Acme thread tolerances

[DayneInfo]

I am looking at my next machine, probably going to be a Joe's. While looking at Acme rods I see that the tolerances of most are .009/ft. This is where it gets confusing to me. Is that .009 accumulative? In other words if I run 3 ft could I end up with .027 out? Or does it mean that any place I measure a foot along the rod it will be less that .009 out?

Dwayne

[widgitmaster]

I'm not sure, but that .009" may be the pitch diameter, as opposed to the lead of the thread (stretch)

Rolled threads come is all levels of quality, and the ones from Enco are the lowest grade in pitch, lead, and straightness! But what can you expect for $1.17/foot of 1/2-10 ACME.

If you want true quality, then get a premium thread from McMaster Carr for $8.88/foot of 1/2-10 ACME.


Eric

[DayneInfo]

I have been concerned about "getting what you pay for". I built my first machine with threaded rod leadscrews. I cut 1" parts and they come out around .007 undersixed. When I cut 6" parts they come out .010 oversized. I am assuming it is inconsistancy in the lead in the thread. I want bettter so I am going to attempt a better/faster/bigger machine. I got the fever...LOL

Dwayne

[DayneInfo]

I just looked at MSN and McMaster. Both say Lead accuracy .009/ft max

[ger21]

Actually, it's plus or minus .009 per foot. It is cumulative, but very unlikely that it would be worst case for 3 consecutive feet. It's probably more likely to get a perfect one. If your using Mach3, it has screw mapping which can compensate, if you have a way to accurately measure the screws deviation.

If you need more accuracy, Nook makes screws with + or - .003 per foot. You can get them at www.mscdirect.com

[DayneInfo]

I was just doing my simple math...Joes machine X Axis = 5ft. 5 x .009 = .045 worst case. I will have to think about it a bit. If some of the people building a Joes machine will test and post up it would be good. See what kind of results they are getting.

[yukonho]

My experience is that the large majority of accuracy issues can be blamed on machine flex or poorly adjusted linear slides. If your machine is rigid enough, then you can simply map the screws in Mach 3, if you find that they are far enough out to be an issue. Most MDF machines would be lucky to be rigid enough to give you +/- .030 precision regardless of what leadscrews you are using, just grab the tip of the bit and give it a tug, you will see.

Colin
Colin

[ViperTX]

I'm sure Joe's machine...since it's made of MDF....has more variation then any ACME screw you can find.

[DayneInfo]

I started wondering about that the other day. Just how accurate can a wooden machine be? I decided I do not need it to be perfect, just nearly perfect 6 inches at a time. I can handle .020 or more if cutting a part 2 foot square. Just do not like to see it in a 2in. part. I also understand about grabbing the bit and pulling, but in my world I hope I am not cutting so hard as to see it moving like that.

Dwayne

[Mr.Chips]

Originally Posted by DayneInfo I was just doing my simple math...Joes machine X Axis = 5ft. 5 x .009 = .045 worst case. I will have to think about it a bit. If some of the people building a Joes machine will test and post up it would be good. See what kind of results they are getting.

How about someone develope a "Standard" cutting routine that would test our machines and see how they compare?

Not interested in putting anyone down but just would like to measure apples without the oranges getting in the way if possible.

Something like cut a 12" square and see the diminsions and squareness,

Have the person testing state how big their cutting area is and perform some kind of test at 80% or so if the cutting area.

Perform some kind of repeated routine to see if you are loosing steps.

I think this would help a lot of people understand the accuracy of their machines.
Hager

[DayneInfo]

In my case I tested by cutting 1, 3, and 6 inch squares. My 1 inch comes out somwhat undersized and the 6 comes out over sized. I can fiddle with Mach3 and set the steps per inch to cut any one size perfect, and I mean perfect. If you test I think you must cut ultiple sizes and maybe even different locations on the machine.

Dwayne