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



## lesrhorer (Sep 6, 2021)

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...


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## lesrhorer (Sep 6, 2021)

*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.


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## lesrhorer (Sep 6, 2021)

Photos:


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## lesrhorer (Sep 6, 2021)

*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.


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## lesrhorer (Sep 6, 2021)

Photos:


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## lesrhorer (Sep 6, 2021)

*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.


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## woodchucker (Sep 6, 2021)

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


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## lesrhorer (Sep 6, 2021)

woodchucker said:


> 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.


woodchucker said:


> 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.


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## woodchucker (Sep 6, 2021)

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 at Grizzly.com
					

<h1>G0678 8" x 30" 1-1/2 HP Variable-Speed Vertical Mill</h1> <h2>A vertical mill that will make a fine addition to any home shop, school or job 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...




					www.grizzly.com


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## lesrhorer (Sep 6, 2021)

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.


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## woodchucker (Sep 6, 2021)

I was just mentioning for parts. I saw the price diff... 

I treat all HF like a kit... I take apart the items clean out the swarf that they left in, then I file, stone, and do whatever to upgrade.
I have had some great HF tools for the price, and some awful. I know there are people especially contractors that won't touch them.. I get it.. they don't have time to run back and forth.

But if you know what  you are in for, and work through it, you can save some money.
DON'T buy the 1/4" torque wrench.. useless... I needed inch lbs. I saw awful reviews, but figured I could work through it if it didn't work out... but no way ... it just doesn't work.

cheers.


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## lesrhorer (Sep 9, 2021)

woodchucker said:


> I treat all HF like a kit...


That's about right, for the larger tools, anyway.  They are inexpensive, and the quality / features reflect that cost.  Failing to realize that will lead to dissapointment.  Many of their tools, however, are decent enough.  Some are even quite nice.  Go in with appropriate expectations and be prepared to make adjustments and / or upgrades, and one will be satisfied.  There are a few things, like the power wiring, on this mill that are truly lousy.  I am going to fix them.  I can do a lot of fixing for the extra $4000 a comparable Grizzly would cost.



woodchucker said:


> I take apart the items clean out the swarf that they left in, then I file, stone, and do whatever to upgrade.


'Not much to do to a grinding wheel or an air compressor fitting, but yeah.  I bought a 3" pneumatic angle grinder that would not admit 3" grinding wheels more than 1/8" thick.  I got out the pneumatic die grinder - also from Harboir Freight - with a carbide burr and in a couple of minutes the shield would admit 3/16" wheels.  I have to chuckle: using one harbor freight grinder to fix another.

I also bought a new compressor pump and 3HP motor for my old 125 gal tank.  The old one was just plain worn out.  (And no wonder, really.  It was over 40 years old.)  The included plumbing just would not seal, no matter what I did.  I actually wound up fracturing the cast iron exhaust pipe trying to get it to seal.  I have never before had a pipe rupture while tightening it with a wrench.  In any case, I junked the whole exhaust system and fabricated a new one out of copper.


woodchucker said:


> I have had some great HF tools for the price, and some awful.


I have had reasonable luck.  I would not go so far as to say everything I bought there has been wonderful, but I have always been able to get the things I have purchased to work reasonably well.  OTOH, I have frequently decided not to purchase the HF tool and gone with a more reputable tool company.


woodchucker said:


> I know there are people especially contractors that won't touch them.. I get it.. they don't have time to run back and forth.


Yeah, fair enough - for some things.  Most of their consumables are decent enough.  Their small hand tools are not nearly as nice as other brands, but often the difference is not significant, while the price is.  I mean, how nice can a sledge hammer be?

I will take your advice about the torque wrench.


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## jwmay (Sep 12, 2021)

The price just jumped $500 on that HF knee mill. You think the Grizzly versions will too?


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## Ken226 (Sep 12, 2021)

lesrhorer said:


> 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.





It looks near identical to the discontinued Grizzly G0729,  6x26 knee mill.   I'm sure they still have some parts available for these.   I still get G4016 lathe parts occasionally,  and it was discontinued before the G0729.

I had the KB Tools version of that mill a long time ago.  G0729 parts were compatible.  They're surprisingly capable for the size.









						6" x 26" 1-1/2 HP Vertical Mill with Power Feed at Grizzly.com
					

<h1>G0729 6" x 26" 1-1/2 HP Vertical Mill with Power Feed</h1> <h2>A simple, solid, and extremely accurate mill.</h2> <p>The G0729 6" x 26" 1-1/2 HP Vertical Mill with Power Feed has 3-axis table movement and nine speeds designed for milling solid materials.</p> <p>This is a great mill for basic...




					www.grizzly.com
				





For example,  a brand new spindle quill,  in-stock and ready to ship....  









						SPINDLE QUILL at Grizzly.com
					






					www.grizzly.com
				




Ouch!  That green stand is over 500$ for a new one.








						STAND at Grizzly.com
					






					www.grizzly.com


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## lesrhorer (Sep 13, 2021)

jwmay said:


> The price just jumped $500 on that HF knee mill. You think the Grizzly versions will too?


Yow!  Boy am I ever glad I pulled the trigger when I did.  They had a Memorial Day special, so I got an additional 10% off.  Phew, I'm glad I dodged that bullet.


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## lesrhorer (Sep 13, 2021)

Ken226 said:


> It looks near identical to the discontinued Grizzly G0729,  6x26 knee mill.   I'm sure they still have some parts available for these.   I still get G4016 lathe parts occasionally,  and it was discontinued before the G0729.


