Help in making a tiny bushing

[Ramv]

Hi,
I am attempting to make a small bushing that has an internal diameter of 3/16" and outer diameter of 6mm. This bushing connects the shaft of a servo motor to a spur gear that drives a worm/wheel arrangement of a telescope.

I am having lots of trouble making this little bushing. I started out with a brass rod of 3/8" and attempted to turn it down to 6mm. What I got looked like something that a rat chewed and spat out. Too much deflection in this tiny rod.
I tried center drilling the 3/16" first but that made the brass rod even more flexy.

I tried to keep the length projecting out of the chuck as small as possible but flexure has been unavoidable.

This part needs to be made precisely to ensure that I dont add error to the drive train that is extremely accurate.

Any advice to this relatively inexperienced lathe operator?

[GlennW]

If it is brass, brass is one of the easiest metals to machine and is used as the baseline for machinability ratings for other metals.

If the brass rod it not protruding more than about a half inch from the chuck or collet, your cutting tool is either not ground/sharpened properly, or it is set to where the cutting edge is above or below the center of the stock.

If you could post a pic of the cutting tool to show how it is ground, I'm sure you will get lots of help with that.

Welcome to the group!

[SteveM]

You didn't say how long the part is. That may be a major factor in the problem as well as the solution.

Brass tends to pull the cutter in, so you should not be using a tool with a sharp, upward pointing edge. A flat tip is better.

If you need precision, boring the center hole will be better than drilling. Drill undersize and then bore to final dimension.

Once you have it bored, you can use a live center to support the end while you turn the OD to the finish diameter. That will keep the part from flexing.

If the part is REALLY long, you could still see flexing in the middle.

Steve

[johnfreese]

Drill and ream the ID. Part off. Press in a mandrel. Finish the OD between centers.

[Magicniner]

I'd use one of my roller boxes, but assuming you don't have one Frank Ford made a great little tool for this kind of thing -

http://www.frets.com/HomeShopTech/Tooli ... lower.html

For a one-off you can improvise and even a bit of wood with a hole through will often provide adequate support for a small brass part, with only just enough sticking out turn a short length to size then finish with the support,

- Nick

[Ramv]

Thanks for all your responses. I have learned from each of them.
Glenn: I am using carbide inserts: I suspect that might be part of the problem. They produce a wonderful finish on large aluminum tubing (2.5" or greater in diameter) but they are not sharp and they definitely are contributing to deflecting the tiny brass rod. I will try it with a sharp HSS bit next.

These are the bits I am currently using:

http://littlemachineshop.com/products/p ... &category=

I also have HSS bits for this so will try them next with a light touch.
These HSS bits are linked here
http://littlemachineshop.com/products/p ... &category=

The part is so thin that even with a live center, I suspect I will see a lot of flexing in the middle. I feel Nick's suggestion will work to eliminate this flex . I will try to improvise some sort of a mini live center.

Will report back on this.
Best regards,

[johnfreese]

The carbide tools you showed will have much higher cutting forces than a properly sharpened HSS tool. I think your original approach would work if you did light OD cuts with HSS.

[spro]

I'd use one of my roller boxes, but assuming you don't have one Frank Ford made a great little tool for this kind of thing -

http://www.frets.com/HomeShopTech/Tooli ... lower.html

For a one-off you can improvise and even a bit of wood with a hole through will often provide adequate support for a small brass part, with only just enough sticking out turn a short length to size then finish with the support,

- Nick

If you go to Frank Ford's site and see the tool holder for nearly the exact thing, there are other possibilities. The base to this fine addition is the "Double Tool Holder". Consider if the other row of tapped holes are use as a slide instead of a mount. The slide spans the holder and drops to provide a thread and calibrated thumb wheel to adjust the steady rollers. As pressure is applied to part the work, the same .001s are adjusted to bring the rollers closer.

[GlennW]

How long is the bushing?

First I read 3/8" diameter, then I see where the bushing is 3/16" dia.

[Ramv]

Hi Glenn,
The bushing exterior is 6mm dia. Interior is 3/16". Length is around 8mm. That dimension is not too critical. I was working with something a bit longer than that to keep the tool away from the chuck as much as possible.

[Harold_V]

Armed with the full description, there's no reason why this item can't be made without difficulty. Rough turn the OD, drill the ID undersized. Bore the ID to size, then turn the OD to size. Part to desired length. It's short enough that deflection shouldn't be an issue (assuming your tool cuts as it should).

I have often admonished readers to not apply insert carbide tooling to their machines, especially if they are small, with fractional horse power motors. They simply don't perform well, although there are instances where there is benefit in their use (hard or abrasive materials, as an example).

If you hope to be independent, learn to grind proper high speed cutting tools. They will generally out-perform carbide in instances just as this.

Harold

[Magicniner]

Armed with a full job description I can see that you shouldn't need any support to make this, just a nice sharp edge and appropriate angles and radii on your cutting tool. Have a look at this -

https://www.youtube.com/watch?time_cont ... _A2xtLF0AU

for some well presented basics on HSS tool grinding.

I'd use an indexable insert tool simply because that is no longer in the least impractical with the range of very sharp edged inserts available with a wide range of coatings and I have one which is suitable hanging on the quick change rack behind the lathe, but HSS would be easier and cheaper for a one-off

- Nick