•  
 
2019 in the workshop
 

Cutting a metric thread

11 May, 1919 - Cutting a metric thread &nbsp;I have chosen to cut an M8 X 1.25 thread where M8 is the diameter and 1.25mm is the distance between two threads. The AL320-G gear chart tells me that the correct gears for a distance of 1.25mm in one revolution (given the fixed 3mm pitch of the lead screw) is A=30 B=32 C=40 D=36.&nbsp; If you divide the initial drive gears (24 and 60) it's 0.4. This is the gearing of the 3-jaw chuck to the motor speed. Then (a/b X c/d) X .4 and this is the ratio of the lead screw to the chuck (the ratio of the saddle horizontal movement to the work piece rotation). That is, with these gears the saddle will move exactly 1.25mm for each revolution of the work piece. Later: Now here's the tricky bit, If the saddle moves 1.25mm and the lead screw has a pitch of 3mm, 1.25 does not divide evenly into 3 and hence the "thread chasing" dial is not much use (because the lead screw advances many uncounted revolutions when you take the saddle back to make another cut.) Clear as mud? <div style="Font-size:5.0pt" 31 May, 1919 - Cutting a metric thread &nbsp;I finally understand this thread chart. &nbsp;The two columns on the left show the gear set required such that the ratio between the saddle and the 3-jaw chuck is the exact distance required every one revolution of the chuck. This relationship only exists when the Half-nut Lever is engaged. The lead screw does not necessarily complete a full revolution (3mm). For example 1.25mm/3.0=.4166 of a revolution. The rightmost columns have nothing to do with threading Metric or US threads. This is used when the half-nut is disengaged and the Longitudinal/Crossfeed lever is engaged. It is useful for setting the saddle travel speed (0.100 mm) and the cross-slide feed rate (0.022 mm). <div style="Font-size:5.0pt" 2024-02-18 Cutting a metric thread &nbsp; I actually calculated the pitch of the lead screw and then, in 2024, I measured it. Lo and behold, 3mm. <div style="Font-size:5.0pt" 11 May, 1919 - Cutting a metric thread &nbsp;From the chart at Tap size Calculator I select the M8 X 1.25 thread. It tells me that the tap drill size is 6.8mm. I need to cut a radius at the shoulder this deep before threading. <div style="Font-size:5.0pt"
20 May, 1919 - Cutting a metric thread &nbsp;This lever engages the half nuts and is used for cutting Metric and US threads. The lever is engaged at the moment. In this position, the relationship between the chuck and lead screw is fixed by the gear set. The lever on top is not normally used for cutting threads. It engages the lead screw to drive either longitudinal or crossfeed cutting. The two levers are interlocked. <div style="Font-size:5.0pt" 20 May, 1919 - Cutting a metric thread &nbsp;In this position the half nuts are not engaged as the handle is up. The Longitudinal/Crossfeed lever is in the neutral position in this picture. The lead screw is turning in either the forward or reverse direction (or not at all) depending on the positioning of the Forward/Reverse Feed handle (way over to the left out of the picture). <div style="Font-size:5.0pt" 31 May, 1919 - Cutting a metric thread &nbsp;This is the Forward/Reverse Feed Handle. The lead screw is engaged by this lever and there are three positions: ・Forward ・Reverse ・Neutral <div style="Font-size:5.0pt" 31 May, 1919 - Cutting a metric thread &nbsp;Herein lies the greatest mystery of all: the usefulness of the thread chasing dial. The dial is permanently engaged with the lead screw, and: ・ when the half-nuts are engaged, the lead screw moves the saddle and the dial as one unit so the dial does not move ・ at all other times, if the lead screw is rotating, the dial moves with it ・ If the lead screw is off, and if the saddle is manually moved using the Longitudinal Feeding Handwheel, the dial moves. The lead screw has a 3mm pitch, i.e, one rotation of the lead screw moves the saddle 3mm. The dial indicates 4 rotations of the lead screw for each mark on the dial. The half-nut lever is designed to easily engage the lead screw at each marked division. <div style="Font-size:5.0pt"
