ACME Leadscrew question

[whmeade10]

I am new to CNC and have been lurking around for 2 or three months just taking everything in and deciding what type of machine to build. I have already ordered the HobbyCNC 305 kit. I am wanting to order the ACME lead screws for the machine but there are many different materials that they come in. For example:

Taken from McMaster-Carr (6 foot length)

Type 316 stainless steel $ 93.87
Zinc-Plated $ 26.55
Grade B7 Alloy $ 38.40
18-8 Stainless Steel $ 59.55
Plain Steel $ 11.48

The Type 316 is way out of my budget, but the others are doable.

What is the most cost effective solution for a home built CNC?
Thanks for checking out my question, this is a great place for CNC info.

[HeritageAmps]

this is what i used:

http://www.use-enco.com/CGI/INSRIT?P...&PARTPG=INLMK3

1/2"-10tpi 6ft $6.99

[txcowdog]

I'm with Heritage... I bought 4 of those carbon steel screws from Enco and at half price you can't go wrong. Excellent accuracy and practically no backlash. An excellent value. Save your money and buy the $6.99 Acme.

[HayTay]

Welcome to CNCzone, whmeade10.

Pick whichever one fits into your price range. I'd go with a tighter tolerance ACME lead screw over a 'pretty' looking one if your going to spend any extra money. If your CNC table is going to be constructed from wood or plastic I'd just buy the cheapest one and be done with it. The cheap plain steel one is working fine for me.

I concur with HeritageAmps and txcowdog, I too purchased my lead screws from ENCO and have no regrets concerning the performance or the price. Spend the money you saved on the ACME screws to purchase lead nuts from DumpsterCNC. In my opinion they're hard to beat.

Whatever you do, stay away from the plated lead screws. If the plating starts to come off (and it will) you're going to start having all kinds of problems with wear and backlash.

Good luck with your build,

[gth629e]

Hey guys,

The different types of steel of the ACME lead screw of course gives different rating for "strength." Of course unless your machine is going to be real heavy or is rated for a lot of cutting force, you shouldn't have to worry about that. Any of those grades will be sufficient strength wise.

Where the grade of the steel has a more pronoun effect on the friction it will cause between the lead screw and the lead nut.

More friction = Less effective power that your stepper/servo motor will deliver to your system. Or in other words the more friction, the less efficient your CNC router will be and it could lead to your motors wearing out sooner.

But, the good news is that most of those steels will all have similar coefficients of friction. As a good general rule to go by is the prettier it looks, or the shinier it is, the less the coefficient of friction will be.

The is important if you your lead nut (the nut going on the lead screw), is steel or bronze. A stainless steel lead screw on a steel nut will have a lower coefficient of friction than a B7 grades steel.

You can look up coefficients of frictions between two material on Google. The LOWER the number, the better. This can be important between two materials as some have a tendency to stick( high CF), and can cause you to have a fair amount of power loss. If you stepper motors are already undersized or close to it, this can cause your machine to bind at high speeds or under the cutting force.

If your lead nut is a bronze or brass, then there will be little difference between any of those grades and the bronze. They will all have very similar coefficients of friction.

What I suggest is using an ani-backlash nut. The cheapest ones are the ones from dumpsterCNC.com. as mentioned above. I have used his products on several of my machines and have never been disappointed. He even offers custom hole patterns for and extra $2.00.

The material he uses is called Delrin or Acetal, the coefficient between delrin and any steel is much lower than even bronze and any of those grades will work fine, so go with the cheapest.


What you should be more concerned about is the type of lead screw not the material. Some of the ones you have listed from McMaster-Carr are called rolled ACME leadscrews.
Also called precision leadscrews and that is why the prices are so high and not the material.

Rolled or precision ACME rods are made for linear motion and thus are more expensive and will have less backlash. However, if you use an anti-backlash nut, like the ones from dumpster. You will do just fine with the plain steel ACME rods that are formed and not rolled. I have made several machines this way and have achieved accuracy and precision of .0005 repeatable. You lead nuts will tend to wear out faster but for the price at which dumpster sales them, its still cheap just to replace them in a couple years lol.

I hope I haven't confused you. I have made several machines of my own. I also own a new website that is still in progress. We started uploading it this week.

Its cncroutersource.com and you can find all this information and much more, and its all free. There is a complete CNC router builders guide that will give you all this kind of information and more. But bear with us as we upload it all, and of course its all free.

