Moving Table vs Moving Gantry; Dremel vs Router

[Marc]

Unless I'm mistaken, that's only 45 oz of torque. What are you planing to cut ?

Marc..

[Kammo1]

Hi I would like to build a machine that will have a cutting envelope of about 30-32 inches X 18 inches and a Z height of about 3 inches. I plan on cutting small decorative signs as well as maybe inlayed pictures and 3D shapes as well. I have seen the Hobby CNC kits and the Xylotex kits but as these were cheap I thought that they might do but if that is the only torque it will deliver it might not be suitable. I plan on using 1/2 acme rod but again what TPI should I get??? I also plan on using a small router like a Rotozip as well as a small Dremel for light delicate work such as inlays. The machine will be 3/4 MDF and a pipe and bearing set-up for all axis.
Also without getting into a A versus B scenario what would be a simple kit to start with
Hobby CNC or Xylotex?? bearing in mind I am a total novice and what something thats easy to build and use, for some strange reason I am tempted to go for the Hobby CNC but again any help in getting it right would be greatful. I must say that since I have visited this site some of the machines you guys have built are superb and this has inspired me to finally now take the plunge and get started but would like to build the machine around parts that I can get easily and also that are going to work. Kammo1

[Marc]

Kammo1
Personaly I would go for the Hobby CNC kit, I think it's your best option and I heard good things about it

Don't forget pictures

Marc..

[Kammo1]

Hi does Dave at Hobby CNC sell the circuit board already wired up so I haven't got to do this myself ? sure would make my life easier if it is all done. Also I found this driver whilst searching the web, would this be suitable to control a 3 axis machine with 200oz steppers. http://www.hobbytron.net/product574.html If I am not misunderstood is this like the Hobby CNC but all in one box ?? and the price indicates to me that it may not be up to Dave's quality but worth some comments. Kammo1

[Marc]

does Dave at Hobby CNC sell the circuit board already wired up so I haven't got to do this myself

I don't beleave so

Also I found this driver whilst searching the web, would this be suitable to control a 3 axis machine with 200oz steppers.

The price sure looks good but I never heard of it

Marc..

[Pete Stenabaugh]

I am surprised at what some of you fellows are talking about here.....Anoel you have yourself listed as a machinist, yet you talk about grinding down a threaded shaft to make the coupler fit? The coupler alignment must be accurate and concentrice or else the shaft will not likely run true and at high speed it will create lot of problems.

As far as speeds go, you fellows likely realize that these small dremels and zip routers etc, all run at very high speeds, but they do not have much power (torque). You need to use a regular variable speed router, like a Hitachi or a Bosch etc. If you plan on trying to route wood at speeds of 5, 12 ipm or so, you are really out of touch with what is required. To properly route soft wood like pine, cedar etc, or even into hard woods like oak, maple etc, you need to have feed rates in the range of 50 to 60 inches per minute, or you risk burning the wood with the high speed routers. Even the variable speed units need to run at least about 15,000 rpm, so trying to slow the router down doesnt help either.

As far a stepper motors go, you need to realize that stepper have their highest torque at 0 rpm when they are stopped. The faster they run, the less torque they have, and then you run into the risk of having them loose steps if they hit anything that tends to jam them up. With the lower torque available at the high speeds, this is easy. Keep your stepper motors to a max of about 250 rpm for cutting, but you can go a bit higher for rapid speeds, if your machine is nice and free moving.

If you think you can get lots of torque out of a stepper by gearing it down (by having a small pulley on the motor and a larger one on the leadscrew) this is also false thinking because to get the speed you need on the lead screw, you need to run the stepper very fast, so it looses torque as previously mentioned. You can get away with this to a degree, but if you plan on doing that then get a BIG stepper that has lots of extra torque right out of the gate.

