand-off-nut modified to grab the wire the rotor. It is 3d printed PETG.

There is an M3 bolt which is supported by two 6mm ball bearings inside the rotor. This thing will be running at 8,000 rpm. Getting a good press fit on the bearings on a 3d printed part was easy. I used the Goldilocks method - make three, vary the bore dimension slightly and one of them is sure to be just right. And one was just right.

I haven't cut anything with this yet, but I expect that the PETG rotor may not last long due to the pounding and would be more durable if I made it from 6061. Getting a good fit for the bearings might be important. I thought it would be useful to see how good a fit I could get in a waste piece of aluminum. I bought a 6MM end mill, drilled a slightly smaller hole in the aluminum and then using my Sherline, ran the mill down the axis of the newly drilled hole and got a very nice looking bore. But he ball bearing is a wee bit loose. This might not be that important because the two bearings one on the front and one on the rear of the rotor are held in place by being squeezed between the head of the bolt on the back and the wire inside the stand-off nut on the front. It runs very smoothly at 8k rpm, to my surprise.

I'm guessing that I should use a 6mm reamer to get a closer fit. I'm also thinking of using blue threadlocker to secure the bearings.

What would you do?

 

[MrWhoopee]

Boring is the appropriate way to get the proper, light press fit. Given your situation, one of the many Loctite products is probably the answer. Which one depends on how much clearance between bearing and bore. My first inclination is #638, green, but, again the amount of space you need to fill will determine.

 

[mikey]

Ummm ... why not just bore the hole? A reamer will also work but selection of the right reamer can be tricky and expensive. Much cheaper and faster to just bore it with boring head and maybe a Micro 100 solid carbide bar if the hole is deepish.

 

[j ferguson]

This is an area, among all the others, where I don't have any experience. Can I get a boring head which can cut a 6mm diameter hole?

 

[Huub Buis]

For tight holes I do not use a 4 flute end mill but a 2 flute slotting mill because they are designed for a "tight fit". This only works if your drill/mill/lathe is aligned OK otherwise the hole will be to large.
An easier way, is the all ready mentioned reamer and locktide. Due to the "long" shaft of the reamer, the hole is less vulnerable for misalignment.
My lathe is more ridged than my mill and turning the hole is an option that is more accurate than my boring bar.
For my boring head I would make (grind a 12 mm HSS round bar) a small boring tool.

 

[MrWhoopee]

This is an area, among all the others, where I don't have any experience. Can I get a boring head which can cut a 6mm diameter hole?

Any boring head can do it, it's all about the boring tool.

 

[homebrewed]

Based on your comment on how the bearings are constrained by the M3 screw and wire, I suspect you may be over-thinking this. The bearings will be captured just fine even if they're a loose fit in their pockets. The force transmitted thru the bearing balls will hold the bearings in place. You _did_ say that the aluminum piece you made worked OK, right?

I don't recommend using set screws to hold the bearings in place. It actually doesn't take much force to distort a bearing and cause it to bind up. Clearances are THAT tight. I know because I made that mistake on something I was making.

You may want to think about adding a counterweight to reduce vibration. As it is, your mounting scheme is unbalanced. Not you, the gizzie .

 

[j ferguson]

hi homebrewed, I too expected it to really vibrate, but for some reason it dorsn't. could be that the wire is somehow dampeing it. I'm sure I'll find out when I start to cut board. the aluminum rotor will have a couple of extra bores for counterweights it I need them.
As you say, the bearings are trapped so maybe that is enough. btw, 6mm ball bearings are really tiny.

john

 

[mikey]

John, do you own a boring head? If not, then now is a good time to invest in one. Boring is a extremely valuable skill that will enable all sorts of projects down the line. Unlike boring on the lathe, which is very much skill-dependent for accuracy, boring on the mill is more equipment and setup-dependent.

Sherline sells a small boring head that works. It isn't very good but it gets the job done for small holes like this.

A vastly superior head is a Criterion S 1-1/2A boring head. It is, in my opinion, the best boring head for a Sherline mill. They are still available as NOS but a good used one from eBay is a better option. You want one that takes the 3/8" shanked boring bars to keep rotating mass weight down.

Then you'll need to find or make a 7/8-20 to 3/4-16 adaptor to fit it to your milling spindle, then buy a boring bar or a set of bars and then, finally, you can learn to bore on the mill. As cumbersome as this sounds, this is the quickest and best way to go in the long run.

If you have questions, PM me.

 

[Inferno]

Here's what I'd do for a one-off.
You can get 6mm end mills pretty cheap.
I'd get a piece of scrap aluminum for testing.
Then I'd chuck up the mm bit in a drill and spin it on a belt sander making sure any taper wad going smaller towards the drill.
A little at a time. Sand, test, sand test, repeat, until I had a bit that would "ream" the hole snug on the mm bearing.
Since you're plunging with it you just need the tips to be the right size. You can go slow enough in aluminum to have a nice clean hole that's just the perfect size.
Lowest cost on tooling...