11/26/2020
Things you should know about cnc conversions
November 26, 2020
Posted by author-avatar Cncconversionplus
I never realized just how big this industry is. I’m talking about the hobbyist machinist. I retired in 2015 after 40 years with the same company. I started out as a machinist trainee working the second shift in Houston. Over the years I moved up in the company and got away from machining and programming but retained the love of making things.
When I retired I found myself looking for something to do like most retirees. One day I decided to buy a small mill and lathe. The natural progression for me was to convert them to cnc. And that is where it started.
In 2015 I could not find many sources for conversion kits. There was an operation located in Humble, Texas. I purchased a kit for a Sieg X2 (Harbor Freight 44991) mill. I never received the kit. It appears that the people making these kits decided to stop. Funny thing, they forgot to let the customers know. Thankfully, PayPal refunded my money.
It was at that point that I decided to make my own. I set out measuring the mill to get the centerline dimensions for the ball screws and other parts. I then sat down at my laptop and, using Autocad I designed the components. I made them, installed them on the mill and then started on the electronics.
Today, I have made and sold over 45 kits for the Sieg X2 and X2D mills. Of course there have been many changes and revisions but today I have the only X2/X2D kit on the market that is truly bolt-on, no machining required. Moreover, I designed the Z axis (head) to lift from the center of the head putting the center of gravity where it belongs, in the center. No more binding!
I grew the business and started buying machines for people, converting them and shipping to their location. Today, I have done a Precision Matthews PM25MV, three Grizzly G0704, a Bridgeport Series II and a few others.
I was asked to do a lathe for a customer. It was a 7 X 12″ Harbor Freight mini lathe. It was pretty straightforward and soon I was asked to do another one. As of today, I have converted two Harbor Freight 7 X 12, one Harbor Freight 8 X 12, a Grizzly G9972z, a Bolton 10 X 24″ and several others. I began offering these machines turn-key as well.
Of course, what is a shop without a large cnc router? So I made one that will hold a 4′ x 4′ piece of wood. I’ve only seen one or two of these larger than this one. It is used by a wood worker and he uses it every day. I was told that he has been up to his ears cutting plexi-glass that you see at every place you go these days. Wish I’d thought of that!!!
The next project is a cnc plasma cutter. The table is being built as we speak. I already have at the other components with the exception of the Plasma torch. I’m excited about this one and I think there is a big market for it.
And recently, I was approached by a wood working shop that makes novelty gavels, Yes, gavels! They sent me a new Jet 12 x 21 wood lathe and I designed a two axis slide that bolts on the lathe and makes gavels by cnc! I’m curious to see how this one works out and if there is a market for them.
If you’re considering a conversion of a machine you have, or one you are thinking about building, take the time to develop a list of things you are wanting to do. There are a lot of kits out there now, some good and some not so. What I find is that most people that contact me are ex-machinists and know what they are looking for. However, I get a lot of calls and emails from people that have never ran a machine before but have decided that they want to get into it.
First of all, expectations are very important. You know what you want to build or at least have some idea. Here are a few things to consider:
Small product specific machines are hard to come by. Know what you want to make. Is this a family of parts? do they vary in size? Machine selection depends on this.
What type of material are you going to use? The mini lathe and mini mills, commonly referred to as benchtop machines are pretty light duty. In fact, if you are going to try to make ultra-precision parts, these are not the machines to consider.
These machines are great with plastic and aluminum and some carbon steels. Although they will, with the proper tooling and programming, machine some steels, it is very slow.
Your skill level as a machinist. If you have experience, know speeds and feeds and tooling, you’ll be better equipped to work through the limitation of these machines.
Your skill level as a programmer. Again, if you know the ins and outs of programming, you’ll be able to work through it.
Tooling knowledge. If you’re familiar with metal working tooling, you’ll be much better off than someone that doesn’t.
Computer Aided Manufacturing (CAM) packages. There are a host of them out there ranging from free to over $5k. Spend some time and investigate their possibilities and options. Note: the most expensive ones are not necessarily the best for you.