We break down every part of a homemade CNC router and walk you through the whole process.

My name is Brian and I am a mechanical engineer who does this stuff for fun lol. If at any time you have any questions. you can contact me or someone at our site with any questions and we will get right back to you!!


I hope i helped. Be patient and HAPPY CNC'n.

~Brian

brian@cncroutersource.com

[whmeade10]

I would like to thank everyone that responded to my question. I believe I will go with the plain steel ACME lead screw and the ani-backlash nuts from dumpsterCNC.com for my first CNC. I am already thinking about my second one and the first one isn't even built yet. Is this normal

My CNC design is based on the JGRO CNC. I say based because it will be made out of steel and aluminum. I will start a build log thread in the CNC Wood Router Project Log when I get along a little farther.

I went and checked out your site Brian, and I have to say, it looks great so far. I have book marked it and plan to go back and check everything out. Keep up the good work. I can't get to much info

[rustamd]

buy 42 feet of acme rod and get free shipping!!! Enco free shipping code for orders over $50:

WBCT7

Expires Oct, 31 2007 But new code should be available in few days

[gth629e]

Glad to hear you are excited about the project. I will try and keep posted on your progress. I do not think you can go wrong with the plain steel ACME rods and the anti-backlash especially for your first machine.

Its very normal to start thinking about your next machine. If all goes well you can use your first machine to help build a much nicer second machine.

So, keep in touch and just shoot us an email if you need any advice or help and of course use the threads on CNCzone.

Just so you know you can also bye a 6' ACME threaded rod from Mcmaster for $6.50. Its listed under ACME threaded rod and not the lead screw section. Part # 93255A431. I have used both Enco and Mcmaster and they both are great companies. So either one is good, just which ever is cheaper and offers the cheapest shipping.

You may also want to think of anything else you need so you may buy it all at once, if you have the funds all at once. That will save you some money on shipping charges.

Good luck!

Brian@cncroutersource.com

CNCroutersource.com

Brian@cnc

[cyclenut44]

What is the difference between 2 start and regular acme treads. Any advantages? It also seems as if most people are using the 1/2 dia. I would like to do a double screw lay out with the 3/8 dia. Is it a power transmission issue? I have the hobbycnc 305 stepper kit on order.

[ger21]

2 start is much more efficient, and also keeps the steppers spinning slower, where they have more torque. When you factor in the added efficiency and the motor having more torque from spinning at 1/2 speed, when you switch from 1/2-10 1 start to 1/2-10 2 start, you basically double your speed, and still have nearly the same force at the screw as you did before, so other than the added cost, it's a no brainer.

The reason most people use 1/2", is because its 1) cheap. and 2) 3/8 is not really useable in longer lengths due to whipping at higher rpm's.

[cyclenut44]

Thanks for the info. I have also been considering rack and pinion for the x/y axis. any thoughts on this?

[gth629e]

Here is the technical definition.

Starts—The number of independent threads on a rod or stud. All general purpose Acme rods and studs are single-start. Precision Acme rods are most commonly single-start, but also come in multiple-starts, which allow the nut to travel faster by moving farther in one turn.

Now and explanation,

The more starts the machine will travel faster by moving farther in one turn. Also, the more starts the less likely there is for backlash but you still need an anti backlash nut. Think of a 2 start has double the threads than a 2 start, its almost like having 2 lead screws in one or two nuts as there is twice the amount of contact between the nut and the lead screw.

Another way to think of it is like this,

A nut on a 3/8-10 1 start lead screw needs to turn 10 times in order to travel a distance of 1 inch on the lead screw.

A nut on a 3/8-10 2 start lead screw needs to turn only 5 times to travel a distance of 1 inch because there are double the amount of threads.

So to find the amount of actual machine travel take the turns per inch (TPI), like the 10 in 3/8-10 and divide by the amount of starts and that’s the amount of travel you will have.


Multi start lead screws are usually more expensive. If the rod itself is not more expensive the nut will be. However, if your budget allows and you can find the anti-backlash lead screw, then it would be best to go with a 2 start over a 1 start, but there are trades that you need to make.

Many people go with a ½-10 lead screw for many reasons

1) Its readily available
2) Anti-backlash nuts are relatively easy to find
3) They are pretty cheap
4) It’s a good trade between mechanical advantage and speed


Speed, linear force, and the span the lead screw will go are 3 trades that will decide you lead screw diameter and turns per inch (pitch).

