Square Column Bench Top Or Knee Mill ?

[wrljet]

In my limited experience with the small mills, such as Grizzly G0704, you don't have good control of the
down feed of the quill. And no hard stop. You can't lower the head on the column very carefully
to get an accurate depth change, because of stiction in the ways. It's hard to move a cutter
down a known amount. You measure something and know you want to take off .003" more.
Now what do you do?

With a knee mill like the Bridgeport you can raise the knee, carefully and against gravity while
watching an indicator, to get an accurate change in cut.

And the working envelope is just so small on the smaller mills.

Bill

 

[xman_charl]

Use this magnet/1 inch dial indicator. Channel is from an old bedframe.

 

[wrmiller]

You measure something and know you want to take off .003" more.
Now what do you do?Bill

That's easy. On these types of mills (and what I do on my mills), you simply move the cutter off the workpiece, lower the head to something below your target depth then raise it to your target depth and lock the head. That way the head is always loaded by gravity against the leadscrew. I was talking to the owner of QMT about this one day on the phone and he says that on these machines he does the same thing.

I have had to take .0006 off the head of a screw on my PM-25 using this method and did it perfectly without touching the material the screw was in.

But this example is for side cutting only. Given the same scenario above for a pocket cut requires that you use the quill on these machines, and I simply keep a slight drag on the quill lock as I'm plunging to keep the quill 'loaded' against the gear. The digital scale tells me exactly how far I've moved. Lock and continue cutting.

 

[Rex Walters]

... simply move the cutter off the workpiece, lower the head to something below your target depth then raise it to your target depth and lock the head. That way the head is always loaded by gravity against the leadscrew.

I'm curious: do you have a graduated dial on the Z-axis crank on your bench top mills? Or have you installed a DRO for the Z-axis? My Grizzly G0762 doesn't have a dial for the Z-axis crank, but I'm considering manufacturing something. I only crank the head to get close, and use the crank on my quill (with a graduated dial) to mill to specific depths. I use a DRO for X and Y.

If you added a dial to Z, please share your design!
--
Rex

 

[wrmiller]

Sorry Rex, didn't design a dial. I have a 3-axis DRO on both of my bench mills. I use a separate digital scale on my the quill for plunge/pocket cuts.

 

[zmotorsports]

I am one of the forum members who have purchased the PM935TV and love it. I had originally ordered a benchtop PM932PDF but just couldn't wrap my head around the fact that it was a benchtop and Chinese at that. I don't mean any offense, I am just anal about my tools and wanted a higher quality machine. I blew my budget to hell and ended up with the PM1340GT lathe and the PM935TV mill. I have owned them for about a year now and fall in love with them all over again every time I use them, which is several times a week. The fit & finish is excellent and they run smooth and true.

The PM935TV uses a full featured Bridgeport head, is a BP clone in a 2/3 body. I fabricated as riser/base for mine to get it up about 8" higher than sitting on the floor and it is perfect for my 6'2" height.

I would follow others recommendations to base your decision on your work envelope and take a good look at what you routinely work on. I do a lot with motorsports, mostly motorcycles any more but lately have turned into a bit of a general job shop with these new machines.

Mike.

 

[Falcon67]

I love my G0519 - but I'd sale it in a minute for a good 9x? knee mill. The square column does many things well. It's fairly accurate but it could use some help. Not even close to the usual specs you see for a Meehanite casting based knee unit. I could hold .005 with care on things and that is pretty good for most work. A 2nd 4" vice will be here today and I'm saving up for a 3 axis DRO. The DRO will make some things easier.

That said, there are some things I'd like to do - possible performance cylinder head work. For that, the square column is there in HP but not even close in tolerance. if I could find a good accurate 9x49 around $6K, I'd be 2/3rds of the way there. Minimum tooling will be in the $3000 range. Don't have the $ right now so just have to wait - had to invest $7K in the house AC this year, so maybe 3-4 years from now. Not many mills crop up around here so there is at least a 80% chance it would be a new purchase. The Enco Turn Pro model was in that price range with the last 30% off+web sale but I picked the 12x36 lathe because we just could not go that much on the mill. Since the AC had to be replaced this year, must have been a premonition LOL.

So it really goes to what you want to do with the tool. Knee mills are much more flexible, especially with the tilt and nod capability.