31 May, 1919 - Cutting a metric thread &nbspThis means that, if the half nuts are disengaged at the end of a threading pass, and the saddle is manually wound back to start the next pass, it is highly unlikely that the half-nuts can be re-engaged in exactly the same spot. The reason is that say a 1.25 Metric thread thread is being cut, and there are 4 rotations of the lead screw/division (4 X 3mm=12mm), then 12/1.25 = 9.6 and this is not a multiple where the half nuts can be re-engaged. In fact, it takes another 5 marks before the half-nuts can be re-engaged (5X12=60) and 60 is a multiple of integer 3.0mm. And then another 5 marks and so on. The blue tape was to measure the saddle movement relative to the lead screw and the threading dial so that I could figure this stuff out. <div style="Font-size:5.0pt" 31 May, 1919 - Cutting a metric thread &nbsp;Another tip I learned this time was to set the Toolpost Slide to almost 30° and to adjust the depth of cut by moving the Toolpost Slide in each pass. A US or Metric thread is 60°. Setting the slide to 1/2 means that one side of the tool is doing most of the cutting. The cross slide remains set at 0 position on the dial. Cutting a thread this way results in a cleaner cut and a cleaner thread (I'm told). <div style="Font-size:5.0pt" 31 May, 1919 - Cutting a metric thread &nbspThe AL320-G is a metric lathe with a metric lead screw and the thread chaser is almost completely useless for US (Inch) threads. The best way to use the thread-chasing dial is either: ・ not at all, or ・ as described in the chapter about cutting the US 28TPI thread. This picture shows what happens when you get the lead screw out of synch. From what I gather (Youtube and forums), most people leave the half nuts engaged and reverse the motor to return the cutting tool back to the start. <div style="Font-size:5.0pt" 03 June, 1919 - Cutting a metric thread &nbsp; I'm now ready to cut the thread. I cut the outer diameter to 8mm minus .04mm (.016") and fit my recently purchased 2mm round tip tool to cut a relief at the thread end. <div style="Font-size:5.0pt"
03 June, 1919 - Cutting a metric thread &nbsp; With the round tip tool touching the work, I set the cross slide to zero. <div style="Font-size:5.0pt" 03 June, 1919 - Cutting a metric thread &nbsp; The tap drill size for an M8 X1.2 thread is 6.8mm. I deduct this from from the O.D. to figure out how far in I should take the round tip tool to cut the relief at the thread end. I need to go in 1.16/2 = 0.58mm and will use the cross slide to do this. <div style="Font-size:5.0pt" 03 June, 1919 - Cutting a metric thread &nbsp; I take the cutting tool in exactly .58mm and then reset it to zero again. <div style="Font-size:5.0pt" 03 June, 1919 - Cutting a metric thread &nbsp; I'm now ready to begin cutting the thread but first I need to make sure that the thread cutting tool is square to the work. <div style="Font-size:5.0pt"
03 June, 1919 - Cutting a metric thread &nbsp; I also need to check that the cutting tool aligns with the centre of the work. <div style="Font-size:5.0pt" 03 June, 1919 - Cutting a metric thread &nbsp; The finished thread. Summary: Setting the toolpost slide to almost 30° makes a cleaner cut. The problem is, by using this technique, I don't know when the the cut is deep enough. There are special thread depth micrometers available but, at over $100, they're not worth the effort. Otherwise, cutting a metric thread is quite easy. Just thought about this: why not simply use the thread pitch gauge to measure the thread depth? End of this project &nbsp; Click to go to home page <div style="Font-size:5.0pt"   
Cutting a US threadNext: Cutting a US thread 2020 New addition to the millPrevious: 2020 New addition to the mill
jAlbum image gallery software & TurtleSkin: Turtle Black, 5.6.7 Download   ·   Help