Hybrid View
-
15-09-2009 #1
I've been practicing machining the ends of my leadscrews using a sacrificial bit of 12mm threaded rod. I am having problems with surface finish and a small amount of taper.
I don't have a steady so have been taking very fine cuts using a brazed tip tool (like the 12mm one in the photo but a 16mm to fit the lathe).
I thought that any movement of the workpiece away from the tool, due to lack of a steady or movement in the spindle bearings, would result in an oversize at the end, and on/under-size nearer the chuck. In fact the reverse is true, the taper being .05 - .07mm in about 30mm.
I've checked the topslide movement relative to a test bar and the topslide runs parallel to the bar within .02mm over a 100mm length. Also the run-out of a test bar in my newly ground chuck jaws is less than 0.02mm over the same length.
If the workpiece was canted in the chuck am I right in my belief that that should not result in a taper, although the machined end would be off centre relative to the rest of the workpiece?
Should I be worried about this level of error, given the age of the machine (90-ish years)?
I am aware there is some movement in the slides, with end float and backlash, and I am assuming this is the main culprit for the somewhat rough surface finish, however slowly I take the feed, although the spindle speed is only 160rpm (I think, I've not measured it accurately yet).
This is clearly too low for such a small diameter workpiece... I need to change the drive pulley arrangement to give me a higher top speed (700rpm?) so I can bring it down if need be with the VFD (and backgear for threading)
-
15-09-2009 #2
Can you support the end with a centre from the tail-stock?
Or, knock-up a tool-post mounted steady; just something to stop the work piece lifting (which can cause the tool to cut deeper as the work rolls towards the tool) and pushing away from the tool.
[edit] have a look at these: http://www.practicalmachinist.com/vb...ghlight=steady
I've checked the topslide movement relative to a test bar and the topslide runs parallel to the bar within .02mm over a 100mm length. Also the run-out of a test bar in my newly ground chuck jaws is less than 0.02mm over the same length.Last edited by BillTodd; 15-09-2009 at 01:35 PM.
-
15-09-2009 #3
I didnt think I could use the tailstock as the part I am cutting is only 50mm long and the saddle is quite large and wont let the tailstock near enough really even with the barrel extended. tbh I've not tried it. Also not sure how centred the tailpost is, I've not tried to adjust it yet.
I'll have a look at making a steady, I have some 20mm aluminium plate that might do the job.
-
15-09-2009 #4
Might be worth experimenting with faster speeds for better finish... 160 sounds slow for 12mm stock (what material?).
EDIT re speeds I should read your post fully before opening my gob!
Incidentally if you are machining a fairly long ballscrew or whatever you might want to make sure the back end of it is supported to stop it flailing around - a collar type arrangement to plug into the back of the spindle hole would do it.Last edited by bikepete; 15-09-2009 at 04:28 PM. Reason: missed reading the bit re speed!
-
Face the end, that will tell you if you have the tool height spot on.
Only possible culprit I can add to the above is angle the tool edge 5 past 12 on the clock. That way it pushes against the work rather than pulling on it. If it shies away from the tool you can fetch it square on the next pass. If angled 5 to 12 you expect a crummy finish and awkward taper.
-
15-09-2009 #6
Hmmm, checked tool height by expedient of trapping a thin steel rule between work and tool - it remains essentially vertical.
Not sure what you mean by "angle the tool edge 5 past 12 on the clock", or rather I understand the words, not the intent... When the tool is square in the toolholder the leading edge is already angled to the work...
BTW, here is a pic of the test piece (its not intended to be a finished item), was just practicing turning to a diameter 12mm->10mm-> 8mm and using a die to thread the end. The 8mm ID thrust bearings and ballraces are a nice fit and a nut screws on nicely.
-
Yes, but carbide will cut with 90 degrees at the tip so you could be above centre and pass that test. Facing the end so it doesn't leave a pip, or snap a pip off, is more of a test. I'd say the tool is either too high or it's lost it's tip. Something rubbing is the usual way to that blistered appearance. :nope:
If the tool leading left hand edge is set perpendicular to the axis of rotation (12 o'clock) you could do well to rotate it clockwise a tadge (12:05). Any rotation counterclockwise (11:55) can be seriously bad news because it pulls on the work rather than pushing against it.
If I got a cut like that I'd start by sharpening the tool
-
15-09-2009 #8
-
16-12-2012 #9
Tickling the swarfe off a bar usually ends up with some weird results, including tapers and poor finishes. Most people seem to be scared to use the correct speeds and feeds, particularly when using carbide tips because you have to give it some welly and take deep cuts, often 2 or 3 full turns of the handle at a time!
Thread Information
Users Browsing this Thread
There are currently 2 users browsing this thread. (0 members and 2 guests)
Similar Threads
-
taper roller bearings vs deep groove ball bearings
By dsc in forum Linear & Rotary MotionReplies: 8Last Post: 10-03-2014, 08:06 AM -
RFQ: Parts need turning
By ams in forum Projects, Jobs & RequestsReplies: 17Last Post: 15-01-2014, 12:27 AM -
Morse taper drills
By John S in forum Marketplace DiscussionReplies: 7Last Post: 08-05-2013, 07:00 PM -
larger home made milling spindles - NMTB/ R8 taper?
By jonnydeen in forum Tool & Tooling TechnologyReplies: 6Last Post: 13-09-2012, 11:17 AM -
FOR SALE: Clarkson Autolock Collet chuck set, Boxed INT 30 / NMTB30 taper
By swarf-boy in forum Items For SaleReplies: 0Last Post: 04-01-2011, 05:28 PM
Bookmarks