What The World's Been Waiting For - Yet Another Power Drawbar Design

[Ron Ginger]

The Z-bot power drawbar works exactly like this See http://littlemachineshop.com/products/product_view.php?ProductID=3034&category=209730719. Brian has one running at the CNC workshop last summer, and we had it running at Cabin Fever in January..

I talked to the guy that developed it and he told me about 600-800 pounds was enough for the Tormach tools.

I built a similar, but simpler one on my Jet knee mill. I have my table move for the z axis, so my quill is free to operate by hand. I put the bellville washers under the drawbar, then I added a simple metal bridge above the top of the drawbar. To change a tool I simply drive the quill up with the hand feed until the top of the drawbar hits the bridge, then push a bit further to compress the springs and pop out the collet.  It works fine, but I am a little light on the bellvilles, so I did have a collet slip once.

[HimyKabibble]

I am running a full size bridgeport clone , 3hp , 10x54 and havent had any problems . I mostly work in aluminum though and rarely do any drilling with that machine so mine may not be a good comparison

The 6-800lb number for r-8 collet came from an articlle in HSM about various collets somewhere in the past 15 years , I have seen 600 lbs mentioned in a thread somewhere in the past also. I would guess your 2500+ lb number is probably more accurate . My manual list a torque on the drawbar of 15-20 lbs which calculates to over 3000lb of clamping force  for a 7/16x20 bolt

look forward to updates when your new parts arrive

Yeah, I started with 900 pounds, and it's scary how easily the tool will move with so little tension.  I just wish I had some direct means of measuring the tension, but I don't.  Estimating from the Belleville specs and deflections is the best I can do.

Regards,
Ray L.

[HimyKabibble]

Ya know, it just occurred to me, it's easy to calculate the maximum torque the spindle motor can exert on the tool, which can serve as a guide for how much torque I need to set the drawbar for.  Since I'm using a VFD, max torque should be at base speed, in the lowest gear, which is 660 RPM.  The motor is 3HP, so the torque is roughly 3 * 5252 / 660 = 24 foot-pounds.  So, the 30 foot-pounds I've been aiming for seems like a reasonable target.

Regards,
Ray L.

[graffian]

I don't have any idea of the force required to lock a tool in a collet, but my matsuura manual
has details of checking the spring stack, at least I think that's where I saw it and the figure of sliglty greater than
2000 lb sounds right.
I did work it out and the air cylinder needed the miniumum air pressure figure written on the machine 71psi, so a 6 inch[or so]
cylinder would be right.
 
I will check the manual tomorrow, if I remember.

[HimyKabibble]

OK, I mounted the new, larger air cylinder, and new Bellevilles.  I am now absolutely convinced this WILL work.  It works just fine as-is, but I'm not able to get quite the "slip torque" I'm looking for on the toolholder.  I'm aiming for 30 foot-pounds, and I'm not at a bit over 20.  A new, longer lower plate and longer lever arm should easily get me there.  At 20 foot-pounds, it works great.  Apply the air, and the toolholder just falls out.  I would guess my 2000# drawbar tension estimate is definitely in the right ballpark.

Regards,
Ray L.

[graffian]

I checked and my drawbar is meant to have 500kg tension. Sure I saw 1000kg somewhere.
Just has a look and there are plenty of drawbar tension checkers made, look up clamprite.

[HimyKabibble]

I checked and my drawbar is meant to have 500kg tension. Sure I saw 1000kg somewhere.
Just has a look and there are plenty of drawbar tension checkers made, look up clamprite.

But isn't that for a CAT30/40 spindle?  Quite a different animal from R8.  Getting the collet in place is easy.  Getting the tool locked in solid enough that it can't spin in the collet under heavy load is much more difficult.

Regards,
Ray L.

[graffian]

BT35!

I suggested looking at the draw bar checkers as they might give some clue to the force usually used.
Are these actually collets you putting in the spindle, I know nothing about R8.

[HimyKabibble]

BT35!

I suggested looking at the draw bar checkers as they might give some clue to the force usually used.
Are these actually collets you putting in the spindle, I know nothing about R8.

Yes, I'm doing this with R8.  I'd much prefer to have a CAT30 or some other spindle designed for ATC, but that ain't in the budget.  Since the tension has to not only locate the collet in the taper, but also hold the tool in the collet, it appears the required tension is higher than for your BT35, perhaps by about 2X.   It works, but takes a fair amount of tension.

Regards,
Ray L.

[simpson36]

Just stubled across this thread . . .interesting project. The arm is a work of art . . .  I want one just to put on my desk and look at!

I don't see where a momentary 2k lb static load on the spindle bearings will do any harm, but the mechanism to prevent that is pretty clever. A sizzor arrangement would also work and not require an additional actuator. Just thinking out loud . . .

The Tormach setup is slick. First I've seen it.  Tormach most certainly does know the drawbar force (or range) required . . it's a liability issue to advise on that, I would imagine. Certainly, the quality and accuracy of the spindle taper and the collet are going to have a significant effect on the force required, as will the presence and type of oil on the holders.

2,000lbs seems like a reasonable number to me for R8.

It will be fun to follow this thread . . . very  project.