# G0704/g0759 Counterweight



## RVJimD (Aug 2, 2016)

I made this counterweight a while ago and sent a few pictures of it to a local friend recently  and thought I would post them here for anyone thinking about something similar.

According to my friend who converted his 759 to CNC the head weighs in at about 65lbs, so my counterweight is an old dewalt tool box with 65lbs of lead shot.  I reworked the cheap pulleys and added two each roller skate bearings.  The original pulleys were dragging a bit on the rope and I could see the grey discoloration indicating the rope was sliding instead of the pulley turning.  It really reduces the up cranking force required and if I decide to put a motor on the Z axis someday the counterweight will really help there as well.










Jim


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## T Bredehoft (Aug 2, 2016)

I think I'd short the weight by maybe 15% so the head doesn't float on the lead screw. If I'd have thought of that  I wouldn't have had to put a windshield wiper motor on mine.


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## RVJimD (Aug 2, 2016)

Tom,  I always lock down the Z for all milling so it not like it is going to move around at that point.  Is there some other reason to have down pressure on the screw?

Jim


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## RVJimD (Aug 29, 2016)

Thought I would continue my G0759 thread.  Today I finally got around to making a knurled knob for the right side table lock of the X axis.  It is aluminum 1.5" OD and about .5" thick.  I drilled the center hole 11/32" which was a press fit onto the stock metric hex of the spring loaded lock handle.  This is much easier for me and quicker, and it is something I use for every milling operation since the table needs all the rigidity it can muster.  You can see two notches in the center hole where I pressed it onto the hex.  I also put a set screw in the knob, but probably didn't need to.




Jim


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## westsailpat (Aug 29, 2016)

Nice Jim , although I do not own that type of machine I very much admire them and have thought of how they could benefit from a counter weight .


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## Greebles (Aug 30, 2016)

Gas struts mounted to the side of the column / head also works well to counter balance the weight of the head. That is the approach I have taken with my PM-727 and X2 mills.


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## stevemetsch (Oct 13, 2016)

I have a 940 but don't have headroom for a pulley system. I have seen gas shocks on this site and would appreciate any photos  and mounting tips. The head nods a lot unless locked and makes the Z axis DRO usless.


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## wrmiller (Oct 13, 2016)

I had both a PM25 (similar to the G704), and a CO 9x40 prior to my current mill, and had Z axis scales on both and used them quite effectively. Once I properly fit the gibb (some scraping was involved in addition to adjusting), my head never moved more than .0004-.0008 (I don't remember which variance was on which machine), when going from loose to locked down. I simply remembered how much the number changed, and in what direction and simply moved to that number and THEN locked it down.


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## Silverbullet (Oct 15, 2016)

I've got the HF MILL DRILL, I'm thinking I can use a pulley on a bearing in the mill column like the delta drill press heads. Now you've got my inventive mind working on a plan ,where's my quill and blueberry ink and birch bark to draw it with. Oh I remember it's in the summer cave.


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## Limo (Dec 21, 2016)

I added a gas shock to my mill. I think I got it from Mcmaster-Carr. 50lb if I remember right.


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## ttt (Jan 6, 2017)

Please take a look a the G0704 manual from Grizzly, page 47, exploded view. It's pretty clear that the head "hangs" on the Z-axis acme screw, on a single thrust bearing. The acme lead screw is held in with two locking nuts on the top only. There no nut holding it in place in the upward direction. If you counter balance the head with the same weight as the head there is a chance that the lead screw or bearing might just jump out of the retainer hole in the top plate. With enough force only of course and only so far as the two ring gears crashing into each other. You will certainly get at least some movement though.

I did convert my G0759 to a CNC/manual combo with ball screws. Since I use the mill mostly in manual mode the ball screw has the downside that unless I lock the Z-axis it gives way too easily; even with the stepper motor somewhat restricting the movement. So right now I am trying to figure the right counter weight to avoid this. Which also depends on how much weight I did add to the head. But I will certainly try to keep it well under the weight of the head itself.


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## RJSakowski (Jan 6, 2017)

RVJimD said:


> Tom,  I always lock down the Z for all milling so it not like it is going to move around at that point.  Is there some other reason to have down pressure on the screw?
> 
> Jim


Actually, I would think that you wanted upward force if anything as the cutting force from drilling or end milling will be pushing up.  With a downward pressure, if you set your tool offset with zero force applied and the weight, now slight, is pushing down, when you start to machine, whatever lash you have will take effect.  If you take a final pass of a few thousandths, there will be only a slight cutting force and the tool will cut deeper.

When you have 65 lbs of downward force, it is sufficient to bias the drive so you won't notice this effect but not so with only a few lbs. of force.

When I set my tool offset on the Tormach, I always make my final motion in the down direction for the same reason, to take up any lash, much like setting the cross  feed on a lathe.


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