Draw bar rattle

[Susquatch]

Maybe another ingredient to rattling is the drawbar section profile itself, not really reflected correctly on my simple sketches. The majority of DB, the lower portion, is skinny by comparison to the upper thicker portion with more mass. So imagine a skinny, flexible stick with a concentrated blob of weight on top. As it spinning & oscillating, its easy to visualize the upper portion tracing a wider path & bending the shaft outward. Left unconstrained in the spindle shaft hole, maybe this (upper) area is where the rattling originates from.

It would be interesting if you temporarily attached a sleeve of material on the DB just where it enters the hole just to test. Like some winds of electrical tape or softer durometer O-ring if you can squeeze it, so the radial gap is filled up. Then tighten DB with moderate torque without the special adapter part. If the noise stops its probably entirely an annulargap/ centering issue.

In a way, I already did this. I put an o-ring on the shaft just to see if it would help center it. The oring ended up twisted like spaghetti. So I abandoned that approach.

In hindsight, the forces involved in tightening the drawbar far exceed the yield strength of a rubber o-ring or tape. To stay in position properly, the O-ring would require a fit that starts centered and is maintained at center during the tightening sequence. If you have that, then why bother with the o-ring?

 

[Susquatch]

It would be interesting if you temporarily attached a sleeve of material on the DB just where it enters the hole just to test. Like some winds of electrical tape or softer durometer O-ring if you can squeeze it, so the radial gap is filled up. Then tighten DB with moderate torque without the special adapter part. If the noise stops its probably entirely an annulargap/ centering issue.

I couldn't resist trying it out anyway.

Without the sleeve, the tape gets mangled and the O-ring gets twisted to rodent dung.

With the sleeve, there is no room for either one - I deliberately made it a tight fit to eliminate the wobble.

While I was in there I also checked for any necking down in the spindle bore. Gave me the heevy jeevies to drop a gauge rod in there so I made a bushing three thou under the bore, attached a wire to it with a lock pin, and gently lowered it all the way through and out the spindle nose. It doesn't neck down at all.

I put my hand on the housing with and without the centering bushing. There is a noticeable difference in vibration. The frequency of the vibration changes with spindle speed.

With each exchange that I have with you guys, I become more and more convinced that it's just the play in the splines that is knocking a bit when the spindle assembly vibrates due to an out of balance (uncentered) draw bar. It certainly has helped to consider other alternatives and evaluate them objectively.

 

[PeterT]

I probably wasn't clear. I was wondering out loud if temporarily filling the annular gap evenly with 'whatever' & just hand tightening the DB nut (without your fix gadget) eliminated the noise, then we would know the rattle is 100% due to centering issue. Whereas the gadget is simultaneously providing centering & pulling axially. But re-thinking this, by you loosely tightening the gadget would probably accomplish the same thing because your part is already made. I was more thinking of people debugging their own machines.

What you are suggesting goes around in my mind. If the DB mass gets tightened off center, especially with the big blob weight on the top portion now acting as a counterweight, it is now taking the spline shaft along for the ride & the rattle source is likely external on the spline mate. And this is where my RF-45 story goes cold case. I know some people tried to tighten the spline gap tolerance but that's not exactly easy. I recall CNC converters claiming no more rattle, but now was that because they replaced the spline shaft altogether or locked/modified when because the entire gearbox head moves in Z, they no longer use the quill action?

 

[Susquatch]

If the DB mass gets tightened off center, especially with the big blob weight on the top portion now acting as a counterweight, it is now taking the spline shaft along for the ride & the rattle source is likely external on the spline mate. And this is where my RF-45 story goes cold case. I know some people tried to tighten the spline gap tolerance but that's not exactly easy. I recall CNC converters claiming no more rattle, but now was that because they replaced the spline shaft altogether or locked/modified when because the entire gearbox head moves in Z, they no longer use the quill action?

Yes, that's the way I see it too.

I looked at the spline clearance when I had the head apart and decided it would be foolish to try and improve it. A nice sliding fit is imperative and loose is actually good for that. It's really only an issue when the spline is not loaded. (Call it backlash if you want). When the spline is loaded it's a zero issue.

I have no idea what cnc guys are doing playing around with that. Using the magic cnc words doesn't change the machine interface one iota. But that is a whole nuther pissing contest. I'd rather stay out of the wind when they start doing that.

 

[slow-poke]

While I was waiting for a 3D print to finish yesterday, I thought what can I do while I'm waiting? .... Oh that little drawbar centering bushing gizmo, thanks Susquatch.

My draw bar didn't slap around, but some times it was centered and sometime it was not, now it's always cenetered.

 

[jcdammeyer]

Yes, that's the way I see it too.

I looked at the spline clearance when I had the head apart and decided it would be foolish to try and improve it. A nice sliding fit is imperative and loose is actually good for that. It's really only an issue when the spline is not loaded. (Call it backlash if you want). When the spline is loaded it's a zero issue.

I have no idea what cnc guys are doing playing around with that. Using the magic cnc words doesn't change the machine interface one iota. But that is a whole nuther pissing contest. I'd rather stay out of the wind when they start doing that.

The spline clearance is really the most annoying thing on my mill. I've often wondered if I can make something that takes out that play while I'm not using the quill. I don't have a drive on the quill, totally for manual work. The knee is the Z axis for CNC.