Mini-Lathe Repair

Oops, sorry the 11 rpm figure is running in reverse, in forward it is 32 rpm and seems to have some torque but how much I dont know, I could do a test to get some idea. Jason, thats a good idea, it had crossed my mind whether to reduce the motor pulley or increase the layshaft pulley or maybe both. There is plenty of top end speed so reducing that is not a worry but gaining some advantage at the bottom end speeds should be worth having.

Ron

Sounds like your mini lathe was made by Real Bull, with a 4mm keyway, rather than the Seig machines with 3mm. So you have very little material to file away, only 0.5mm all round.the keyway. Obviously more, if you have to open up the bore. In which case it might be worth making up a 4mm tool with which to broach the wider keyway.

Howard

Is anyone with experience of the multi vee belts and pulleys able to confirm my thinking above. It seems obvious to me that the fit of the belt to the pulley should be as above but it would be nice to have it confirmed or otherwise.

Edited By Ron Laden on 13/12/2018 19:20:27

Hi Ron, if you google "Gates poly v pulley dimensions" or "Gates poly v pulley profile" or the like, you may find the design guide Gates originally published for their poly V belts and pulleys. There used to be full data in it about pitch, angles, diameters etc to enable cutting exact fit pulleys. Sorry I don't have the paper version any more or I would scan and post it. The Gates Rubber Co may help if you enquire. Forgot to mention I designed all sorts of drives with this system and also Gates PGGT synchronous toothed belts when I worked for a large US based photocopier company years ago.

Edited By Jeff Dayman on 13/12/2018 19:39:08

I think you want the Vee of the valley to go to full depth but have flat tops to the crests, my thinking is the belt won't go right down into the vee so will not bottom out and as the tops of the vee are blunt they won't go all the way into the belt. So you will only have flank to flank contact.

Probably been in MEW at some time maybe someone who takes it could direct you to an issue number.

The drive on any V belt should be on the sides, not on the bottoms, or tops, of each element.

Ron, look at MEW 246, it is as I said.

Thanks guys

Jeff got some good detail from the Gates site, thanks for the that.

Jason thanks for the advice and pointer to 246, just what I needed.

Ron

Having had a measure up for the new poly vee belt pulleys I need to make, it is not possible to change the ratio (apart from a small amount)  to gain a bit more mechanical advantage for the bottom end. The layshaft pulley is 45mm and it could be increased to 48mm but no more as it would clash with the spindle spacer, I would have liked to increase it to say 55mm but there isnt the space. Also the recommended minimum pulley diameter for the belt is 20mm and the motor pulley is close to that now. So I will have to stick with the current ratio in fitting the new belt drive.

However if one was to get a bit more adventurous it would be possible to change the layshaft pulley to a gear and fit an intermediate stepped combination gear/pulley between the motor and layshaft. It would mean making a bracket and spindle for the intermediate which could be fitted to the rear face of the head, there isnt the space to fit it within the current assembly. I can see that working quite well and it could offer ratio options but I dont think I will go there just at the moment.

Edited By Ron Laden on 15/12/2018 09:20:36

Picture below of my old thread angle gauge, can you get similar with angles not thread related, one with 40 degrees would be ideal. I found one with a single 40 degrees but it is from the States and carriage is £17.

I have to make a lathe tool for the poly vee pulleys and would like to get the angle as good as possible. If nothing is available then I will have to make something but I dont have much in the way of tooling re measuring/setting angles, not accurately anyway.

For a one off why not drawn it out on a bit of stiff paper or light card then use a stanley knife to cut out the vee and make a little rectangular tool.

Simple if you put a line at right angles to your papers edge 100mm long, then mark 36.4 or 36.5 would do each side and make the triangle.

Typical me, complicating things, thanks Jason I have some decent graph paper which I can paste to some thin card. I think I was getting a bit hung up on how good the angle needs to be, using the above it should be pretty close anyway for making the tool.

Edited By Ron Laden on 17/12/2018 14:12:21

Ron Laden:

Axminster do various angle gauges including one by Veritas at £7.20.

Another idea would be to turn a 40 deg groove in a piece of stock using acute angled left and right hand tools and setting the topslide to 20 deg either way; that method would allow you to make gauges to a variety of angles.

Ron, you can apparently buy insert tooling for cutting these pulley profiles. Never tried one so don't know how easy to find or use. Found a link to an ISCAR Insert page.

John

Ron,

Try drawing the angle out with the side opposite the angle at 40 mm along a base of 110 mm.

The angle subtended is then 19.98 degrees, and the geometry is so much easier to do.

Regards

Brian

Thanks Ega/John and Brian

I have just made a one off gauge and ground a 55 degree tool back to 40 degrees, so will see how it goes.

John, thats interesting finding insert tooling, I guess it stands to reason they would be available.

Ron

Edited By Ron Laden on 17/12/2018 16:49:59

Ron,

I would think your drawn and cut gauge will be fine. I was castigated recently in a thread on boiler bushes where the OP was asking if BSB (55 degree) or BSC (60 degree) threads would be used in a commercial boiler bush. I replied saying if you make your male fittings 55 degree they will fit either. Apparently this was viewed as dangerous by one authority. I did calculate the difference in width of the thread form and it amounted to only a few tenths. In your case with the belt material being compliant a degree or two either side is not likely to be the end of the world. There is the perfect world and the real world! Nothing wrong in striving for perfection where it's needed but little point in agonising over it where it isn't!

Paul.

ON a similar lathe I broke the plastic gears by being hamfisted (an overbrutal attempt to face a piece of large dia scrap sourced from a scrapyard).

I replaced mine with metal and added some grease nozzles just above. I can't say I mind the noise and, as I did the breaking early on in my journey, I can't say that I remember it being particularly quiet before.

The other thing that happened as part of that repair was that I blew the electronics loose wire and ended up replacing the power devices with higher current ones.

when my lathe jams up (parting off mainly) nothing blows, though I do try and hit the big red button as soon as possible.

This is just my experience and not a recommendation. One thing I will say though is that I had to remove the spindle to change the gears and on my particular lathe it was pressed in SOLID. YOu need to take care in how you remove it and avoid excessive use of an engineers friend.

Iain

Thanks Paul,

You are quite right I was thinking the tool needs to be spot on, obviously good if it is but a tad over or under wont be detrimental.

I tried the tool I have made on some alu and it cuts really nicely I just have to wait for some 45mm 6082 round bar to arrive so I can produce the pulleys.

Ron

Had a practice at cutting the vee,s for the pulleys.

40 degree tool, 0.092" vee centres, 0.082" deep, skimmed the o/d to put a small flat on the crests. With a magnifier and back light the belt ribs looks to have good contact with the flanks, though you cant see that in the pic as the belt is inside out so curving away from the bar. I used a blunt knife blade wrapped with a used piece of 1200 paper to polish the flanks. However if you look at the vee,s at the top of the pic you can see that the tool needs adjusting, the 40 degrees angle on the tool is good but its off centre.

Could be a better picture.