Now let’s talk precision. The mini mills and lathes that can be bought at Harbor Freight and others ARE NOT PRECISION, they’re just not. First of all, the ways (the part of the machine that moves) are not ground. They are machined. Secondly, they have straight gibs. The gib is a an insert that goes between the mating ways. This is how you adjust the tightness of the machine. Straight ways use set screws along the side that you simply tighten until you think the slack is out of the machine. The problem is, if you tighten them too tight, the stepper motor will stall. Too loose and you have slack that causes chatter, loose tolerances and other issues. Having said that, you can achieve tolerances on location of holes and such within .007″. This may sound close, but if you’re making mating parts there could be an issue. Finally, most of the kits on the market, including mine, use a C-7 class ball screw and nut. C-7 is a loose fitting ball screw and nut, but they are durable and in-expensive. There are ways around the ball screw and nut tolerance. If the machine has the room, you can put double, opposing ball nuts and simply dial out the slack of the ball screw and nut. With this option, you can reduce the slack and hold tolerances within .003″. For most hobbyists, this is close enough. I do offer an upgrade to premium ball screws and nuts…it’s not cheap.
For closer tolerances still, upgrade to a machine that has tapered ways. These are usually the larger benchtop mills like the Precision Matthews PM25-MV or the Grizzly G0704. About 3 to 4 times the price of the mini mills, these machines with the tapered ways and double ball nuts can hold tolerances of .0015″ which is pretty close for almost anything a hobbyist makes.
Mini lathes are a little different. Most all lathes have straight gibs. There are a few with tapered gibs but are pretty pricey. The ultimate lathe has linear guide ways. A linear guide is a precision ground bar (rail) that can be round or square matching “trucks.” the truck is what moves back and forth along the rail and are very closely matched. Not only are the more precise, they can move much faster due to less drag. Machines with these rails are pretty pricey as well.
Some of the benchtop mills have linear guides also. Again, these can get pricey.
Now let’s discuss options:
Lathe Threading
To thread on a lathe you need an encoder on the spindle. An encoder counts the revolutions that the chuck is turning. This signal is used to tell the stepper how fast to move to cut the proper thread lead.
You can add an encoder to the machine as it is and thread successfully. However, you have to change belt pulleys to increase or decrease the spindle rpm that is desired.
A variable frequency drive can be installed on the machine and control the spindle rpm via computer. However, the VFD will not work with the spindle that comes on the machine as these are usually 110 volt. To control a spindle motor, first of all it has to be three phase and these are always 220 volts are larger to be controlled by the VFD. Also, the VFD has to match the motor you decide on.
With the VFD properly matched to a motor, you can thread at any rpm you choose. Another advantage of spindle control is constant surface footage or CSS. CSS means that the spindle will turn the correct rpm at any diameter to assure that the correct surface footage is maintained. For example, in facing a part, the larger diameter is going faster than the center of the part. With css, while facing a part, the spindle will speed up as it gets closer to center. CSS improves finishes and increases tool life.
Mill threading
Threading on a mill can be done in a few ways. One, you can use a thread mill and mill the threads. Another is using a tap. A tap is a tool that cuts the threads by screwing it in to the part.
Tapping can be done manually or on the machine. If you use the machine, you can use a tap driver that floats or use the “rigid tapping option”
Rigid tapping also requires a spindle control feature as described in the lathe.
Other options
Coolant – either flood or mist coolant are available. You can turn on either system via computer input but it has to be set up properly.
Air – believe it or not, I use an old re-purposed CPAP to blow my chips away from the cutter. It works well with plastic and aluminum but not great on steel as the chips are heavier. Air adds NO lubricity, it simply blows the chips away. It is easy to use and less messy than coolant.
With all that said, hobby cnc machining is fun. If you’re not trying to do production work, these machines are pretty durable, cheap and easy to convert. They are meant to have fun with and that they can do.
So whatever machine you decide on, just be informed of their limitations and match the machine to your needs.
Contact us at CNC Conversions Plus for consultation and selection. We also write programs and select tooling. We can do the entire machine turn-key or just the kits alone.
