BT30 Drawbar Rookie Question

[fostereast]

I've found a few BT30 spindle assembly section views on the web, and it looks (to me) like the stud gripper and possibly drawbar spin with the spindle? Can someone confirm the interaction with the stud gripper, drawbar and spindle.

Similar Threads:

• Power drawbar on a 1100 with BT30 spindle
• Problem- Rookie lathe question
• basic question for rookie
• X3 Drawbar Question
• Rookie question: How to make a stepper motor run using only a battery ?

[DLF]

Hi,

The drawbar uses a spring, usually made of bellevile washers, and pulls the pullstud gripper to retain the tool holder in the spindle. The drawbar and gripper assembly spin together with the spindle.

The power transfer is made via the spindle dogs which engage the toolholder notches. The spindle taper assures tool concentricity to the spindle.

To release the tool the draw bar is pushed down. It is not wise to release the tool while the spindle is running. Just my 0.02$

Sent from my SM-A520F using Tapatalk

[fostereast]

Thanks for the detailed description. For Through Spindle Coolant, how is the coolant introduced?

[DLF]

Thanks for the detailed description. For Through Spindle Coolant, how is the coolant introduced?

This I do not know. Never seen one, so I can not say. This is a question for the big boys in the VMC forum. I do not think you will see TSC on a retrofitted benchtop mill

Sent from my SM-A520F using Tapatalk

[Azalin]

Some have spiral coolant flow channels machined on the exterior wall of the housing. But, in my opinion cooling is not that necessary unless the spindle runs at very high speed like 15k and over. Many machine spindles uses ac bearings and there is not much statical load that makes bearings to make heat.

Sent from my MI 5s Plus using Tapatalk

[DLF]

Some spindles have channels for compressed air. This is used for ejecting the tool in the event the tool sticks. Shouldn't stick, but in reality if the spindle runs hot the taper will expand and draw the tool a few microns further into the spindle making the tool stick when the spindle is cold.
If it doesn't stick that much the compressed air will help release.

But IMO the biggest benefit of compressed air is for taper cleaning. Will help blow any foreign particles from the cone which will prevent the toolholder from seating properly.
Not so much of a problem when machining metal, but usefull for wood and plastics which produce dust. A blast of air prio to inserting the tool will clean the spindle

Sent from my SM-A520F using Tapatalk

[bhurts]

Thru spindle coolant is usually done via a rotary union mounted on the top of the spindle shaft. The shaft is hollow and the coolant flows into the spinning shaft via the union. The union has a bearing that is between the spindle shaft and the main body. That allows the shaft to spin while the union stays stationary. The coolant is run into the side of the union via a compression fitting. So the coolant flows thru a hose into the rotary union body then thru the bearing bore into the spindle shaft continuing thru the tool holder and tool.

Ben

Sent from my E6810 using Tapatalk

[CS900]

Hi,
The power transfer is made via the spindle dogs which engage the toolholder notches. The spindle taper assures tool concentricity to the spindle.

A bit off topic, but is this true? I know on 40 and 50 tapers there is clearance on the tangs. I was always told they are used for orienting the tool, and the torque transmission is made from the locking tapers. Maybe this isn't true for a 30 taper as there's quite a bit less surface area on it?

[DLF]

A quick wikipedia search reveals this:

"NMTB is a "self releasing" or "fast" taper. Unlike the more acute self holding tapers above, such tapers are not designed to transmit high torque; high torques are carried by driving keys engaging slots on the flange"

Not quite BT, but very similar in form and function. For milling machines the taper power transfer function is assured by the dogs. The taper will slip if not for the flanges which engage.

For ATC function the dogs actually complicate matters, the spindle needing indexing. So the flanges are there for a reason, not just for looks.

Sent from my SM-A520F using Tapatalk

[SCzEngrgGroup]

A bit off topic, but is this true? I know on 40 and 50 tapers there is clearance on the tangs. I was always told they are used for orienting the tool, and the torque transmission is made from the locking tapers. Maybe this isn't true for a 30 taper as there's quite a bit less surface area on it?

Drive dogs are completely unnecessary until you get up around a minimum of 3-5 HP or more. The taper is MORE than enough to handle the power of any benchtop machine.

Regards,
Ray L.

[mcardoso]

On the topic of rotary unions, they are a very impressive feat of engineering. They function by precision lapped carbide faces which rub under spring pressure. Many commercial spindles can run 100bar (1450psi) through a spindle as high as 30,000 rpm. No rubber seal could handle those kinds of loads. Also quite expensive.

[DLF]

Drive dogs are completely unnecessary until you get up around a minimum of 3-5 HP or more. The taper is MORE than enough to handle the power of any benchtop machine.

Regards,
Ray L.

Let's suppose for a moment you are right.

Then why don't we see low power BT spindles without drive dogs? If they are not mandatory from an engineering perspective then costwise there would be no reason to manufacture and install them. And yet even the cheap china spindles have them. So, I wonder...

Sent from my SM-A520F using Tapatalk

[CL_MotoTech]

I haven't run a drive dog in my R8 spindle in years. I've never had an issue. It makes changing tooling manually a good bit faster. 2.2kw spindle motor btw.

[SCzEngrgGroup]

Let's suppose for a moment you are right.

Then why don't we see low power BT spindles without drive dogs? If they are not mandatory from an engineering perspective then costwise there would be no reason to manufacture and install them. And yet even the cheap china spindles have them. So, I wonder...

Sent from my SM-A520F using Tapatalk

Because they are built to handle the maximum torque BT is capable of. But if you don't NEED that much torque, you don't NEED the dogs. The incremental cost of adding the dogs is pennies.

Regards,
Ray L

[footpetaljones]

Let's suppose for a moment you are right.

Then why don't we see low power BT spindles without drive dogs? If they are not mandatory from an engineering perspective then costwise there would be no reason to manufacture and install them. And yet even the cheap china spindles have them. So, I wonder...

Sent from my SM-A520F using Tapatalk

I have an Austrian made Emco F1 that has a BT30 spindle without drive dogs. Emco even made tool holders without drive dogs for it.

[DLF]

I have an Austrian made Emco F1 that has a BT30 spindle without drive dogs. Emco even made tool holders without drive dogs for it.

Cool. I was not aweare of such spindles. My bad

Sent from my SM-A520F using Tapatalk

[fostereast]

Thanks for all the advice so far. The further along I get with this, the more I think it's an exercise to learn a little about bearings and seals, and maybe too much cost and effort to actually make. Below is a rough layout of the pieces I've put together so far. Almost all areas of the design still need finalizing, but just two are the focus for today. I'm planning to convert a PM-940 and for several reasons would like a BT30 spindle. I thought that as I go through the design process, I would look at Through the Spindle Coolant, even if it becomes to expensive/complicated to build.

Questions:

1) Regarding the bearings: Is a spring force pre-load more appropriate than a physical spacer pre-load?

2) Regarding the coolant flow through a drawbar: I'm assuming the end of the drawbar seals against the stud. How is this accomplished? (maybe a deform-able washer or a spring loaded solid washer?)

I was thinking of using "wave" springs if spring pre-load is better, and am not sure how to seal the drawbar to the pull stud

[footpetaljones]

On the machines I've worked on:

1. Never seen preload set by springs. Always a spacer.

2. Drawbar doesn't seal on the pullstud. The outside of the gripper has an oring to prevent coolant leaking back out of the top of the spindle.