Why not to Buy Cheap Jet CNC Mill?

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SteveHGraham
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Why not to Buy Cheap Jet CNC Mill?

Post by SteveHGraham »

I feel like I need to try to pick up a CNC mill during the next year or two. I have questions.

First thing...I do not want to build my own mill. I converted a mini-lathe, and while it was fun, the end product was not a great machine. I've seen conversions on various websites, and they all seem like crap to me. "I've been working on my converted Grizzly for 3 years, and I finally have the backlash down to twenty thousandths." Doesn't sound good.

It looks like Tormach and Haas are popular, but they are not all that cheap. Somewhere not far from $30K to get going, with the Haas costing more than a Tormach. These machines have small work areas.

I noticed that Jet makes CNC knee mills. These are not that expensive. You can put one in your garage for under $20K. Naturally, I'm wondering: why do people buy little Tormachs and Haases if they can get 9x49 CNC knee mills for less. I'm sure there are lots of good reasons.

What are those reasons?
Every hard-fried egg began life sunny-side up.
Ianagos
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Why not to Buy Cheap Jet CNC Mill?

Post by Ianagos »

At least one I have is show me a knee mill with an atc.

But a lot of people grab the tormach I think because it can be moved with a engine hoist (good luck moveing a Bridgeport with one) and they are faster as well as upgradable as you go. So they start at about 20k then you can add a power drawbar and later an atc. They are also better at marketing and support.

A haas is much better but I think they are starting at least 35k with me wanting a vf2ss at around 55k so that’s a bit more but you get a real machine with real support that can really do work. If I was in this game a tormach wouldn’t even be an option as it costs as much as a real machine once you get the atc and enclosure etc. Wait a second I am in this game and guess what I have, a Fadal Vmc and a converted CNc knee mill (what got me started). Both of those machines have cost me less than a tormach still.

If you are willing to move a heavy machine and have 15k ready to spend go get a used Fadal or haas and live the life of having a real machine.


Edit to mention power considerations:

My Fadal runs off a 60a 240v breaker.
(With a 10 hp motor)

Haas has single phase options as well.
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SteveHGraham
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Re: Why not to Buy Cheap Jet CNC Mill?

Post by SteveHGraham »

I'm not spending $55K unless I get diagnosed with a brain tumor.

Haas makes a very small mill for under $30K. My understanding is that it's very stripped down, but I don't know too much about it.

How satisfied are you with the knee mill? My guess: not very, since you bought a Fadal.
Every hard-fried egg began life sunny-side up.
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NP317
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Re: Why not to Buy Cheap Jet CNC Mill?

Post by NP317 »

When I was managing the Univ. of Washington (Seattle) Mechanical Engineering Student Shops, I purchased 4 Southwestern Industries TRAK 2-axis CNC mills.
https://www.southwesternindustries.com/ ... knee-mills
They were perfect for that environment, based on the Bridgeport knee mill structure, robust, and relatively impervious to student damage. (Really!)

With X & Y CNC control, and conversational programming, they could be used manually, just like the Bridgeport knee mill there, and the program could be operated simply by turning the electronic Y-axis hand wheel. The program was executed at the speed of rotation of the wheel, forward or backward, with the Z tool above the part. Once proven, CNC and the Z axis could be engaged. They were wildly popular with all users, and enhanced machining capabilities, while allowing significant learning by ENGINEERS!

I machined the Stephensen valve gear radius links for my Allen Ten wheeler, on those UW machines. Quite a simple operation. Spoiled me forever.
I contemplated purchasing one for my new home shop, but it was just above my set budget. Bummer, because the ability to do accurate curves in X-Y was wonderful.
Anyway, maybe they are affordable for you.
~RN
Ianagos
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Why not to Buy Cheap Jet CNC Mill?

Post by Ianagos »

SteveHGraham wrote:I'm not spending $55K unless I get diagnosed with a brain tumor.

Haas makes a very small mill for under $30K. My understanding is that it's very stripped down, but I don't know too much about it.

How satisfied are you with the knee mill? My guess: not very, since you bought a Fadal.

I didn’t say you should spend 55k (that’s just what I want) I think you should spend 15k on a used Vmc. About same price as a knee mill and it will mill circles around it. Fusion 360 Cad and cam is free and that will allow you to do great stuff without having to buy an expensive software package.

The knee mill was slow and not so accurate within a few thou but without an enclosure it was messy and also limited in spindle rpm ( I have the same problem on the Fadal now).
But the worst is that just like most cnc knee mills the quill was the z axis therefore you had to move the knee a lot. Especially when changing tools. Overall it sucked I just didn’t realize how much it sucked at the time.


For those I need a hand wheel people the Fadal and haas both have a jog wheel that works but once you learn some Gcode they are just a waste of your time.
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Dave_C
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Re: Why not to Buy Cheap Jet CNC Mill?

Post by Dave_C »

Steve,

You mentioned that you have worked on your Grizzly mill and still have .020" backlash! Well, this is going to be an issue on cheap machines as well as it cost money to eliminate backlash and build an accurate machine.

You'll get exactly what you pay for no matter whom you buy from. Light mills are just that, they have less weight and are less rigid so therefore accuracy suffers as a result.

