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ThreadExpress Accuracy and Durability

This video ThreadExpress 2.2 - Part 3. Accuracy and Durability ThreadExpress 2.2 Part 2.    • ThreadExpress 2.2 - Further Revelation   ThreadExpress 2.2 Design revelation b...    • ThreadExpress 2.2 Design revelation b...   ThreadExpress 2.1 - A new design mach...    • ThreadExpress 2.1 - A new design mach...   ThreadExpress 2.0 Introduction: Thread Machining At...   • ThreadExpress 2 - Thread Machining At...   Memo re ThreadExpress pitch setting and accuracy. Mechanical projects like this are full of potential pitfalls and anything that is rushed could easily result in an unhappy build/builder who complains online, and ThreadExpress could easily be sunk like many other projects, on its maiden journey. Lack of thread cutting accuracy is a potential issue that could do just that. Although the ThreadExpress assembly is relatively simple for what it does, some details are important. For example, a spindle center line that is out of parallel to the slideway, or a spindle nut that has too much play, or sliding yoke pivots that have too much play or positional errors, or the ratio arm pitch set position on the transfer plate scale, could all cause ThreadExpress to cut inaccurate threads. If all these details are made and set correctly, ThreadExpress can cut comparatively accurate threads. Understandably some enthusiasts will want to carefully scrutinize the accuracy aspect of ThreadExpress. As part of the development of ThreadExpress I have done a bit of testing and study on this subject. Here are some points to consider. We assume threads cut by traditional methods are highly accurate. We need to consider this before we make comparisons, and we will find the bar is not set very high. Most threads to be cut are for use as fasteners and fasteners are typically not very accurate. A thread cut with a die will typically drift off pitch quite a lot, but this is seldom realized because nuts are generally short and loose fitting. For example, a M6 x 1mm pitch die cut into high tensile or stainless steel will typically run off pitch 0.05 to 0.15 mm per 10 mm or 0.005 to 0.015 mm per thread. This is fine for engagement in a nut, but not in a deep tapped hole. Threads cut in a lathe via single point threading are much more accurate but are still often full of errors. For example, my two lathes have inspection certificates allowing leadscrew errors of 0.035mm per 35mm and 0.02mm per 50mm respectively. But in actual reality errors are often much beyond this. For example, I will be publishing a video where I show one of my lathes has a leadscrew error of 0.14mm over 170mm. Many lathe leadscrews and half nuts are not cleaned before screwcutting and the half nuts are closed on to lead screws contaminated with swarf and dirt resulting in inaccurate seating on the usual acme thread and scoring wear and errors to the leadscrew. As mentioned ThreadExpress can cut comparatively accurate threads. This was tested in the video above: ThreadExpress Accuracy and Durability. Where it was shown to have the accuracy of an average lathe leadscrew. However, it cannot cut high precision threads suitable for precision indexing such as grinding micrometer threads. Re thread pitch/lead setting via the pitch/lead scale. I have found on my prototype that with fine graduation lines and an initial scale position calibration (setting the slotted scales into calibrated positions) line alignment is surprisingly accurate and good enough for the general machining of fasteners etc. However, when precision cutting of longer threads is desired, the actual setting can be checked and tweaked via first testing the pitch setting with a long travel indicator, digital caliper, or the mill Z DRO. I have found this actually only takes a few minutes to tap the adjustment up or down a few thou and check the long travel pitch before doing precision work. However, this is on my prototype and results may differ with different builds, time will tell. My prototype was quickly thrown together back in 2009 and has a fairly rough spindle thread with a fairly loose fitting nut. I plan to rebuild this portion and do more tests. It may be that the slight errors I was measuring in the video linked above will be less. Cliff