My machine is cutting parts to small

[clocksandblocks]

Hey every one, this is probably a redundant thread and I'm sorry but I tried to search for answers to my problem with no avail. I just finished building my first CNC router using Ebay (I know I know!) ballscrews. Its just for playing around in the garage so I figured these would be fine.

Well after getting everything set up and the machine cutting out parts I noticed they are all coming out undersized. When I tell it to cut out a 2" circle, the circle actually comes out to be 1.915 in diameter. The ballscrews I bought are metric (16mm in diameter) From Ebay seller "Linearmotionbearings2008" Who sells TONS of these ballscrews there. I was skeptical when I was told they have a thread pitch of 5 turns per inch because if they are metric in diameter I would assume the pitch is metric as well. after cutting a few parts out I can say for sure they are NOT 5tpi...

Is there a way to calculate the tpi on these ballscrews now that the machine is up and running? Or does any one know the real TPI on these ballscrews? Thanks for any help! I have attached a picture of my homemade CNC router I built that is cutting the undersized parts.

-Jerome

[tulsaturbo]

You need to set your motors SPI (steps per inch). If you are using Mach3, its a simple procedure. Below is a video that should help:

YouTube- Mach 3 Steps Calibration

YouTube- Mach 3 Steps Calibration

[mactec54]

clocksandblocks
He probably ment 5mm, 5 turns per inch would be .200 per turn
5mm pitch would be .19685029 per turn enough to make your parts cut small But you can set it up as tulsaturbo has already said in the above post

[clocksandblocks]

Thanks for the info guys. I use EMC2, is there an easy way to do this calibration in that software as well?

-Jerome

[Andre' B]

Lay a ruler up to the ball screw and see how well the 5mm marks line up with the thread.

If it looks good use mactec54's number of 0.19685029 inches per turn.

Divide your steps per turn by the inches per turn and you get steps per inch.

Assuming you are using 10x micro stepping drivers.
200 step per turn motors x 10 micro steps = 2000 steps per turn

2000 / 0.19685029 = 10160.005 steps per inch.

[mactec54]

clocksandblocks

How did you do it to get the 5 turns per inch that you have now, if that is what you have entered, then just change it to 5.08000000002 & that will make it for 5mm pitch
You most likely don't need all the zero's on the end

[clocksandblocks]

I asked the Ebay seller I bought the ballscrews from what the pitch was and he said 5tpi. He isn't returning my emails now so I will try changing the numbers to the metric pitch, that sounds like it should do the trick.

[kawazuki]

Originally Posted by clocksandblocks I asked the Ebay seller I bought the ballscrews from what the pitch was and he said 5tpi. He isn't returning my emails now so I will try changing the numbers to the metric pitch, that sounds like it should do the trick.

If he's saying its a 5tpi screw then it possibly measures 15.875mm dia or 5/8" imperial which would tie up? Even if it is a metric 5mm pitch the conversion doesn't tie in with your stated undercut? So maybe there is some lash in the set-up that also needs addressing?

Mactec54 - then just change it to 5.08000000002 & that will make it for 5mm pitch. You most likely don't need all the zero's on the end

How did you arrive at the 0002 at the end? 25.4mm / 5 = 5.08!

[mactec54]

kawazuki

5mm is .1968503937 divide that x 1.00 will give you 5.08000000002 if you read the post you would of seen that I said he did not need all the zero's on the end so he would of put in 5.08, His screw is 16mm not 5/8 dia

He had it set up for .200 this would make it cut undersize when it should of been for .196850

[mactec54]

clocksandblocks

Is the part number for your ballscrew RM1605 or RM1610 this tells you the dia & the pitch

[kawazuki]

Originally Posted by mactec54 kawazuki 5mm is .1968503937 divide that x 1.00 will give you 5.08000000002 if you read the post you would of seen that I said he did not need all the zero's on the end so he would of put in 5.08, His screw is 16mm not 5/8 dia He had it set up for .200 this would make it cut undersize when it should of been for .196850

Just because someone says its 16mm doesn't mean it is 16mm - not when the seller says its 5tpi. Could you visually tell the difference between 16mm and 15.875mm?

Anyway why set the machine up with metric screws and talk in inches? Set it as metric - each unit is 1mm, the screw moves the object 5mm for every rotation? For every 1mm the screw rotates 0.2 of a turn or 10 steps. Use a
G20 or G21 in code if you prefer working in imperial or native units, the converse obviously applies if the screw actually is imperial!

Anyway - all the above rambling doesn't add up to 1.915" as opposed to 2"

[mactec54]

kawazuki

You only need to go to Ebay & look at what the screws are it tells you they are metric, I think clocksandblocks just has some setting up to do to get it right