First let’s look at the trade between the linear force mechanical advantage and the travel speed. The travel speed will directly determine your cutting speed which is rated in inches per minute for CNC routers (IPM). Many people do not care about the cutting speed when they build their first machine and then later get tired of their machine only moving at 20 IPM. The lower the IPM rating of your machine the longer it will take.

The cutting force is how much force your machine will have when cutting a piece, remember, just the movement of the machine itself will require some force. If you have a nice linear motion system with some sort of linear bearing (rails and bearing blocks, homemade linear bearings from radial bearings etc) then it should not take much force to move your machine.

For example let’s say the y and z axis gantry assembly of your machine weighs lets say 40 lbs, and you have some sort of ball bearings for your linear bearing system. Since the coefficient of friction for ball bearings is around .02 and you will have 2 linear systems for each axis, it would only take about 1 lb to move your gantry assembly. The rest of the force would be the cutting force.

The pitch of your lead screw or the TPI of your lead screw will determine both of the speed and the cutting force.

The more turns per inch of a lead screw means there will be more mechanical advantage and MORE cutting force but LESS speed.

The less turns per inch means MORE speed and LESS cutting force.

If you have the 305 kit, you have 4 steppers and I’m guessing will be putting 2 steppers on the x axis. This is 610 oz on the x axis which is the longest axis. Depending on the size of your machine, your will probably not need a mechanical advantage of a ½-10 lead screw.

So I would go with a 10 tpi and 2 starts or an 8 tpi 2 start. In this case speed will be better than cutting force.

Remember that the rating at 305 oz is rated at 0 RPM’s of the stepper motor. Stepper motors have the most force at 0 RPM’s.

This brings us to the second consideration the span. Stepper motors max RPM may be anywhere from 800 to 1500 RMP’s. Let’s say you are using micro steps and your stepper motor max RPM is 500 RPMs because the torque of the motor drops off too much after that.

With a 3/8-10 2 starts you have 5 turns per inch and at 500 RPM your machine will travel at 100 IPM which is pretty good rating for a hobby CNC.

500 Revolutions per minute
--------------------------------- = 100 Inches per Minute (100 IPM)
5 Revolutions per inch

This would still leave you a lot of cutting force so that if you were cutting a very hard material, you would be less likely to bind and have your steppers miss steps.

But there is bad news at 500 RPM’s a diameter of 3/8 inches would want to “wobble” over spans larger than 36”. No matter how good we are nothing is perfectly lined up.

So if your machine is small, a 3/8 diameter would work, but if the span of any axes spans more than 36” (48” MAX) I would strongly recommend that you go with a ½ inch diameter lead screw. Not because of the force, but rather because of the risk of the lead screw whipping and wobbling at higher RPM’s.

My suggestion would be use a ½-10 2 start lead screw and avoid this conflict all together.

You can buy anti-backlash nuts for this at dumpster CNC and its only a couple bucks more than a 3/8-10 2 start nut, they even offer a ½-10 5 start (really fast).

However, its hard for me to say without knowing the size of your machine, and if you have 2 lead screws on the x axis and it does whip and wobble, it will be twice as bad.

You can find all this information and more at my website http://www.cncroutersource.com/ and it’s all free. Check out the builders guide section http://www.cncroutersource.com/homemade-cnc-router.html Book mark it and be patient as we upload the pages. The website is only a few days old.

You can send me your design specs and I can give more detailed advice. Just send any questions or anything to Brian@cncroutersource.com

P.S

To find how much linear force you will get from your lead screw and motor assembly, use this formula.

1. Convert ball screw/lead screw turns per inch to pulley radius:
Example:
½-10 2 start = 5 turns per inch convert this 1 inch/5 turns per inch = 1/5= .2 = pitch

So pitch = .200

Then divide pitch by 2pi or 6.28
Example:
0.200 / (2pi) = 0.200 / 6.283 = 0.0318" (equliv. radius pully)

2. Convert motor torque into linear force: divide torque of your motor by the puller radius
example:
Torque = 100 Oz.-In.
100oz-in / .0318 in = 3144 oz. (linear force)
3144 / 16 = 196 lb (linear force)

3. Multiply result by the efficiency:
ball screws have efficiency of 0.8
Acme Lead screws have efficiency of about .6 (for a Acetyl nut on a steel lead screw)

196 * 0.6 (efficiency for ball screw) = 117.6 lb linear force.