Central Machinery (Harbor Freight) 8 x 26 Knee Mill Upgrades

[lesrhorer]

I am pretty pleased with my Central Machinery mill, given its cost. I do wish it had a variable speed motor (future upgrade?), rather than cone pulleys and V-belts, and I do wish it had just a few more inches of vertical travel on the knee. Of course, no mill can really call itself a mill if it doesn't have a DRO (OK, so I am a milling machine snob!), and operations on the X-axis are just absurd without a motor drive. The milling itself is bad enough without an automated X-axis, but moving the bed around between operations is just plain tedious and way, way too slow. Given all that...

 

[lesrhorer]

Phase I: X-axis DRO Scale
I have seen a lot of people claim mounting the X-axis scale is a problem, especially on the CM 8 x 26 mill. The main complaint seems to be an inability to mount the scale on the front of the mill, and mounting the scale on the back of the mill means losing a fair amount of precious Y travel.

I really didn't have many problems. I made sure to buy an oversized 500mm scale so that the scale mounts would clear the motor drive stops. The only real issue was the read head mount had to be integrated with the motor drive limit switch. The limit switch could barely be moved low enough without lowering the scale more than would have been acceptable, but it just did fit well enough to prevent binding. The steel mounting plate provided with the limit switch went into the scrap pile.

 

[lesrhorer]

Photos:

 

[lesrhorer]

Phase II: Motor Drive

Because the bracket for the motor drive limit switch and for the X-axis scale had to b e integrated into one unit, the motor drive and the X-axis scale were installed semi-simultaneously. Installation of the drive itself was pretty straightforward. I had a helper hold the drive unit in place on the lead screw shaft while I drilled two holes through the housing into the mill body. The housing was then mounted with two included bolts.

Once the housing was mounted, I drilled and tapped three 1/4-20 holes for set screws into the brass drive shaft included with the drive. Two of the holes were drilled close to the drive gear in order to fit into the 5mm keyway already milled into the lead screw shaft. The third hole was drilled outboard on the drive shaft to fit into a later-to-be installed 5/8" shaft extension which will allow me to re-install the hand wheel.

The spring-loaded stops simply replaced the old stops on the side of the mill bed. I did have to mill off the bottoms of the stops just slightly to prevent them from bottoming out against the T-nuts in the very shallow side slots.

 

[lesrhorer]

Photos:

 

[lesrhorer]

DRO Display:

Mounting the DRO display was mostly just a matter of drilling two 1/4" holes in the side of the mill housing where the pulleys are mounted and using a pair of 1/4-20 hex bolts to hold the display in place. The mounting base did tend to rub against the pulley cover whenever I changed speeds or tools, so I put a pair of 0.060" brass shims between the mounting base and the aluminum housing.

 

[woodchucker]

what axis is the U axis? is that for your quil?
I didn't know HF had a knee mill.

 

[lesrhorer]

what axis is the U axis? is that for your quil?

That is correct. I am not certain I will ever actually install a quill scale, but if I do, the display will handle it. The extra cost for the 4th axis was minimal.

I didn't know HF had a knee mill.

Yes. They have a little bench-top mill, of course, but they also have one of the few mid-size knee mills I have come across. It is much larger and heavier, not to mention more expensive, than a bench-top mill, but nowhere nearly as large or heavy as a full size knee mill. It's also only about 1/10th as expensive as a Bridgeport. What's more, it isn't nearly as much of a pain as a round-column mill.

 

[woodchucker]

very similar to my Clausing in size I assume.
Also similar or exact match to the Grizzly... if you ever need parts and can't get them from HF..

8" x 30" 1-1/2 HP Variable-Speed Vertical Mill

<h1>G0678 8" x 30" 1-1/2 HP Variable-Speed Vertical Mill</h1> <h2>A vertical mill for any home shop, school, or production shop!</h2> <p>The outstanding features of the Grizzly G0678 8" x 30" 1-1/2 HP Variable-Speed Vertical Mill include fold-up safety handles, one-shot lubrication, tilting...

www.grizzly.com

 

[lesrhorer]

It is similar to the Grizzly, but definitely not exact. First of all, the Grizzly costs considerably more than twice as much, and is more than 100 lbs heavier. OTOH, the Grizzly offers some significant advantages for the cost. The CM has a 6 x 26 bed, while the Grizzly is 8 x 30. The Grizzly has infinitely variable speeds with a tachometer, while the CM has 9 discrete speeds and no tach. The Grizzly includes a stand with a chip tray. I had to make my stand, and there is no chip tray on the base. The power switch on the Grizzly is much nicer, and I like the quill oiler much better on the Grizzly. The Grizzly's hand wheels have folding handles, although I am not all that wowed by this feature. I think the Grizzly's knee travel is much greater. If I had the money, it would be worth the additional cost just for that. The Grizzly motor is only 1.5 HP, however, while the CM is 2 HP, and the CM's 2885 RPM top speed is 28% faster than the Grizzly's max of 2250. The low end torque of the CM is undoubtably much, much greater than the Grizzly, since it is geared down. At the lower speeds, the Grizzly might stall rather easily. There is no way the CM mill would stall at 240 RPM without destroying something. The CM's high end torque is without doubt also greater.