Gas Strut Counterbalance for the Mini Mill

I made a counterbalance for my bench top mill using a gas strut to replace the worn out torsion spring.

I tried adding a 30mm link to change the force vector but it only made things worse at the top of travel and not much improvement at the bottom.

When milling close to the vice, any vibration would cause the spindle/cutter to drop, gouging the work piece and pulling the column out of perpendicular. So far I havent stripped the plastic gear on the motor shaft.

• Gas strut, K4045 length 450mm, stroke 205mm and I had it charged to 180 Newtons. (approx 18kgs) Mill head weighs 17kgs. The column swivel bolt is 475mm down from the top edge.
• 2m of stainless steel wire rope, 1.5mm dia. I used 1.28m, i.e. 64 cm per side.
• 2 x M6 eye bolts.
• 4 x 3mm dia ferrule crimps.
• 2 x 45mm x 37.5mm x 15mm aluminium plate
• 1 x 73mm x 55mm aluminium plate.
• 5 x M6 x 30mm counter sink screws.
• 2 x M8 allen head screws.
• 3 x M6 brass nuts, brazed together for the pump body clevis mount.
• 2x M6 brass nuts and 1 M8 nut brazed together for the piston clevis mount.
• 1 x 6mm dia steel rod, 60mm long.
• 1 x M3 grub screw.
• 2 x 12mm equal angle aluminium, 40mm long.
• 4 x sheet metal screw for the aluminium angle.
• 2 x pulleys, 25.4mm flange dia, 6mm ID, 22mm groove dia (absolute minimum)

My first thought was a pulley system with a 18kg counter weight hanging off the back somewhat like gym equipment. I then had a look around to see what was available, nothing locally and only kit imports. A few Youtube users were using 2 gas struts with a single wire rope.

I figured a single gas strut with a wire rope on either side would be a better choice to balance the forces acting on the strut, as well as a safety margin. If one rope breaks the other is strong enough to continue supporting the spindle head.

I first bought a K4046 strut with 2 x 8mm clevis, but then discovered that it was too long.

Next was the K4045 which fitted, but the clevis ends were too long, so I brazed together some nuts and was happy with the resulting assembly.

The aluminium blocks were from a failed casting, the sand was too wet and steam ejected a lot of the melt out the riser.

This accounts for the porosity(black speckles) on all the aluminium parts.

The flex clamps prevent any sideways or forward / backward movement of the gas strut. If the forces aren't aligned vertically with the body, it will bend the piston rod.

Both blocks ( 37.5mm wide, 45mm long) are clamped together and drilled as a pair, 3mm dia holes for the wires and 18mm dia for the pump body.

There are little casting flash mounds inside the vertical column that will need to be ground flat, I used a dremel flex shaft for the upper ones. The lower one I was unable to reach and ended up sliding the bottom clamp up to clear.

I cut a 8mm chamfer on the bottom clamp to clear the screw protruding at the back and a 2.9mm deep 7mm wide slot to clear the gear rack screws on the front. Alternatively one could replace the M6 x 12mm gear rack ones with a shorter M6 x9mm.

The top flex clamp (layer 2) had 2 holes drilled and tapped M8 to enable it to be screwed to the layer 3 mount.

This block functions as a connection between the pulleys and the flex clamps, as well as providing a mounting point for the strut.

The hole for the strut clevis was recessed such that the eye was in the center of block. Next the layer 3 block was clamped to this one, turned upside down and drilled in the vise. This is to ensure the cable holes line up.

From the edge of the block to the cable holes is 21.5mm, which is the minimum size the pulleys can be.

I first made these pulleys in the lathe from a piece of 25.4mm brass bar, they are 9.5mm thick.

Then I took a section of 72mm x 30mm x 15mm, cut the openings for the pulleys and cross drilled 6mm dia for their axle which is held in place with a grub screw.

Once assembled it was clamped to the layer 3 block to position the pulley grooves over the cable holes in layer 3.

As per usual, vise, clamp, drill and tap. Tapped M6 in the pulley block, drilled 6mm in layer 3 block.

First the 2 flex clamps are aligned and tightened, then layer 3 is screwed to layer 2 and M6 strut clevis screw installed. Finally the pulley block is fastened to layer 3 with 2 x M6 csk screws and you're ready to thread the wire rope through on one side to determine the length needed.

I used 7cm for the loop at the bottom clevis and fed the wire through the rest of the rest of the layers.

Next I compressed the strut to have about 15cm of slack, then slid the assembly into the mill with the spindle raised to the top.

Then I looped the wire around the eye bolts which had been installed by this stage and used a sharpie to mark the cable at the eye bolt. Using a tape measure, I measured 64cm and cut the cable. This was then crimped and the second cable marked at 64cm, so that I could make sure both sides were fairly equal in length.

The beauty of using eye bolts, is that you can change the tension of the 2 sides so that they sound similar when twanged.

I strongly suggest using either aluminium strip or leather to prevent the mole grips from damaging the cable in any way.

Once everything was installed, I used bike chain oil to spray the cable and pulleys, this will allow the individual strands to slide over each other as they navigate the tight radius of the pulleys.

As a final test, I lowered the spindle down as far as it would go, and then wound it up as fast as possible.

There was absolutely no drama with cables slackening, wires jumping off the pulleys or block layers 2,3,4 lifting off the column, just the gentle sigh of the strut.

To say I'm well pleased with this upgrade would be putting it mildly.