Thread Gearing on a BT800

This forum is dedicated to those hobbyists with the 3-in-1 metalworking machines. Mill-Drill-Lathes. Tips, techniques, modification and use of these machines is topical.

Moderator: Harold_V

TechTony
Posts: 54
Joined: Wed Aug 31, 2016 7:11 pm

Thread Gearing on a BT800

Post by TechTony » Mon Oct 31, 2016 8:23 pm

I guess I'm a glutton for punishment ... because I'm back. I've never geared a lathe for threading and I'm really regretting not getting a lathe with a quick change gear box. The documentation that came with this thing is total crap. I gave it the ol' college try tonight and as soon as I turned it on I knew something was wrong and didn't go any further. The lead screw spins way too fast for it to be correct. I'm stumped. I've included images of what I configured, and the charts included by the manufacturer.

I'm trying to thread 13 TPI

Documentation: http://www.boltontools.net/pdf/BT800.pdf

My interpretation of the documentation:
Speed: 2nd Gear (II)
Gear A: 72T
Gear B: 127T
Gear C: 120T
Gear D: 39T


Input welcomed. Thanks ahead of time.
20161031_193535.jpg
20161031_193843.jpg

TechTony
Posts: 54
Joined: Wed Aug 31, 2016 7:11 pm

Re: Thread Gearing on a BT800

Post by TechTony » Mon Oct 31, 2016 8:27 pm

For clarity about what I think I know...
gears.jpg

Torch
Posts: 1543
Joined: Wed Mar 17, 2010 7:58 am
Location: Muskoka

Re: Thread Gearing on a BT800

Post by Torch » Mon Oct 31, 2016 8:52 pm

B and C should be the same gear -- doesn't matter if its the 127 or the 120, but don't mix and match. 127/120 is used only to thread metric threads on an imperial lead screw.

A is 72, as shown and D is 39. Set the headstock lever to II.

Take a very very light cut -- just scratching the surface -- and compare the scratches to a thread pitch gauge to make sure everything is set correctly.

TechTony
Posts: 54
Joined: Wed Aug 31, 2016 7:11 pm

Re: Thread Gearing on a BT800

Post by TechTony » Mon Oct 31, 2016 9:46 pm

After looking at the diagram a little bit more I flipped gear D around so that it is in line with gear B (so it's Gear A -> Gear B -> Gear D) - which is what it sounds like you were trying to convey with your don't mix & match statement. Unfortunately, you have to mix and match because the 2 large gears (B & C) are manufactured as a single piece. Lead screw still spins way too fast for comfort but I'll give it a shot.

Harold_V
Posts: 17618
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Thread Gearing on a BT800

Post by Harold_V » Mon Oct 31, 2016 10:23 pm

I can't help but think that the gears A and D should be reversed.

Harold
Wise people talk because they have something to say. Fools talk because they have to say something.

Torch
Posts: 1543
Joined: Wed Mar 17, 2010 7:58 am
Location: Muskoka

Re: Thread Gearing on a BT800

Post by Torch » Tue Nov 01, 2016 4:20 am

Good, you have it correct now (provided you shifted the headstock lever to the II position), and Yes, I realize B and C are a compound gear, and do not separate. That's typical of this style gear train. But don't use both sets of teeth! If both A and D are riding on the same set of teeth -- B OR C -- then it's just an idler gear that has no effect on the ratios. However if you are cutting metric threads on a machine with an inch lead screw, then engage A with B and C with D to get the necessary 127:120 conversion ratio.

Your chart is consistent with other change-gear machines such as the Grizzly G9729, so I believe A and D are in the correct order. I think you are just surprised with the speed the lead screw is turning as compared to the speed when it's configured as a feed screw. But realize that now you are attempting to move the table 1/13th of an inch for every revolution of the chuck instead of a few thousandths of an inch per revolution. It has to spin like an airplane propeller in comparison!

TechTony
Posts: 54
Joined: Wed Aug 31, 2016 7:11 pm

Re: Thread Gearing on a BT800

Post by TechTony » Tue Nov 01, 2016 7:29 am

That's a pretty accurate assertion. Thanks Torch! Since you seem to have some familiarity with this type of machine, I don't suppose you can point me in the right direction as to why by indicator dial never spins? From what I've seen on youtube, I would think that it needs to engage with the lead screw somehow - mines a good 2" away from it and appears to be permanently mounted there. As far as I know it doesn't toggle in and out like some of the higher end lathes I've seen footage of. I've only had this machine for a few months now and am still getting acquainted. Thanks again!

Torch
Posts: 1543
Joined: Wed Mar 17, 2010 7:58 am
Location: Muskoka

Re: Thread Gearing on a BT800

Post by Torch » Tue Nov 01, 2016 8:31 am

A picture might help ;-)

The thread indicator may be mounted with a single bolt. Loosen the bolt and roll the thread indicator into the leadscrew to engage. Bear in mind that the thread indicator does NOT move when engaged and the lead screw is turning. IE: if you engage the half nut while the carriage is at the far right of the bed the thread indicator will stay at whatever number all the way to the chuck. However, with the half nut disengaged, the carriage completely stationary and the lead screw turning, the thread indicator dial WILL turn.

Personally, after going to great lengths to get my thread indicator to work with my lead screw cover in place, now I find I never use the darn thing anyway. It can't be used with metric threading (because of the 127:120 gearing) and can't be used with certain inch threads either. Rather than having one technique for one thread pitch and a different technique for another thread pitch, I just go with the lowest common denominator and leave the half-nut engaged until the thread is complete. But, I have variable speed reversible motors. That may not be an option for all.

TechTony
Posts: 54
Joined: Wed Aug 31, 2016 7:11 pm

Re: Thread Gearing on a BT800

Post by TechTony » Tue Nov 01, 2016 3:00 pm

I don't guess I can post the image online from my phone, but I've attached it. I'll post more angles of it this evening but here's the one image I have available currently.
Attachments
20161031_194159.jpg

Torch
Posts: 1543
Joined: Wed Mar 17, 2010 7:58 am
Location: Muskoka

Re: Thread Gearing on a BT800

Post by Torch » Tue Nov 01, 2016 5:31 pm

<lol> There's the problem all right. You see the 1/4 round cutout in the steel block? That needs to be positioned around the leadscrew so the gear can engage. Undo that one bolt, position things correctly and reinstall the bolt. Rotate it slightly around the bolt to fine tune the gear lash.

TechTony
Posts: 54
Joined: Wed Aug 31, 2016 7:11 pm

Re: Thread Gearing on a BT800

Post by TechTony » Tue Nov 01, 2016 6:37 pm

You were right - again. I loosened the bolt, repositioned the dial, cranked it up - it now engages with the lead screw, as expected, but the dial doesn't spin like I would expect it to. Tried loosening and tightening the screw in the middle of the dial and got nowhere. I do see the threads of the lead screw turning the gear on the dial block and the threads/teeth are fully seated.
20161101_182717.jpg

TechTony
Posts: 54
Joined: Wed Aug 31, 2016 7:11 pm

Re: Thread Gearing on a BT800

Post by TechTony » Tue Nov 01, 2016 7:11 pm

Took the block apart, found the fit to be a bit tight. Reamed it, greased it, and reassembled. All is working well now. Thanks so much for the assist.

Post Reply