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-   -   need advice on leadscrew (http://www.routerforums.com/cnc-routing/42253-need-advice-leadscrew.html)

flyforever 07-09-2013 11:27 AM

need advice on leadscrew
 
I currently run a 14x25" cnc router with .1 or ten rev per inch leadscrews.
When I was cutting balsa, I had no trouble running the machine at 70ipm to cut precise parts with a high degree of repeteability.

Now I am cutting mostly 3/8 mdf parts with pocket and holes. I need precision, since these parts make up a large assembly that needs to fit and move with a degree of precision.

My machine, which has 380oz and 425 steppers, can barely cut at 10ipm at depths of no more than .060 in order to maintain a high degree of precision without losing steps.

The .1 lead screw is the main cause for not being able to go faster without losing torque. The two most popular choices for lead screws are 2 and 5 starts. Once I commit to a size, I will need to take it to a machinist for stepping and threading the ends. The wrong decision can cost me money, a lot of money, since the lead screws are not plug and play.

I have very little experience cutting mdf on cnc routers, and the biggest bit I can use is 1/4". I try to use .187 mostly in order to nest as many parts as possible on one board. Also, the pocket at times are round, and this size bit makes it convenient.

I hope some of you can give some good advice on whether I should go 2 or 5 starts on the lead screws, the x and y axis. The z is not an issue: I am happy with the current leadscrew.


thanks
tony

geotek 07-09-2013 01:59 PM

Sounds like you must have other issues, maybe the stepper drivers. Unless your cutting bits are very dull, I don't think you would have so much tool pressure that you couldn't run 10ipm. Going to a coarser thread would increase the torque needed from the motor. This is a good idea when you are having to run a stepper at high speed, but you say you're having problems at only 100RPM. So going to a coarser thread would make the problem worse.

flyforever 07-09-2013 02:12 PM

That's what I thought, so I purchased a gecko 201x and a new stepper. It made no difference. A dull tool is a possibility, given that at such a low feed rate the tools get blue. It's a bit of chicken and egg syndrome right now.
tony

geotek 07-09-2013 09:48 PM

This may sound silly, but make sure your tool is spinning clockwise. I don't know what type of spindle you have, but it's very easy to run a 3ph spindle backwards. Once I had a bad conductor in the cable going to my spindle. The spindle would spin up, but half the time it was running in reverse. That's real bad for the tooling.

rwbaker 07-10-2013 06:24 AM

I'm with everyone else but for a different reason. Changing the lead screw on a CNC has a whole down-hill effect that ends up with you having to recalculate the entire project. Try everything else, including, is your current lead screw slipping, before making this drastic change. Have you calculated what degree of accuracy and repeatability is currently possible?

Good luck - Baker

bgriggs 07-11-2013 11:16 AM

If you have room for 1/2" acme leadscrews you can get a coupler that does not require the ends to be machined......

Before I go much further, why don't you post a picture of your machine and I will advise from there.

Bill

flyforever 07-15-2013 06:28 AM

Quote:

Originally Posted by bgriggs (Post 342925)
If you have room for 1/2" acme leadscrews you can get a coupler that does not require the ends to be machined......

Before I go much further, why don't you post a picture of your machine and I will advise from there.

Bill

As I write this, I have already given the two new lead screws to a machinist,who will cut the end steps for me. I decided against the couplers, given that they would have reduced the working area.

From my reading on many forums, I noticed that a .1 lead screw is used for very high precision works such as pcb or fine metal working. But with this resolution, a stepper motor simply runs out of torque after 500 rpm. This lead screw at 500 rpm equals 50 ipm.

I am hoping that a .2 will be a good compromise. Yes, it's costing me some money, but mdf is a tough material, and more torque was needed---all things being equal.


Ironically, the new backlash nuts have a different size bolt circle, so now I am making some adapters to mount the new nuts to the existing holes.

I am also putting a hitachi router, which replaces the bosch colt. The hitachi is stronger and provides much lower rpm than the colt.

Thanks for the advice, though. It keeps me going.

tony

geotek 07-15-2013 08:55 AM

Like I said before, I think that there must be other problems in your machine. Going from 0.1" lead to a 0.2" will double the speed, but it will also require twice as much torque. I have a 24x48 CNC router of my own design. It uses 5mm pitch ball screws all around. The 5mm pitch limits my speeds to about 150ipm, but that's fast enough for a small machine. The 425ozin motors can produce over 400lbs of tool pressure. You may want to consider ball screws if the latest change does not fix your problem. Anti-backlash nuts can produce a lot of friction.

kolias 07-26-2013 12:03 PM

3 Attachment(s)
On my 4 axis cnc, I used 10mm Lead ball screws, 276oz-in motors, 90VDC Sherline motor attached to Taig-200 spindle and cut 3/8" thick aluminum with problem as per attached pictures.

I donít think your screws / motor is the problem

flyforever 08-01-2013 06:55 PM

Here's my status so far:
New leadscrews: from .1 to .2, essentially halving the motor rpm for same distance.
This has resulted in higher torque at higher speeds.
New router: removed the Colt, and replace it with a Hitachi mv12.
This has resulted in lower spindle speed and more stability.

First cut on MDF
My speeds have doubled from barely 20 ipm to 50ipm. The tool cuts quietly and it doesn't get too hot.

I am happy so far.
To those of you who felt that the leadscrews were not the issue, I suggest that you read a bit about step motors loss of torque at appx 1000 rpm. With a .1 lead screw, this is 100 ipm. With a .2 lead screw, this is at 200ipm.

All things being equal, I was able to increase the speed to 50 ipm( and perhaps higher) while retaining enough torque and not miss steps.

Finally, there's one more issue that simply wont go away. All my pockets fall short .015 thousands on the x axis( the short one), and I know I don't have that much slop in the machine. It's a puzzle....

Thanks for all the input. It gave me a lot to think about.....
tony


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