Large work envelopes cost more, servo control cost more but are quitter and don't lose steps.

So maybe the real question is this {what will it take to make you happy with the end results of the mill you buy}? If you spend $10,000 and the results aren't what you wanted then you just wasted $10,000. A Tormach PCNC1100 starts at less then $9,000, then add the stand some freight and your at about $12,000 but this machine will give you a 10 x 20 work area which is pretty nice and some decent Z travel. And if you don't crowd it too hard it will produce some decent results. (so I'm told)

I spent $10,000 doing my G0678 Knee mill conversion but I'm happy with the end result. However, I used Clearpath servos and zero backlash ball screws. I only have an 8 x 20 work envelope and only 4" of Z to work with, but for the space I had, the parts I needed to make and so on it was the right solution for me. I say "for me" and as long as I am happy I don't really care what someone else thinks about my machine.

So what will it take to make you happy?

Dave C.
I learn something new every day! Problem is I forget two.
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SteveHGraham
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Re: Why not to Buy Cheap Jet CNC Mill?

Post by SteveHGraham »

I didn't say I converted a Grizzly. I was talking about the things people who convert them say. They have all sorts of problems.

It sounds like a CNC knee mill is a really bad idea!
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Ianagos
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Re: Why not to Buy Cheap Jet CNC Mill?

Post by Ianagos »

SteveHGraham wrote:I didn't say I converted a Grizzly. I was talking about the things people who convert them say. They have all sorts of problems.

It sounds like a CNC knee mill is a really bad idea!


Yea I honestly think a cnc knee mill is the worst of the three. It’s just trying to be something it’s not. While they are very versatile manual machines and I think a well equipped shop should have one they just aren’t good for cnc.
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Dave_C
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Re: Why not to Buy Cheap Jet CNC Mill?

Post by Dave_C »

While they are very versatile manual machines and I think a well equipped shop should have one they just aren’t good for cnc.
Really, says they guy who probably does not own one! :roll:
G0678 CNC Knee Mill
G0678 CNC Knee Mill
They have their draw backs but "IF" you know how to approach them they can produce very good results.

At least mine does!

Dave .
I learn something new every day! Problem is I forget two.
Ianagos
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Re: Why not to Buy Cheap Jet CNC Mill?

Post by Ianagos »

Dave_C wrote:
While they are very versatile manual machines and I think a well equipped shop should have one they just aren’t good for cnc.
Really, says they guy who probably does not own one! :roll:
0522181311 (2).jpg
They have their draw backs but "IF" you know how to approach them they can produce very good results.

At least mine does!

Dave .
I said earlier I do own one? And a real VMC
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Dave_C
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Re: Why not to Buy Cheap Jet CNC Mill?

Post by Dave_C »

I said earlier I do own one? And a real VMC
Well if you own a VMC then that explains why everything else would be less than acceptable to you. Most of us of the "HOME MACHINIST" variety machinist either don't have the room, power supply or funds to own such a machine.

But back to Steve's issue of what to buy, just because a machine is a bed mill does not mean it is an accurate machine! There are lots of bed mill such as the Grizzly bed mills that use an open C type column and such a design is not very rigid. Tormach solved that issue by closing the front of the column and using a channel to run the ball screw and nut up and down. Theirs is a much more rigid design than the Grizzy bed mills, the harbor freight bed mills and all that fall in the same class as those two.

If any of you own one of these try this! Put in a piece of round stock in a collet, put a DTI and mag base on the table with the tip touching the stock and then just push on the head with your hand and watch what happens to the DTI. The higher you go up the column the more deflection you will see.

Accuracy cost money, as does a VMC! LOTS of money.

And back to what I said before, I am happy with my mill so to each their own.

So Steve, what would it take to make you say you are happy with your mill? A VMC? :D

Dave C.
I learn something new every day! Problem is I forget two.
Ianagos
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Re: Why not to Buy Cheap Jet CNC Mill?

Post by Ianagos »

Dave_C wrote:
I said earlier I do own one? And a real VMC
Well if you own a VMC then that explains why everything else would be less than acceptable to you. Most of us of the "HOME MACHINIST" variety machinist either don't have the room, power supply or funds to own such a machine.

But back to Steve's issue of what to buy, just because a machine is a bed mill does not mean it is an accurate machine! There are lots of bed mill such as the Grizzly bed mills that use an open C type column and such a design is not very rigid. Tormach solved that issue by closing the front of the column and using a channel to run the ball screw and nut up and down. Theirs is a much more rigid design than the Grizzy bed mills, the harbor freight bed mills and all that fall in the same class as those two.

If any of you own one of these try this! Put in a piece of round stock in a collet, put a DTI and mag base on the table with the tip touching the stock and then just push on the head with your hand and watch what happens to the DTI. The higher you go up the column the more deflection you will see.

Accuracy cost money, as does a VMC! LOTS of money.

And back to what I said before, I am happy with my mill so to each their own.

So Steve, what would it take to make you say you are happy with your mill? A VMC? :D

Dave C.

Question is how much did you pay for your knee mill? I paid 4K for my Fadal and like I said works off a 60a 240v subpanel. I will admit it takes up more space. 1 car spot in a 2 car garage is plenty of space for it.

In the end it really depends on what the op wants to do.
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