[.xXACEXx.]

What is one to do if they don't know what speed they want to cut at...say for instance they are cutting wood today..and for several weeks then some guy comes over and says "hey can you trim my cast iron exhaust manifold it only needs to be resurfaced?" first you need slower rpm ,then you need slower feed rates,

while i am a machinist and have been for years its the whole "homebrew cnc" thing thats has me sorta stumped am i correct that assuming a router that can run 100 inches a minute,cant run smooth at 2 inches a minute?

sorry if this is off topic too much...

[ger21]

You design for the fastest speeds you plan on using. You can always slow down. Unless you have a very low resolution setup, movement while going slowly should still be smooth. Even with 2 turns per inch screws, if you use Geckos with 10 microsteps, you still have 4000 steps per inch. To move 2 inches per minute, the PC will send 8000 steps in 60 seconds, or 133 steps per second. So if the motor is moving 133 steps every second, it's going to be pretty smooth.

[.xXACEXx.]

this sorta leads into another question,as to using a low threads per inch leadscrews.i have been a "lurker" for a very long time,and have noticed a push for screws in the range of 10 threads per inch with 5 starts..which would be .500 inch per rev.


if i can design a machine to have enough power to snap a 1/4 inch carbide e.m. while cutting,but still have a rapid of 300 inches a minute why would i not want to do so??...surely a homebuilt machine would never be able to use the "extra" resolution of a say "1/4 x 20" lead screw? . and the slower pitch of a "1/4 x 20" screw would make the rapid slow (unless you have some really great motor,driver,power supply combo going on ,right?)

so the question really has to do with speed ,what are some possable reasons to use ..say "1/4 x 20" leadscrew VS. a "1/2 x 10 x 5 starts"?? at.050 per revolution and 200 steps per revolution and micro stepped at 10 microsteps thats .

..hmmm... .050 / 200 steps=.00025 / 10 (microsteps) = .000025 ...

thats better than my (not yet started) home built machine would be able to hold, not to mention the speed would suffer because of the high frequency of steps,so i understand that part (my math may be off a lil bit still a lilbit confused on the microstepping) so i understand why any one would use the higher lead screws but are there any other reasons i hadn't thought of??


motors and drivers have every thing to do with it...as i am looking at a certian "tiny" drive and some day will be needing motors,power supply.ect.ect.


this is just a general question ..as i have not started designing yet ,thanks again!!

[ger21]

so the question really has to do with speed ,what are some possible reasons to use ..say "1/4 x 20" leadscrew VS. a "1/2 x 10 x 5 starts"??

Because you have a budget of about $5. But even that's not a good reason, because you can get cheap 1/2-10 acme for close to that.

Other than for budget reasons, the only reason to use lower lead screws is to increase resolution. Many will tell you that you can't count on microsteps for resolution, although some will say you can. A compromise is to use 1/2 steps when figuring resolution. So 1/2-10 5 start with 1/2 stepping gives you .00125. (your math was wrong, 1/2-10 5 start is .5" per rev, not .05. It's 2 turns per inch) So if .001 is good enough for you (and you can build a machine accurate to .001), then that's the way to go. But keep in mind that you'll need more power to spin those screws than for a single start screw, so you'll probably spend more on motors and drives.

Which leads you to the fact that speed costs money, and everything is a compromise.

[.xXACEXx.]

i was giving the example of the "1/4 x 20" lead screw which is .250 inch by .050 per rev..not the example for the "1/2 x 10 x5 start" but my math is prolly still wrong as i dont quite get the microstep .10 microsteps is one full step broken down to 10 steps right?? so a 200 step motor would actually have 2000 steps ??

is that math wrong for the .050 inch per rev? at 200 steps per rev. with 10 micro step? i actually thought i understood it better but maybe not...for instance lemme try again...


1/4 x 20 =.050 per rev,devided by 200 steps per rev..= .00025 (right so far?)

then ..micro stepped by 10...= .000025 per (micro)step <---(this is the part i dont think i have right)

then if you can run 25000 steps per second in mach3 thats...=.625 inches per second

which is 37.5 inches per minute ??

at that speed i can see why anyone would want more speed if cutting anything other than steel..

again im prolly wrong ,and i am cheap so i was wanting to use .500 x 10 threads per inch ,but can see where a little figuring would come in handy to get where ya want to go..as far as top speed ...and you say that even if you can rapid at 100 inches per minute you can still go slow while cutting ,thats a good thing..i thought the motors might be sorta "steppy" while running at a slow speed...like ..

step....step...step....step...

[rfrenzl]

Its all quite simple.

TPI x steps per revolution of the motor x micro-step = steps/inch

In your case:

10 x 200 x 10 = 20000 steps per inch

IPM = (number of steps per seconds sent x 60s/min) / steps per inch
= 25000 x 60 / 20000 = 75 ipm

With a 20 tpi lead screw, just divide this by 2 which is your 37.5 ipm.

I don't know of any material you can cut at this speed other than plastic. The spindle speed is such that you will never get a good finish with much over 8 ipm of tool speed on most small machines. Just look at any surface speed chart or try cutting something (anything) at 37 ipm. You'll see what I mean. If you are doing rapids of only 10"-15", this is still respectable.

As stated earlier, more TPI means higher resolution. Less TPI means more speed at the cost of needing more power. A 10 TPI screw is a good option, a 5 TPI is about as high as I'd go. There are a lot of ACME screws out there that will work great. I've used my share and it seems that those around 10 tpi have the best overall performance.