The BEST way to get high feed rates, with only having your stepper run slow to maximize the torque is by using a rack and pinion drive. This is what the professional units use. Just think, a pinion gear that has a pitch diameter of 1" such as a 12 dp gear - the circumference of this gear, at the pitch line is 3.14", so if this gear turns at 100 rpm, then the resultant feed rate is 314 inches per minute.....that is how you get high feed rates out of steppers. The only draw back is that the resolution is lower, at about 0.001" per step, if you use the Xylotex controller and gear the stepper 2:1 to the pinion gear, but that is fine for wood working (200 steps x 8 = 1600..........then gear it 2:1 gives you 3200 steps on the pinion. The pinion is 3.14" diameter, so 3.14/3200 = 0.00098" per step).

Get your steppers from www.homeshopcnc.com this is likely the best and the cheapest place to deal. A very good reliable place to shop, Rick is a good guy. Dont use steppers that are under 500 oz in. you are only wasting your money and fooling yourself. These are not expensive motors, get the big ones. I use the 1780 oz in, which he has now replaced with even bigger ones, but you cant run these on a Xylotex.

Pete

[delstanton]

I see a large disadvantage in size with an X-Y table versus an X table gantry machine, with Y and Z on the gantry.

Suppose you machine must cover a 4 x 8 foot panel. With the X-Y table you will need 4 x 8 foot motion of the table - a BIG machine. You have a 4 x 8 ffot workpiece that you need to move 4 feet one way and 8 feet the other way, a 8 X 16 foor envelpe. Of course this is disregarding the structures necessay to support the X and Y axis, outside that envelope.

Also the structure to support an X-Y table is more complicated. Looking at large metal workinjg machines there is no such thing as a large X-Y table milling machine, the large machines are all gantry types. I believe as you proceed with design of an X-Y table machine you will appreciate the superiority of the gantry approach.

You design the gantry to be as stiff as required to maintain accuracy. Looking at successful gantry machines will give you a good sense of what is required.

Del

Originally Posted by chuckknigh For a homebuilt unit, which is a more robust design? I can see benefits to both approaches... My understanding of them is based, primarily, on photos of other people's machines, and of Cranky's 7th Sojourn and Routezilla designs. Both look well engineered, and both would be excellent designs to emulate. (OK...copy) But which, for someone starting with a "blank sheet of paper" is the better design to emulate? I *like* Routezilla better -- something about it appeals to me. But, my concern is the potential for the gantry to rack while the machine is under load. Is this a real concern, or is it more of a theoretical thing? I'd be using twin rails -- one on each side with fully supported homemade linear bearings -- and a screw drive mounted in the center. THANKS! -- Chuck Knight P.S. For the cutter head, would it be better to go with a Dremel/RotoZip, or a small router? I already have both a Dremel and a trim router, so it's not a matter of using "what I already have." And then there's this router: http://www.harborfreight.com/cpi/cta...emnumber=44914 I've seen it on sale for $20 from time to time...a truly exceptional price. Something cheap that I could dedicate to this project, next time it goes on sale.

[lgroulx200]

