Bench top mill questions?

[thequintessentialman]

Been reading a few of the back posts and still don't fully understand how to determine the working size of a mill. If money and space were not mitigating factors then no problem but I'm a little limited on both. Say I wanted, among other things to do some milling on an aluminum block roughly 9"long, maybe 3"tall, and 1.5"wide. (Compare to a 0% AR10 billet for size ref). Right now that's the largest thing I can think of I would work on but every time I get a bigger chain saw I find a bigger tree. Can a bench top mill handle something that size? I've been wanting one of these things for a while and suddenly have some bonus money coming in from work so figured this may be the time go move on it.

Next question would be what are some decent quality brands bench top (or stand supported) mills that could at some point be retrofitted as CNC? I'm dabbling with a 3D printer at the moment and figure at some point I'll be able to cast some of my own billets. (We got to have high goals, right? )

 

[GeneT45]

Your first question is somewhat complicated. A mill (benchtop or otherwise) has a "work envelope" which is basically how far it can move in each direction. Typically the table is bigger than the work envelope - you can't move the very end of the table under the cutter. Which brings us to the next part of figuring out what you might need - fixturing. If you want to work on a 9" x 3" x 1.5" piece - you might start thinking about a work envelope of 9" x 9" x 9" - You might need to orient that long side in X, Y, & Z. Using the AR10 for ref - to cut the FCG pocket, bore the buffer tube hole, and cut the deck flat. You might get away with just X or Y & Z... BUT, you also need room for your tools.

The hardest thing on a small mill is usually the Z-axis. You'll want room for your part in the "tallest" orientation you'll have to machine, PLUS a drill chuck, PLUS a drill, PLUS the height of your vise! Yikes! For the price of a few extra tool changes you might be able to eliminate the chuck and clamp your drill directly in a collet, you can buy short drills, you might be able to mount your workpiece closer to the table by clamping to an angle plate.

In the X and/or Y You want room to get your endmill on either side of your workpiece as well as have extra table to allow for the additional space taken up by clamps. And again with the right clamps you can economize on what space you have. I'd shop based on Z axis movement. X will typically always exceed Z and you can usually use X when your Y doesn't have enough travel.

I'm sure that's a bit confusing, but hopefully you get the gist.

Recommendations: Precision Mathews has pretty good bang for the buck. If you can afford it, their small knee mill takes up about the same space as a benchtop mill on a stand and is a better machine: PM mini knee mill . They have some conventional benchtop mills as well.
I was going to mention Industrial Hobbies as well - they made a pretty hefty benchtop mill that could be purchased with CNC, or a CNC retrofit kit could be purchased later. The only other benchtop I've used is the Rong-Fu style and I'd encourage you to skip those and look for something with a square column.

I hope that helps at least a little.

GsT

 

[Aaron_W]

I've found small mills to be less forgiving on size than a small lathe. On a lathe, pretty much if it fits you are ok, but small vertical mills already compromise rigidity so a part that just fits is probably going to be challenging. I'd aim for a work envelope at least 2x the size of your likely largest projects.

You could probably make a large mini mill like the Grizzly G0781 4x18" mill (12x5x7" travel) work for a 9x3x1.5" part, but it would be far from desirable being just barely adequate. I would not recommend it.

The medium size 7x27 (-ish) bench mills are popular because they are a manageable size and weight for many people with small shops but still have a pretty useful work envelope. This would include Grizzly G0704, Precision Matthew PM25, PM727, PM728.

The many Rong Fu RF-31 clones are also popular and just slightly larger (8x29"), Grizzly G-0705 or Jet JMD-18 are examples of this style of mill. These "round column" mills have some drawbacks, but can be very affordable on the used market.

Any of these should be adequate for a part that size, and leave you some room to go bigger if needed.

Also consider how the work will be done though. If you needed to drill into the long end of your 9x3x1.5" part, you will need a lot more vertical height than working the 3" and 1.5" sides. Smaller mills tend to have pretty good side to side (X axis) travel, but not so much in and out (Y axis) or vertical (Z axis). For example most of the 7x27" mills I listed have 12-16" spindle to table height which on the lower end wouldn't leave you much room for tooling.

Something else to consider, if you have the space. Small mills often cost as much or more than a used Bridgeport type mill.

 

[C-Bag]

For example most of the 7x27" mills I listed have 12-16" spindle to table height which on the lower end wouldn't leave you much room for tooling.

It is really hard to figure out sometimes by spec alone. But the one thing a round column mill can do that a knee can’t do is swing the head away from the table for long/large work. I’ve not done that with my mill drill but I have done it with 24” radial arm drill. Where I was able to rest a 4’ piece on the floor and clamp it to the side of the table and extend the drill arm over and drill and tap the piece. The UniDrill has 6 3/4” Z where the RF30 has 5”. This is where imagination and creativity along with good tooling and extensive scrap pile get er done.

 

[OTmachine]

I have one of these which knocks down into smaller moveable pieces. It has been pretty useful and is accurate.

 

[Aaron_W]

It is really hard to figure out sometimes by spec alone. But the one thing a round column mill can do that a knee can’t do is swing the head away from the table for long/large work. I’ve not done that with my mill drill but I have done it with 24” radial arm drill. Where I was able to rest a 4’ piece on the floor and clamp it to the side of the table and extend the drill arm over and drill and tap the piece. The UniDrill has 6 3/4” Z where the RF30 has 5”. This is where imagination and creativity along with good tooling and extensive scrap pile get er done.

Many knee mills can swing the head like that.

 

[C-Bag]

Many knee mills can swing the head like that.

Doh! Yup you’re right…..I was thinking of the benchtop knees.

 

[Weldingrod1]

The Rockwell and Clausing small knee mills can't swing the head to access long work. I have a Rockwell...

Sent from my SM-G715A using Tapatalk

 

[Charles scozzari]

Good morning, I feel for the money you'll spend on a bench top mill you can find a used Bridgeport for the same money or less that will take what ever you throw at it. Granted you'll need a phase converter but overall I would consider a Bridgeport a better choice.

 

[matthewsx]

If I could fit a Bridgeport or clone I would definitely have gone that route.

However, I simply can’t so I bought an RF-31 built in Taiwan for Harbor Freight. I haven’t done many projects yet but I’m confident I can get around the round column issues when I need to.

Honestly, I think unless you’re in the market for larger “bed” mills like an RF-45 clone the round columns are the best bang for the buck. There are lots around, prices are good and parts are available.

Now, I do have access to bigger machines at work if I need more capacity but for a hobby shop I feel like this is a good compromise. I have less than $1000 into mine and know what that buys on the new market.

John