With the exception of the "bolt-on" parts, indeed it does.  Once again, the Grizzly for some reason used a smaller, 110V motor.  I don't know how well it worked, but I really like this 220V motor.  It is smooth, quiet, and runs cool.  It would have been nice if this mill came with the X-axis motor drive and the pump oiler, and the wiring on this mill is just stupid.


Ken226 said:


> I had the KB Tools version of that mill a long time ago.  G0729 parts were compatible.


That is good to know.  Thank you both, Ken and Woodchucker.


Ken226 said:


> They're surprisingly capable for the size.


I am mostly pleased.  The Y-axis handwheel always makes this awful chattering noise.  The X-axis is perfectly smooth by hand or at modest speeds with the motor,  but at high speeds it sometimes makes an ungodly screech while moving.  I have been unable to find the source, and it is worrisome.  That chatter can't be good.


Ken226 said:


> For example,  a brand new spindle quill,  in-stock and ready to ship....
> 
> 
> 
> ...


That's good to know.


Ken226 said:


> Ouch!  That green stand is over 500$ for a new one.
> 
> 
> 
> ...


I think I will pass.  It cost me some labor, but the metal for the stand I made was under $50.


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## lesrhorer (Sep 17, 2021)

*Phase III: Z-axis Scale*
Technically, the Z-axis scale was not difficult.  In practice it was a royal pain.  First of all, as one might expect, the knee is very stiff, especially when raising the table.  With a little use, it is beginning to free up some, but winding the table all the way up and down is a chore.  Once.  Twice starts to make the muscles a little sore.  Ten times is just downright painful.  The knee column is tapered - a lot.  It is around 2" wider at the bottom than the top.  This might have presented far less of a problem if I had a 24" caliper, but I don't, so an accurate measure of the column width is not practical.  What's more, the scale length is much greater than the knee travel, so the shim size was much greater than the indicator deflection when moving the table to its full extent.  My estimation of the difference was nort very good, it turned out.  Ergo: lots and lots of adding a shim and then moving the table up and down again.  Not only that, but situating the indicator where I could read it turned out to be an exercise in frustration.  The thick layer of Bondo on the mill means the indicator base does not stick well to the column.  The indicator kept not only moving but even falling off.  Thank goodness my reflexes are still better than my eyesight.  Having everything on the side of the mill column in the way didn't help.  I finally got smart (well, OK, not so stupid) and removed everything off the side of the column.  I was then able to mount the indicator on the bed directly, which fixed that issue.

Because the scale port faces the mill cutter, I figure the scale guard is definitely essential in this case.  If it were facing away or downward, as is the case with the X and Y axes, I think it would be acceptable to forego shielding the scale, which I had to do in the case of the X-axis, and I think  the same may be the case for the Y-axis.  The Z-axis scale must mount vertically, however, and it's not really practical to face the scale port away from the cutter.  There is no means of mounting the cover directly to any feature of the mill, so I had to fabricate a pair of brackets.  I had them 3-D printed, the upper one 7/8" taller than the lower one.  This turned out to be a bit more slanted relative to the scale than I had planned, but there is still plenty of clearance across its entire length.


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## lesrhorer (Sep 17, 2021)

Photos:


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## lesrhorer (Sep 17, 2021)

*Phase IV: Drawbar*

The drawbar which came with this lathe was really pathetic.  The threads on both ends were poorly formed.  Screwing the drawbar into some of the the R8 tools was all but impossible.  In one case, I actually had to resort to using a pipe wrench!  The flange nut which cinches the bar felt gritty, and was far too stiff when either cinching or removing the tool.  On some of the tools, the drawbar would not fit far enough into the tool to allow the flange nut to be cinched down, meaning the wool was loose in the spindle column, and thus unusable.  I did buy a quality HSS 7/16-20 tap and reamed all the tools.  This helped substantially, but the feel of both the bar itself and the cinch nut were still "crunchy".  I also bought a 7/16-20 die and ran it over the bottom threads.  This allowed the bar to seat fully and smoothly into all the tools.

The roll pin holding the top nut onto the bar actually fell out.  This meant to thread and unthread the bar I had to spin the cinch nut all the way up to lock the top nut and then break it loose when I was done.  The bar itself is 7/16" diameter, of course, but the top thread is some sort of weird, nonstandard, oversized thread, although still 14 TPI.  The cinch nut was quite soft, and its flats even stripped during one of the really aggressive operations (with the pipe wrench.)  Fortunately, it was right after  that the tap and die arrived, so I was able to ease the threads, making the torque on the nut far more reasonable, and even stripped, I could still tighten and loosen the nut.

There appears to be a harmonic balancer on the spindle drive pulley.  In any case, there are two cap screws that stick up above the pulley.  The cinch nut was shorter than these two screws, making access to the nut limited and sometimes a bit difficult.  Furthermore, the top nut is larger (3/4") than the cinch nut (18mm), preventing the use of a box-end wrench on the cinch nut.   I purchased an 18mm flare nut wrench, which helped with both issues, but not perfectly, especially when the cinch nut was threaded all the way up to lock the top nut.

I machined a new flange nut out of drill rod and tapped it to a standard 7/16-14 thread.  The old nut is 15mm tall.  The new one is 24mm tall.  I bored a 1/4" deep relief hole in the bottom of the new flange nut.  I turned the old threads down to a proper 7/16-20 and added some relief behind the threads.  The maximum depth for the old nut was 32mm from the bottom of the top nut.  The new one can thread down 44mm, yet still with more threads engaged than the old nut, and several mm greater travel.

I actually have two drawbars on hand, both with the same issues, except for the loose roll pin, so I upgraded both of them.  I hardened and tempered the new nuts, shielding the threads so they are less brittle.  I welded the top nuts in place.  The cinch nut how moves as smoothly as silk.


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## lesrhorer (Sep 17, 2021)

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