Originally Posted by Pete Stenabaugh I am surprised at what some of you fellows are talking about here.....Anoel you have yourself listed as a machinist, yet you talk about grinding down a threaded shaft to make the coupler fit? The coupler alignment must be accurate and concentrice or else the shaft will not likely run true and at high speed it will create lot of problems. As far as speeds go, you fellows likely realize that these small dremels and zip routers etc, all run at very high speeds, but they do not have much power (torque). You need to use a regular variable speed router, like a Hitachi or a Bosch etc. If you plan on trying to route wood at speeds of 5, 12 ipm or so, you are really out of touch with what is required. To properly route soft wood like pine, cedar etc, or even into hard woods like oak, maple etc, you need to have feed rates in the range of 50 to 60 inches per minute, or you risk burning the wood with the high speed routers. Even the variable speed units need to run at least about 15,000 rpm, so trying to slow the router down doesnt help either. As far a stepper motors go, you need to realize that stepper have their highest torque at 0 rpm when they are stopped. The faster they run, the less torque they have, and then you run into the risk of having them loose steps if they hit anything that tends to jam them up. With the lower torque available at the high speeds, this is easy. Keep your stepper motors to a max of about 250 rpm for cutting, but you can go a bit higher for rapid speeds, if your machine is nice and free moving. If you think you can get lots of torque out of a stepper by gearing it down (by having a small pulley on the motor and a larger one on the leadscrew) this is also false thinking because to get the speed you need on the lead screw, you need to run the stepper very fast, so it looses torque as previously mentioned. You can get away with this to a degree, but if you plan on doing that then get a BIG stepper that has lots of extra torque right out of the gate. The BEST way to get high feed rates, with only having your stepper run slow to maximize the torque is by using a rack and pinion drive. This is what the professional units use. Just think, a pinion gear that has a pitch diameter of 1" such as a 12 dp gear - the circumference of this gear, at the pitch line is 3.14", so if this gear turns at 100 rpm, then the resultant feed rate is 314 inches per minute.....that is how you get high feed rates out of steppers. The only draw back is that the resolution is lower, at about 0.001" per step, if you use the Xylotex controller and gear the stepper 2:1 to the pinion gear, but that is fine for wood working (200 steps x 8 = 1600..........then gear it 2:1 gives you 3200 steps on the pinion. The pinion is 3.14" diameter, so 3.14/3200 = 0.00098" per step). Get your steppers from www.homeshopcnc.com this is likely the best and the cheapest place to deal. A very good reliable place to shop, Rick is a good guy. Dont use steppers that are under 500 oz in. you are only wasting your money and fooling yourself. These are not expensive motors, get the big ones. I use the 1780 oz in, which he has now replaced with even bigger ones, but you cant run these on a Xylotex. Pete

Hi Pete,

I agree with many of the points you have made. But you can still make a DIY router with small steppers, threaded rod etc. and still have a excellent working machine. Do you have any pictures of your machine?

Larry

[Pete Stenabaugh]

Larry, no I do not have any photos yet, it is still in the design stage, using solidworks. I do have the partial assembly model, but there is not too much detail to be seen yet.

Unfortunately there are too many other things in my face right now to be able to spend much time on it. I hope to be able to get back on the design in the next couple of weeks.

This is going to be a very rigid and heavy duty machine, capable of anything the commercial units can do, although I have limited the length to an 8 foot table, due to space limitations in my garage. It will be able to cut 49" wide by 78" long, so if a person wanted to increase the cutting lenght it is only a matter of a longer frame, and some more rack gearing to make the frame 9'-6". I really wanted to do this, but in reality I think if I need to cut more than 78", I will have to move the material. The table is designed to handle up to 49" wide material (mdf) at any length.

If you are interested in something like this, keep in touch. If you contact me off the group and send my your personal email address, I will email you (and any others that are interested) a pdf of what I have designed so far. I think you will be impressed. I got the basic idea from a web site that was selling plans, but I did not llike some of their design ideas, so I took it up a few levels to make it a true "Peter-built" machine.

You are correct about the smaller DIY routers. They are quite adequate with small steppers etc., but the same problem exists with feeds and speeds. Fellows that build these units typically mount small dremel or similar tools on the gantry, due to the limited size of the machine. With these small high speed tools, there still exists the issue of feed rate as I mentioned before, however if a person is doing small detailed work, engraving, filgree details etc, then the high speed router is fine with small diameter cutters. With this type of engraving then you should still try to attain at least 20 to 30 ipm of feed. If you are only getting feed rates around 10 or 15 ipm, you will have trouble with your cutter burning and getting overheated. The other issue that exists is that with the slow feed rates, it is more difficult to get the material to clear the cutter, which again causes overheating and melting of materials like plastic and foam. If you do this type of thing, only use single or maximum 2 flute cutters, and use ones that are designed for cutting plastic, or for engraving. Regular machining end mills are not ground the same, and do have the high helix required for this type of work. Get cutters from "Engraving" suppliers, and you will have better success.

Pete