ACME Nuts and Lead Screws: Design Considerations
This section covers other important considerations regarding ACME nuts and lead screws. It is important to consider implementation of your design and not just the design itself.Matching Leads
It is important to match the lead of the lead screws and nuts. Single start ACME nuts and screws adhere to ANSI standards. This means that if you are using a single start assembly, the nut and lead screw should match nicely even if bought from different manufactories.
Multiple start lead screws do not adhere to any standards. It is good practice to purchase the nut and the lead screw from the same manufacturer when using a multiple start screws. This will ensure a nice fit. Many times you can mismatch the lead screw and nut from different sources and they may work. Usually as the number of starts increase, the less likely the nut and lead screw will match if purchased from different locations.End Machining
End machining is the practice of turning down the ends of the lead screw to a nominal diameter that is less than the minor diameter of a lead screw. End machining allows the use of bearings to be used to support the lead screw. Most manufacturers and sellers of lead screws offer end machining services, but expect to pay a pretty penny. You can also find lead screws in standard lengths that have already been machined.
If you are building a small router and can not afford end machining, another solution is to use cheap sleeve bearings and shaft colors on each side of the sleeve bearing. This solution is cheap and it works but is not recommended for fast or large machines.
If you plan on machining the screws yourself, it is important to support the lead screw properly while turning the ends to avoid bending the lead screw. Also try and protect the threads.
More on bearing supports and end machining may be found in the end supports section.
It is important to keep your lead screw assembly operating to the most of its abilities. If at all possible, it is best to try and keep the lead screw dust free. Although lead screws can operate in harsh condition, that doesnt mean they should. Lead screws that are caked with dust and contaminants are less efficient and will wear much quicker. One way to protect the lead screw is by using bellows or boots, but they expensive. Most builders invent their own way of keeping the lead screws out of harms way. For static anti-back lash ACME nuts, check the tension or compression load and readjust if necessary when possible.Lubrication
Lubricating the lead screw will increase the life and the efficiency. However, sometimes it is possible to over lubricate the heavy grease which allows contaminants to then stick to the lead screw. For plastic ACME nuts and steel screws, it is recommended to use a dry spray lubricant, very light layer of lithium grease, or a small amount of oil. For bronze on steel, heavier grease may be used.
When the contaminants and grease buildup become excessive, it is recommended to remove the sludge and reapply new lubricant.Nut Mounting Types: Flanges, One Piece
A nut is useless if you cant properly mount it. There are two types of nuts, one piece and two pieces. The one piece nuts are made of one piece of material. They have a hole pattern on one end to properly mount the nut.
Two piece nuts have threads on the inside, for the lead screw, and threads on the outside for a mounting flange to attach. Many times, users buy the nut that is often shown with a flange and fail to order the flange. Make sure that you choose the right flange size to fit the nut that you choose. Many times the flange cost as much or more than the nut itself so keep that in mind. Often it is the bronze nuts that require separate flanges while the plastic nuts are one piece.
The Builder's Guide
Check out the Builder's Guide. The Builder's guide walks you through the important decisions and design considerations when building your own CNC router.
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